Mechthild Donner
email: mechthild.donner@inrae.fr

| Hugo De Vries

How to innovate business models for a circular bio-economy?

Keywords: economy, business models, circular economy, co-creation, innovation, sustainability

Shifting from a linear to a circular bio-economy requires new business models. The objective was getting insights into the uncharted research field of business model innovation for a circular and sustainable bio-economy within the agrifood sector.

Eight European cases valorising agricultural waste and by-products by closing loops or cascading were studied regarding their innovation drivers and elements, via interviews, on-site visits and secondary data. In this domain, the findings highlight that business model innovations are depending on the (i) macro-environmental institutional-legal conditions and market trends, (ii) driven by internal economic, environmental and/or social objectives, but especially strongly linked to (iii) other actors often from different sectors seeking synergies and (iv) value co-creation via combined organisational and technological innovations. Business models for a circular bio-economy thus depend on various action levels and need radical combined organisational and technological innovations for a most efficient usage of agricultural waste and by-products. This also means new business configurations instead of linear innovation strategies currently still being dominant due to economic viability.

| INTRODUCTION

In a context of limited natural resources, climate change and growing population, circular economy has become a popular concept for sustainable development and of increasing interest for policy makers, companies and the civil society. It aims to shift from the current linear 'take-make-dispose' model to closing loops by recycling and reusing products, components and materials, and by reducing waste to a minimum (Ellen MacArthur Foundation [EMF], 2013; [START_REF] Murray | The circular economy: An interdisciplinary exploration of the concept and application in a global context[END_REF]. It is thus considered as a new and alternative way to reconcile economic growth with the use of natural resources and to develop sustainable economic systems, but with a narrower focus on economic and environmental dimensions [START_REF] Geissdoerfer | The circular economy-a new sustainability paradigm[END_REF]. The implementation of circular economy is still in an early stage but driven worldwide by diverse policies, concerning macro, meso and micro action levels [START_REF] Ghisellini | A review on circular economy: The expected transition to a balanced interplay of environmental and economic systems[END_REF]. As an emerging concept [START_REF] Velenturf | Circular economy and the matter of integrated resources[END_REF], there is not yet a common definition of circular economy [START_REF] Kirchherr | Conceptualizing the circular economy: An analysis of 114 definitions[END_REF][START_REF] Korhonen | Circular economy: The concept and its limitations[END_REF], it is rather used as an 'umbrella term' [START_REF] Homrich | The circular economy umbrella: Trends and gaps on integrating pathways[END_REF]. The Ellen MacArthur Foundation conceptualises and defines circular economy as 'an industrial system that is restorative or regenerative by intention and design' (EMF, 2015).

In the past years, the importance of circular economy for the agrifood sector has been highlighted [START_REF] Barros | Mapping of research lines on circular economy practices in agriculture: From waste to energy[END_REF]EMF, 2015;[START_REF] Esposito | Towards circular economy in the Agri-food sector. A systematic literature review[END_REF]FoodDrinkEurope, 2020;[START_REF] Jurgilevich | Transition towards circular economy in the food system[END_REF].

In Europe alone, around 90 million tonnes of food and 700 million tonnes of crops are wasted each year (http://agrimax-project.eu/), and the worldwide food production and supply chains consume about 30% of the global energy production (Food and Agricultural Organisation of the United Nations, 2015). Agricultural waste and by-products are usually defined as plant or animal residues that are produced during various agricultural operations and that are not or not further processed into food or feed (Organisation for Economic Co-operation and Development, 1997). Waste streams are produced in all parts of the agrifood system or chains, mainly due to over-dimensioned, spatially or temporarily in-adapted processes and handling schemes.

However, agricultural waste and by-products can be converted into valuable resources, resulting in new value-added bio-based products such as bio-energy, bio-fertilisers, bio-materials or bio-molecules [START_REF] Venkata Mohan | Waste biorefinery models towards sustainable circular bio-economy: Critical review and future perspectives[END_REF]. Creating value from or valorising agricultural waste and by-products is challenging due to the heterogeneity and perishability of agri-resources, and the seasonal and territorial fluctuations in volumes and quality [START_REF] Donner | A new circular business model typology for creating value from agro-waste[END_REF]; this even more holds for urban food waste. Moreover, different valorisation opportunities exist in alternative sectors, with differences in value, as shown in the value pyramid for biomass valorisation [START_REF] Rood | Food for the circular economy[END_REF]. Cascading approaches, that is, a multiple and diversified use of waste streams through consecutive production processes [START_REF] Ghisellini | A review on circular economy: The expected transition to a balanced interplay of environmental and economic systems[END_REF] via, for example, biorefineries require then careful consideration [START_REF] Donner | A new circular business model typology for creating value from agro-waste[END_REF]. Research on circular bio-economy approaches for valorising agricultural waste for new products is relevant but still presenting important gaps, including new circular business models, as highlighted in the recent literature review by [START_REF] Barros | Mapping of research lines on circular economy practices in agriculture: From waste to energy[END_REF].

A shift from a linear to a circular and sustainable bio-economy requires a radical change at a system level (Science Advice for Policy by European Academies, 2020). Circular economy is thus strongly linked to innovation, as important policy measures and socioeconomic changes, including new technologies and products, are needed for a transition. On the other hand, circular economy can also be a driver for eco-, responsible or sustainable innovation in production and consumption (Boons & McMeekin, 2019). Companies need to rethink and redefine how they understand and do business and how they generate and offer value to customers [START_REF] Centobelli | Designing business models in circular economy: A systematic literature review and research agenda[END_REF][START_REF] Pieroni | Business model innovation for circular economy and sustainability: A review of approaches[END_REF]. Hence, business model innovations are needed that offer new products or services and/or new market opportunities (EMF, 2013); especially between businesses, new interactions emerge [START_REF] Stewart | Circular economy in corporate sustainability strategies: A review of corporate sustainability reports in the fastmoving consumer goods sector[END_REF]. Therefore, business managers and researchers increasingly explore how new, sustainable and circular business models can create economic growth while reducing negative effects on the natural environment and society (e.g. [START_REF] Bocken | A review and evaluation of circular business model innovation tools[END_REF][START_REF] Boons | Business models for sustainable innovation: State-of-the-art and steps towards a research agenda[END_REF][START_REF] De Giacomo | Business models for environmental sustainability: Contemporary shortcomings and some perspectives[END_REF][START_REF] Schaltegger | Business models for sustainability: Origins, present research, and future avenues[END_REF][START_REF] Stubbs | Conceptualizing a "sustainability business model[END_REF].

| LITERATURE REVIEW AND THEORETICAL FRAMEWORK

| The business model concept

Despite an increasing research interest in business models and business model innovations, a sound theoretical foundation is still missing [START_REF] Pucihar | Drivers and outcomes of business model innovation-Micro, small and medium-sized enterprises perspective[END_REF]. In general, a business model describes how a firm does business, that is, the activities of a firm, the way it operates and how it creates value for its stakeholders [START_REF] Casadesus-Masanell | From strategy to business models and onto tactics[END_REF][START_REF] Magretta | Why business models matter[END_REF][START_REF] Teece | Business models, business strategy and innovation[END_REF]. The notion of value is central to a business model and has been broadened from economic value to environmental and social value within the emerging literature on new and sustainable business models (e.g. [START_REF] Birkin | A new business model for sustainable development: An exploratory study using the theory of constraints in Nordic organizations[END_REF][START_REF] Schaltegger | Business cases for sustainability: The role of business model innovation for corporate sustainability[END_REF], hence including the three pillars of sustainability (people, plant and profit;[START_REF] Elkington | Cannibals with forks: The triple bottom line of 21st century business[END_REF]. The concept of circular business models is more recent and builds on management literature and on circular strategies from the resource efficiency field [START_REF] Nußholz | Circular business models: Defining a concept and framing an emerging research field[END_REF]. It thus integrates principles and practices from the circular economy with the goal to achieve a more resource effective and efficient economic system (EMF, 2015). Accordingly, Mentink (2014, p. 35) defines a circular business model as 'the rationale of how an organization creates, delivers and captures value with and within closed material loops'. Linder and Williander (2017, p. 2) define it as 'a business model in which the conceptual logic for value creation is based on utilizing the economic value retained in products after use in the production of new offerings', thus highlighting the return flow from the user to the producer. Sustainable and circular business models are closely related and can be considered as a sub-category of business models [START_REF] Antikainen | A framework for sustainable circular business model innovation[END_REF].

However, circular business models offer new perspectives by developing strategies to close, slow, intensify, de-materialise or narrow resource loops (Bocken, De Pauw, Bakker, & Van Der Grinten, 2016; Ünal, Urbinati, Chiaroni, & Manzini, 2019).

| Business model innovation

In contrast to business models, the concept of business model innovation (BMI) is less well understood, perhaps because its literature is more recent [START_REF] Foss | Fifteen years of research on business model innovation: How far have we come, and where should we go[END_REF]. There is general agreement that business model innovation (BMI) is continuously needed due to market liberalisation, increased competition and changing socioeconomic conditions. Nowadays, products can easily be copied and local markets overtaken by international competitors [START_REF] Taran | A business model innovation typology[END_REF]. BMI can thus increase the resilience of firms to changes in its environment and become a source of a firms' competitive advantage [START_REF] Mitchell | The ultimate competitive advantage of continuing business model innovation[END_REF]. Some authors go even further by considering that BMI can reshape industries, contribute to more sustainability [START_REF] Geissdoerfer | Sustainable business model innovation: A review[END_REF] and possibly even 'change the world' (Massa & Tucci, 2013, p. 438). However, BMIs are risky, as they are time-consuming and require investments, for example, in R&D, resources or equipment. Therefore, a systemic and holistic thinking to innovation is recommended, instead of an individual and isolated approach [START_REF] Amit | Creating value through business model innovation[END_REF]. BMI consists of changing a business model by creating, diversifying, acquiring or transforming it [START_REF] Pieroni | Business model innovation for circular economy and sustainability: A review of approaches[END_REF]. [START_REF] Massa | Business model innovation[END_REF] distinguish between two types of BMI. The first refers to the design of novel business models for newly created organisations (business model design), the second to the reconfiguration of an existing business model (business model reconfiguration), in which managers acquire new or reconfigure existing resources to change their business model.

Regarding the degree of innovation, one can distinguish between radical and incremental. While radical innovation describes a high degree of novelty or a breakthrough, that is, an innovation which breaks with previous structures, procedures, activities or products in a firm, an incremental innovation has a low degree of novelty, is less risky, and does not break with previous products, processes or organisation methods but significantly improves them [START_REF] Souto | Business model innovation and business concept innovation as the context of incremental innovation and radical innovation[END_REF]. Moreover, [START_REF] Bocken | A literature and practice review to develop sustainable business model archetypes[END_REF] describe eight archetypes of sustainable business models, classified in higher order groupings that show three main types of innovation: technological, social or organisational. They suppose that firms can use one or several archetypes as references for shaping their own transformation.

| Drivers of business model innovation

There are few studies on BMI drivers, which may be 'many, different in nature, placed at different levels, and be external or internal to a firm' (Foss & Saebi, 2017, p. 217). Hence, BMI can be triggered either by internal (technology-push or inside-out) or external (market-pull or outside-in) opportunities or threats [START_REF] Bucherer | Towards systematic business model innovation: Lessons from product innovation management[END_REF]. Innovations due to changes in the external environment have been described in earlier studies, such as changing demands of stakeholders (e.g. [START_REF] Ferreira | The transition from products to solutions: External business model fit and dynamics[END_REF], changes in the competitive environment (e.g., De Reuver, [START_REF] De Reuver | Business models dynamics for start-ups and innovating e-businesses[END_REF], opportunities brought about by new, for example, information and communication technologies (e.g. [START_REF] Pateli | Technology innovation-induced business model change: A contingency approach[END_REF], or other general trends such as consumer awareness, circular economy or CSR (corporate social responsibility) [START_REF] Todeschini | Innovative and sustainable business models in the fashion industry: Entrepreneurial drivers, opportunities, and challenges[END_REF]. The ability to innovate a business model in response to major external environment changes has been characterised as 'dynamic capability' [START_REF] Teece | Business models and dynamic capabilities[END_REF][START_REF] Zott | The business model: Recent developments and future research[END_REF]. [START_REF] Bocken | Barriers and drivers to sustainable business model innovation: Organization design and dynamic capabilities[END_REF] have identified internal drivers for sustainable BMI within large multinational corporations at three levels: institutional (e.g. norms and believes that affect organisational behaviour), strategic (e.g. collaborative innovation) and operational (e.g. people capability development). Finally, [START_REF] Schaltegger | Business cases for sustainability: The role of business model innovation for corporate sustainability[END_REF] highlight the link between a firms' overall strategy and its BMI for sustainability: (c) proactive strategies lead to business model redesign and fully integrate sustainability issues in their products and processes.

| Circular business model innovation

Circular business models are in general driven by the objective to reconcile commercial value creation with resource efficiency strategies [START_REF] Nußholz | Circular business models: Defining a concept and framing an emerging research field[END_REF]. However, circular business model (innovation) is a recent field of research [START_REF] Bocken | A review and evaluation of circular business model innovation tools[END_REF][START_REF] Ferasso | Circular economy business models: The state of research and avenues ahead[END_REF][START_REF] Lopez | Business model innovation for resource-efficiency, circularity and cleaner production: What 143 cases tell us[END_REF][START_REF] Lüdeke-Freund | A review and typology of circular economy business model patterns[END_REF][START_REF] Pieroni | Business model innovation for circular economy and sustainability: A review of approaches[END_REF] and especially in the context of agriculture, still reveals a gap in literature [START_REF] Barros | Mapping of research lines on circular economy practices in agriculture: From waste to energy[END_REF]. Circular BMI depends on the larger business ecosystem and a broad range of actors and stakeholders [START_REF] Antikainen | A framework for sustainable circular business model innovation[END_REF][START_REF] Brown | An economic model of circular supply network dynamics: Toward an understanding of performance measurement in the context of multiple stakeholders[END_REF][START_REF] Evans | Business model innovation for sustainability: Towards a unified perspective for creation of sustainable business models[END_REF][START_REF] Lüdeke-Freund | Sustainable entrepreneurship, innovation, and business models: Integrative framework propositions for future research[END_REF]: 'Circular business model innovations are by nature networked: they require collaboration, communication, and coordination within complex networks of interdependent but independent actors/stakeholders'. (Antikainen & Valkokari, 2016, p. 7). Therefore, an analysis of circular BMI should be linked to an innovation system perspective. [START_REF] Boons | Business models for sustainable innovation: State-of-the-art and steps towards a research agenda[END_REF], by reviewing literature on sustainable innovation, have identified three levels of analysis at the system boundaries: (i) an organisational, focused on individual firms and their own value adding activities; (ii) an inter-organisational, concerning the interrelationship with other actors that co-create and share values; and (iii) a societal, considering the wider landscape for transition and interrelationship with other organisations to produce a shared societal value. However, research on circular economy has mostly been limited to one action level [START_REF] Barreiro-Gen | How circular is the circular economy? Analysing the implementation of circular economy in organisations[END_REF]. Furthermore, [START_REF] Antikainen | A framework for sustainable circular business model innovation[END_REF] have offered a framework for sustainable circular business model innovation. In this framework, they integrate-apart from the business level, which consists of the nine business model canvas building blocks proposed by [START_REF] Osterwalder | Business model generation: A handbook for visionaries, game changers, and challengers[END_REF]-the 'business ecosystem level' (referring to trends and drivers and to a stakeholder involvement having a direct impact on the business model) and the 'sustainability impact' (environmental, social and business requirements and benefits). The idea is to come to a continuous sustainability and circularity evaluation of the business model innovation, in order to optimise the processes.

| Towards a theoretical framework

The considerations above about business models and their innovations are summarised in a theoretical framework (Figure 1) that is used as basis for analysing and discussing our results. It consists of two levels, the business level itself and the wider context, that is, the business eco-system level. At the business level, one recognises the internal drivers for innovation [START_REF] Bocken | Barriers and drivers to sustainable business model innovation: Organization design and dynamic capabilities[END_REF][START_REF] Foss | Fifteen years of research on business model innovation: How far have we come, and where should we go[END_REF] and the main features of the circular business model innovation. The latter refer to the (i) business model itself [START_REF] Massa | Business model innovation[END_REF], (ii) business model or canvas model elements [START_REF] Osterwalder | Business model generation: A handbook for visionaries, game changers, and challengers[END_REF], (iii) innovation types [START_REF] Bocken | A literature and practice review to develop sustainable business model archetypes[END_REF], (iv) degree of innovation [START_REF] Souto | Business model innovation and business concept innovation as the context of incremental innovation and radical innovation[END_REF] and (v) innovation strategy [START_REF] Schaltegger | Business cases for sustainability: The role of business model innovation for corporate sustainability[END_REF]. At the business eco-system level, the external drivers are to be considered; if adapted to the specific bio-economy context, the Reseda (2017) methodology for analysing by-product valorisation pathways is most relevant. Also, the stakeholder involvement is crucial as revealed in the sustainable circular BMI framework by [START_REF] Antikainen | A framework for sustainable circular business model innovation[END_REF].

| METHODOLOGY

Our study was realised within the European H2020 project NoAW (No Agricultural Waste), driven by a 'near zero-waste' society requirement and aiming to develop innovative approaches for the conversion of increasing volumes of agricultural waste and by-products into eco-efficient bio-based products. In particular, co-products from wine, cereals and manure were addressed. Although the focus of the project was on technological development aspects, one working package was dedicated to the challenge of how to design new business and marketing strategies for a cross-sectoral valorisation of agricultural waste and by-products.

For this socio-economic working package, a qualitative research approach was defined, which is the dominant methodology for analysing business models so far, to explore them as current phenomena in their given contexts, as well as the antecedents and consequences of their configurations [START_REF] Ehret | Business models: Impact on business markets and opportunities for marketing research[END_REF]. The case study method was chosen, as 'an empirical enquiry that investigates a contemporary phenomenon in depth and within its real-life context, especially when the boundaries between phenomenon and context are not clearly evident' (Yin, 2009, p. 18). This method is particularly appropriate for developing a new theory and answering questions of why, what and how, and it allows better understanding of the nature and complexity of a phenomenon [START_REF] Voss | Case research in operations management[END_REF].

It is also suited to generate relevant knowledge for managers [START_REF] De Massis | The case study method in family business research: Guidelines for qualitative scholarship[END_REF]. In business model research, a scarcity of case studies has been stated, making it 'challenging for firms to understand how to innovate their business models, identify and design alternatives, then assess and select the most adequate one' (Evans et al., 2017, p. 598).

Eight cases were studied in 2016-2019, four from France, two from Germany, one from Italy and one from the Netherlands. Multiple case study augments the external validity [START_REF] Voss | Case research in operations management[END_REF]. The cases were selected for the following reasons. First, because they offered sufficient elements for an analysis of their innovation. The selected cases were also representative as they consisted of different types of business models [START_REF] Donner | A new circular business model typology for creating value from agro-waste[END_REF], were centred on the three different focal product chains of the research project (wine, cereals and manure), and their agricultural waste and by-product valorisation relied on different technologies with different product outputs: either via a simple closing loop (e.g. bioenergy production 1 )

F I G U R E 1 Theoretical framework 1
Mostly via anaerobic digestion, that is, processes by which microorganisms break down biodegradable material in the absence of oxygen, used to produce bioenergy.

or a cascading (diversified used of waste streams) approach implying many actors. Another reason to investigate these business cases was the relative availability and openness of the managers to share more detailed information and organise site visits. As the cases were from project-partner countries, partners could provide additional information for the cases and their contexts. Nonetheless, we had to limit the number of interviewees and interviews considering the realities of (especially small and medium-sized) enterprises, characterised by limited time and resources that can be allocated for external activities (Ünal et al., 2019).

The data collection was done through field and desk research.

The field research involved an on-site visit for each case and at least one semi-structured interview with one or several members of the company, with different positions, for example, CEO, R&D or marketing managers (Table 1). The interviews had an approximate duration of 2 h. Key informants were also regularly contacted by e-mail for further clarifications. The interview guide included questions about the macro-environmental political-legal (policies, laws and regulations), economic (markets and subsidies), social, technological and environmental conditions, as well as micro-level business aspects such as historical (origin, triggers and development of the initiative), organisational (governance, partnerships and logistics), technological (type and maturity of technologies used, examples of by-product valorisation and outputs), marketing and financial (investments and cost-benefit structure) elements. The site visits focused on the presentation and explanations of the technologies, the observations of the different agricultural waste and by-products used for valorisation (from animals or crops) and the logistics (transport, storage and weight). These visits had durations between 1 and 4 h and were mainly guided by the interviewed persons, sometimes with support of technical experts. The data from these field visits, mostly in form of photos or notes, were discussed after the visits by the authors and provided additional insights especially regarding the amounts of waste valorised, the innovativeness and technology-readiness-levels (from pilot-scale to full implementation). The primary data were triangulated with secondary data from desk research, that is, academic articles if available, online articles, videos and internet sites of the companies, and internal documents such as reports or presentations directly received from the companies.

Interview and secondary data were transcribed and analysed according to the content method, which relies on an analytical interpretation of data and is used to make 'replicable and valid inferences from texts' (Krippendorff, 2004, p. 18;[START_REF] Berg | An introduction to content analysis[END_REF]. A hand-coding was done for each case (within-case analysis). This was done by the leading author and then verified and discussed with the other author.

In a first step, general features assigned to the businesses were studied. This was done to provide first insights and illustrate major issues for each case regarding its main actor(s), resources and transformation processes, value proposition, key partners, customers and strategic approach (Table 2). Next, data were analysed regarding the more specific innovation elements as main focus of this work. For processing this coding, the theoretical framework (Figure 1) was used as basis.

T A B L E 1
Here, we first identified the main trends and drivers at a macro-level and stakeholders involved. Second, the contents regarding the origin, evolution and key milestones of the businesses over time, and in particular their main internal drivers to start or further develop their business, were identified. Third, the innovation of the business model itself and its nine buildings blocks [START_REF] Osterwalder | Business model generation: A handbook for visionaries, game changers, and challengers[END_REF] 4).

| RESULTS

This section starts with a more general presentation of the characteristics of the eight cases studied, as described in the methodology, including their main actor(s), resources and transformation processes, value proposition, key partners, customers and strategic approach (Table 2). This provides a first introduction to each case. Next, following the theoretical framework, in section 4.1, results about the internal drivers for innovation and innovations at the business model level are given (Table 3). In section 4.2, insights from the business ecosystem level including stakeholders involved and external trends and drivers are shown.

| Internal drivers and innovations at the business model level for the eight cases

In Table 3, T A B L E 2 Presentation of the characteristics of 8 business cases valorising agricultural waste and by-products Results from the business ecosystem level analysis including general trends and drivers are described at a political, legal, economic, social, technological and environmental level.

The political level in terms of public national bio-or circular economy strategies and policies plays a crucial role for developing and maintaining agricultural waste valorisation activities. In particular, biogas production is strongly depending on incentives and subsidies, for example, in form of feed-in tariffs (Germany, e.g. Case 1)-being limited to 20 years and constantly decreasing, or co-financing for the construction of biogas plants (France, Case 8) or moving from subsidies for agricultural waste to urban waste (Italy, topic in discussion, e.g. Case 2). Closely related to the political level are legal conditions that define the rules and boundary conditions for agricultural waste recovery, management and valorisation pathways. For example, the removal of the obligation to deliver wine by-products to a distillery in France in 2014 has an impact on the waste quantities to be collected (e.g. Case 4), or in Italy, it is legally not allowed to mix agricultural waste with food waste. In Germany, the Renewable Energy Sources

Act in 2012 has impacted the biogas production, as the use of maize as feedstock for anaerobic digestion has been limited to 60%. At an economic level, the demand and general performance of markets for Figure 2 summarises the main findings: external trends and drivers, stakeholders involved, internal drivers for innovation and innovations at the business model level.

| DISCUSSION AND IMPLICATIONS

Here, the results and their conceptual and management implications are discussed while coming back to the three questions from the introduction.

(i) With regard to the first question 'What types of circular business model innovations exist in the agricultural sector?', results

indicate that business model innovation for a circular economy in this sector depends on various case-specific drivers (Figure 2), wherein the business ecosystem plays a crucial role, as earlier highlighted by [START_REF] Antikainen | A framework for sustainable circular business model innovation[END_REF]. These drivers lead to different types Our findings indicate that new circular business models, corresponding to sustainable and circular bio-economy strategies, are changing the representation of the three circles (Figure 4). First, Figure 4 shows the new conceptual framework for circular business model innovations connected to bio-technological innovations in the agrifood sector, in which the co-creation zone is emerging. The main findings (cf. Figure 2) are here integrated.

results
The proposed new circular business model conceptual framework reflects well the results of our case studies on circular business model innovation in the agricultural sector, in particular for an efficient usage of new resources. It helps in clarifying insights from former studies on circular economy and sustainable business model innovation, highlighting the increased importance of the wider ecosystem and the more complex interactions between the macro-and micro-level [START_REF] Antikainen | A framework for sustainable circular business model innovation[END_REF], but also the meso-level of cooperation and partnerships (e.g. [START_REF] Boons | Business models for sustainable innovation: State-of-the-art and steps towards a research agenda[END_REF]. On the other hand, it becomes evident that the implementation of circular bioeconomy at the business model level is often still immature, although various businesses show the need and/or willingness to jointly adapt and innovate in order to switch the linear to a circular logic. The described cases are relatively recent evolutions; hence, circular BMI can indeed be considered as recent field of research [START_REF] Ferasso | Circular economy business models: The state of research and avenues ahead[END_REF].

(iii) Our third question 'What would be needed for more radical innovations in the agrifood domain leading to more circularity?' is also 3 and 4). This also asks for social and organisational innovations-as well as product and technology innovations-that are redesigned such that applicability could be maintained and handled after first, second, third and so forth usages; even suppliers become clients and vice versa, hence the underlined interactivity (double arrow) between these in Figure 4. This is fully in line with the circular economy defined by EMF (2015) as 'an industrial system that is restorative or regenerative by intention and design'. New organisational networks and technology innovations are needed for large-scale re-usage of energy and water, however, need to go together with logistic innovations (Cases 3 and 5). The social innovation dimension is less visible but may require involvement of citizens at local level. We summarise the main management recommendations and potential implications in the following Table 4.

| CONCLUSION

In this article, eight different European business cases, contributing to the transition to a circular and sustainable bio-economy via agricultural waste and by-product valorisation, have been studied regarding their drivers and elements of business model innovation.

Insights from the case studies highlight that business model innovations for a circular bio-economy in the agrifood sector are depending on various drivers and elements. They have to appreciate the (i) macro-environmental institutional and legal conditions and market trends, (ii) are driven by economic, environmental or social objectives or a combination of those, but are especially strongly linked to (iii) other actors often from different sectors seeking synergies and

(iv) value co-creation via combined organisational and technological innovations (Figure 4). These links between different action levels [START_REF] Barreiro-Gen | How circular is the circular economy? Analysing the implementation of circular economy in organisations[END_REF]) and between business model and technological innovations [START_REF] Ferasso | Circular economy business models: The state of research and avenues ahead[END_REF] have not been clearly

shown before in literature on circular economy business models, possibly due to the new area of research [START_REF] Pieroni | Business model innovation for circular economy and sustainability: A review of approaches[END_REF].

Our results also indicate that business models in the agrifood

  Our research objective is to get insights into the drivers and elements of business model innovation within the agrifood sector and contributing to the circular bio-economy transition via agricultural waste and by-product valorisation. Eight European circular business cases are studied that convert agricultural waste and byproducts into value-added products, with particular attention to their business model and technological innovations. The underlying questions are: (i) What types of circular business model innovations exist in the agricultural sector? (ii) Is agribusiness model innovation for a circular bio-economy different from the linear economy? And (iii) What would be needed for more radical innovations in the agrifood domain leading to more circularity? With our study, we contribute to the recent discussion on business models for a circular bio-economy and fill the specific gap in the agricultural sector. We also propose a new conceptual framework for circular business model innovation connected to bio-technological innovations in the agrifood sector, which shows the complexity and interactions at different action levels.

  (a) defensive strategies with slight degrees of business model adjustment or adoption might be motivated only by a need to comply with legislation and protect the current business model, (b) accommodative strategies go along with a change and improvement of the business model by addressing environmental and/or social objectives;

  the internal drivers and elements of innovations at the business model level are presented for each case. A difference is made between innovation of the business model itself, either as a new creation or reconfiguration of an existing model, and of innovative business model elements, for example, novel products, technologies or forms of organisation. Each case is then classified according to its main innovation type(s), degree and innovation strategy.Case 1, initially a farmer, was driven by the question of how to create economic value out of agricultural waste, in the early 2000s when the biogas boom started in Germany. In 2004, when the biogas plant was already in operation for years and enlarged, he noticed that the area to spread digestate 2 and nutrients would become a limiting factor. Therefore, he decided to dry digestate in a new special largescale production facility and to produce and market a pellet fertiliser himself. This allowed him to divert nutrients from agriculture into other sectors like viti-or horticulture and to expand his business; this required new scale-adapted supplies, distribution and market outlets with stakeholders. Apart from this, he delivers the (often lost) heat from his biogas plant to the nearby (eco-)village via appropriate, new infrastructure, thanks to a joint initiative with the local community. Also, he is building a new partnership with a local e-car sharing company for utilising locally produced electricity.Case 2 was created in 2013 as a university spin-off in order to enable and facilitate a market uptake of new technologies for agricultural by-product valorisation. It is actually fully operational after the creation of a first spin-off that had to stop after one year, mainly for financial reasons. The spin-off is well embedded in a large cooperative providing resources, housing and local assistance, while receiving advice on biogas production and potential new biotechnological ways to valorise their co-products; this allows mutually benefiting from know-how and new opportunities. The university is now intended to fully spin-out the company. The company attracts new employees and continues to operate within the cooperative however also as an international consultancy firm in the agricultural sector. The key technological innovation is the combined biogas and PHA production-a biodegradable polyester which potentially may, for example, replace fossil-based plastics-via a new biotechnological concept exploited at pilot-scale.Case 3 was based on the willingness of local cooperatives to diversify their activities in a synergistic way and to define a joint local sustainable bio-economy development strategy. The aim of this process of clustering was to create synergies by common technological investments, to enter new markets via cereal and sugar cane byproduct valorisation activities, to close energy and material cycles and increase farmers' revenues. The biorefinery can be characterised by its high technological and organisational innovation capability. Its concept is adopted by the regional government as a unique and attractive example internationally, and fuels teaching and research activities, even leading to the creation of an industrial chair on bio-economy, hence providing scientific feedback to the case.Case 4 was created in 1969 in order to ensure the existence of the wine cooperatives in the region. In 1970, a new law obliged the winemakers to deliver their waste from the production for distillation.The valorisation of grape marc for distillation in one larger unit was thus a collective response to this legal obligation. Since 1994, the business started a diversification of new products and ingredients towards new markets, the food and petfood industry and nutraceutical companies, hence evolving into a small 'biorefinery' with the inclusion of novel bio-and extraction technologies. In 2007, a geographical expansion was decided upon in order to reach critical mass of waste, 2 Digestate is the remaining part of organic matter treated by anaerobic digestion, rich in nutrients and nitrogen, and therefore an excellent organic fertiliser. But the management of quantities is regulated because a surplus of nitrogen and phosphate damage the environment.

  and demand chains substantially. This case serves as example for national reports on valorising by-products from wineries, receives support from the region and provides input to various teaching courses, overall making it a private-public partnership focused case.In Case 5, the key trigger in 2002 was to cluster local companies that could benefit from joint waste management, natural resources use and logistics. The objective was to create synergies between agrifood-related companies on a large-scale business park. At the beginning, the focus was on logistics. During the planning process, efficient usage of energy and water became more and more important. Today, cross-sector cooperation and interaction exists between big data-centres that use the electricity produced by greenhouses and deliver their heat for greenhouses. The park is even organising tourist excursion for promotional and educational reasons. The case has shown to transform barren land into a fully operational business park with only private means, hence serves internationally as an exemplary case within the presentations about agriparks.In Case 6, three people met with sought complementary skills and with a common objective to implement a circular economy with new technological developments around an existing biogas plant, in order to keep it profitable. The entrepreneurs were thus discussing about closing loops, circular economy, individual and common companies' demands and supplies, and how to create and use synergies. The joint business was founded in 2016 with the ambition to attract investors and partners like the public utility for electricity and for the drying of herbs via locally produced heat. The ambition was to create 'modules' to be marketed such as algae and humus mainly via a technology development hub, followed by the production of fruit, vegetables and fish locally in a CO 2 neutral manner by the use of biological nutrient cycles. The aim in Case 7 was to bring people from different sectors together and inform them about value-adding valorisation opportunities for cereal by-products. In 2015, the actual joint association was created collecting by-products of various cereals for different applications of hulled grain such as rice, spelt, buckwheat and barley in a new sector, in particular the building sector. This association has become a catalyst attracting many individual actors because it has developed quality schemes, a portfolio of technical characteristics, treatments and construction procedures, and networks of actors; in addition, the know-how is continuously updated thanks to cooperation with nearby research centres. It is thus working as an intermediate between two usually separate sectors, namely, agriculture and construction. Case 8 is the only public initiative with the objective to cluster different public and private actors for sustainable development and circular economy practices in a rural area. It was initially based on a local cluster of small organic food enterprises. With the increasing demands for sustainability and circular economy, including the valorisation of urban food waste and agricultural by-products, the cluster was reoriented and expanded. It now covers also biogas production and waste treatment, benefiting from financial support of local public, commercial and environmental bodies. The initiative is original in the sense that it brings various actors with different orientations together under the circular economy header.From these cases, it can be concluded that circular BMI can be triggered by diverse case-specific reasons, either by economic, environmental and/or social objectives or a combination of those, or because of legal obligations. But all cases have in common that they F I G U R E 2 Summary of the main findings aim to convert agricultural waste into value added products (bio-materials, food, fertilisers, feed and bio-energy) and to efficiently use all resources from the agricultural production (by-products, water and energy). Very often, this is done by bringing together complementary knowledge, resources, novel (bio-)technological know-how and in particular also actors. Innovative cross-sector and/or public-private cooperation as well as local clustering of actors have been evolving allowing jointly co-creating circular economy concepts.4.2 |The business ecosystem: An overall view on stakeholders involved and external driversRegarding the types of stakeholders directly involved or concerned by initiatives turning agricultural waste into value, the case studies show that their variety and scope is rather broad. They encompass direct local actors such as farmers that deliver agricultural residues or buy bio-fertilisers, associated processing companies, various types of suppliers, grid operators, consumers, wholesalers, members of an association or cooperative, public actors, local residents, employees, financial partners or investors and research institutes. External stakeholders are in several cases (Cases 2, 3, 4 and 5) also international public and private interest groups willing to learn via cooperation or consulting from the European businesses. Challenges (regarding stakeholders) concern above all awareness and education campaigns, particularly towards local residents often being against biogas or biorefinery installations due to odour and noise emissions or fear of health risks (Cases 1, 3, 4 and 6). In order to convince and/or involve suppliers, members, wholesalers, consumers and residents, reliability, product quality and a good and transparent communication are expected and important. Also, local benefits and participatory actions with residents may overcome these barriers.

  the existing or potentially entering materials, products or services, competitiveness in terms of quantities and prices, long-term investments in technological facilities (e.g. Case 3) or an agri-logistic network (Case 5) are determining conditions. Here, actors in all countries indicate that is it not only necessary to analyse existing market trends but also estimate future developments (e.g. market opportunities, environmental restrictions and social perceptions). The social level concerns, for example, the implication of the civil society and awareness raising for bio-based products and services, something which is only in its infancy stage. First examples are the (bio-)energy villages (Case 1) and the co-construction of houses with biomaterials (Case 7, France). In contrast, technological conditions for agricultural waste conversion into biogas are in general rather well developed and mature technologies are available on the markets, but continuous improvement and learning by users (e.g. for biogas installations) is needed (e.g. Cases 1, 6 and 8). The technological conditions for agricultural waste conversion in a cascading manner for multiple and diversified end-products are less well developed; at laboratory or pilot scale, first potential technological concepts are elaborated (Cases 2, 3, 4 and 6). However, mature technologies at appropriate scales and targeted at a wide range of functional properties of products are still to be researched and explored(authors, 2020). At the environmental level, agricultural waste and by-products partly suffer from a high sensitivity towards climate change and extreme weather conditions like elevated temperatures, periodic flooding and extended periods of dryness (e.g. wine by-products and livestock-manure). Also, soil conditions are impacting the agricultural waste conversion strategy or valorisation pathways like for organic production (Case 8).

  of innovations for adding value to agricultural waste at the business level, concerning either the overall business model or single business model elements. Table3has shown that there are two ways of innovating the overall business model. The first are entirely new start-ups in the form of limited liability companies or an association, with proactive strategies and directly focusing on agricultural waste and by-product valorisation (Cases 2, 5 and 7); they all have already either a simple network structure itself or are embedded in a network. The second are business reconfigurations and evolutions from rather classical farm or food processing activities to specialised companies and integrated business parks choosing for circular economy approaches (Cases 1, 3, 4, 6 and 8). The latter follow mainly accommodative strategies, as they change and improve the business model by addressing environmental and/or social objectives. However, Cases 3 and 6 can also partly be considered as proactive as they create new circular economy clusters. Concerning single business model elements, innovations were found in the form of new or higher value-added products, applications, materials or ingredients, all based on the principle to turn agricultural waste into new value propositions[START_REF] Perey | The place of waste: Changing business value for the circular economy[END_REF][START_REF] Zucchella | Circular business models for sustainable development: A "waste is food" restorative ecosystem[END_REF]. But there are also combined new value propositions (product, service and/or technology (platforms), via co-creating new partnerships and (public-private) cooperation with joint investments and for a better resource efficiency, new customers or distribution channels. Overall, technological often precede organisational and social innovations, but a combination is also increasingly appearing. The degrees of innovation for the cases are four times radical and three times incremental for both technological and organisational innovations, whereas there is only one radical social innovation. This is not surprising as within the domain of agricultural waste valorisation, technological developments at least for high-value adding conversion pathways are still ongoing and often not yet in mature stages or asking for scaling-up.However, also organisational innovation types are imperative to realise opportunities for agricultural waste valorisation since the complexities and diversities in resources, products, technologies and markets for individual companies are too substantial (Cases 1, 2, 3, 5, 6, 7 and 8). These then require follow-up joint technological (or even logistical and social) innovations to remain competitive in economic terms because social and environmental values are generally not compensating economic value today.(ii) These observations lead us to a new conceptual framework (Figure4) for discussing our second question: 'Is agribusiness model innovation for a circular bio-economy different from the linear economy?' This framework partly fills in the gap in circular BMI literature in the context of agriculture as mentioned by[START_REF] Barros | Mapping of research lines on circular economy practices in agriculture: From waste to energy[END_REF].A traditional linear agribusiness model can be positioned within three circles (Figure3). The first and tiniest circle is the circle of 'control' in which the individual business is able to manage and steer its business strategy and operations. The second, somewhat larger circle, is the circle of influence in which the key customers, clients and consumers are situated. It is entitled 'circle of influence' due to the mutual exchanges and adaptations of actions. The largest circle is the circle of 'appreciation' of the wider ecosystem and contextual macroenvironment; here, the wider social and economic trends and global agricultural and trade policies and laws are to be appreciated and accepted as the framework conditions for business operations.

  challenging and very much depending on local playing grounds, regulations, incentives, actors, resources, products and local-to-global markets. Even more, it asks for 'dynamic capability' to cope with major external environment changes as outlined by[START_REF] Zott | The business model: Recent developments and future research[END_REF] and[START_REF] Teece | Business models and dynamic capabilities[END_REF].For local circularity in the agrifood domain, downsizing of technologies for in-field application, capability of handling agricultural resources variability and mixed scales (in volumes and in potential market values), new entrepreneurship and interventions of intermediates, new public-private cooperation models and in some F I G U R E 4 New conceptual framework for Circular Business Model Innovation in the agrifood domain Source: own design cases connections with local or nearby communities seem important (Cases 2, 6, 7 and 8). Consequently, as compared with linear chains focusing on a straightforward main product or value proposition, here, both the organisational and the technological complexity are substantially enlarged. In order to avoid chaos and well steer business, leadership is asked next to shared ambitions and a well-organised management process for changing or developing a new business model. More diverse market-focused, flexible, eco-friendly technological innovations as compared with technologies for high throughput, high volume, single resource orientations are looked for (Cases 1, 2,

  domain are obliged to innovate themselves towards new configurations in order to close material loops and switch to a circular economy. These new configurations should incorporate both organisational and technological innovations in order to handle the increased complexity of dealing with diverse resources and sustainable needs. Radical innovations are thus needed addressing concomitantly economic, environmental and social challenges, but also to handle the complexities. Until now, radical technological and organisational innovations in the agrifood sector are still rare; they have often difficulties reaching marketable scales and becoming economically viable. Fully closing cycles for agri-resources and creating value for (new) markets via cascading pathways for all main and co-products is far from easy. Hence, business model innovation should concern innovations of the business concept itself, including both radical technological and organisational innovations in which the relations between new technology developments, business change and new ways of cooperation arise. no. 688338. The authors especially thank the colleagues from WP5 who contributed to the data collection and the enterprises for the interviews.

  Research methodology characteristics

	Case	Country	Type of business model	Data collection
	1	Germany	Small biogas plant	The biogas plant owner is a customer of a project
				partner, who is consultant within the biogas domain.
				Regular exchanges, one group face-to-face interview
				with the plant owner and one joint field visit with
				the project partner.
	2	Italy	Small upcycling entrepreneur	The firm is a project partner. Regular exchanges, one
				group interview and discussion with 3 persons, joint
				field visit.
	3	France	Large environmental biorefinery	Preliminary face-to-face and e-mail exchanges, one
				telephone and two face-to-face interviews with the
				director of the open innovation platform and field
				visit.
	4	France	Large agricultural cooperative	The firm is a project partner. Regular exchanges, four
				interviews, two with R&D and two with marketing
				manager, including field visit.
	5	Netherlands	Large agripark	The lead firm of the park is a project partner. One
				group interview with CEO and operational manager,
				regular e-mail exchanges, field visit to get a general
				overview of the park.
	6	Germany	Small agripark	Two interviews (1 telephone and 1 face-to-face), one
				field visit.

7

France Small support structure First exchange about their main activities followed by in-depth interview, a field visit and several e-mail exchanges.

  Internal drivers and elements of circular business model innovation

	Case 8	Case 8 Support Cluster public and structure private actors for	Local cluster of rural sustainable	different development and	stakeholders circular economy	Organic urban food	and agricultural Creation of a	waste handling cooperative	processes, platform for local	anaerobic sustainable and	digestion agricultural	development via	a circular	economy	approach	Biogas, network	Value proposition:	thematic	workshops and	meetings on	circular economy,	eco-conception,	waste prevention	Local authorities, and valorisation,	enterprises and industrial ecology	research Public-private	partnership				Local enterprises						Support for	networking,	niche strategy	(organic)
	Case 7	Case 7 Support Enable cross-sector structure cooperation for	Association by-product	valorisation		Cereal husks; new	separation (de-Association	hulling),quality founded to	schemes and create cross-	treatment sector synergies	procedures among local	actors for cereal	by-product	valorisation, role	of intermediator,	Cross-sector facilitator	cooperation Value proposition:	between farmers Husks valorised	and eco-for new value-	constructors added products:	eco-insulation	and decoration	(former use:	Farmers, research heating or animal	institutes, other litter)	associations Partners: cross-	sector	cooperation			Architects, house	constructors					Networking
	Case 6	Case 6 Agripark Interest in jointly developing a	Consulting and value-adding	service company circular economy	approach	Manure, energy	crops; anaerobic Development from	digestion, drying a farm with	biogas and herb	drying company	to an integrated	food-energy park				Consulting for	waste collection Value proposition:	& valorisation, technology	fertiliser, humus, development	electricity & heat hub, creating	'modules' to be	marketed (e.g.	algae and humus	Public partner projects)	(county), several Partners: public-	companies and private	research partners cooperation with	partly shared	infrastructure		Grid operator,	famers, private	customers and Technological	consumers (radical but still	pilot scale),	organisational	Technology	development	hub, networking
	Case 5	Case 5 Agripark Create multiple benefits by	Agribusiness park clustering actors	manager locally; efficient	usage of	Combined heat resources	electricity and Reconfiguration:	water from selling	recirculation ground and	systems logistics to an	innovative eco	-/agribusiness	park			Heat, electricity,	logistics Partners: Cross-	sector	cooperation for	energy and water	efficiency,	interchange	between Data-	Vegetable centres that use	producers electricity from	(greenhouses) greenhouses and	and traders; produce heat	private partners them	in water and New channel:	energy recycling tourist excursions	Data-centres using	electricity and	producing heat Organisational	for greenhouses (radical)			Networking,	economies of	scale
	Case 4	Case 4 Agricultural Collective response cooperative to a legal	Union of wine obligation	cooperatives		Waste and by-	products from Originally distillery	wine; extraction cooperative,	and distillery evolvement to a	processes specialized	enterprise for	wine waste and	by-product	valorisation		Compost,	ingredients for Value proposition:	food, pharma & Product	pet food diversification to	industries a large portfolio	going from low to	high added value	via new	Cooperatives and technologies	research						Enterprises in	different sectors,	consumers Technological	(incremental and	radical but some	at laboratory	Innovation, mixed scale)	market sectors
	Case 3 Case 3	Environmental Diversify activities biorefinery and define a	Eco-industrial common	cluster with sustainable	various firms development	Focus on cereals strategy	and sugar cane Originally sugar	by-products; full factory and	range of distillery,	agricultural, stepwise	forestry and reconfiguration	marine residues; to an agro-	different bio-industrial and	refinery biorefinery park	processes	Diverse plant by-	products, Technological	applications in innovation	the domain of platform	agriculture, food, Partners: ongoing	bio-materials, governance	chemical formalisation,	industry, energy integration of	Agrifood various members	enterprises, around different	research types of	synergies (water,	energy, products,	R&D)		A large variety of	enterprises for	different Technological	products (radical mixed	industrial and	laboratory scale),	New markets for Organisational (radical) large volumes of	by-products
	Case 2	Case 2 Upcycling Enable and entrepreneur facilitate market	University spin-off uptake of new	technologies for	waste conversion	Cow manure &	wine; anaerobic Newly created firm	digestion, new as university	extraction spin-off,	methods currently	considering to	become a	consultancy	company		Electricity,	fertilisers, PHA Value proposition:	for bio-materials, combined biogas	pilot-scale & PHA	equipment, production with a	consultancy new bio-	technological	concept,	Cooperative, two consultancy;	other shared	universities, infrastructure	research centres between farm	in EU and spin-off			Electricity	corporations,	operators of Technological	biogas plants (radical but pilot	scale),	Organisational	Innovation, (incremental)	upscaling, pilot-	scale	demonstration,	consultancy
	Case 1	Case 1 Biogas Create economic plant value out of	Farmer with a agricultural waste	biogas plant		Pig manure &	vegetables; Stepwise	anaerobic reconfiguration	digestion, new of the BM from a	drying system farm to a modern	biogas company,	created as limited	liability company			Biogas, electricity,	bio-fertiliser, Product: dried bio-	heat fertilizer via	special	production	facility	Partners: e-car	sharing company	Local farmers & (electricity),	community, technology	technology supplier, local	supplier, e-car community for	company heat	infrastructure	Customer: citizens	Public suppliers, in bio-village (heat) private	households Technological and	(electricity), organisational	wholesaler (incremental),	(fertiliser) social (radical)	Enlarge product	portfolio for	mixed market	sectors
		Internal	driver(s)	Main actor(s)			Resources and	transformation Innovation of	process BM itself								Value proposition	Innovative	BM	elements					Key partners							Customers		Main	innovation	type and	degree	Strategic	approach

T A B L E 3

  suggest that the rather simple representation of the control cir-

		cle should be adapted as many individual business models are to be
		integrally considered with complementary new key resources (agricul-
		tural waste, biotechnologies and complementary skills). Next, an addi-
		tional second circle is now emerging in which joint activities and
		synergies with partners in a cluster are taken into account, leading to
		new value propositions (all cases). This means that the 'sharing' of
		business activities is now becoming the dominant circle of a circular
		business system, still incorporating many tiny small circles rep-
		resenting the individual business 'control' units; this zone we entitle
		'co-creation zone'. The results show that co-creation is observed in
		joint, efficient handling of key resources, cascading processes for mul-
		tiple resources and products (Cases 2, 3, 4 and 6), shared logistics and
		infrastructure (Cases 2, 3, 4, 5 and 6), common value propositions and
		agreements (all cases), new cross-sector partners and networks (Cases
		2, 3, 5, 6, 7 and 8), joint (training) events, matching complementary
		knowledge and know-how (Cases 2, 3, 6, 7 and 8). The third circle-
		called 'influence'-also changes due to (i) a wider zone of influence of
		a cluster towards other multiple customers, channels and segments or
		clusters and markets as well as (ii) a joint responsibility for interac-
	firms (Cases 1 and 2), innovating their business model towards new	tively reaching solutions for a circular and sustainable bio-economy. It
	'green' products (e.g. targeted fertilisers), new markets and/or a new	is here where new value propositions connect to new opportunities,
	'intermediate' organisational type (e.g. consultancy and pilot-scale
	offerings). However, this institutional-legal driver can also be consid-
	ered as an opportunity to adapt, by heading towards a new clustered
	circular business model with organisational-technological innovations
	locally (Case 6, 8 and Case 3, at a larger scale). Environmental drivers
	can have a similar effect, leading to new collaborations and synergies
	between different local actors (Case 5). Also a 'sharing or co-creation
	by nature' business model such as a cooperative (Case 4) can be
	driven by changing environmental conditions, like periods of extreme
	droughts negatively impacting production yields, or the need to stay
	competitive and resilient in case of insufficient available local
	resources. Giving just responses to environmental and social chal-
	lenges is not given for granted and may need time to mature; eco-
	nomic robustness is not yet always reached (Cases 7 and 8). Hence,
	business model adaption and/or innovation for more circularity
	because of external changes is far from trivial due to many more
	uncertainties and options in products, markets, regulations and actors
	as compared with changing linear chains.

providing a kind of continuum between co-creation and influence. We therefore have made visible in Figure

4

that 'partners in a cluster', which are directly involved in the circular business model, and stakeholders, which are indirectly involved but still influence the business, mutually interact. This well aligns new value propositions and new, sustainable, opportunities.

Finally, the fourth and largest circle is in its overall meaning not modified. The wider political, legal, environmental and social context (the business eco-system) remains to be appreciated; however, it should be interpreted in a more sustainable way to foster both F I G U R E 3 Simplified scheme for (agri) business in a linear chain Source: own design, inspired by Shell "Scenariosan explorer's guide" (2008; www.shell.com/ scenarios) economic development and preserving natural resources. For example, the limits or uncertainties about biogas subsidies are an external driver for innovation, which seems to be appreciated (accepted) by

  T A B L E 4 Management recommendations and implications, in particular for the agrifood sector

			Potential management
	No.	Recommendations	implications
	1	Investing in and learning	Putting in place management
		from circular BMI case	processes that analyse
		studies in different	how business models
		external settings/areas,	evolve under various
		both in agrifood and other	external & internal factors,
		sectors related to the	in particular also
		wider bio-economy	considering seasonality,
		(Tables 1, 2 and 3)	heterogeneity of
			agricultural resources,
			quality decay and so forth.
	2	Exploiting case studies with	Motivating stakeholders to
		other stakeholders in the	be involved in reaching
		bio-economy, either	appropriate circular BMI in
		directly involved or	the bio-economy via a
		impacted by valorising co-	generic approach and a
		products and waste, for	common framework.
		getting insights while
		utilising a common
		framework (Figures 1 and
		2)
	3	Exploiting the conceptual	A continuous monitoring of
		framework (Figure 4) to	outcomes for guiding and
		consistently explore and	optimising next actions to
		optimise sustainable bio-	most efficiently utilise
		economy system	natural resources.
		approaches as compared
		with linear chains
		(Figure 3)
	4	Creating a European-wide or	Creating a management
		even global platform for	learning network across
		sharing best practices in	Europe for circular BMI
		circular business model	and how to avoid wasting
		innovations in the bio-	natural resources.
		economy

FranceMedium-sized support structure One group interview with four persons (director of the association, of the cluster of enterprises, of a waste treatment firm, of an international firm) and joint discussion, field visit.

ACKNOWLEDGEMENTS

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement

European Union's Horizon 2020 research and innovation programme, Grant/Award Number: 688338


Prof Sylvain Y Danto
email: sylvain.danto@u-bordeaux.fr

Dr Frédéric Désévédavy

Yannick Petit

Jean-Charles Desmoulin

Alain Abou Khalil

Prof. F Clément Strutynski

Dr Marc Dussauze

Frédéric Smektala

Thierry Cardinal

Dr Lionel Canioni

J-C Desmoulin

J-C Petit

Dr T Desmoulin

Cardinal

Smektala

Prof Y Petit

Prof A L Abou Canioni

Khalil

Photowritable Silver-Containing Phosphate Glass Ribbon Fibers

Keywords: phosphate, glasses, fiber, photosensitivity, laser inscription

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Photosensitive fibers represent a ubiquitous platform for Fiber Bragg gratings (FBGs). Their discovery paved the way for the development of a multitude of in-fiber optical components (wavelength-selective reflectors, gain-flattening filters, chirped FBGs for pulse compression and dispersion compensation, interferometers…) ultimately forming the backbone for long-haul modern telecommunication systems or for fiber-based sensors. [START_REF] Croteau | Photosensitive Fibers[END_REF] When it comes to industrial applications, silica glass vastly dominates the field of photosensitive fibers. Tuning of their photosensitivity usually relies either on pre-drawing co-doping (intrinsic photosensitivity), or on post-drawing hydrogen-loading (extrinsic photosensitivity). [START_REF] Croteau | Photosensitive Fibers[END_REF] Beside silica, other materials may be employed for the fabrication of photosensitive fibers, including polymers, fluorides or chalcogenides glasses. [START_REF] Peng | Polymer optical fiber gratings[END_REF][START_REF] Bernier | [END_REF][4] Polymers find applications too in fluorescent optical fibers for highenergy particles detection, imaging, and sensors. [5][6][7] Phosphate glasses however inherently lack photosensitivity, therefore strongly limiting their amenability for Bragg's mirrors inscription into fibers. Various photo-sensitization approaches have been developed, such as ion-exchange, [8] Germanium doping [9] or UV inscription combined with photo-thermal annealing; [10] yet critical limitations (technical complexity, sensitivity, scattering loss) persist to date. Best results were obtained with the use of IR-femtosecond laser pulses and phase-masks, enabling for FBGs inscription into rare earth doped phosphate glass fibers. [11,12] Here we report on a novel strategy for developing intrinsically photosensitive fibers, a strategy that relies on the direct drawing of tailored silver-containing zinc-phosphate glass materials.

Zinc-phosphate glasses possess good thermo-chemical durability, excellent linear optical properties and form a competing matrix for the heavy loading of homogeneously-dispersed silver ions. [13][14][15] Besides their interaction with a high-repetition-rate femtosecond laser yields the formation of locally distributed silver clusters with fluorescence and 2 nd -order nonlinear optical properties. While extensively studied in bulk form, attempts for the fabrication of luminescent silver-containing fibers glass remain scarce to date, [START_REF] Agafonova | Proc. SPIE 9141 Optical Sensing and Detection III 2014[END_REF] including to our knowledge no report on the use of phosphate glasses.

Then, going a step further in the shaping of silver-containing phosphate glasses, we introduce an innovative approach for the design of flat-substrate, sharp corner-edges fibers. Due to surface energy minimization, fiber optics' geometry has long remained narrowed down to cylindrical shapes. Yet approaches have emerged recently to overcome this limitation, which involve the drawing of stacked polymers sheets [START_REF] Chocat | [END_REF]18] or of hollow silica preforms, subsequently flattened under vacuum. [19] Our method consists in the direct, homothetic shaping of a rectangular phosphate glass preform into tens-of-meters-long ribbon fibers. Key to this breakthrough is the precise control of the viscosity-regime applied to the glass during the drawing. Building on this result, we show that ribbon fibers represent a powerful platform for the Direct Laser Writing (DLW) of silver clusters motifs with elaborated profiles within the glass matrix.

The manufacturing of intrinsically photosensitive silver-containing zinc-phosphate ribbon fibers offers several important features as fully described below. To efficiently treat our research endeavor we selected the most suitable materials for the targeted application, developed the fiberization technique and adopted innovative geometry. Raman spectroscopy, together with absorption and fluorescence spectroscopies were appropriately exploited to establish the correlation between the properties of native glass composition and the fiber properties.

Results and discussion

Cylindrical silver-containing zinc-phosphate glass fibers

The selected tailored glass compositions with their glass transition temperatures (T g ), density and refractive index are specified in Table 1. It consists of the glasses 40P 2 O 5 -55ZnO-1Ga 2 O 3 -

4Na 2 O (hereafter PZG-4N) and 40P 2 O 5 -55ZnO-1Ga 2 O 3 -2Na 2 O-2Ag 2 O (hereafter PZG-2N2A).
The substitution of Na 2 O with Ag 2 O (2% mol) aims to demonstrate the effect of the drawing on the luminescence properties of the glass. The materials were fabricated using a standard meltquench technique in a platinum crucible (see Experimental section). Thermal properties are the main indicator for evaluating the potential of the glasses for optical fiber manufacturing. The T g of the PZN-4N and PZN-2N2A glasses are T g = 382 °C and T g =385 °C respectively. Above all the absence of discernable crystallization events on the Differential Scanning Calorimetry (DSC) scans up to 700 °C attests their suitability for thermal drawing.

Fibers were drawn from a macroscopic scaled-up preform, which is elongated through localized thermal heating while being hold vertically (See Experimental section). Phosphate-based preforms are depicted on Figure 1 ((a) PZG-4N, (b) PZG-4N and PZG-2N2A). It consists of cylindrical macroscopic glass rods of 8 mm in diameter and ~7 cm in length (note: the horizontal base of the preforms, due to the shape of the brass mold, have no influence on the drawing process). Alternatively rectangular preforms (Figure 1b) were designed, as will be further discussed below. We assessed the drawing ability of both glass compositions. They were brought to their softening temperature regime while temperature, preform-holder motion and capstan velocity were continuously monitored to prevent catastrophic mechanical failure. Following this procedure, each glass was drawn into ~40-meters-long fibers with diameters ranging ϕ = 200 µm, down to 100 µm. Figure 1c depicts a bundle of PZG-2N2A fibers, as well as a bottom-neck preform and two tens-of-centimeter-long capillaries under UV excitation (254 nm). No signs of heterogeneities or inclusions were visually apparent. In order to confirm the optical transparency of the fibers, attenuation measurements were performed by the cut-back method at the wavelengths 1064 and 1550 nm on initial sections of ~1.20 meters.

Table 2 Fiber loss at λ = 1064 nm and λ = 1550 nm (measured by the cut-back method)

Glass Composition (mol%)

1064 nm

[dB.m -1 ] 1550 nm [dB.m -1 ] PZG-4N 40P 2 O 5 -55ZnO-1Ga 2 O 3 4Na 2 O - 1.8 9.7 PZG-2N2A 40P 2 O 5 -55ZnO-1Ga 2 O 3 2Na 2 O 2Ag 2 O 1.6 8.5
Loss are similar with wavelengths for both compositions (Table 2). We find 1.8 dB.m -1 and 1.6 dB.m -1 at 1064 nm, and 9.7 dB.m -1 and 8.5 dB.m -1 at 1550 nm for the glasses PZG-4N and PZG-2N2A respectively. We believe the increase in loss observed at 1550 nm relates to the first overtone of the hydroxyl absorption and to the multi-phonons cutting edge of the glass (centered on 3 µm for a sample of 0.5 mm in thickness). Loss measurement proves that the resistance toward devitrification of the selected materials effectively prevents the nucleation and growth of parasitic crystallite centers during the drawing process. Although minimum loss remain relatively high for practical applications, we must note that no purification protocol, other than the use of high-purity precursors, was introduced at this stage. Furthermore measurements were conducted on bare mono-material fiber, with no core/clad geometry or protective polymer cladding involved.

Raman spectroscopy was carried out on the PZG-2N2A bulk, preform and fiber samples in order to assess an eventual structural deviation induced by the drawing process (λ Excitation = 532 nm).

The three normalized Raman signatures precisely overlap (Figure 2a). The Raman spectra are dominated by two broad envelopes centered on 705 and 1175 cm -1 , assigned respectively to symmetric P-O-P stretching modes and to symmetric stretching of PO 2 -in PØ 2 O 2 -tetrahedral units (where Ø is a bridging oxygen) expected for a derived structure of metaphosphate. [20] Less intense peaks at 1000 and 1050 cm -1 can be attributed to symmetric and asymmetric stretching modes of PØO 3 2-units forming the end of the phosphate chains. [21,22] At lower frequencies, the large envelopes peaking at 350 and 550 cm -1 are mainly due to bending modes of phosphate units with probably weak contributions coming from gallium and zinc oxides vibrational modes.

Similarly, the fluorescence properties of the PZG-2N2A glass before and after drawing have been compared (Figure 2b). Remarkably we observe that the fluorescence emission of the bulk glass is fully preserved in the fiber sample. The PZG-2N2A glass exhibits an excitation band centered on λ = 245 nm (due to the absorption band associated with Ag + ions) and it emits intrinsic fluorescence mainly around λ = 368 nm under UV excitation. This emission has been attributed to the d 10 ↔d 9 s 1 transition of Ag + ions homogenously dispersed throughout the glassy matrix. [13] Figure The preservation of the luminescence properties of the PZG-2N2A glass is a crucial result as it indicates that neither clustering nor reduction of Ag + ions species has occurred during the drawing process, although the material is being hold above T g for approximately one hour. It ensues from maintaining the material under oxidizing conditions (O 2 atmosphere), which prevents the reduction and diffusion of the Ag + ions.

Silver-containing zinc-phosphate glass ribbon fibers

The thermo-viscous flowing ability of the PZG-2N2A glass, associated with the precise control of the drawing conditions, allows for the fabrication of intrinsically photosensitive phosphatebased fibers. Now we proceed to overtake a more sophisticated and original task, namely the direct preform-to-fiber drawing of a planar, sharp corner-edges device. To succeed a stringent viscosity-regime trade-off is imposed to the glass medium during its shaping. Drawing parameters are finely tuned to allow the material to reach its necessary lowviscous fluid state while, in the meantime, it is being maintained in a high-viscosity regime so as to slow down the kinetics of surface energy mechanisms driven during the shaping process. This operation allows preventing transverse features to evolve toward cylindrical shape.

DLW on silver-containing zinc-phosphate glass ribbon fibers

Direct femtosecond laser writing is a powerful method to implement active compounds into zinc phosphate glasses. [14,15] Herein, taking advantage of the flat, uniform geometry of the ribbon fibers, we generate silver ions clustering in photosensitive PZG-2N2A glass. A ribbon fiber was truncated and exposed to irradiation (Figure 4a). DLW was performed typically 50 µm below the surface by linearly moving the fiber sample at controlled speeds along its longitudinal axis x: ionization degree). [23] The clusters are localized at the envelope of the focusing voxel. Under exposure to UV radiations (λ = 405 nm), the Ag m x+ clusters emit homogenous fluorescence with a broad spectrum covering the visible range centered on λ = 480 nm (Figure 4b). In order to highlight the Laser-induced modifications, we have performed differential microtransmission measurements on structured zones with respect to the pristine glass as a function of the irradiation dose. Six curvilinear patterns were processed for the three irradiance values I = 5.1 TW.cm -2 , 6.1 TW.cm -2 and 6.7 TW.cm -2 and for the two sample velocities v = 10 µm.s -1 and 100 µm.s -1 (corresponding at each position to 2.0 10 6 to 2.0 10 5 deposited pulses respectively).

Figure 4c shows the wide-field fluorescence imaging of the Laser-induced silver clusters. We observe that emitted luminescence, and then that the related concentration of Laser-induced are in good agreement with previous observations made on the silver zinc-phosphate glass. [23] Finally we note a modification of the baseline of the differential micro-transmission spectra over the whole spectral range, from the ideal 100% value down to 95%, typically. We believe the observed decrease in transmitted light intensity which affects the whole spectral domain is due to scattering effect.

The in-fiber Laser-structuring represents an original method for implementing complex lightmanipulating architectures. To illustrate its potential we have schematically inscribed optical micro-ring resonators directly in ribbon fibers (Figure 5a), without having confirmed and characterized their optical functionalities yet, such demonstration being beyond the scope of this article. The first structure (left-side) consists in a closed resonator ring side-coupled to two light input/output bus waveguides, one of which being addressed from the side of the fiber.

Alternatively (right-side) a Mach-Zehnder interferometer made of two arms split from a single source is coupled to one side of the resonator ring. To get confident about the potentiality of this route to generate photonics structures, we aimed at verifying the waveguiding properties of the photo-inscribed Ag m x+ clusters features. To assess such guiding abilities, far field experiments were conducted using a Yb:KGW Laser operating at 1030 nm in CW low-power regime as a light source, hence preventing any glass structuring and multi-photon fluorescence excitation (see Experimental). For the purpose of the demonstration, the propagation characteristics were investigated on a bulk sample. This assumption is reasonable since all the here-above described experiments highlight the fact that both physico-chemical properties and Laser-writing ability are fully preserved from bulk samples to ribbon fibers. A 7-mm-long linear pattern was produced in a silver zinc-phosphate bulk glass (with 4% of molar Ag 2 O), typically at 100 µm below its surface. Then the sample was rotated 90° in order to allow for Laser injection from above and for the collection of the received guided light from beneath (Figure 5b). Micrograph of the bulk glass and of the fluorescent photo-written tube under broad field UV illumination at 405 nm is depicted on Figure 5c. The light from a tightly-focused Laser diode at 405 nm was injected from the top surface of the structured bulk sample while imaging of the induced fluorescence was performed at the bottom plane (Figure 5d). Note that spectral filtering was employed to cancel the incident Laser diode light. The transversal dimensions of the collected spot of photo-luminescent signal from the Laser-written tube are L ~ 8 µm and l ~ 4 µm. Furthermore it exhibits a double-line shape that is typical of Ag m x+ clusters features. [23] In order to get a clearer demonstration of waveguiding abilities of the structure, the same operation was performed with a tightly-focused CW lowpower 1030 nm Laser (Figure 5e), for which no fluorescence is excited. Upon the latter injection, we observe a transmitted Laser light localized on the photo-written features, which demonstrates unambiguously their waveguiding properties. Indeed, since the top injection plane and the bottom imaging plane are 7-mm distant, we claim that the observed light at the bottom imaging plane cannot come from the collection of free propagating injected light. To support the guiding observation we claim, the refractive index variation between the pristine glass and the Ag m x+ clusters areas was estimated using a phase-contrast microscopy method (see Experimental), which gives access to the local index modification (providing that the structure thickness had been previously measured by high-resolution confocal microscopy). It showed an increase of the refractive index in the modified region of Δn ~5.10 -3 over the visible spectral range, with a spatial distribution corresponding to that of the silver clusters. Further effort is currently engaged to characterize the beam modes, loss and the magnitude of the photo-induced index modification to accurately regulate the propagation of light through the device. Laser-structuring allows for the inscription of perennial silver cluster-based waveguides in fibers, thanks to the thermal and optical stability of the photo-induced clusters. [14,24] It provides unique opportunities for applications in nonlinear optics, cavity-enhanced spectroscopic sensing, filtering or fullyintegrated tunable photonics.

Conclusion and perspectives

Here we have detailed the methodology for the manufacturing of silver-containing zinc phosphate glass fibers. The fabrication of photosensitive, photo-writable silver-containing phosphate-based ribbon fibers stem from judiciously adjusting both the materials composition and its geometry. We validated in first place the aptitude of the glasses to be shaped into fibers.

Loss measurements verify the effective transparency of the fiber in the NIR region. Notably, as evidenced by photo-luminescence measurements, Ag + ions remain homogenously dispersed upon drawing within the glass matrix. The intrinsically photosensitive fibers proposed here authorizes for large amount of silver ions doping without clustering, hence reducing the need for germanium doping or hydrogen-loading. Following we have presented a straightforward manufacturing method of elongated ribbon structures. Cost-limited, photosensitive fibers in a flat format is anticipated to ease the design of high-sensitivity fully-distributed fiber-based dosimeter sensors. Already, silica-based optical flat fibers have been proposed as the basis for such purpose [19] while high irradiation dose studies have been initiated on bulk silver zinc-phosphate glasses. [25] More sophisticated interconnected fiber-based 2D screen may eventually be envisioned for applications in the field of high-dose therapy dosimetry where image reconstruction instruments are needed. The in-fiber Laser-structuring scheme enables the integration in a single device of an entire array of unconventional device architectures on a flexible substrate platform. Beyond, we reported recently on structured light-induced DLW of nontrivial fluorescent Ag m x+ topologies obtained using non-Gaussian beam profiles. [26] This result could be exploited for the implementation of innovative photonic architectures relying on the spatial distribution of photo-induced patterns with linear and nonlinear optical properties.

Additionally we have shown that, upon subsequent post-irradiation annealing of the glass above T g , Ag m x+ clusters can grow into larger entities, up to metallic nanoparticles. [23] It would allow for the in-fiber sub-micrometric 3D patterning of metal-dielectric composites within low-T g glass fiber, a promising approach for engineering highly-efficient plasmonic sensing devices or increasingly complex electronic circuits.

Future work will consist in establishing improved glass purification and fiber manufacturing protocols in order to satisfy technological specifications. Other work will consist in developing core-clad fiber structures to propose innovative laser materials and geometry able to handle high flux of photons without premature ageing and, by taking advantage of nonlinear optical processes, to favor all-fiber short-length monolithic systems. Beyond, our approach could lead to a powerful alternative to bottom-up synthesis methods for producing scalable in-fiber 3D nanostructured materials, with functionalities spanning sensing, photonics, or even metamaterials devices.

Experimental Section

Glasses and preforms synthesis The zinc-phosphate glasses were melted using the standard meltquench technique. High-purity precursors (ZnO, Zn(PO 3 ) 2 , Ga 2 O 3 , NaPO 3 , AgNO 3 ) were weighted in powder forms and mixed together in a platinum crucible. The mix was ramped up (1 °C.min -1 ) at 1100 °C and then kept at this temperature for 12 hours. Following the liquid was poured on a brass plate to freeze the melt. Preforms were produced by casting the glass in a specifically designed brass mold, pre-heated at ~ T g -10 °C and annealed at T g -40 °C for 12 hours.

Fiber drawing Thermal drawing was performed using a dedicated 3-meters-high optical fiber draw tower composed of an annular electrical furnace with a sharp temperature profile, a diameter monitor, a tension dancer and a collecting drum. The preform was slowly fed into the furnace and the temperature was gradually increased at a rate of 10 °C.mn -1 up to ~700 °C under continuous oxygen gas flow (3 l.min -1 ). The preform-holder motion and capstan rotation velocity were controlled in real-time to produce the targeted fiber diameter. Following this procedure, meters of fibers were drawn with diameters ranging from 250 µm, down to 75 µm.

Direct Laser Writing

Figure 1

 1 Figure 1 Phosphate-glass preforms and fibers (a) Undoped preform under white light (b)

2

 2 Figure 2 Silver-doped phosphate glass PZG-2N2A (a) Normalized Raman spectra of the bulk,

Figure 3

 3 outlines our approach to ribbon fiber processing. The procedure starts with the preparation of a macroscopic scale rectangular preform (panel a). For this a glass slab is grinded and optically polished to the proper dimensions (typically: 10-mm wide, 0.4-mm thick, and 80-mm long). The preform is then thermally scaled down into tens-of-meters mechanically flexible flat fiber. Optical and SEM micrographs in panel b depict the longitudinal and cross-sectional views of the fiber (left and right panels respectively) and of its dimensions.

Figure 3

 3 Figure 3 Thermal drawing of ribbon fibers (a) Preform, bottom-neck preform and fiber samples

(

  see Experimental). Motifs were made of multiple series of lines parallel and perpendicular to the fiber main axis. Laser structuring places stringent requirements on the surface and optical quality of the substrate. Therefore, in this regard, successful DLW carried out in PZG-2N2A fiber first stresses the flatness and homogeneity of the device. Silver ions clustering is triggered by exposing the glass to a high-repetition-rate pulse train from a tightly focused NIR femtosecond laser. It results from multiphoton ionization associated with cumulative thermal effects, allowing for the reduction of Ag + ions, their diffusion and aggregation into Ag m x+ clusters (m: number of atoms, m < 20;

Figure 4

 4 Figure 4 DLW in ribbon fibers (a) Glass PZG-2N2A fiber sample before (under white light) and

  TW.cm -2 5.1 TW.cm -2 clusters, increases with the cumulative deposited dose. For clarity we show the differential micro-transmission spectra only for the velocity v = 100 µm.s -1 . They exhibit an intense absorption band centered at λ = 324 nm (E = 3.82 eV) (and a much weaker one around λ = 290 nm (E = 4.27 eV)). The amplitude of the multicomponent absorption band around λ = 324 nm grows linearly with the deposited dose. These bands have been assigned to Ag m x+ clusters and

Figure 5

 5 Figure 5 Waveguiding evidence in silver zinc-phosphate bulk glass (a) Ring resonators and

  DLW on ribbon fibers was performed with a Yb:KGW femtosecond oscillator (T-Pulse 200, Amplitude Systemes, up to 2.6 W, 9.1 MHz, 390 fs at 1030 nm). The irradiation duration and transmitted irradiance were controlled by an acousto-optic modulator, enabling the accumulation of N = 10 5 -10 6 pulses with energies from 50 to 150 nJ. The positioning and displacement of the sample were performed with a high-precision 3D translation stage (XPS-50 stages, Micro-Contrôle). Irradiations were carried out by focusing laser pulses with a microscope objective (Mitutoyo, APO PLAN NIR, 20, NA 0.4). In order to highlight waveguiding abilities of the photo-written features, a CW low-power 1.03-µm Laser was injected inside the tube using a microscope objective (Olympus, 20, NA 0.5). An identical bottom microscope objective was used to collect the transmitted (guided) light, and to conjugate the imaged plane on a CCD camera. With the considered 7-mm long sample, the injection and collection planes of these two objectives were optically distinct, ensuring that any collected light at the bottom plane could only be guided light.Characterization instruments Thermal analysis was performed by differential thermal analysis (DSC 404 PC from Netzsch Inc.). About 20 mg of powdered glass was inserted into a Pt pan and placed in the DSC chamber along with an empty reference pan. Characteristic temperatures were measured as the inflection point of the endotherm at a heating rate of 10 °C/min (precision ±2°C).Refractive indices were measured using Abbe refractometer. Micro-absorption measurements were performed in bright-field using a 'CRAIC Technologies' microscope equipped with a Xenon lamp and a condenser as a white light source. A conjugated 10 magnification microscope objective was used to collect the transmitted light through the laser-induced structures. Raman spectra were recorded with a Xplora Horiba micro-Raman spectrometer using a 532 nm excitation line with 10 mW of incident power. Raman spectra in the range of 200-2000 cm -1 were recorded with a resolution of 2.5 cm -1 . The macroscopic luminescence spectra (emission and excitation) were recorded at room temperature with a SPEX Fluorolog-2 spectrofluorimeter (Horiba Jobin-Yvon). The excitation source was a 450 W xenon lamp enabling continuous excitation from 200 nm to 800 nm. The signal was detected and amplified by a Hamamatsu R298 photomultiplier. The samples were grounded into powder to cancel geometric effects. Refractive index variation between the pristine glass and the Ag m x+ clusters areas was estimated using a phase-contrast microscopy method with commercially available equipment's from PHASICS Inc.

Table 1

 1 Investigated glasses and related properties

			T g	ρ	n (639 nm)
	Glass	Composition (mol%)
			[±2 o C]	[±0.01 g.cm -3 ]	[±0.001]
	PZG-4N	40P 2 O 5 -55ZnO-1Ga 2 O 3 4Na 2 O -	382	3.21	1.567
	PZG-2N2A 40P 2 O 5 -55ZnO-1Ga 2 O 3 2Na 2 O 2Ag 2 O 385	3.30	1.570

Acknowledgements

The authors wish to thank Frédéric Adamietz for micrographs of the preforms and Arnaud Royon for the helpful discussions. Funding for this work has been provided from the French Government, managed by the French National Research Agency (ANR Grant #40611), by the Programme IdEx at the University of Bordeaux, the Cluster of excellence LAPHIA and by the Aquitaine Region.

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Table of Content figure (55 × 50 mm or 110 × 20 mm) Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013. Title Photo-writable Silver-containing Phosphate Glass Ribbon Fibers

Table of contents entry

The thermal drawing of photosensitive silver-containing zinc phosphate glass ribbon fibers is demonstrated. Structural and luminescence properties of the native glass are preserved. The flat fiber geometry allows for the convenient, accurate Laser inscription of complex luminescent silver clusters patterns within the glass matrix. These fibers offers a promising platform for the design of highly-efficient sensing, photonics or metamaterials devices.

Keywords