Benjamin Cabanes
email: benjamin.cabanes@st.com

Philippe Galy
email: philippe.galy@st.com

Benoit Pascal Le Masson
email: pascal.le_masson@mines-paristech.fr

Benoit Weil
email: benoit.weil@mines-paristech.fr

Pascal Le Masson

Management for Radical Innovation in Science-based Organizations: a New Framework Based on Design Theory

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Managing R&D staff for innovation is not a new topic in technology and innovation management [START_REF] Shepard | Nine dilemmas in industrial research[END_REF][START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Roth | A Critical Examination of the Dual Ladder[END_REF]. However, in an innovation-intensive capitalism [START_REF] Le Masson | Strategic management of innovation and design[END_REF], characterized by the acceleration of the rupture of dominant designs and by hypercompetitive environments (Ilinitcht et al., 1996), the role of members of technical staff (i.e. researchers, scientists and engineers) is increasingly crucial for science-based organizations. They have to be able to anticipate and to accelerate the renewal of knowledge, competencies and expertise in a context of uncertain dominant design, while controlling resources. Moreover, the renewal of R&D practices for radical innovation highlights new challenges and new issues [START_REF] Allen | Formal versus informal knowledge network knowledge networks in R&D: a case study using social network analysis[END_REF]Bigliardi et al., 2011;[START_REF] Wu | Innovation mechanisms and knowledge communities for corporate central R&D[END_REF]. For example, Bigliardi et al. (2011) show that open innovation strategies significantly change organization of the R&D functions and the way to manage R&D personnel. [START_REF] Allen | Formal versus informal knowledge network knowledge networks in R&D: a case study using social network analysis[END_REF] highlight the positive role of informal networks in the development and the transfer of knowledge within the R&D function. More over, [START_REF] Wu | Innovation mechanisms and knowledge communities for corporate central R&D[END_REF] discuss new management practices, such as creating competence around emerging technologies that form product platforms. Thus, Dell 'Era and Verganti (2009) explain that managing radical innovation requires technological competences but also socio-cultural and design capabilities. However, we observe three paradoxes regarding organizational practices about the management of technical staff facing innovation issues. First, the main technical staff management system, used by science-based organizations, is still the same for over 50 years. The dual ladder system [START_REF] Shepard | Nine dilemmas in industrial research[END_REF][START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Allen | The dual ladder: motivational solution or managerial delusion?[END_REF]) is always the main management incentive instrument to motivate technical performance. This managerial device is a reward system and an organizational arrangement for career advancement, that is both stimulating to the technical staff and productive for the organization. The dual ladder system offers a technical career path (technical ladder) as an alternative for managerial career progression (managerial ladder) in order to recognize technical experts in organizations and to secure a pool of technical and scientific talent for science-based organizations [START_REF] Shepard | Nine dilemmas in industrial research[END_REF][START_REF] Shepard | The Dual Hierarchy in Research[END_REF]. Secondly, although many sciencebased organizations have adopted this system, there is currently a consensus from researchers and practitioners that dual ladder fails to respond to the difficulties encountered in the management of the technical staff [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Gunz | Dual ladders in research: A paradoxical organizational fix[END_REF][START_REF] Allen | The dual ladder: motivational solution or managerial delusion?[END_REF][START_REF] Roth | A Critical Examination of the Dual Ladder[END_REF][START_REF] Katz | Exploring the dynamics of dual ladders: a longitudinal study[END_REF][START_REF] Gupta | Managing human resources for innovation and creativity[END_REF][START_REF] Katz | The influence of supervisory promotion and network location on subordinate careers in a dual ladder RD&E setting[END_REF][START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF][START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF]. Finally, there are no consensus both in academic literature and in managerial practices in organizations regarding new managerial solutions to improve the management of technical staff. Thus, in our view, this management instrument needs a critical reevaluation.

Recently, new publications [START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF]Bigliardi et al., 2011;[START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF][START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF] bring new insights to develop new analytical frameworks and to improve the implementation of practices for managing technical staff. These works highlight that rather than to focus of the recognition, legitimization and promoting issues, a key challenge is to take in account the role of the members of technical staff in the organization. However, from our point of view a theoretical framework, able to specify the role of technical staff for radical innovation and associated management devices, is missing. What is the value of the knowledge, competences, and expertise for radical innovation? How to build and mobilize technical staff knowledge for radical innovation? How to organize technical staff collective action for radical innovation? What are the roles of technical staff for radical innovation? What are the management devices to support the renewal of knowledge, competencies and expertise in a context of uncertain dominant design?

In this paper, we propose using recent advances in design theory (Hatchuel andWeil, 2003, 2009;[START_REF] Le Masson | Strategic management of innovation and design[END_REF], 2015) to discuss the relevance of the dual ladder system and its limitations in situations of radical innovation. Indeed, recent advances in design theory [START_REF] Le Masson | Strategic management of innovation and design[END_REF], 2015) highlight new perspectives to model the management of radical innovation as an innovative design process. The present research is based on a qualitative and exploratory case study [START_REF] Eisenhardt | Building theories from case study research[END_REF][START_REF] Yin | Case Study Research[END_REF] exploring the role of technical staff facing radical innovation in STMicroelectronics, one of the European leaders in semiconductor industry. From theoretical perspective we highlight that design theory allows to characterize the role of technical staff according to two modalities of intervention: rulebased design and innovative design [START_REF] Le Masson | Strategic management of innovation and design[END_REF]. We demonstrate that design theory is a powerful framework to go beyond the existing frameworks and we contribute to theoretical debates that attempt to explain the implementation of organizational arrangements and management practices in R&D. From a practical perspective, we propose to highlight a new organizational arrangement with its associated governance principles. This new structure does not aim to replace the dual ladder system but proposes to complete this management device taking into account the role of experts and the conditions of technical staff collective action for radical innovation management.

Literature review and theoretical frameworks

Current doctrine on technical staff management: the dual ladder

At the end of the 50s, [START_REF] Shepard | The Dual Hierarchy in Research[END_REF] had already pointed out that science-based organizations had become increasingly dependent on technological innovation. [START_REF] Shepard | The Dual Hierarchy in Research[END_REF] highlights that power, within the industrial firm, tends to shift to those who possess the skills most needed for survival and growth: from manufacturing to sales and finally to R&D. However, the shift of power to R&D has been hampered by the important difficulty of evaluating the laboratory's actual and potential business contribution to the firm [START_REF] Shepard | Nine dilemmas in industrial research[END_REF]. Moreover, hire best scientists and leave them alone has never constituted a guarantee of successful R&D management. The difficulty of evaluating R&D performance, the limitation of the laboratory's responsibility to only questions of technological feasibility, and the differences in expectations, values, and organizational traditions between the technical staff and the rest of the company constitute the main challenges for managing R&D for innovation. [START_REF] Shepard | Nine dilemmas in industrial research[END_REF] identifies nine dilemmas in industrial research. Among these dilemmas, technical staff management (scientists, engineers, researchers) seems to be the most important, particularly the question of reward of technical staff. Indeed, rewarding members of technical staff for good performance presents a difficult administrative issue. In the main industrial organizations, success is measured largely by the size of the organization one controls. However, all the best members of technical staff are not able to supervise, manage and administer work. Talented and creative researchers will not be most profitably employed in such work. Finally, members of technical staff are in doubt concerning their career development:

"Scientists and engineers are themselves somewhat ambivalent about organizational advancement. On the one hand, it may be regarded as the only generally accepted evidence of their value. On the other hand, they fear loss of professional status and competence" [START_REF] Shepard | Nine dilemmas in industrial research[END_REF] To solve this problem, the literature highlights two major management devices to reward members of technical staff for technical and scientific performance without removing them from technical work.: the concept of technical direction and the dual hierarchy or technical ladder [START_REF] Shepard | The Dual Hierarchy in Research[END_REF]. It is clear that practitioners and academics focused on the second one. The dual ladder system is based on the view that:

"the best scientists are lost when they are rewarded by being made managers" [START_REF] Shepard | The Dual Hierarchy in Research[END_REF] Hence, the dual ladder system offers a technical career path (technical ladder) as an alternative for managerial career progression (managerial ladder) (Figure 1). On the one hand, the managerial ladder ensures promotion, status, recognition, salary, power and influence through the ranks of supervision and management. On the other hand, the technical ladder ensures promotion through special advantages witch can be obtained without supervisory responsibilities: autonomy, freedom, high salaries and specific titles (senior member of technical staff, senior scientist, fellow, etc.).

Figure 1. The classic dual ladder system of career advancement As mentioned above, the dual ladder is a reward system and an organizational arrangement for career advancement, that is both stimulating to the technical staff and productive for the organization. Indeed, in science-based organization, innovation requires talented and experienced researchers, scientists and engineers that are able to be creative, current, and productive in their respective fields. The movement of brilliant members of technical staff toward the managerial ladder will seriously decrease the organization's pool of creative technical talent [START_REF] Katz | Exploring the dynamics of dual ladders: a longitudinal study[END_REF]. Thus, the dual ladder aims to secure a pool of technical and scientific talent for science-based organizations. Moreover, as mentioned by [START_REF] Gastaldi | Vers un modèle d'analyse des instruments de gestion des ressources humaines. Le cas de la double échelle scientifique et managériale[END_REF], the dual ladder also allows to recognize technical experts in organizations, to provide career development opportunities for members of technical staff, to reduce turnover of technical staff, to preserve and develop expertise, competencies and know-how in the organization, to establish equity between technical staff and managers, to highlight the importance of the R&D department in the organization and to attract young engineers by offering them interesting careers.

Although many organizations have adopted this system, there is currently a consensus from researchers and practitioners that dual ladder fails to respond to the difficulties encountered in the management of the technical staff [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Gunz | Dual ladders in research: A paradoxical organizational fix[END_REF][START_REF] Allen | The dual ladder: motivational solution or managerial delusion?[END_REF][START_REF] Roth | A Critical Examination of the Dual Ladder[END_REF][START_REF] Katz | Exploring the dynamics of dual ladders: a longitudinal study[END_REF][START_REF] Gupta | Managing human resources for innovation and creativity[END_REF][START_REF] Katz | The influence of supervisory promotion and network location on subordinate careers in a dual ladder RD&E setting[END_REF][START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF][START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF]. Based on a literature review, some common ones include the following:

• (1) The dual ladder system does not specify a definition of the role of technical staff in the organization.

Freedom in research is supposed to be the main principle of upward movement in the technical ladder. However, in industrial organization, research should not be dissociated from the business strategies of the firm. The dual ladder system does not give definition of a strategic role of members of technical staff, with the exception of a technical and scientific consulting role. Technical staff works and activities seem to be dissociated from the strategic ambitions of the firm [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Aryee | Career orientations, perceptions of rewarded activity, and career strategies among R&D professionals[END_REF][START_REF] Van Wees | Dual Ladder in Balance[END_REF][START_REF] Turpin | Occupational roles and expectations of research scientists and research managers in scientific research institutions[END_REF][START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF][START_REF] Lelebina | La gestion des experts en entreprise[END_REF][START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF]. • (2) The technical ladder position is a reward position rather than a strategic position in the organization.

Without a clear definition of a strategic role of the members of technical staff in organization, technical ladder position is restricted to being a simple reward for past service and not an opportunity to take part in the strategy of the firm. Ensure a status and advantages is not enough. The attractiveness of proposed roles must be considered, in particular their content and their legitimacy in the organization in view of strategic issues [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Van Wees | Dual Ladder in Balance[END_REF][START_REF] Turpin | Occupational roles and expectations of research scientists and research managers in scientific research institutions[END_REF][START_REF] Gastaldi | Vers un modèle d'analyse des instruments de gestion des ressources humaines. Le cas de la double échelle scientifique et managériale[END_REF][START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF][START_REF] Lelebina | La gestion des experts en entreprise[END_REF]. • (3) The nature of the dual ladder system is a static organizational structure. The reward aspect of the dual ladder and its capacity to isolate members from the rest of the organization make it a convenient shelf for managers who are looking for advice and skills. But, for technical staff such nomination does not provide an influential position in the firm. In addition, the dual ladder ignores the evolution of expertise within the organization. If a field of expertise becomes obsolete in the organization, how should we manage the members of technical staff who are no longer useful? The dual ladder is focused on researchers and engineers who are ready to be recognized and rewarded. However, management issues have also to include management arrangements able to dynamically manage areas of expertise and experts. • (4) The technical ladder position is an ambiguous status in the organization. Even in the best-case scenario, the promotion and reward by the technical ladder is at once a symbol of success and of managerial incompetence. A brilliant scientist with good leadership skills belongs logically in the managerial ladder. Thus, the technical ladder is for talented scientists who are lacking a managerial potential. Moreover, it's often the only top positions of the technical ladder which are interesting in terms of prestige. Generally, the technical ladder is unattractive because advancement is too slow for ambitious researchers [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Katz | Exploring the dynamics of dual ladders: a longitudinal study[END_REF][START_REF] Debackere | Strategic career development for R&D professionals: Lessons from field research[END_REF][START_REF] Aryee | Career orientations, perceptions of rewarded activity, and career strategies among R&D professionals[END_REF][START_REF] Van Wees | Dual Ladder in Balance[END_REF][START_REF] Tremblay | Determinants of career path preferences among Canadian engineers[END_REF] • (5) The dual ladder is an organizational arrangement focused on individual needs and not on collective action between individuals. As a human resource instrument, dual ladder is only focused on career evolution issues of individual. It must be noted that the dual ladder system does not address the fundamental issues such as the coordination between actors, social interactions, learning process, and managerial procedures to achieve a common objective. However, many organizations try to create and organize a technical staff community or technical staff network only based on the dual ladder system. Technical ladder in the organization does not necessarily produce an organized community whose members are able to work together. • (6) The dual ladder system does not resolve the problem of organizational influence or power. The dual ladder system tends to remove technical staff from the main stream of organization and contributes to lower the prestige of the technical ladder. Moreover, freedom is not enough, technical staff need power to remain free [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Katz | Exploring the dynamics of dual ladders: a longitudinal study[END_REF][START_REF] Van Wees | Dual Ladder in Balance[END_REF][START_REF] Debackere | Strategic career development for R&D professionals: Lessons from field research[END_REF]. • (7) Technical promotions are validated through justifications of past contribution without reflection about future expectations. Engineers, scientists and researchers have to justify the high level of their technical contributions to be admitted as a member of technical staff. In most of the case, criteria are the numbers of patents, numbers of scientific publications, numbers of PhD supervisions, external reputation, international network, success in development of complex advanced technology, etc., and are based on the result of past activities [START_REF] Gunz | Dual ladders in research: A paradoxical organizational fix[END_REF][START_REF] Bailyn | The hybrid career: an exploratory study of career routes in R&D[END_REF]. • (8) The technical ladder is a technical staff recognition system based on existing and already known competencies, knowledge and expertise. Manage R&D for innovation assumes to ensure that engineers, scientists and researchers are able to anticipate the renewal of knowledge, competencies and expertise in a context of uncertain dominant design, while controlling resources. The dual ladder system doesn't allow to anticipate the future key core competencies of the firm and fails to motivate technical staff to develop new expertise. Finally, the dual ladder is most a reward system for past technical success rather than a motivation system for future developments.

• (9) The technical ladder could become a reward for organizational loyalty rather than technical contribution.

Over the time, the dual ladder concept used by organizations tends to diverge from the initial vision and expectations. At the beginning of the implementation of a dual hierarchy, all criteria for promotion are well followed, but they gradually become abandoned. If the initial concept is degraded, then impacts on the organization are also degraded [START_REF] Katz | Exploring the dynamics of dual ladders: a longitudinal study[END_REF][START_REF] Bailyn | The hybrid career: an exploratory study of career routes in R&D[END_REF][START_REF] Debackere | Strategic career development for R&D professionals: Lessons from field research[END_REF]Gastaldi andGilbert, 2007, 2008;[START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF]. • (10) The dual ladder does not address innovation management issues. The dual ladder is solely based on the fact that scientists and engineers motivations allow to the firm to innovate. This is a necessary, but not sufficient. Collaboration issues, strategic alliance, coalition, partnership, design activities, business strategy, teamwork, creativity, market valuation and business model generation are not addressed by the dual ladder.

Nevertheless, it is necessary to recognize that today, and in spite of the many challenges facing the dual ladder, this management device is still the most recommended management device in both the practitioner and academic literature [START_REF] Igbaria | Career orientations and career success among research, and development and engineering professionals[END_REF][START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF].

Human resource strategy, R&D and innovation: What theoretical framework?

In response to the difficulties facing the management of technical staff, several theoretical frameworks have been proposed in various literatures. A distinction can be made between two research orientations. On the one hand, some researches bring new insights to examine implementations and adaptations of HR practices in R&D department [START_REF] Angel | R&D managers' adaptation of firms' HRM practices[END_REF]Ensign and Hebert, 2009;[START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF]. On the other hand, other researches revisit the role of technical staff facing new managerial practices such as open innovation [START_REF] Beugelsdijk | Strategic human resource practices and product innovation[END_REF]Bigliardi et al., 2011;[START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF] Based on procedural and distributive justice framework [START_REF] Greenberg | The Quest of Justice on the Job: Essays and Experiments[END_REF][START_REF] Folger | Organizational Justice and Human Resource Management[END_REF][START_REF] Wiesenfeld | Is more fairness always preferred? Selfesteem moderates reactions to procedural justice[END_REF], [START_REF] Angel | R&D managers' adaptation of firms' HRM practices[END_REF] revisit the fact that the human resource management (HRM) practices of firms must be adapted to the R&D departments' specificities. The authors emphasize that adaptation of HRM practices may be viewed as unjust differential treatment and can negatively affect the technical staff performances. However, these results contradict the specialized literature [START_REF] Shepard | The Dual Hierarchy in Research[END_REF][START_REF] Allen | The dual ladder: motivational solution or managerial delusion?[END_REF][START_REF] Robinson | Corporate Creativity[END_REF][START_REF] Kim | Career orientations of R&D professionals in Korea[END_REF]. For [START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF], the lack of consensus between those favoring the non-adaptation of HRM practices and those who are for the adaptation, show that a theoretical framework allowing us to unveil the tensions and conflicts of interest experienced by different practitioners is missing.

From a sociologist perspective, [START_REF] Ensign | Competing explanations for knowledge exchange: technology sharing within the globally dispersed R&D of the multinational, multidivisional firm[END_REF] suggest that there are five competing theories to explain the success of technological knowledge sharing within the the globally dispersed R&D function: (1) economics, (2) technological, (3) organizational, (4) geographic, and (5) sociological. They highlight that certain factors are primary and other supporting. From their point of view, sociological considerations and social interactions seem to be the most likely key factors in explaining the success of knowledge exchange.

Recently, new researches [START_REF] Gand | La gestion des parcours d'experts en entreprise: pourquoi la simple gestion des carriers est insuffisante et comment la completer? Proposition d'un cadre d'analyse[END_REF][START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF] bring new insights to develop new analytical frameworks in order to explain the difficulties of the implementation of practices for managing technical staff. [START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF] propose to draw upon convention theories (Boltanski and Thévenot, 2006) for the purpose of explaining the mismatch between managerial practices and rationalization process, that they observe in knowledgeintensive organizations. According to these authors, mismatch comes from tensions provoked by different cohabiting social logic that may even be in competition with each other within the same organization. They identify three common worlds in the implementation of managerial practices and devices in R&D: (1) the world of inspiration, (2) the world of market, and (3) the world of industrial. This work has the merit of to explain concrete problems faced organization. Then, it allows to go beyond the existing frameworks that take an adaptation vs. a non-adaptation approach, to show how and under what occasions arrangements were made between the different logics. However, all these researches bring explanatory approaches more than prescriptive approaches and fails to propose new managerial solutions to improve the management of technical staff, in particular to go beyond the dual ladder system.

Based on innovation management literature, other studies tried to link strategic human resource practices and innovation. [START_REF] Beugelsdijk | Strategic human resource practices and product innovation[END_REF] explore the relationship between 12 HR practices and product innovation. In his analysis, he distinguishes radical innovation (new for industry) and incremental innovation (new for the firm). His results suggest that training, schooling, performance-based pay and job autonomy is beneficial for incremental innovation. Nevertheless, incentive systems and training programs have no effect on radical innovation.

Other researchers discuss how the adoption of open innovation paradigm [START_REF] Chesbrough | Open Innovation: The New Imperative for Creating and Profiting from Technology[END_REF] has changed the organizational structures of R&D and has modified the practices used in managing technical staff (Bigliardi et al., 2011;[START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF]. There findings suggest that open innovation reduces the role of senior scientists in innovation process. According to us, two aspects of open innovation have influenced the model for managing researchers, scientist and engineers in science-based organizations. First, in open innovation paradigm, the R&D function is no longer the "interceptor and promoter" of technological innovation in the firm [START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF]. Second, innovation process based on the adoption of external knowledge and technologies tends to reduce the influence of scientists and researchers [START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF]. The R&D department, who adopted open innovation practices, are reducing the role of technical staff as knowledge producers while expanding the mission of integrating knowledge that originates outside the firm. In this context, new professional figures are emerging in the organization: "integration expert" (Bigliardi et al., 2011;[START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF]. Their roles are to select and integrate external knowledge and successfully use it in new products and processes.

As a first result, in industrial research, the role of engineers and the business innovation team become more important, whereas the role of senior scientists is weakened. (Bigliardi et al., 2011) Another proposition is that the adoption of open innovation leads to the creation of a new career path, thus overcoming the inadequacy of the traditional dual ladder system. [START_REF] Petroni | Open innovation and new issues in R&D organization and personnel management[END_REF] propose a new career path: the "open dual ladder". According to the authors, the main difference is that in the open dual ladder, technical staff can later move to other positions such as managerial position. These works highlight that rather than to focus of the recognition, legitimization and promoting issues, a key challenge is to take in account the role of the members of technical staff in the organization. However, these conclusions are problematic. First, the alleged novelty of "integration expert" role is not convincing. Indeed, [START_REF] Allen | Information flow in research and development laboratories[END_REF] already identified and popularized the role of the gatekeeper (long before the concept of open innovation). Gatekeepers are technical professionals who are able to link scientists within the firm and scientists outside the firm in order to exchange knowledge and information aiming to improve the performances of R&D activities. That is exactly what it is expected from a senior scientist. Second, build an innovation strategy, only based on seeking knowledge outside the firm, is a reductive vision. This means that organizations are incompetents to generate new knowledge by themselves, i.e. to create innovation by themselves. Finally, the concept of "open dual ladder" does not seem a credible alternative to the technical ladder. Indeed, the dual ladder system does not restrain R&D personnel to move toward managerial responsibilities.

In science-based industries, contemporary competition by innovation is characterized by the acceleration of the rupture of dominant designs [START_REF] Le Masson | Strategic management of innovation and design[END_REF] and by hypercompetitive environments (Ilinitcht et al., 1996).

To succeed in this hypercompetitive environments, continuous improvement of existing products and technologies (incremental innovation) is not enough. Firms have to pursue strategies and have to be organized for radical innovation [START_REF] Mcdermott | Managing radical innovation: an overview of emergent strategy issues[END_REF]. Radical innovation is defined by strong change both in technology and market diffusion, and requires new knowledge, competencies and expertise. From our point of view, a theoretical framework, able to specify the role of technical staff for radical innovation and associated management devices, is missing: What are the roles of technical staff for radical innovation? How to mobilize and organize technical staff for radical innovation? What are the management devices to support the renewal of knowledge, competencies and expertise in a context of uncertain dominant design? The lack of consistent theoretical frameworks does not allow us to unveil an organizational arrangement for the management of technical staff facing radical innovation.

To go beyond the existing framework presented above, we propose to use design theory to formulate new research hypotheses on the management of technical staff facing radical innovation. The intersection of R&D personnel management and design theory is promising because we believe that issues of the management of R&D personnel requires an in-depth understanding of their design reasoning and their design activities.

Toward a new theoretical framework based on recent design theory

Design theories attempt to model the interaction between knowledge and innovation, and lead to develop management devices with the objective to organize and rationalize design processes [START_REF] Simon | The sciences of the artificial[END_REF][START_REF] Pahl | Engineering design: a systematic approach[END_REF]Hatchuel andWeil, 2003, 2009;[START_REF] Le Masson | Strategic management of innovation and design[END_REF], 2015). Recent advances in design theory [START_REF] Le Masson | Strategic management of innovation and design[END_REF], 2015) highlight a new way of thinking about innovation far beyond a mere combination of existing knowledge to include the renewal of knowledge and the expansion of expertise. Accordingly, design theory allows new perspectives to model the management of radical innovation as an innovative design process. Moreover, design theory, in particular C-K design theory (Hatchuel andWeil, 2003, 2009), as already been successfully used in the study of radical innovation processes [START_REF] Elmquist | Towards a New Logic for Front End Management: From Drug Discovery to Drug Design in Pharmaceutical R&D[END_REF][START_REF] Elmquist | The value of 'failed' R&D project: an emerging evaluation framework for building innovative capabilities[END_REF][START_REF] Lenfle | Exploration, Project Evaluation and Design Theory: A Rereading of the Manhattan Case[END_REF]. For these reasons, we decided to rely on design theory as an analytical framework to study the issues of technical staff management for radical innovation.

The main objective of design theories is to model design activities in R&D department. Some academic researches emphasize that R&D departments are based on a design rationale through the application of a system of rules that provides engineers the capacity to develop a variety of objects [START_REF] Pahl | Engineering design: a systematic approach[END_REF]Hatchuel andWeil, 2003, 2009;[START_REF] Le Masson | Strategic management of innovation and design[END_REF], 2015). The design theory developed by [START_REF] Pahl | Engineering design: a systematic approach[END_REF], called Systematic Design, propose to model design activities through four stages: Functional definition, conceptual design, physical and morphological definition and detailed definition. In this approach, design process is linear and aims to standardize methods allowing to enhance the effectiveness of new product development. This design theory became the reference model for many industries and helped science-based organizations facing mass production challenges. However, the application of a system of rules is only efficient within a stable object model. Indeed, rules-based design theory fails to describe design activities outside a dominant design. Managing design activities for radical innovation involves the exploration of new values, new business model, new market and new technologies. Today, firms have to accelerate the renewal of their products and technologies in order to provide radical innovations. Thus, managing the renewal of object identity is become a fundamental issue for design theory. To go for beyond the limits of Systematic Design theory, C-K design theory (Hatchuel andWeil, 2003, 2009;[START_REF] Le Masson | Strategic management of innovation and design[END_REF], 2015) provides a new formal framework in order to understand and to explain the mechanisms underlying the renewal of object identity.

In C-K design framework, two logics of design activities are identified. On the one hand, the main purpose is to maximize product development efficiency. The collective design capabilities repose in identifying, preserving and reusing an invariant system of rules as much as possible. In this context, exploration activities are not the objective [START_REF] Le Masson | Testing whether major innovation capabilities are systemic design capabilities: analyzing rule-renewal design capabilities in a case-control study of historical new business developments[END_REF]. This logic of design activities consists in being focus on invariant rules, such as invariant target (same customer requirements, stable market), invariant competencies (reuse technical skills and existing knowledge), invariant people resources (same people and identical network) and invariant of possible risks (anticipated risks). In C-K design theory, this perspective is characterized as rule-based design [START_REF] Le Masson | Strategic management of innovation and design[END_REF]. In this logic of design activities, generate innovation is possible. However, innovation is limited to a continuous improvement of existing products and technologies, i.e. incremental innovation. On the other hand, collective design activities are organized to intentionally renew the existing system of rules. The main challenges are characterized by capabilities of investigating new types of specifications, new competencies, new knowledge, new markets, new partners and by capabilities of identifying new types of risk in order to provoke the renewal of object identity [START_REF] Le Masson | Testing whether major innovation capabilities are systemic design capabilities: analyzing rule-renewal design capabilities in a case-control study of historical new business developments[END_REF]. In C-K design theory, this perspective is characterized as innovative design [START_REF] Le Masson | Strategic management of innovation and design[END_REF]. From this perspective, managing radical innovation involves to create organizational arrangements allowing the development of innovative design capabilities to renew the existing system of rules. Therefore, in a design perspective, it is possible to analyze capabilities for radical innovation by identifying the underlying system of rules design and the capabilities used to manage the system, whether to preserve the system (rules-based design) or to renew it (innovative design).

Our research questions emerge from this theoretical framework: (1) What is the role of members of technical staff in rules-based design and innovative design activities? (2) What are the organizational arrangements aiming the development of innovative design capabilities to renew the existing system of rules?

Research Methodology

Research design

This research is a qualitative and exploratory case study [START_REF] Eisenhardt | Building theories from case study research[END_REF][START_REF] Yin | Case Study Research[END_REF] exploring the role of technical staff facing radical innovation in STMicroelectronics, one of the European leaders in semiconductor industry. STMicroelectronics has been chosen because of its ability to develop radical innovations and its capacity to invent new organizational practices [START_REF] Le Masson | Absorptive Capacity For Radical Innovation: A Case Study In The Semiconductor Industry[END_REF][START_REF] Kokshagina | Risk management in double unknown: theory, model and organization for the design of generic technologies[END_REF]. We have several reasons to believe that technical staff management for radical innovation is an important issue for STMicroelectronics. First, semiconductor industry is subject to radical innovation (transistor miniaturization, sensors and actuators, internet of things, energy optimization, bioelectronics, etc.). Second, STMicroelectronics is a leading technology innovator with approximately 8,300 people working in R&D (engineers, researchers, scientists, etc.). And finally, the acceleration of innovation may have led the firm to invent new practices and new organizational models. With a turnover of $6.90 billion (2015), STMicroelectronics (Franco-Italian group) is among the world's largest semiconductor companies such as Intel, Samsung and TSMC. In 2015, the company has approximately 43,200 employees and almost one fifth of people work in R&D and product design. The company spends about 21% of its revenue in R&D and owns a substantial patent library (~15,000 owned patents in ~9,000 patent families and 500 new patent filings). This research is based on an exploratory single case study in order to look at the applicability of mainstream management theories and for the testing of new theoretical propositions [START_REF] Eisenhardt | Building theories from case study research[END_REF][START_REF] Yin | Case Study Research[END_REF]. This study should be considered as a powerful example or as an illustration rather than representative selection. Moreover, this case study should be appropriate for theorizing about the phenomenon of technical staff management facing radical innovation in this early stage of the research process [START_REF] Siggelkow | Persuasion with case studies[END_REF]. Indeed, our main objective is to initiate theoretical reflections and develop further avenues for research on this specific issue, which is still in early stages [START_REF] Siggelkow | Persuasion with case studies[END_REF]. Finally, this study is an initial step in a larger research process, aiming to understand organizational arrangements for the management of technical staff for radical innovation.

Data collection process and data analysis

Data collection was conducted in STMicroelectronics -more specifically in the Technology R&D department, located in Crolles (France) -, from January 2015 to November 2015. We conducted more than 35 semi-structured interviews with different types of actors (researchers, senior scientists, R&D managers and directors and human resource manager) and we took part in internal meetings and working groups on the topic of technical staff management facing radical innovation. This material was crossed with four main written sources: (1) internal documents about technical staff organization (white paper about the technical staff organization, communication kit), (2) standard operating procedure (SOP) about technical staff organization, (3) the Member of Technical Staff charter and (4) the Technical ladder matrix.

The documentation has essentially allowed us to corroborate and supplement the information provided during the interviews. In addition, we organized reflective seminars with interviewees to discuss the findings. The data analysis procedure was as follows. First, we realized summaries of interviews to compile collected information. Then, we made a coding through categories (Technical staff roles, different types of innovation activities, managerial practices and model of governance) in order to conceptualize and combine the data. Finally, we determined the general themes to identify patterns across all data collected. Moreover, the role of members of technical staff is not restricted to a role of technical and scientific consulting. Indeed, base on several interviews we distinguish two main roles: expert as a resource and expert as a strategist (Table .1).

Description of the findings

Expert as a resource and expert as a strategist

To be efficient, the role of the members of technical staff has to be shared by all actors of the firm. In my opinion, the role of members of technical staff must be understood according to different modalities of intervention. First, we can define expert as a resource. The role of expert is to provide a solution facing a technical problem. As a manager, I need to know that I can find an expert for a specific issue. Secondly expert as also to be strategist. 3). Each office is managed by a Fellow, and all members of technical staff are invited to participate in activities of one or several offices. This participation is not obligation, but this is an evaluation criteria allowing to evolve within the technical ladder.

Moreover, a technical staff charter has been created in order to specify the roles and the assignments of the members of technical staff. This charter is also signed by R&D managers and HR managers.

Theoretical and managerial implications

From theoretical perspective we highlight that design theory allows to characterize the role of technical staff according to two modalities of intervention: rule-based design and innovative design [START_REF] Le Masson | Strategic management of innovation and design[END_REF]. This distinction is not found in a judgment on the results, but in the reasoning and expertise mobilized. In rule-based design, expertise mobilized by members of technical staff are known and stabilized. They are working in a stabilized dominant design and they are the guarantors of the validity of knowledge, competencies and expertise. In this logic of design activities, it's possible to innovate. However, innovation is usually limited to a continuous improvement of existing products and technologies, i.e. incremental innovation. In innovative design, the challenge is different. Members of technical staff are in charge to renew knowledge, competencies and expertise to transform the identity of objects (i.e. to provoke the rupture of dominant designs). Hence, collective design activities are organized to intentionally renew the existing expertise. In this case, the main objectives are to manage and organize the investigation of new types of specifications, new competencies, new knowledge, new markets, new partners allowing to provoke the renewal of object identity. That way, we demonstrate that design theory is a powerful framework to go beyond the existing frameworks and we contribute to theoretical debates that attempt to explain the implementation of organizational arrangements and management practices in R&D.

From a practical perspective, our case study highlights a new organizational structure with its associated governance principles: The Technical Staff College. This new structure does not aim to replace the dual ladder system but proposes to complete this management device taking into account the role of experts and the conditions of technical staff collective action for radical innovation management. In a certain way, the existence of dual ladder system is a necessary condition to implement a governance structure for the technical staff, such as the Technical Staff College. On the one hand, the dual ladder system secures a pool of talented researches and scientists and allows to recognize technical experts in organizations. On the other hand, the case study in STMicroelectronics highlights new managerial solutions allowing to improve the efficiency of technical staff management, in particular for the management of radical innovation. First, this organizational device specifies the role of technical staff in organization. Second, it defines a strategic mission for the technical staff. It describes managerial solutions able to manage the dynamic evolution of expertise within the organization. In addition, it takes in consideration collective action issues between individuals, in particular by providing managerial instruments to organize coordination between actors, social interactions and learning process. And finally, it allows to address innovation management issues.

As [START_REF] Bobadilla | Managing scientific and technical experts in R&D: beyong tensions, conflicting logics and others of worth[END_REF], we suggest that to go beyond the dual ladder system, that implies that the new managerial solutions must be focused on the development of expertise rather than the experts themselves. The Technical Staff College, studied in our case, is above all an organizational device for managing expertise and innovation. However, we suggest that these types of managerial device must not to be considered as being informal devices. On the contrary, in our case study we observe that the management of expertise and innovation involve formal and well-defined organizational arrangements. To our opinion, the effectiveness of such managerial systems is conditioned to a systematic approach. Additionally, a well-defined organizational arrangement is need in order to rebalance influences and power relations within the organization. Furthermore, we observe that the process which led up to the creation of the Technical Staff College is a major issue for the acceptance, the sustainability and the effectiveness of such new structure. Indeed, during more than one year, people from different departments (HR, R&D management, members of technical staff, etc.) have developed a common vision of expertise management, that is indispensable to sustain the commitment of everyone involved.

While the findings presented in this research provide an in-depth understanding of roles and missions of members of technical staff in a science-based organization, they offer a limited basis for generalization. These results must be interpreted carefully, but we think that they are a fist step in the further exploration of organizational models for the management of technical staff in science-based organizations. Besides, we believe these results can be regarded as management recommendations on issues such as best practices and the extension of management theory. Finally, in order to reinforce and to validate these findings, it could be interesting to pursue other researches in other firms both in same industries and in different industries. More over, the studied phenomenon is very recent to the organization. Also, it seems necessary to complete this research with another study, perhaps in one or two years, enabling us to rigorously evaluate performances and implications of this new structure.

From

  2014 to 2015, a working group, composed by human resources managers, members of technical staff (only the Fellows), R&D managers and innovation directors have decided to lead an in-depth reflection about the organization of the experts' community in STMicroelectronics. The debate was based on two assumptions. First, the dual ladder system doesn't allow to organize an experts' community. Secondly, members of technical staff are not well engaged in the management of their community. Further to this reflection process, they decided to create a new organizational arrangement in charge to coordinate activities and missions of the technical staff: The Technical Staff College. The Technical Staff College is composed by three main components (Figure 2): • Board of Fellows: In charge to coordinate the activities of the Technical Staff College. • Technical Staff Community: In charge to participate in activities related to the missions and roles of the Technical Staff College. • Advisory Board: People from different ST organizations (HR, training, marketing, management, innovation managers, etc.) invited by technical staff as advisor for specific topics.

Figure 2 .

 2 Figure 2. Governance of the Technical Staff College (internal documentation, STMicroelectronics)

  At STMicroelectronics (ST), people use the term experts (i.e. technical and scientific experts) to designate technical staff who are recognized by the company as professionals with a high level of expertise. These professionals are member of the technical ladder, that has been created in 2011. ST technical ladder is composed by four levels (member of technical staff, senior member of technical staff, Fellow and Company Fellow). The evolution within the technical ladder depends of several criteria such as technical competencies (expertise, technical diversity and problem solving), business impact (technical vision, company competitiveness), innovation (number of publications and patents, innovative solution) and communication (internal and external network developer, communication influence, knowledge sharing). The HR function play a role of support rather than a role of leader.In contrast, the leader role is ensured by members of technical staff themselves. By leader role, we mean the capability to manage both members of technical staff and expertise development. For example, managing networks of experts, expertise and experience sharing seminars, animation of the community of experts through technical workshop are not the HR's responsibility. The technical ladder system does not produce an organized community whose members are able to work together. Members of technical staff have a key role in the effective operation of the expert community.

	As a HR manager of the technical ladder, my objective is support the development the technical career
	path. My main preoccupation is to recognize talented engineers who should be a member of the technical
	ladder for their career advancement. The selection is based following specific criteria such as innovation
	and technical competence (technical achievement, patent, relationship with universities & academia,
	etc.), sharing technical knowledge and technology reputation (communication skills, leadership on
	technical projects, etc.) and business impact (customers network, business acumen). In accordance with
	R&D managers, people, who are interested to become a member of technical staff, have to create a full
	application file. Then, I organize the selection in order to evaluate the performances of the candidate
	through a Technical Advisory Committee (TAC), which is composed by managers, HR and Fellows. (HR
	manager, STMicroelectronics).

The HR function have to support experts in their activities. They have to suggest strategies to support expert development through training opportunities for example. HR manage expert career but they are not in charge of expertise management. The management of expertise is the role of members of technical staff. The technical ladder provide essential support such as autonomy and freedom. However, the community of experts has to organize and manage the expertise, and especially the renewal of expertise to be able to innovate.

(Company Fellow, STMicroelectronics)

Table 1 .

 1 They have to be able to pursue research and innovation strategies to prepare the future of company.(R&D Director, STMicroelectronics) Roles of experts in organization4.2 Distinguish two innovation regimes: incremental innovation and radical innovationLinking innovation issues to the roles of technical personnel requires distinguishing two innovation regimes: incremental innovation development and radical innovation development (Table.2). Incremental innovation consists in continuous improvement of existing products and technologies. When markets, customers, technological requirements and expertise are well identified, new products or technologies development are also considered as incremental innovation development.It should not be assumed that manage incremental innovation is an easy process. Improving technologies and products is a main challenge for semiconductor industry. Investments are important and research programs are plentiful. However, markets, customers, partners and technical issues are well identified. On the one hand, experts have to be able to provide provide sustainable expertise and competencies for products and technologies development. On the other hand, experts must contribute to generate new expertise and competencies for disruptive or radical innovations.But incremental innovation is not enough. To prepare the future of the company, members of technical staff have also to pursue strategies to renew expertise or to develop new knowledge. For example, we develop research programs on silicon photonics, energy harvesting and neuromorphic technologies. Open innovation should not be viewed as a panacea for radical innovation R&D collaborations plays a central role in the development of new knowledge and expertise. However, the role of members of technical staff cannot be restricted in selecting and integrating external knowledge. Members of technical staff have for mission of influencing research orientations and technology developments outside the firm rather than integrating knowledge that originates outside the firm. Innovation involve to be the first on a new technology. If a company pursues a such strategy, based on the integration of external knowledge, it will never competitive. Moreover, in our industry, intellectual property (patent for example) is also an important challenge. If the company doesn't create its own new knowledge, it will never be able to create a strategic patents portfolio.(Senior Members of Technical Staff, STMicroelectronics). radical innovation exclusively based on the adoption of external knowledge and technologies is too restrictive. Indeed, key issues are the capabilities in formulating new concepts and in investigating new types of value (technological and business value). Contrary to what has been argued by other studies, we observe that within STMicroelectronics, contemporary innovation management practices don't reduce the influence of scientists and researchers. On the contrary, scientists and researchers are key resources for the development of radical innovations. Their roles are complexes and primordial. Their high level of expertise allows them to direct and guide original research programs and to use external resources such as university laboratories and public research centers.

		knowledge creation / skills, methods and knowledge development / dissemination of knowledge /
		training / transmission of experience and expertise / coordinate, manage and organize scientific
		work / animation of experts network / represent the company: publications and conferences
		prepare specific studies / carry out	directing research strategy / participate in the
		targeted scientific investigations / find	definition of new research programs / propose new
		technical solutions / provide substantive	ideas allowing to prepare the future / technology
	Roles of experts	arguments / result analysis / Risks	intelligence / technological benchmarking / being a
	in organization	evaluation / handle technical incidents /	force of proposals to senior management / make
		provide advice and support for technical	suggestions to improve innovation strategies of the
		issues / certification issues / work with	firm / be able to explain and manage the technology
		customer for technical issues/ problem	strategies of the company / manage a innovation
		solving / manage delicate crisis situations	ecosystem (partners, universities, customers and
		/ provide continuous improvements for	suppliers) / develop strategies to create disruptive
		existing products	innovations
		Expert as a resource	Expert as a strategist

However, applications, markets, customer needs, products

or technology platform are not yet fully understood, or at least partially unknown. So, a part of our job consist in contributing to create new expertise on new areas, in developing new collaborations and partnerships, even if there are not yet a products roadmap (Fellow, STMicroelectronics).

Technical Staff Valorization & RH relation •

  The charter comprises three subsections: "Members of Technical Staff have specific expertise missions to grow STMicroelectronics excellence", "Management supports the Members of Technical Staff specific missions in STMicroelectronics", "HR promote the development of the Technical Staff Community". The part dedicated to the members of technical staff in is composed by ten commitments, divided into three part: "Members of Technical Staff provide sustainable expertise and competencies for STMicroelectronics business needs", "Members of Technical Staff contribute to generate new expertise and competencies for disruptive innovation", and "Members of technical Staff are committed to the management of the Technical Staff Community". For example, the commitment number 10 is the following: "Members of Technical Staff contribute actively to the development of the technical community through the Technical Staff College, the offices of the Technical Staff community and Technical Advisory Committees". Coordinating the relationship between the different communities of technical experts • Promoting the technical ladder in the organization

	Incremental innovation development	Radical innovation development
	Office 1: Technical Benchmarks & Competition	Office 3: Technical Vision & Prospective
	Analysis	• Providing prospective and strategic
	• Conducting technological benchmarks and	analysis about disruptive technologies
	evaluating the performance of competing	• Exploring new applications and possible
	technologies	uses from mastered technologies
	• Mapping the strategic expertise needs	• Anticipating and identifying technological
	• Securing intellectual property by filing	challenges of the future
	patents
	Office 2: Knowledge Management & Best Practice	Office 4: Disruptive Innovation Exploration
	• Developing methods and procedures to	• Creating new expertise and competencies
	improve knowledge management in the	in order to generate new concepts & rules
	organization	systems
		• Supporting disruptive initiatives that could
		initiate new expertise and new pertinent
		concepts
	Support
	Office 5: University & Laboratory Collaboration
	• Developing expertise and network by participating in conferences and by developing projects and
	partnerships with laboratories, universities (ex: courses, ANR project, PhD, European project, etc.)
	Office 6: Technical Workshop & Training
	• Transferring and disseminating knowledge through workshops and trainings (ex: Techday)
	Office 7:

Table 3 .

 3 Technical Staff Offices and missions (internal documentation, STMicroelectronics)


Adrien Girard

Hélène Gehan

Aurelien Crut

Alain Mermet

Lucien Saviot

Jeremie Margueritat
email: jeremie.margueritat@univ-lyon1.fr

Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy

Keywords: Vibration, Lamb, gold NPs dimer, mechanical properties, Raman, Brillouin Article

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During the last two decades many works have focused on the characterization of the acoustic vibrations of nanoparticles (NPs). Many materials were considered (dielectric spheres, [1][2][3][4] metallic NPs, [5][6][7] quantum dots 8-10 ) using different optical spectroscopy techniques such as pump-probe experiment, [11][12][13][14] and low frequency Raman or Brillouin scattering. 7,15-18 They mostly addressed the localized acoustic vibration modes of non-interacting nano-objects, yielding many results regarding the effect of the NP crystallinity, 7,15 size, 16 shape, 17,18 and surrounding medium. 19,20 More recently, results on the physical properties of ordered NP assemblies were obtained. 5,[21][22][23][24] In analogy with their atomic counterparts, such assemblies are commonly called super-crystals (where atoms are replaced by spherical NPs) and also sustain long wavelength acoustic waves. These correspond to coherent motions of the NPs around their equilibrium positions, propagating with a velocity governed by the mass of the spheres and the elastic properties of the ligands that bind them together. Various pump-probe experiments [25][26][27][28][29] have enabled the observation of such elastic waves, showing that they propagate at speeds lower than in their atomic crystal counterparts due to heavier masses and looser bonds. However, probing such sound waves in supercrystals through the classical Brillouin scattering technique has not yet been reported, most probably due to their small scattering cross-sections.

Yet since more than 40 years, the limitation due to poor scattering efficiencies in light scattering experiments is known to be possibly lifted by placing the object of interest in the close vicinity of a metallic surface, like in surface enhanced Raman scattering. 30 Such a possibility was recently demonstrated for low frequency vibration modes of NPs. 31,32 In the present paper the possibility to enhance the scattering by ultra low frequency modes was investigated, using gold NPs where collective electron oscillations (localized plasmons) and collective atom oscillations (localized vibrations) take place at the same time within the same object, which thus acts both as enhancer and scatterer. In particular, using an excitation wavelength corresponding to the optical resonance of gold NPs in close vicinity enables to measure new low frequency features that are characteristic of the mechanical coupling between the NPs.

The solutions of gold NPs used in this study were synthesized following a seeded-growth procedure already reported in the literature 33,34 (see Supporting Information for a detailed description). Thanks to this method it was possible to tune the gold NPs diameters from few to hundred of nanometers. After synthesis, polyvinylpyrrolidone (PVP) was used to stabilize the as-synthesized gold NPs in order to transfer them in ethanol, ensuring colloidal stability over several months. For optical measurements, drops of gold NP/PVP solutions were deposited onto indium tin oxide substrates and let dry overnight, leading to the formation of a polymer film with a thickness of approximately 100 µm in which the NPs are embedded. Meanwhile, drops of the same solutions diluted one hundred times in ethanol were deposited onto carbon grids for morphological characterization using a transmission electron microscope (TEM). The average NP diameter in each solution was found to vary from 12 to 100 nm, with relatively low size dispersion (see Supporting Information for a detailed description).Typical images of gold NPs from a solution with average diameters of 3 100 nm are presented in Figure 1 A and B. For this sample the size dispersion is about 10%.

The entanglement of adsorbed PVP chains of neighboring NPs during the drying step leads to the formation of disordered clusters, with a 1 to 2 nm inter-particle separation due to the polymer coating that prevents a direct contact between the particles. As an example the dimer of Figure 1B shows an inter-particle distance (d) of about 1 nm. Figure 1: A. TEM image of diluted gold 100 nm sample. B. 100 nm NP dimer showing d ≈0.8 nm inter-particle spacing. C. Experimental setup allowing to measure both micro-absorption (transmission geometry, white lamp) and micro-Raman spectra (backscattering geometry, 532/647 nm laser lines) of gold NPs from a unique sample spot (laser spot ≈0.6 µm 2 ). D. From top to bottom, absorption spectra of samples with 12 to 100 nm diameter showing the dipolar resonance of isolated particles and the plasmonic coupling band. Red dashed lines correspond to absorption spectra of 12 and 100 nm NPs in liquids before deposition. Green and red vertical lines correspond to the 532 and 647 nm excitation wavelengths used for Raman spectroscopy, respectively.

in Figure 1C, allowing to correlate plasmonic and vibrational resonances of the assemblies.

Absorption spectra were recorded in transmission configuration. The white light beam was focused onto the sample surface using a condenser with a numerical aperture of 0.65 and the transmitted light was collected with a ×100 objective (NA = 0.95) and focused onto the entrance slit of a spectrometer equipped with a thermo-electric cooled CCD.

Figure 1 D shows the absorption spectra obtained for seven films of PVP containing NPs with different average diameters, together with those of the initial solutions of PVPcoated NPs for 12 nm and 100 nm diameters. A typical band around 540 nm is observed in all spectra and is attributed to the dipolar surface plasmon resonance of isolated NPs, which is confirmed by its presence on the spectra obtained in liquids. At larger wavelengths, between 600 and 750 nm, one additionally observes a more or less well defined shoulder that is not observed in the initial solution (Figure 1 D)(see also Supporting Information).

The origin of this additional feature lies in the large density of NPs and the presence of NPs clusters with short interspacing distance (d) (Figure 1C), leading to a strong electromagnetic coupling between neighbouring NPs and thus the appearance of additional resonances in the optical response of the assemblies [35][36][37][38] (referred to in the following as "coupling plasmon band") . In the simplest case of NP dimers, it was shown that the energy of the coupling plasmon band redshifts as the gap size decreases. 39 For NP assemblies, the energy of the coupling plasmon band and its relative intensity depend on different parameters such like the distribution of interparticle spacing, the ratio of interacting versus non interacting NPs and the size of the NPs. These multiple dependences explain why no clear trend in the intensity of the plasmonic coupling band is observed as the diameters of the NPs increase in Figure 1 D. Nevertheless the absorbance is always non-negligible around 647 nm implying that significant electromagnetic interaction between NPs take place in all samples.This is in agreement with Figure 1 B which underlines the possibility to form supermolecules, among which the basic one is the dimer, with an average interparticle separation of about 1 nm fixed by the PVP coating shell. In the following, we focus on how tuning the excitation wavelength to the plasmonic coupling band of these NP assemblies allows to detect the inelastic light scattering from interacting NPs vibration modes. Low frequency Raman scattered light was collected in the backscattering geometry using a ×100 objective (NA =0.95) and focused on the entrance hole of a six pass tandem Fabry-Pérot interferometer that can be operated at two distinct wavelengths. These wavelengths were chosen to match either the dipolar resonance of the isolated NPs or the plasmonic coupling band of the interacting NPs and generated using the 532 nm line from a solid state laser and the 647 nm line from a krypton laser (vertical green and red lines in Figure 1D).

Figure 2 shows the Raman spectra of a sample containing gold NPs of 49 nm diameter for both excitations. At 532 nm the Raman spectrum shows no other contribution than the quadrupolar = 2 Lamb mode at 22 GHz, in excellent agreement with its expected frequency for free nanospheres f ( = 2) = 0.85v T D =21.6 GHz using the transverse velocity of bulk gold v T =1250 m.s -1 and the average diameter (D) measured by TEM. When tuning the laser wavelength to 647 nm close to the resonance with the plasmonic coupling band, the quadrupolar vibration mode is still observed at the same frequency. However, the spectra of Figure 2, normalized with respect to the acquisition time and laser power used, show that the measured intensity of the = 2 mode is at least 1000 times larger with the red excitation than with the green excitation, evidencing the strong enhancement in the former case (see Supporting Information). The strong enhancement observed is probably due to the higher sensitivity of the absorption cross-section to the small displacement of the nanoparticles surface at 647 nm.

Additionally, the resonant excitation with the coupling plasmon band also leads to the observation of a second sharp and intense band in the low frequency part of the spectra around 9 GHz. Its frequency stands largely below that of the traditionally observed lowest frequency Raman active mode, 40 i.e. the quadrupolar = 2 mode. This ultra low frequency range is reminiscent of previous pump-probe spectroscopy measurements on super-crystals in which a mode correlated to the propagation of acoustic waves between the mechanically interacting NPs was observed. 25,27,28 However the independence of the ultra low frequency peak with scattering angle (Figure 2 inset) confirms the confined (rather than propagative) character of the vibration mode observed.

The ultra low Raman spectra obtained with varying NP diameters from 12 to 100 nm are presented in Figure 3A. For all spectra the typical acquisition times were of the order of one hour except for the sample containing NPs of 12nm diameter (24 hours), which yielded a low scattering efficiency, as illustrated by the lower signal-to-noise ratio and the emergence of a 26 GHz Brillouin signal from the underneath substrate. Note that the usually intense Brillouin signal is completely screened by the intense ultra low frequency band otherwise.

Both the quadrupolar and the ultra low frequency band frequencies decrease with increasing NP diameters(Figure 3B). The dependence of the quadrupolar frequency measured with Frequency shift as a function of the NP inverse diameter for the ultra low frequency band at 647 nm(black squares), the quadrupolar =2 vibration bands at 532 nm (green squares) and 647 nm (red squares). Red full line is the theoretical quadrupolar frequency f =0.85v T /D and the black line is a fit of the ultra low frequency band with zero intercept. =1 fundamental frequency, calculated with complex frequency model 6 in PVP matrix, is indicated with a green line.

the 532 nm excitation wavelength, i.e. exciting the dipolar resonance, scales as 0.85v T /D as predicted by the free sphere Lamb model. 41 With the 647 nm excitation the frequency variation slightly deviates from the Lamb model prediction, in particular for the NPs of 25 and 18 nm diameters. These deviations will be explained thereafter at the light of the interpretation of the ultra low frequency band.

The ultra low frequency band position also scales inversely to the NP diameter, but with a lower slope as compared to the =2 mode. This means that this vibration mode is somehow related to the vibration of the NPs. Only spheroidal and torsional =1 Lamb modes can reach such a low frequency. For a free sphere the frequencies of these modes are zero since there is no restoring force. They become non-zero for NPs embedded in a host matrix, and correspond to rattling (constrained translation) and libration (constrained rotation)

vibrations respectively. This latter mode is never observed in Raman due to its torsional nature and is discarded in the following. According to selection rules 40 the rattling mode of a single NP is not Raman active. However it is instructing to compare the frequency of the ultra low frequency mode with the rattling mode of a single NP since it is well known that in the case of strong enhancement with hot-spots, Raman selection rules may be lifted. To do so we used the complex frequency model described elsewhere 6 considering a gold particle in an infinite PVP matrix. The transverse and longitudinal velocities as well as the density of bulk gold were used. The density of PVP used for this calculation was ρ = 1.2 g.cm -3 and its longitudinal and transverse sound velocities (v L =2989 m.s -1 and v T =1443 m.s -1 ) were deduced from Brillouin experiments on gold-free PVP films (see Supporting Information).

The calculated spheroidal ( = 1, n = 0) frequencies are reported in Figure 3B (green line).

However, these calculated frequencies do not reproduce the experimental frequencies. The ultra low frequency mode observed can therefore not be simply assigned to the spheroidal = 1 eigenmode of an isolated NP even if it appears close enough to the experimental values.

Another possibility for the rattling mode to become Raman active is a change of the symmetry of the system. This occurs for example when placing a NP on a surface 42 or, when considering a supermolecule like a dimer of NPs 43 in which the frequency of the vibration will be necessarily impacted. Two dimer modes can be built from the rattling modes of the individual embedded NPs, namely the in-phase and the out-of-phase rattling motions of the NPs. From symmetry considerations, only the out-of-phase mode is expected to be Raman active. Since the separation between the two NPs oscillates in the vibration, we expect a strong coupling of this mode with the coupling plasmon band. The ultra low frequency feature could thus be the signature of the mechanical coupling between both NPs (mediated by the PVP matrix) resulting in dimer supermolecule modes.

This ultra low frequency vibration should therefore depend on the elasticity of the interstitial polymer (PVP). In order to check this assumption we have followed the evolution of the low frequency Raman spectra as a function of temperature. We have measured the Raman spectra of the sample containing gold NPs of 49 nm diameter from ambient temperature T=24 • C down to T=-180 • C. In this temperature range, the PVP matrix remains in the vitreous state but its elastic properties are modified. The corresponding Raman spectra are gathered in Figure 4A while Figure 4B compares the relative frequency shifts of the Raman bands with those of a NP-free PVP bulk film drop-cast in the same conditions as the NP containing samples. Upon cooling all vibration frequencies increase. For a free sphere, the expected frequency shift of the quadrupolar =2 mode results from the variation of the density ρ and from the elastic constant C 44 of gold, which accounts for the relative variation:

∆f f = 1 2 ( ∆C 44 C 44 -∆ρ ρ ).
The coefficient of thermal expansion of gold is α gold V =1.4×10 -5 K -1 , which results in a mass density variation of only ∆ρ ρ = α gold V ∆T = 0.3% over the considered temperature range. From Ref. 44, the relative variation of C 44 is about 5.7% in the 0-300 K range. This leads to ∆f f ≈2.7%, in very good agreement with the measured variation ∆f =2 f =2 =2.8%. Therefore the moderate frequency shift of the =2 mode is mostly related to the temperature-induced gold stiffening, in agreement with calculations using the complex frequency model which also show that its frequency is weakly affected by the elastic properties of the environnement. Conversely, the ultra low frequency band (black squares) strongly deviates from this trend with a relative shift ∆f f =27.5%. This further suggests that this mode cannot be interpreted as a Lamb mode of a single NP. In order to evaluate the polymer contribution, the longitudinal Brillouin mode of the PVP without NPs was monitored and is reported in Figure 4B(red squares). Over this temperature range, a relative increase of the Brillouin longitudinal mode frequency by 21% is observed. Adding the experimental frequency variation due to the gold elastic parameters (21+2.8≈24%) results in a value close to the measured one. Therefore, the ultra low frequency mode is strongly related to the PVP which acts as a binder between NPs.

Since we have strong evidence that the lowest frequency vibrations observed in the Raman spectra are due to the coupling between NPs, we consider in the following the case of dimer vibrations, which is the simplest way to take into account interparticle coupling between adjacent NPs in supermolecules. This does not necessarily imply that the measured Raman intensity comes from dimers only. Cyan and yellow squares are data from ref. 45 and ref. 46 The vibration of interest can be modeled in a first approach by the symmetric stretch of two masses connected by a spring of effective stiffness K ef f . Within this simple picture the dimer vibrationnal frequency can be approximated by :

ω = 2K ef f M (1)
where M=ρ 4 3 π( D N P 2 ) 3 is the mass of one NP. The effective stiffness constants K ef f evaluated from equation 1, ranging between 370N.m -1 and 2700N.m -1 , as a function of the NPs diameters are summarized in Figure 5 and compared to previous measurements performed using pump-probe spectroscopy on isolated dimers containing 80 nm, 46 28 and 10 nm diameter gold particles. 45 The values of K ef f deduced from these measurements are of the same order of magnitude as ours (Figure 5), even if in these references the authors claimed a direct contact between the gold NPs. Moreover the observed linear behavior is in good agreement with studies conducted on crystals of coupled large polystyrene spheres in contact. 1 However, the spring model cannot be expected to provide a good description of the observed low frequency mode as the non-zero frequency of the NP rattling mode was shown to be a consequence of the full embedding of the NPs. 43 Moreover, this model only provides a rough approximation of the dimer nano-contact as it does not allow taking into account subtle effects such as the elastic stiffening of the surrounding medium due to the interaction with the NP surface. [47][48][49][50] More importantly, the spring model assumes a purely stretching motion of rigid spheres, while the actual rattling motion involves a dipolar-type deformation of the nanospheres.

In this work the entanglement of the PVP molecules form a solid vitreous matrix all around the sphere. In order to fully take into account the presence of such an embedding matrix we have used finite element modeling (Figure 6 A). In this approach, a gold dimer is set within a spherical embedding medium of larger dimensions with the elastic properties of PVP. A spherical perfectly matched layer prevents any unrealistic effect due to the reflection of acoustic waves at the system boundaries. An oscillating unidirectional boundary load is applied on one NP side in the direction of the dimer long axis and varying the frequency of The calculations were performed using the same parameters as those used for estimating the rattling mode frequencies of single NPs (Figure 3). As an illustration the dimer eigenmode (relevant for this study) is sketched Figure 6B. for a 12 nm diameter NP. This mode corresponds to an out-of-phase translation of the NP centers, i.e. a symmetric stretch of the dimer, with a slight dipolar deformation of the NPs. They are mainly translated along the dimer symmetry axis, which confirms their link with the spheroidal = 1 mode of a single sphere in a matrix. As shown in Figure 6C. the calculated size dependence of the out of phase mode frequency is found to perfectly match the experimental one. Note that the excellent agreement between measured and computed vibrational frequencies is not preserved if simulations are performed on more complex multimers (e.g. trimers), a posteriori justifying the assumption of dimeric acoustic interactions made in the modeling. In order to appreciate the effect of the different parameters used in the model, a typical variation of 10% around the values used here (d=1 nm, C P V P 11 =10.7 GPa, ρ P V P =1200g/m 3 , and ν P V P =0. 35) was applied to each of them. These calculations (detailed in the Supporting Information)

show that d, ρ P V P , and ν P V P only have a weak impact on the relative frequency shift of the dimer vibration mode (less than 1%) compared to the C P V P 11 that induces a variation of 5%.

These results confirm that the dimer vibration mode that originates from the mechanical coupling of the = 1 rattling mode is essentially sensitive to the elastic properties of the surrounding medium. Basically what we have observed with the rattling mode at ultra low frequencies is a classical coupling scheme where two identical modes of moieties couple to generate an out-of-phase mode and a (herein non-observed) in-phase mode, the frequency of the out-of-phase mode being larger than that of the non-coupled moieties. In the present experiment with gold NPs the effect of this coupling was observed thanks to the resonance between the Raman excitation wavelength and the plasmonic coupling excitation. Therefore when the =2 mode is detected with the 647 nm line, i.e. in resonance with the plasmon coupling band, the scattered signal is mainly due to the NPs that are present in resonant structures, i.e. mechanically coupled NPs thus explaining the small deviations observed for the = 2 mode frequencies in Figure 3 B. For the smallest NPs almost no coupling plasmon band is observed (Figure 1D) which means that there is only few coupled NPs and weak enhancement is observed, thus the signal measured is mainly due to the mechanically isolated NPs like in the case of an excitation at 532 nm. Once again, thanks to the plasmonic enhancement it is thus possible to demonstrate the effect of the mechanical coupling on the vibration modes of mechanically coupled NPs.

In summary, we have successfully investigated the vibrations of polymer-connected gold NP supermolecules using ultra low frequency Raman spectroscopy, optical absorption measurements and finite element simulations. Tuning the excitation wavelength to the plasmonic coupling band has led to a great enhancement of Raman signals and allowed us to observe for the first time the inelastic scattering by an ultra low frequency vibrational mode of embedded and mechanically coupled gold NP supermolecules. Using finite element simulations, this vibration mode was assigned to an out of phase symmetric stretch of the NPs composing a dimer. We have demonstrated that this mode takes its origin in the spheroidal = 1 vibration mode (rattling) of embedded isolated NPs. The plasmon-enhanced spectroscopy of the vibrations of mechanically coupled NPs allows studying the elastic properties of the surrounding medium, as the dimer vibration mode was found to be sensitive to its change of elastic properties induced by temperature. The strong enhancement observed by exciting in resonance with the plasmonic coupling band opens the door to studying the vibrations of a single dimer and correlating its Raman spectrum with its exact morphology.

French Agence National de la Recherche and the Fédération André Marie Ampère 2013 (FRAMA). The authors declare no competing financial interest. We thank the Centre Technologique des Microstructures at Villeurbanne (CTµ) for access to the microscopes platform for TEM characterizations. We also thank E. Cottancin for his help for the morphological characterization by TEM.

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Figure 2 :

 2 Figure 2: Raman spectra of the Au49nm sample recorded at 532 nm (resonance with the surface plasmon resonance (black curve and scale)) and at 647 nm (resonance with the plasmon coupling band (red curve and scale)). The spectra were normalized with respect to the acquisition duration and power. The inset shows spectra of the same sample recorded in backscattering geometry (θ k inc , ksca =180 • )(pink full line) and forward scattering geometry at θ k inc , ksca =18 • (black dashed line).

Figure 3 :

 3 Figure3: A. Low and ultra low frequency Raman spectra of NPs assemblies with varying NPs diameters (12-100 nm). B. Frequency shift as a function of the NP inverse diameter for the ultra low frequency band at 647 nm(black squares), the quadrupolar =2 vibration bands at 532 nm (green squares) and 647 nm (red squares). Red full line is the theoretical quadrupolar frequency f =0.85v T /D and the black line is a fit of the ultra low frequency band with zero intercept. =1 fundamental frequency, calculated with complex frequency model 6 in PVP matrix, is indicated with a green line.

Figure 4 :

 4 Figure 4: A. Au49nm Raman spectra upon decreasing temperature from 24 • C down to -180 • C from bottom to top. B. Relative frequency shifts normalized to those of ambient temperature for the = 2 (empty squares), the ultra low frequency mode (black squares) and the PVP longitudinal Brillouin mode (red square).

Figure 5 :

 5 Figure5: Size dependence of the effective stiffness constant K ef f evaluated from the spring model (black triangles). Cyan and yellow squares are data from ref.45 and ref.46

Figure 6 :

 6 Figure 6: A. Schematic view of the simulation geometry. B. Representation of the dimer out of phase vibration mode. C. Calculated dimer out of phase (red) frequencies as a function of the inverse NP diameter compared to experimental frequencies (black squares).

  7) Portales, H.; Goubet, N.; Saviot, L.; Adichtchev, S.; Murray, D. B.; Mermet, A.; Duval, E.; Pileni, M.-P. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 14784-9. (8) Saviot, L.; Champagnon, B.; Duval, E.; Ekimov, A. Phys. Rev. B 1998, 57, 341-346. (9) Saviot, L.; Champagnon, B.; Duval, E.; Kudriavtsev, I.; Ekimov, A. J. Non. Cryst. Solids 1996, 197, 238-246. (10) Ivanda, M.; Babocsi, K.; Dem, C.; Schmitt, M.; Montagna, M.; Kiefer, W. Phys. Rev. B 2003, 67, 235329. (11) Cardinal, M. F.; Mongin, D.; Crut, A.; Maioli, P.; Rodríguez-González, B.; Pérez-Juste, J.; Liz-Marzán, L. M.; Del Fatti, N.; Vallée, F. J. Phys. Chem. Lett. 2012, 3, 613-619, PMID: 26286157. Nano Lett. 2012, 12, 5292-5298. (16) Bachelier, G.; Margueritat, J.; Mlayah, A.; Gonzalo, J.; Afonso, C. Phys. Rev. B 2007,

	76, 1-6.
	(17) Margueritat, J.; Gonzalo, J.; Afonso, C. N.; Mlayah, A.; Murray, D. B.; Saviot, L. Nano
	Letters 2006, 6, 2037-2042.

(12) Baida, H.; Christofilos, D.; Maioli, P.; Crut, A.; Del Fatti, N.; Vallée, F. J. Raman Spectrosc. 2011, 42, 1891-1896. (13) Crut, A.; Maioli, P.; Del Fatti, N.; Vallée, F. Ultrasonics 2015, 56, 98-108. (14) Crut, A.; Juvé, V.; Mongin, D.; Maioli, P.; Del Fatti, N.; Vallée, F. Phys. Rev. B 2011, 83, 205430. (15) Portales, H.; Goubet, N.; Sirotkin, S.; Duval, E.

(18) Large, N.; Saviot, L.; Margueritat, J.; Gonzalo, J.; Afonso, C. N.; Arbouet, A.; Langot, P.; Mlayah, A.; Aizpurua, J. Nano Lett. 2009, 9, 3732-8.

Micro-absorption and micro-Raman spectra were measured at the same position as sketched

Acknowledgement

This work was supported by the ANR NanoVip project, grant ANR.13.JSIO.0002 of the

Supporting Information Available

Details of experimental procedures and characterization data for the samples measured as well as details for finite element simulations can be found here. This material is available free of charge via the Internet at http://pubs.acs.org/.