François-Marie Martin
email: francois-marie.martin@inrae.fr

Philippe Janssen

Laurent Bergès

Blandine Dupont

André Evette

Higher structural connectivity and resistance against invasions of soil bioengineering over hard-engineering for riverbank stabilisation

Keywords: longitudinal connectivity, riparian vegetation, forest corridor, plant invasions, biotic resistance, riverine landscape management

Riparian corridors play an important role for the maintenance of regional biodiversity and ecosystem functions. Riparian forests are even the only semi-natural vegetation strips remaining in many agricultural or urbanised landscapes. In such landscapes, the spatial continuity of riparian vegetation is frequently broken by the construction of stabilisation structures engineered for erosion control. Here, we examined the effects of different riverbank stabilisation structuresfascines (soil bioengineering), ripraps (hard engineering), and mixed-technique (lower-bank ripraps with upper-bank plantings) -on the structural connectivity of their respective riverbanks. We first revisited previously studied stabilisation structures to extend their vegetation sampling to their adjacent riverbanks. Then, for each type of stabilisation structure, we compared community composition, richness and abundance of native and invasive alien species (IAS), and cover of vegetation strata (herbaceous, shrub and tree) between stabilised embankments and their upstream and downstream banks. Results indicated that, although the composition of fascine banks differed from that of their adjacent riverbanks, they fitted nicely in the structural continuity of their riparian surroundings. Differences were likely explained by the proportion of fast-growing woody species (e.g. willows) planted in fascines, which also induced strong reductions in IAS richness and abundances; i.e. propagule "sinks". Conversely, ripraps broke the structural continuity of riverbanks and were heavily dominated by IAS while mixed-technique banks displayed intermediate characteristics. Consequently, we argued that fascines may be the riverbank stabilisation structures displaying highest ecological benefits in terms of habitat quality and connectivity and should be preferred over the other investigated engineering techniques.

Introduction

At the interface between aquatic and terrestrial ecosystems, riparian zones are landscape elements with disproportional ecological, recreational and aesthetic importance [START_REF] Naiman | The ecology of interfaces: Riparian zones[END_REF][START_REF] González | Integrative conservation of riparian zones[END_REF]. Although they only represent 1.4% of continental land surfaces, riparian zones and related floodplains contribute to more than 25% of all terrestrial ecosystem services [START_REF] Tockner | Riverine flood plains: present state and future trends[END_REF]. Under natural conditions, rivers and their riparian areas are disturbance-dependent dynamic systems that are longitudinally, laterally and vertically linked by hydrogeomorphological processes occurring in spatiotemporal hierarchies [START_REF] Ward | Ecological connectivity in alluvial river ecosystems and its disruption by flow regulation[END_REF][START_REF] Tabacchi | Development, maintenance and role of riparian vegetation in the river landscape[END_REF]. The freedom space required by such dynamic systems to function properly is however often in conflict with human activities. This is why, despite their paramount importance, riverine ecosystems have undergone a very long history of human modifications [START_REF] Labonne | History of bioengineering techniques for erosion control in rivers in Western Europe[END_REF] and are therefore highly degraded and threatened worldwide [START_REF] Tockner | Riverine flood plains: present state and future trends[END_REF][START_REF] Dudgeon | Freshwater biodiversity: importance, threats, status and conservation challenges[END_REF][START_REF] González | Integrative conservation of riparian zones[END_REF][START_REF] Tonkin | Flow regime alteration degrades ecological networks in riparian ecosystems[END_REF].

Flow regulation by dams and channel stabilisation by dikes are among the various anthropogenic pressures that alter the functionality of riparian zones [START_REF] Best | Anthropogenic stresses on the world's big rivers[END_REF]. By nature, these heavy civil engineering infrastructures alter flow, sediment and disturbance regimes, and lead to the degradation, loss or fragmentation of natural riparian habitats. As such, these constructions are directly and indirectly responsible of strong reductions in biodiversity and ecosystem functions over space and time [START_REF] Ward | Ecological connectivity in alluvial river ecosystems and its disruption by flow regulation[END_REF][START_REF] Dudgeon | Freshwater biodiversity: importance, threats, status and conservation challenges[END_REF]. By modifying the structure and composition of native riparian communities, they also favour the establishment and spread of invasive alien plant species (hereafter, IAS) in riparian corridors [START_REF] Tickner | Riparian plant invasions: hydrogeomorphological control and ecological impacts[END_REF][START_REF] Pyšek | Contrasting patterns in the invasions of European terrestrial and freshwater habitats by alien plants, insects and vertebrates[END_REF]. Overall, these cumulative changes in both abiotic and biotic conditions may lead to catastrophic shifts in ecosystems properties [START_REF] Scheffer | Catastrophic shifts in ecosystems[END_REF], therefore compromising efforts to restore riparian vegetation [START_REF] Richardson | Riparian vegetation: degradation, alien plant invasions, and restoration prospects[END_REF].

When riverbanks need to be stabilised to protect human stakes from erosion, "hard" civil engineering techniques like ripraps or concrete walls are often chosen. Still, the use of naturebased solutions such as soil bioengineering techniques (e.g. fascines or vegetated crib-walls) is increasingly promoted [START_REF] Li | Biotechnical engineering as an alternative to traditional engineering methods: A biotechnical streambank stabilization design approach[END_REF][START_REF] Von Der Thannen | Development of an environmental life cycle assessment model for soil bioengineering constructions[END_REF]. Soil bioengineering seeks to mimic the natural functioning of riparian vegetation, i.e. strengthening riverbanks through the use of the soil fixation properties of roots and protecting the soil via a carpet effect as well as by slowing down water flow [START_REF] Li | Biotechnical engineering as an alternative to traditional engineering methods: A biotechnical streambank stabilization design approach[END_REF][START_REF] Lavaine | European Tamaricaceae in bioengineering on dry soils[END_REF]. Recently, several studies have demonstrated the effectiveness of soil bioengineering structures in resisting shear stress (e.g. [START_REF] Pinto | A method for selecting suitable technical solutions to support sustainable riverbank stabilisation[END_REF][START_REF] Recking | Design of fascines for riverbank protection in alpine rivers: Insight from flume experiments[END_REF] as well as their ecological benefits for riparian biodiversity (e.g. [START_REF] Cavaillé | Functional and taxonomic plant diversity for riverbank protection works: bioengineering techniques close to natural banks and beyond hard engineering[END_REF][START_REF] Janssen | Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains[END_REF]. Nevertheless, as far as we know, no study has yet assessed the ecological benefits of these stabilisation techniques on the structural connectivity of riparian habitats, including the effects on the continuity of IAS populations. This information is yet essential to help stakeholders and river managers choose the engineering structure that will best fulfil the various functions associated with embankments (e.g. consolidation, restoration, recreation).

Connectivity is basically the degree to which the landscape facilitates or impedes species movements among habitat patches [START_REF] Taylor | Connectivity is a vital element of landscape structure[END_REF]. Two types of connectivity are usually distinguished: (i) functional connectivity, which refers to the behavioural responses of dispersing individuals to landscape structure and elements; and (ii) structural connectivity, which is related to the continuity and spatial arrangement of habitat patches in the landscape, independently of species attributes [START_REF] Tischendorf | On the usage and measurement of landscape connectivity[END_REF][START_REF] Baguette | Landscape connectivity and animal behavior: functional grain as a key determinant for dispersal[END_REF]. Obviously, although a structural connection may exist between habitat patches (i.e. a corridor), it does not necessarily mean that it is functional and thus used by all species [START_REF] Beier | Forks in the road: Choices in procedures for designing wildland linkages[END_REF]. Nonetheless, the spatial continuity of habitats is recognized as of prime importance for the control of gene and individual fluxes and is therefore crucial for the maintenance of populations and metapopulations [START_REF] Taylor | Connectivity is a vital element of landscape structure[END_REF][START_REF] Mitchell | Linking landscape connectivity and ecosystem service provision: current knowledge and research gaps[END_REF]. As such, the overwhelming importance of riparian zones in supporting regional biodiversity and ecosystem functions is, to a large extent, linked to their role as habitats and dispersal corridors [START_REF] Naiman | The role of riparian corridors in maintaining regional biodiversity[END_REF][START_REF] Tabacchi | Development, maintenance and role of riparian vegetation in the river landscape[END_REF][START_REF] Van Looy | Order and disorder in the river continuum: the contribution of continuity and connectivity to floodplain meadow biodiversity[END_REF]. Around 70% of vertebrate species use riparian corridors at some point of their life-cycle (Raedeke, 1989cited in Naiman et al., 1993), and evidence is piling up on the role of the spatial continuity of riparian forest in maintaining the dispersal dynamics of many species groups (e.g. [START_REF] Lambeets | Integrating environmental conditions and functional life-history traits for riparian arthropod conservation planning[END_REF][START_REF] Golfieri | Towards a more comprehensive assessment of river corridor conditions: A comparison between the Morphological Quality Index and three biotic indices[END_REF]. This is especially relevant when we consider that riparian forests account for much of the remnant forests in many transformed landscapes [START_REF] Lees | Conservation value of remnant riparian forest corridors of varying quality for Amazonian birds and mammals[END_REF][START_REF] Fremier | A riparian conservation network for ecological resilience[END_REF].

Considering that, (i) the construction of engineered embankments on riverbanks leads to severe alterations of the structure of riparian habitats and impacts species whose movements are dependent on riparian connectivity and that, (ii) the time required for plant communities to recover and for the continuity of riparian vegetation to be restored varies with the local context but also with the level of revegetation of embankments [START_REF] Tisserand | Diversity and succession of riparian plant communities along riverbanks bioengineered for erosion control: a case study in the foothills of the Alps and the Jura Mountains[END_REF], we expect a strong influence of the characteristics of stabilisation techniques on connectivity. By revisiting several engineered riverbanks which were used to study the direct effect of structures' type on riparian biodiversity and habitat quality [START_REF] Cavaillé | Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers[END_REF][START_REF] Cavaillé | Functional and taxonomic plant diversity for riverbank protection works: bioengineering techniques close to natural banks and beyond hard engineering[END_REF][START_REF] Janssen | Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains[END_REF], we aimed to assess the relative effect of civil engineering and soil bioengineering structures on the structural connectivity of their respective local riverbanks. Specifically, we focused on three sets of engineered embankments -willow fascines (pure soil bioengineering), mixed-technique embankments (lower-bank ripraps with upper-bank plantings), and ripraps (pure civil engineering) -to answer the following questions: (1) do stabilisation structures break the floristic continuity of communities along riparian corridors at the local scale? (2) Do they affect patterns of native and IAS diversity? And (3) do they break the structural continuity of vegetation strata? These questions relate to different components of structural connectivity relevant for the dispersal dynamics of riparian plant and animal species. As such, they may help enlighten stakeholders and managers regarding the ecological impacts and benefits of stabilisation structures and may help improve the management of riparian ecosystems.

Materials & Methods

Study area and sampling design

To investigate the effects of stabilisation engineering structures on the structural connectivity of riverbanks, we developed a dedicated sampling design. In the summer 2017, we sampled 15 riverbank sites spread across the Rhône-Alpes region in central-eastern France. Sites hosted either willow fascines (n = 5), mixed-technique embankments (i.e., lower-bank riprap combined with soil bioengineering on the upper parts of the bank; n = 5) or ripraps (n = 5). For a given site, we collected data at the level of the engineered embankment (e.g. riprap) as well as on its adjacent riverbanks, i.e. upstream and downstream. A site therefore encompassed three sampling areas, hereafter referred to as: the upstream bank, the structure's bank, and the downstream bank (Figure 1). At the scale of each study site, we gathered information on various environmental variables and, at the scale of each sampling area, we measured bank steepness and sampled plant communities (see following sections).

To the best of our abilities, we tried to select sites that shared similar environment and position to obtain comparable situations, especially regarding species pools. Consequently, all sites were located in densely populated areas, along rivers running in plains or large valleys, between 160 and 510 m a.s.l., and in globally similar climatic areas. To be selected, sites had to host structures with a minimal length of 50 m in order to avoid strong edge-effects in samples (Figure 1). Because we wanted the plant communities of engineering structures to be approximately in the same successional stage, we also chose structures that had been built 12 to 20 years prior to the study. We also ensured that the vegetation on site was not regularly managed.

Environmental variables

To control site homogeneity, several environmental variables were measured. Site coordinates and elevation were measured in situ using a high-accuracy GPS (GeoExplorer 6000, Trimble ®, California, USA), while river widths were measured using an Impulse 200 rangefinder (Laser Technology Inc., Colorado, USA). In the middle of each bank, steepness was also estimated using the same laser rangefinder for all main slope knickpoints located between the water level and the top of the bank. Measurements were later averaged to get a mean slope for each bank.

The construction date and effective length of structures were gathered from interviews. The Strahler rank of rivers (i.e. tributary hierarchy) as well as sites' aspect (northness) were retrieved from ArcGIS 10.7.1 (ESRI, 2019). We also used GISs to create two landscape-based categorical variables: context and stake. Context is a two-level factor describing the dominant landcover class in a 500 m radius around each site (i.e. mainly "semi-natural" or mainly "suburban"). Stake is a three-level factor stating what landscape feature or object is being protected by the structure: i.e. a "bridge" (road, railway etc.), an "urban area" (street, house etc.), or a "semi-natural area" (forest, pasture or crop). Finally, we derived climatic variables (i.e. mean annual temperature and annual sums of precipitation) from the WorldClim model [START_REF] Hijmans | Very high resolution interpolated climate surfaces for global land areas[END_REF] and adjusted for the effect of elevation following [START_REF] Zimmermann | Predictive mapping of alpine grasslands in Switzerland: species versus community approach[END_REF].

Vegetation survey

Vegetation was surveyed using pin-point transects [START_REF] Bonham | Measurements for terrestrial vegetation[END_REF]. To sample the lateral gradient of riparian vegetation, we equidistantly placed three 25-m-long parallel transects alongside the river on the bottom, middle, and upper part of banks (Figure 1). Every 50 cm along each transect, a 2-m-long pole was vertically placed. If an individual of any plant species touched the pole below the height of 1 m, it was recorded as belonging to the herbaceous stratum. If a plant actually or virtually touched the pole axis at a height of 1 to 5 m, it was recorded as belonging to the shrub stratum, while any individual virtually crossing the pole axis at a higher height than 5 m was accounted for in the tree stratum. This method was chosen because it is time-efficient, robust against observer biases, and it gives good estimates of richness, abundance and cover for the most structuring plant species in the community [START_REF] Buckland | Line transect methods for plant surveys[END_REF][START_REF] Bonham | Measurements for terrestrial vegetation[END_REF]. The status (i.e. native, alien, or invasive alien) of each plant species was subsequently retrieved from the official list of the French National History Museum [START_REF] Gargominy | TAXREF v13, référentiel taxonomique pour la France : méthodologie, mise en oeuvre et diffusion[END_REF]. To summarize, we thus sampled 135 transects in total (3 transects per bank, 3 banks per site, 5 sites per type of engineering technique, 3 engineering techniques).

Statistical analyses

All analyses were performed using R version 3.6.2 (R Development Core Team, 2019) and the 'vegan' [START_REF] Oksanen | Package 'vegan': Community ecology package (v.2.5-6)[END_REF] and 'stats' packages. Data exploration was performed following [START_REF] Zuur | A protocol for data exploration to avoid common statistical problems[END_REF], and homoscedasticity was further checked using Bartlett tests.

Preliminary analyses were performed on environmental variables to assess whether fascine sites, mixed-technique sites and riprap sites were reasonably comparable (Online Resource 1). For quantitative variables (i.e. structure's age and length, river width and section's rank, site's latitude, longitude, elevation, slope, northness, average temperature and sums of precipitation), variations among the three groups of sites were investigated using either Kruskal-Wallis tests or Welch's ANOVAs, when data were homoscedastic or heteroscedastic, respectively [START_REF] Sheskin | Handbook of parametric and nonparametric statistical procedures: Third Edition[END_REF][START_REF] Shieh | Sample size determinations for Welch's test in one-way heteroscedastic ANOVA[END_REF]). Dunn's or Games-Howell's tests for multiple comparisons were used when significant differences were found in the former and latter cases, respectively [START_REF] Sheskin | Handbook of parametric and nonparametric statistical procedures: Third Edition[END_REF][START_REF] Shingala | Comparison of post hoc tests for unequal variance[END_REF]. The p-values of Dunn's tests were adjusted using the Holm-Bonferroni method. Mean slopes were additionally compared among the banks of each type of site to describe the topographical continuity of riverbanks. For the two categorical variables (namely context and stake), relationship among groups of sites and factor levels were examined using Pearson's Chisquare tests of independence [START_REF] Sheskin | Handbook of parametric and nonparametric statistical procedures: Third Edition[END_REF].

To explore the floristic component of riparian connectivity, i.e. variation in riparian community composition among upstream bank, structure's bank and downstream bank, we used Non-Metric Multidimensional Scaling (NMDS) with Bray-Curtis distances [START_REF] Legendre | Numerical ecology[END_REF]. First, highly uncommon species were removed based on sensitivity analyses ran to optimize NMDS stress values (Online Resource 2). Then, we performed a separate NMDS for each of the three groups of sites (i.e. fascine, mixed-technique and riprap) after ensuring the absence of multivariate spread in the data [START_REF] Anderson | Distance-based tests for homogeneity of multivariate dispersions[END_REF]. Within each group, we used Permutational Multivariate Analysis of Variance (PERMANOVA; cf. [START_REF] Anderson | Permutational Multivariate Analysis of Variance (PERMANOVA)[END_REF] with 999 permutations to test for significant differences in species composition between upstream, structure's and downstream banks. When significant differences were found, pairwise comparisons were then performed.

To understand the effects of stabilisation structures on biodiversity, we investigated variations in plant diversity patterns. Within each group of sites, overall plant richness (total number of species) and abundance (total number of species occurrences) were compared between the upstream, the structure and the downstream parts of riverbanks. Additionally, we also compared the richness and abundance of IAS (invasive alien plant species) using both absolute and relative measurements (i.e. proportions of IAS in total richness and abundance). Differences were investigated using Kruskal-Wallis tests or Welch's ANOVAs coupled with Dunn's or Games-Howell's post-hoc tests. To analyse the structural dimension of riparian vegetation connectivity, we used the same tests to highlight possible differences in herbaceous, shrub and tree cover within the three groups of sites.

Results

Environmental variables

Kruskal-Wallis tests and Welch's ANOVAs indicated that the three types of stabilisation structures (namely fascine, mixed-technique and riprap) were not located in similar environments. Mixedtechnique sites indeed appeared to be located along rivers with larger widths (p-value = 0.015) and Strahler ranks (p-value = 0.014) than the other two types of site. Their structure length also tended to present a marginally significant difference (p-value = 0.082). Structures also differed regarding mean slope (p-value = 0.022), with ripraps having significantly steeper slopes than fascine banks (adjusted p-value = 0.009) and mixed-technique banks having intermediate values between these two extremes (for further details see Online Resource 1). Regarding the continuity of slopes within each type of site, no significant difference in mean bank slope was found for the fascine (p-value = 0.522) and mixed-technique sites (p-value = 0.41). However, a significant difference was found for riprap sites (p-value = 0.017). In these sites, upstream banks possessed milder bank slopes than both riprap (adjusted p-value = 0.021) and downstream banks (adjusted p-value = 0.011).

Chi-square tests also showed that the type of engineering structures was not randomly associated with context (p-value = 0.005) or stake (p-value = 0.016). Fascine sites were more often associated with semi-natural context and stakes, while the two other techniques appeared to be more frequently associated with suburban context and urbanized stakes such as bridges or urban areas (for further details see Online Resource 1).

Plant community composition

Across all sampled sites, a total of 295 plant species were found. All NMDS analyses gave satisfactory stress values (< 0.2) when 2 dimensions where kept, with a maximum value of 0.176 for the NMDS on mixed-technique sites and a minimum value of 0.094 for fascine sites. Overall, the only significant difference in species composition between upstream, structure and downstream banks was found for the fascine sites (p-value = 0.004; Figure 2). Pairwise comparisons indicated that the composition of fascine banks significantly differed from that of their downstream banks (adjusted p-value = 0.018), but not from their upstream banks (adjusted p-value = 0.104) (for further details see, Online Resource 2).

Interestingly, among the 25 most abundant species on fascine banks, there was no IAS while there was two for the mixed-technique banks: i.e. Reynoutria spp and Solidago gigantea. In contrast, the 3 most abundant plant species on riprap banks were all IAS: i.e. Buddleia davidii, Reynoutria spp. and Parthenocissus quinquefolia.

Diversity patterns and vegetation strata

Regardless of the type of structure, no significant difference in total richness was found between upstream, structure and downstream banks (Figure 3a). A marginally significant difference in total abundance was however found among banks for mixed-technique sites (p-value = 0.088), and a significant difference was found among banks for riprap sites (p-value = 0.04). Riprap banks indeed appeared to have a less abundant cover of plant species than their upstream banks (adjusted pvalue = 0.025), and marginally so than their downstream banks (adjusted p-value = 0.059).

Regarding IAS richness (absolute or relative), the only significant difference was found in fascine sites (p-value = 0.032), where downstream banks appeared to host a higher number of IAS than fascine banks (adjusted p-value = 0.022; Figure 3b). Regarding IAS abundances however, two opposite patterns were found within fascine and riprap sites. Although differences were only marginally significant for IAS absolute abundances, significant differences in IAS relative abundances were found among fascine type (p-value = 0.048) and riprap type riverbanks (p-value = 0.021; Figure 3b). Fascine banks indeed displayed a lower IAS relative abundance than both their upstream (adjusted p-value = 0.033) and downstream banks (adjusted p-value = 0.039). Conversely, riprap banks displayed a IAS relative abundance marginally higher than their upstream banks (adjusted p-value = 0.092) and significantly higher than their downstream banks (adjusted pvalue = 0.024). Overall, no significant differences in mean herbaceous, shrub and tree covers were found for any type of site (Figure 4). Nevertheless, a marginally significant difference in mean shrub cover in mixed-technique sites (p-value = 0.093) has been found, as well as for the mean tree cover of riprap sites (p-value = 0.0511).

Discussion

In this study, we examined the effects of three types of riverbank stabilisation structures (namely fascine, mixed-technique and riprap) on the compositional, diversity-related and structural continuity of riparian vegetation. Our results showed that, unlike ripraps, fascine banks displayed interesting features for restoring the structural connectivity of riparian vegetation, particularly in terms of vegetation cover and resistance against plant invasions. To a lesser extent, mixedtechnique banks also presented characteristics that could be beneficial for the riparian connectivity of the urbanized landscapes in which they are primarily located. These observations may be useful to improve the management of riparian corridors at the local and regional scales.

Effects of stabilisation structures on the spatial continuity of riparian vegetation

Our results indicated that soil bioengineering and civil engineering structures have different effects on the floristic component of riparian connectivity. On the one hand, fascine banks harboured plant communities that differed significantly from the communities of their downstream banks, and substantially from that of their upstream banks (Figure 2). Fascine banks also displayed significantly lower IAS absolute richness and relative abundance compared to their adjacent riverbanks (Figure 3a,b). These observations are likely related to the very nature of fascines, which are soil bioengineering structures that use a large amount of living plant materials. In our study area, lower parts of fascine banks were composed of live fascines (bundles of willow twigs) while willow cuttings were planted in upper parts. Fascines and cuttings consisted mostly of two willow species: Salix purpurea, S. viminalis. Other woody species, such as Corylus avellana or Cornus sanguinea, were also planted on the upper banks. Such profusion of fast-growing woody plants thus creates highly competitive assemblages that may filter out less adapted species (i.e. light demanding species), allowing the establishment of a different pool of herbaceous species between fascine structures and adjacent riverbanks. Furthermore, we showed that these competitive conditions seemed to constrain the richness and abundance of IAS. The most abundant IAS in the upstream and downstream banks of fascine sites were Reynoutria spp., Parthenocissus quinquefolia and Robinia pseudoacacia (Online Resource 2). On fascine banks however, competition for light and soil resources by planted native species likely reduced the vigour and spread of these IAS and enhanced the biotic resistance of the community (e.g. [START_REF] Byun | Interactions between abiotic constraint, propagule pressure, and biotic resistance regulate plant invasion[END_REF][START_REF] Dommanget | Fast-growing willows significantly reduce invasive knotweed spread[END_REF]. Fascine may thus potentially act as "sinks" for IAS propagules along riverbanks. On the other hand, if no differences in plant community composition were found for mixed-technique and riprap sites (Figure 2), riprap banks were far less abundantly covered by vegetation than their adjacent riverbanks (Figure 3a), and were significantly more dominated by IAS (Figure 3b). Ripraps are indeed known to be prone to invasions, both by terrestrial [START_REF] Cavaillé | Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers[END_REF] and aquatic species [START_REF] Borcherding | Non-native Gobiid species in the lower River Rhine (Germany): recent range extensions and densities[END_REF][START_REF] Reid | Geomorphic and ecological consequences of riprap placement in river systems[END_REF]. They may thus potentially act as "sources" of IAS propagules within the longitudinal gradient of the river network.

Regarding the vegetation structure component of riparian connectivity, no significant differences between any of the compared riverbanks were highlighted. However, it is interesting to discuss some of the trends found in the dataset (Figure 4). While the vegetation structure of fascine banks tended to fit nicely in the continuity of their adjacent riverbanks, riprap banks seemed to possess lower herbaceous and especially tree covers than their upstream and downstream banks. Besides, mixed-technique banks appeared to present higher shrub cover than their adjacent riverbanks. The absence of statistical significance may simply be due to our limited sample size, with only 5 replicates for each type of engineered embankments. Indeed, these tendencies make sense: because of their mineral nature, ripraps often represent poor substrates subject to high temperature fluctuations, hence limiting the installation and survival of many native temperate species [START_REF] Cavaillé | Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers[END_REF][START_REF] Aguilera | Mapping microhabitat thermal patterns in artificial breakwaters: Alteration of intertidal biodiversity by higher rock temperature[END_REF]. Consequently, ripraps usually represent harsh environment hosting lower specific and functional diversity as well as lower vegetation cover than natural or bioengineered embankments [START_REF] Cavaillé | Functional and taxonomic plant diversity for riverbank protection works: bioengineering techniques close to natural banks and beyond hard engineering[END_REF][START_REF] Reid | Geomorphic and ecological consequences of riprap placement in river systems[END_REF][START_REF] Janssen | Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains[END_REF][START_REF] Wollny | Dominance of competitors in riparian plant species composition along constructed banks of the German rivers Main and Danube[END_REF]. The lack of more significant differences in tree cover may additionally stem from the fact that the surrounding banks did not possess substantial tree covers themselves (Figure 4), as confirmed by preliminary analyses of environmental parameters showing that ripraps tends to be located in more urbanized environments (see Online Resource 1). As such, these stabilisation structures may represent degraded riparian habitats nested in a slightly less degraded continuum. A similar cause may explain the seemingly higher shrub cover of mixed-technique banks compared to their adjacent banks. This type of engineered embankment appeared to be mostly used in suburban areas characterized by relatively low covers of woody species. Since mixed-technique banks incorporate plantings of woody plant species, they may restore a certain level of shrub and tree covers amid segments of fragmented riparian corridors. This is in accordance with observations made in other settings [START_REF] Cavaillé | Functional and taxonomic plant diversity for riverbank protection works: bioengineering techniques close to natural banks and beyond hard engineering[END_REF], although the quality of the terrestrial and aquatic parts of such habitats has been criticized [START_REF] Cavaillé | Influence of riverbank stabilization techniques on taxonomic and functional macrobenthic communities[END_REF][START_REF] Janssen | Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains[END_REF].

Lessons for riparian corridors management

The results presented in this study may be useful to inform stakeholders on one key aspect of the environmental impacts of engineering structures for riverbank stabilisation, namely structural connectivity, and help them decide which technique is most suited to reach their goals. Overall, we reckon that fascines are the structures with the lowest impact on the local structural continuity of riparian corridors. Although the floristic composition of fascine banks differs substantially from that of their adjacent riverbanks, which might affect the dispersal dynamics of some host-specific taxa (e.g. [START_REF] Menéndez | Metapopulation structure depends on spatial scale in the host-specific moth Wheeleria spilodactylus (Lepidoptera: Pterophoridae)[END_REF], we argue that this drawback is quite counter-balanced by other features of fascine embankments. Fascines did not create discontinuities in the slope of riverbanks nor in the stratification of riparian vegetation which are very important elements for the population dynamics of many riparian species [START_REF] Baguette | Landscape connectivity and animal behavior: functional grain as a key determinant for dispersal[END_REF][START_REF] Fahrig | Non-optimal animal movement in human-altered landscapes[END_REF][START_REF] Gilbert-Norton | A meta-analytic review of corridor effectiveness[END_REF] and the maintenance of most riparian ecosystem functions [START_REF] Naiman | The ecology of interfaces: Riparian zones[END_REF][START_REF] Wissmar | Restoration and management of riparian ecosystems: a catchment perspective[END_REF][START_REF] Del Tánago | Riparian Quality Index (RQI): A methodology for characterising and assessing the environmental conditions of riparian zones[END_REF][START_REF] Gurnell | Plants as river system engineers[END_REF]. Moreover, the potential ability of fascine banks to constrain IAS richness and abundances is valuable for the habitat restoration function of engineered riverbanks [START_REF] Li | Biotechnical engineering as an alternative to traditional engineering methods: A biotechnical streambank stabilization design approach[END_REF][START_REF] Cavaillé | Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers[END_REF] as it means that fascine may enhance riparian connectivity without promoting the spread of IAS (cf. [START_REF] Richardson | Riparian vegetation: degradation, alien plant invasions, and restoration prospects[END_REF][START_REF] Vilà | Plant invasions in the landscape[END_REF].

Mixed-technique banks also exhibited interesting features since the only discontinuity they created in our study was the seemingly increase in shrub cover and total plant abundance of their riverbank section, although these differences lacked statistical significance. Yet, as pointed out by [START_REF] Janssen | Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains[END_REF], the restoration value of this technique in terms of terrestrial and aquatic biodiversity is lower than that of pure soil bioengineering techniques, especially willow fascines. Incidentally, our results suggest that mixed-technique banks may have a lower potential to control IAS populations than fascines, possibly because the civil engineered part of their banks may be favourable to colonisation by Reynoutria spp. or Solidago gigantea, as observed in the studied riverbanks (Figure 2 and Online Resource 2). This civil engineered part possibly also explains the intermediate riverbank steepness that mixed-technique banks exhibited compared to fascine (mild slope) and riprap banks (steep slope; Online Resource 1). This is ecologically important since, for many species, steep slopes may represent dispersal barriers and may reduce longitudinal [START_REF] Tabacchi | Continuity and discontinuity of the riparian vegetation along a fluvial corridor[END_REF][START_REF] Richards-Zawacki | Effects of slope and riparian habitat connectivity on gene flow in an endangered Panamanian frog, Atelopus varius[END_REF]) and lateral connectivity [START_REF] Elosegi | Effects of hydromorphological integrity on biodiversity and functioning of river ecosystems[END_REF][START_REF] Wollny | Dominance of competitors in riparian plant species composition along constructed banks of the German rivers Main and Danube[END_REF].

Overall, ripraps appeared to be the most impacting technique for the structural connectivity of riverbanks. Compared to their adjacent riverbanks, ripraps tended to have lower vegetation covers, to be proportionally more dominated by IAS, and to exhibit steeper bank slopes. Previous studies partly using the same sites also showed that ripraps have lower woody vegetation cover, native species diversity, functional diversity, and higher IAS diversity than mixed-technique or fascine embankments [START_REF] Cavaillé | Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers[END_REF][START_REF] Cavaillé | Functional and taxonomic plant diversity for riverbank protection works: bioengineering techniques close to natural banks and beyond hard engineering[END_REF]. These observations suggest that ripraps may represent poor riparian habitats and corridors for many species. The fact that ripraps appeared to be mostly located in riverside suburban environments, i.e. in areas characterized by low woody vegetation cover and strong propagule pressure from IAS [START_REF] Meek | A river runs through it: Land-use and the composition of vegetation along a riparian corridor in the Cape Floristic Region, South Africa[END_REF][START_REF] Vilà | Plant invasions in the landscape[END_REF], is not a relevant explanation for these differences since we compared ripraps with their adjacent riverbanks (and not explicitly with fascine or mixed-technique sites), and since mixed-technique sites were also located in suburban environments (Online Resource 1). Rather than satisfactorily explaining the lower ecological value of ripraps, their location in degraded environmental contexts may be viewed as a plea for favouring engineering techniques that fulfil ecological restoration functions on top of riverbanks stabilisation functions (e.g. [START_REF] Li | Soil bioengineering and the ecological restoration of riverbanks at the Airport Town, Shanghai, China[END_REF][START_REF] Rey | Soil and water bioengineering: Practice and research needs for reconciling natural hazard control and ecological restoration[END_REF]. Avoiding a function of propagule source for downstream IAS invasions should be of special concern for riverine corridor management, particularly when stabilisation structures are meant to be located at the head of watersheds. The ecological impacts and benefits of riverbank stabilisation structures are not the only aspects that matter choosing an engineering technique for erosion control. Still, in cases where environmental considerations are important for stakeholders, evidence suggest that fascines should be preferred over the other techniques examined in this paper as they present highest ecological benefits in terms of habitat quality and continuity. Ripraps, on the other hand, seem to be quite poor habitats and may represent strong barriers possibly impeding the movement of many riparian species. Mixed-technique banks, with their intermediate position between these latter two techniques, should be favoured when ecological benefits are sought-after but banks require quick consolidation since ripraps provide immediate resistance against shear stress while the consolidation abilities of soil bioengineering techniques (e.g. fascines, cribwalls) increase over time [START_REF] Pinto | Monitoring methodology of interventions for riverbanks stabilization: Assessment of technical solutions performance[END_REF][START_REF] Recking | The limits of mechanical resistance in bioengineering for riverbank protection[END_REF], simultaneously with their habitat quality [START_REF] Janssen | Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains[END_REF][START_REF] Tisserand | Diversity and succession of riparian plant communities along riverbanks bioengineered for erosion control: a case study in the foothills of the Alps and the Jura Mountains[END_REF].

Perspectives

This study represents the first attempt to highlight the effects of riverbank stabilisation engineering structures on different components of the structural connectivity of riparian zones. However, other aspects of riparian connectivity still deserve attention to better enlighten management decisions, such as the aquatic components of riparian structure. Indeed, the continuity of aquatic habitats is likely to be affected by stabilisation structures as well (cf. [START_REF] Reid | Geomorphic and ecological consequences of riprap placement in river systems[END_REF][START_REF] Cavaillé | Influence of riverbank stabilization techniques on taxonomic and functional macrobenthic communities[END_REF]. The edaphic component of riparian connectivity (or vertical connectivity) should also not be neglected because of its importance to explain longitudinal biodiversity patterns [START_REF] Tabacchi | Continuity and discontinuity of the riparian vegetation along a fluvial corridor[END_REF][START_REF] Tabacchi | Development, maintenance and role of riparian vegetation in the river landscape[END_REF][START_REF] Tickner | Riparian plant invasions: hydrogeomorphological control and ecological impacts[END_REF]. Similarly, the effects of stabilisation structures on the topographical continuity of riverbanks would deserve proper investigation and could be another stone in the garden of ripraps. The environmental characterisation of our study sites indeed showed that riprap was the only technique that disrupted the continuity of riverbank slopes (Online Resource 1). Another interesting perspective would be to extend such environmental impact assessment studies to incorporate the lateral dimension of riparian connectivity. The width of riparian corridors is indeed known to strongly influence species movements and assemblages [START_REF] Lees | Conservation value of remnant riparian forest corridors of varying quality for Amazonian birds and mammals[END_REF][START_REF] Valle | The influence of connectivity in forest patches, and riparian vegetation width on stream macroinvertebrate fauna[END_REF][START_REF] Fremier | A riparian conservation network for ecological resilience[END_REF]. Consequently, continuities and discontinuities restored or induced by riverbank engineering structures are assumed to have distinct effects on biotic communities depending on the state and width of riparian corridors, as well as on the position of stabilisation structures along the river network [START_REF] Swan | Metacommunity theory meets restoration: isolation may mediate how ecological communities respond to stream restoration[END_REF]. Assessing the functional connectivity of riverbanks with engineered sections, with actual dispersal data or through network modelling, would also be a necessary step to quantify the direct impacts of stabilisation structures on the spatial dynamics of key species for conservation, such as umbrella species or protected species (e.g. [START_REF] Van Looy | A scale-sensitive connectivity analysis to identify ecological networks and conservation value in river networks[END_REF][START_REF] Sutherland | Modelling non-Euclidean movement and landscape connectivity in highly structured ecological networks[END_REF]. Another informative research avenue would be to study people's perception of engineered embankments in the landscape, especially to assess whether it matches the ecological value of each structure (e.g. [START_REF] Zhao | Visual ecology: exploring the relationships between ecological quality and aesthetic preference[END_REF][START_REF] Arsénio | Riparian vegetation restoration: Does social perception reflect ecological value?[END_REF].

Fig. 1

 1 Fig. 1 Illustration of the sampling design of sites in an imaginary landscape mosaic. The red box indicates the location of the engineered embankment and purple lines represent sampling transects

Fig. 2

 2 Fig. 2 NMDS ordinations of the floristic communities of upstream, structure's and downstream riverbanks in a) fascine sites, b) mixed-technique sites, and c) riprap sites. For each ordination, the stress value is given and the projections of the 10 species that are the most correlated with the NMDS dimensions (i.e. the most structuring species) are displayed. Abbreviations: Agro_sp = Agrostis sp.; Alli_peti = Alliaria petiolata; Alli_ursi = Allium ursinum; Arrh_elat = Arrhenatherum elatius; Brom_race = Bromus racemosus; Budd_davi = Buddleia davidii; Conv_arve = Convolvulus arvensis; Conv_sepi = Convolvulus sepium; Cory_avel = Corylus avellana; Desc_cesp = Deschampsia cespitosa; Elym_cani = Elymus caninus; Frax_exce = Fraxinus excelsior; Glec_hede = Glechoma hederacea; Hede_heli = Hedera helix; Phal_arun = Phalaris arundinacea; Phyl_sp. = Phyllostachys sp.; Popu_nigr = Populus nigra; Prun_sp. = Prunus sp.; Robi_pseu = Robinia pseudoacacia; Rubu_caes = Rubus caesus; Sali_alba = Salix alba; Sche_arun = Schedonorus arundinaceus; Soli_giga = Solidago gigantea; Urti_dioi = Urtica dioica; Vici_sati = Vicia sativa; Viol_rivi = Viola riviniana

Fig. 3

 3 Fig. 3 Differences in (a) total plant species richness and abundance (total number of species occurrences), and (b) IAS absolute and relative (i.e. compared to total values) richness and abundances. Samples sharing the same letter are not significantly different

Fig. 4

 4 Fig. 4 Differences of cover of the different vegetation strata (herbaceous, shrub and tree). Samples sharing the same letter are not significantly different

Acknowledgement

We are deeply grateful to Paul Cavaillé, Nathan Daumergue, Gilles Favier, Delphine Jaymond, Martin Fargeat and Timothée Herviault for their help at various stages of this work. We also thank Renaud Jaunatre for his helpful comments on the manuscript. This work was part of a project (Trame bleue : espaces et continuités) funded by the European Union through the European Regional Development Fund, the Auvergne-Rhône-Alpes region and the Agence de l'Eau Rhône-Méditerranée-Corse. The work in this study complied with current French legislation.

This work was part of a project (Trame bleue : espaces et continuités) funded by the European Union through the ERDF, the Auvergne-Rhône-Alpes region and the Agence de l'Eau Rhône-Méditerranée-Corse.

Conflict of Interest:

The authors declare that they have no conflict of interest. The datasets used in this study will be deposited in a public repository upon acceptance of a final version of this manuscript.


Yannick Petit
email: yannick.petit@u-bordeaux.fr

Gustavo Galleani

Guillaume Raffy

Jean-Charles C Desmoulin

Veronique Jubera

Del André

Andrea Simone Guerzo

Lionel Stucchi De Camargo

Thierry Canioni

Cardinal

A Del Guerzo

A Camargo

L Canioni

T Cardinal

Three

Keywords: silver clusters, rare earth ions, femtosecond laser inscription, FLIM spectroscopy, resonant transfers, Dexter process, FRET process, CIE chomaticity diagram

INTRODUCTION

Photonics has been clearly identified as a key enabling technology. The development of such penetrating technology relies on its ability to integrate and miniaturize components and devices. Therefore, the development of innovative materials together with the fabrication processes remain a central issue. Glass materials have shown, thanks to the development of fiber technology for instance, that they are excellent candidate for optical applications. New solution should found for enhancement and promoting 3D architectures. It requires developing intense local physical effect that could be structured in 3D. Direct laser writing are offering numerous solutions for shaping and inscribing functionality in Glass [ [START_REF] Gross | Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications[END_REF]]. Moreover, especially designed glass composition can offer additional possibility for implementing multiple functionalities such as luminescence for instance [(2)]. Still now, structuring efficient emission in various spectral range remains an important challenge. This last decade, we demonstrated that femtosecond direct laser writing (DLW) in a high optical quality photosensitive silvercontaining zinc phosphate glasses allows for implementing 3D fluorescent structure formed by the aggregation of silver ions and silver atoms within the glass volume [(3)] that could be a solution for perennial three-dimension high-density data storage [(4)- [START_REF] Park | Five-dimensional high-density perennial optical data storage based on ellipse orientation and fluorescence intensity in silver-containing Zincphosphate glasses[END_REF]]. The cluster exhibit a broad excitation spectrum from 280 nm up to at least 550 nm. This local precipitation of silver cluster is preceded by the localized creation of a space charge separation at the root of a perennial high-amplitude (~10 8-9 V/m) static electric field, leading to the local breaking of centrosymmetry and thus allowing for an efficient electric-field induced second-harmonic generation (χ (2) eff ~ 0.8 pm/V) [( 9), [START_REF] Papon | Femtosecond Single-Beam Direct Laser Poling of Stable and Efficient Second-Order Nonlinear Optical Properties in Glass[END_REF]]. Moreover, the spatial structuring of the incident laser beam led us to achieve the first demonstration of bulk structuring by means of optical vortexinduced DLW [(11)], locally producing correlative optical contrast in terms of silver cluster fluorescence and of effective second-order nonlinear optical properties. Finally, these innovative laser-induced multi-scale structures were tuned into silver metallic nano-crystallites by means of thermal annealing, creating a composite metal-dielectric material with plasmonics properties [(12)]. The local cluster formation by DLW was recently shown to be partially or fully inhibited by original dual-color DLW with the co-illumination of a CW UV beam with the fs IR structuring beam, opening interesting potentialities in terms of super-resolution DLW [(8)]. The co-doping of glasses with both rare earth ions and noble metal ions has been increasingly reported [( 13)-( 16)], for optoelectronic devices such as white light emitting diode, three-dimensional display and solar cell [( 13),( 15)- [START_REF] Lin | Molecular-like Ag Clusters Sensitized near-Infrared down-Conversion Luminescence in Oxyfluoride Glasses for Broadband Spectral Modification[END_REF]]. Indeed, efficient energy transfer between silver species and europium ions was described and achieved in various materials such as glasses and zeolite materials [( 13),( 14),( 19)- [START_REF] Guo | Ultraviolet Light Induced White Light Emission in Ag and Eu^3+ Co-Doped Oxyfluoride Glasses[END_REF]]. In glasses, beside the traditional melt technique to incorporate silver species with rare ions, different methods of preparation was described, such as the ion-exchange processes. The modification of the quantum yield of fluorescence emission has been described thanks to the presence of silver cluster and silver nanoparticles [(13)]. An enhancement of the rare earth emission by energy transfer from silver clusters to Eu 3+ ions have been observed in various matrices such as in silicate [(20), [START_REF] Ghafoor | Energy Transfer between Silver Clusters and Europium Eu 3+ Ions in Photo-Thermo-Refractive Glasses[END_REF]] or borate [(25)]. In most of the investigations there are no control on the spatial distribution of the rare earth ions emission and the 3D organization of the luminescent structure is not considered. In the specific case of photonic integrated circuits, the issue of the 3D organization of robust luminescent structure is crucial. The goal is then not solely the efficiency of the luminescent structure but also the engineering of luminescent 3D photonic structure. It requires to be able to foster locally efficient transfer or enhancement mechanisms between noble ion based aggregates and rare earth ions as for instance between silver species and europium ions. In this article, we report on the co-doping of a silver-doped zinc phosphate glass with Europium Eu 3+ rare earth ions and the 3D structure of the luminescent properties. The absorption, excitation and emission properties of the pristine co-doped glasses were investigated and microluminescence properties were characterized after femtosecond laser direct writing and the local precipitation of fluorescent silver clusters. The origin of the interaction between the silver clusters and the europium ions has been investigated in order to elucidate the potential contributions of radiative and non-radiative transfers. A local field enhancement effect due to hypothetical formation of metallic nanoparticles, is also discussed.

EXPERIMENTAL SECTION

Glass synthesis and sample preparation. The europium-doped silver-containing zinc phosphate glass with the composition 99.2[54.1 PO5/2 / 37 ZnO / 1.4 GaO3/2 / 7.5 AgO1/2] -0.8 Eu2O3 (mol. %) was prepared using classical melt/quench technique using platinum crucible. After annealing at 30°C below the glass transition temperature the glass is cut to 1 mm thick sample with typically 1 cm lateral dimensions and polished to optical quality. Hereafter, the codoped glass sample is referred to as PZn:Ag-Eu. For nanoparticles development effect investigation, the glass was heat-treated at 400 °C, typically 20 °C above the glass transition temperature for 30 minutes.

Transmission, excitation and emission spectroscopies. The transmission spectroscopy was performed with a Carry 500 (differential transmission spectrometer from Varian) from 250 nm to 800 nm. The excitation and emission spectroscopies were performed with a SPEX Jobin Yvon fluorescence spectrometer equipped with double monochromator and Peltier cooled PMT.

Femtosecond direct laser writing. DLW was performed using a Ytterbium fs oscillator (Amplitude Systèmes, T-pulse 200, 9.1 MHz, 390 fs at 1030 nm), focused with a microscope objective (Mitutoyo, M Plan Apo NIR, 20× NA 0.4). The transmitted irradiances were controlled by an acousto-optic modulator, enabling focused energies up to 100 nJ and associated focused irradiances up to 20 TW/cm 2 . The positioning and displacement of the sample were performed with a high-precision 3D translation stage (XMS-50 stages, Micro-Contrôle). A laser-matter interaction matrix was performed, by producing to 18 structured squares, each composed of 10 lines of 100 μm spaced by 10 μm, associated to 6 irradiances spanning from 2 to 6.7 TW/cm 2 , and 3 translation speeds of 10, 50 and 100 μm/s, respectively corresponding to the local accumulation of 3.1×10 6 , 6.2×10 6 and 3.1×10 5 pulses.

Absorption micro-spectroscopy of the structured sample. Transmission measurements

were performed in bright-field using a 'CRAIC Technologies' microscope equipped with a Xenon lamp and a condenser as a white light source and with a spectrometer equipped with CCD . A conjugated 10x magnification microscope objective was used to collect the transmitted light through the laser-induced structures. For such micro-transmission measurements, the reference spectrum was recorded through the pristine glass, exactly in the same geometrical conditions for the measurement through the laser-induced structures, which excludes any artifacts from distinct collection issues. As a consequence the resulting transmission measurements gave a direct experimental access to the local differential micro-extinction due to the laser-induced structures. Such a local differential micro-extinction, interpreted in terms of linear absorption coefficient, is a spatially averaged value over a 50´50 μm 2 surface, successively overlapping each of the 100´100 μm 2 laser-induced structures.

Fluorescence micro-spectroscopy of the structured sample. Fluorescence spectrum measurements were performed in an epi-collection scheme, with a high spectral resolution micro-spectrometer (LabRAM HR800 from Jobin-Yvon, with a 150 grooves/cm grating for the spectral dispersion of the fluorescence in the near-UV and visible region) for three different conditions. First, a 325 nm excitation with a UV He-Cd laser source (30 mW, TEM 00 , Kimmon) was focused with a UV-enhanced Cassegrain objective (36´ NA 0.5), and fluorescence was collected over the range of 330 nm -750 nm. Second, a 405 nm excitation with a near-UV laser diode (100 mW, TEM00 , OBIS from COHERENT) was focused with a Olympus refractive objective (100´ NA 0.9), and fluorescence was collected over the range of 450 nm -750 nm. Finally, a 532 nm excitation with a laser diode (100 mW, TEM00 , monomode, Coherent) was focused with the same Olympus refractive objective (100´ NA 0.9), and fluorescence was collected over the range of 550 nm -750 nm. Great care was taken to provide spectrally corrected fluorescence spectra for each of the three conditions. Time-resolved fluorescence micro-spectroscopy of the structured sample. The Fluorescence Lifetime Imaging Microscopy (FLIM) analysis was performed with a Picoquant MT200 microscope equipped with two MPD single photon avalanche diodes (SPADs) and a PicoHarp300 timing board for Time Correlated Single Photon Counting (TCSPC) operation. The pulsed laser source was a frequency-doubled 6 ps coherent MIRA 900 laser, tuned at 435 nm, with a repetition rate set to 4.75 MHz via an acousto-optic pulse picker. The laser was reflected to a 100´ 1.40NA oil objective (Olympus UPLSAPO100XO) by a 80%T/20%R spectrally flat beam splitter, and the fluorescence was collected by the same objective and transmitted by the same beam splitter to the confocal optics and detector after excitation light was rejected by a band-pass filter (HQ510/80, chroma with a 470-550 nm transmitted spectral window). The laser was scanned by means of a piezoelectric stage in a grid pattern of 156 nm steps and 0.6 ms/pixel. The shown FLIM images were calculated using the so-called "fast-FLIM" algorithm, where for each pixel, the reported lifetime was calculated as the first statistical momentum of the histogram distribution of the single photon arrival times, minus a reference time taken at the onset of the decay curve. Note that the shown FLIM images include both temporal and intensity information of the collected fluorescence, as the color code depicts the fast-FLIM average arrival times on the one hand and the brightness levels depicts the number of collected events on the other hand. Alternatively, curve fitting with a multiexponential model was performed on decays with better statistics, corresponding to the binning of pixels of larger regions of interest. In the latter case, tail fitting could be applied thanks to the shortness of the instrumental response function (about 40 ps FWHM).

RESULTS AND DISCUSSION

Photoluminescence in Bulk Glass. The absorption spectrum in the range 250-800 nm obtained for the PZ: Ag/Eu 3+ glass is shown in Figure 1. The UV edge is around 270 nm mainly due to silver ions absorption as previously observed for silver ions containing zinc phosphate glass [( 26)]. Additional characteristic absorption bands of Eu 3+ ions are observed at 318, 362, 380, 393, 464 and 526 nm assigned to the transition from the ground state 7 F0 to excited states 5 H3, 5 D4, 5 G2, 5 L6, 5 D3, 5 D2, respectively. There is no evidence of Eu 2+ formation in the glasses and no absorption band above 300 nm, which could be attributed to surface plasmon resonance or silver cluster species such as (Ag2) + or (Ag3) 2+ for instance.

Figure 1.

Linear absorption coefficient of the pristine PZ:Ag-Eu glass, mostly showing the UV cut-off due to the silver ion insertion below 300 nm, as well as the much weaker and narrow 4f-4f transition lines of the Eu 3+ ions (see insert).

Fluorescence spectroscopy have been measured under different UV excitation wavelengths to investigate the emission properties of silver and Eu 3+ ions and the results are shown in Figure 2(a). Under 235 nm excitation, two broad emission bands are observed due to two different intrinsic environments of silver with peak emission at 290 nm (site A) and 380 nm (site B), as well as the characteristics emissions of the 4f-4f transitions of Eu 3+ : 5 D0-7 Fj (J=0-6). The site A emission is assigned to Ag + ion with the 3 DJ→ 1 S0 transition, while site B emission is assigned to (Ag + )2 pairs with the S1, T1→S0 transition [( 27)]. Note that the considered silver ion concentration makes site B is largely preponderant compared to site A [( 2)]. With the increase of the excitation wavelength to 260 nm, almost exclusively site B emission as well as the characteristics emission of Eu 3+ is observed. Excitation at UV light about 235 nm or 260 nm, promotes the electron to the excited state of both sites A and B, or site B, respectively. Thus, the excited electrons can relax to the ground state radiatively, but also non-radiatively including relaxation paths involving energy transfers to the neighboring Eu 3+ ions. The occurrence of energy transfer from silver sites to Eu 3+ ions is favored by the energy level accordance between the excited states of the silver sites with the 5 L6 level of Eu 3+ ions. Under excitation at 365 nm, only the line-shape characteristics emissions of the Eu 3+ transitions at 578 nm, 590 nm, 612 nm, 652nm and 702 nm corresponding to the Eu 3+ ions emission 5 D0 → 7 FJ (J= 0-4) are observed.

Based on the photoluminescence spectra of the glasses presented in Fig. 2, the color of samples under the three distinct excitations at 235 nm, 260 nm and 365 nm were characterized by International Commission on Illumination (CIE) chromacity diagram (Fig. 2b). The colors shift from purplish pink to red with the increase of excitation wavelength from 235 nm to 365 nm, which demonstrates that the luminescence color can be systematically tuned by changing the excitation wavelength. The photoluminescence excitation spectra is shown in Figure 3. By monitoring the emission at 290 nm (site A), a broad excitation band centered at 230 nm is observed attributed to Ag + ion. At 340 nm (site B), two overlapping broad excitation bands are observed centered at 230 nm and 260 nm, from the Ag + ion and (Ag + )2 pairs, respectively. When monitoring the Eu 3+ emissions at 612 nm or 700 nm, besides the two broad excitation bands of silver originating from the energy transfer of Ag + ions and (Ag + )2 pairs energy transfer to the rare earth ions, several characteristic f-f transitions sharps lines are observed. Under UV-light, Ag + ions and (Ag + )2 pairs can be excited to the 3 Dj and S1 energy levels, respectively, and then the nonradiative S1→S0 and 3 DJ→S0 transitions may transfer the excitation energy non-radiatively to Eu 3+ ions, such populated Eu 3+ excited levels leading then to the observed typical Eu 3+ fluorescent emission. Such non-radiative energy transfer is understood as a non-radiative resonant relaxation from the Silver species (donor) to the Eu 3+ ions (acceptor), without any electron transfer. Indeed, the overlapping of excitation bands evidences the potential ability of energy transfer from silver ionic species to the Eu 3+ ions. Furthermore, no fluorescence emission signature of Eu 2+ ions was observed, which corroborates the absence of electron transfer in the present energy transfer. The micro-emission spectroscopy of laser induced silver clusters are presented in Figure 6 for two different excitations wavelengths at 325 nm and 405 nm as a function of the sample motion velocity for 6.75 TW.cm -2 irradiance (Fig. 6a and6c) and as a function of the irradiance for a fixed sample motion velocity of 10 µm/s (Fig. 6b and6d). At 325 nm (Figs. 6a and6b), the emission spectra consist of two major contributions, one broad emission ranging from 400 to 700 nm attributed to silver nanoclusters and sharp emission peaks within the 575 to 715 nm region assigned to Europium ions. The decrease in sample velocity (Fig. 6a) and the increase of the irradiance (Fig. 6d) corresponding to a raise of the deposited dose promote the increase of the broad emission band intensity assigned to the formation of Agm n+ species and promote also the emission of the europium ions. The most intense emission of Eu 3+ around 612 nm is attributed to the hypersensitive electric dipole transition ( 5 D0→ 7 F2). This emission is strongly influenced by the chemical environment of the Eu 3+ ions in the host network. On the other hand, the 5 D0→ 7 F1 transition occurs via the magnetic dipole mechanism and its intensity is not sensitive to the chemical environment of Eu 3+ ions. Therefore, the emission intensity ratio of these two transitions can be used to highlight a site evolution of the rare earth ion. In the present case, one can notice that the spectral shape of the 4f-4f transition remains constant and the intensity ratio between the 5 D0→ 7 F2 and 5 D0→ 7 F1 also remains at constant value of 4.1 ± 0.1. Thus an absence of major modification of the chemical environment at the vicinity of the Eu 3+ after DLW can be assumed.

Figure 6: Micro-emission fluorescence spectroscopy of the laser/matter interaction matrix for some laser-induced squares. Excitation at 325 nm (reflective obj. 36x, NA 0.4): (a) dependence with sample translation velocity (irradiance = 6.7 TW/cm 2 ), and (b) dependence with irradiance (translation velocity = 10 μm/s). Excitation at 405 nm (refractive obj. 100x, NA 0.9): (c) dependence with sample translation velocity (irradiance = 6.7 TW/cm 2 ), and (d) dependence with irradiance (translation velocity = 10 μm/s).

Micro-fluorescence emission spectra bring interesting behavior in terms of display and colorimetry. Indeed, the fluorescence emission analysis under UV excitation at 325 nm and associated projection in the XYZ tri-chromatic coordinates in the CIE chromaticity diagram shows that the chromatic coordinates can be continuously controlled in 3D by means of laser irradiation, namely from red (almost pristine glass with quasi-null action of the laser inscription) to white light emission, as shown in Fig. 7. In the case of the largest irradiances (I = 6.7 TW/cm 2 for patterns A6, B6 and C6 depicted in Fig. 7(a)), the trichromatic coordinates get very localized in the white emission domain whatever the three used velocities suggesting that the broadband silver cluster emission dominates the reddish contribution of Eu 3+ ions in the resulting trichromatic behavior. Similarly, in the case of the lowest velocity v = 10 µm/s (namely, the largest number of cumulated pulses with patterns C3 to C6 depicted in Fig. 7(c)), the deposited dose is also high enough to produce enough efficient fluorescent silver clusters to overcome the Eu 3+ emission and dominate the resulting tri-chromatic behavior. However, a larger velocity of 50 µm/s tends to limit the amount of silver clusters and its fluorescence emission amplitude, locally allowing for a control a the tri-chromatic coordinates between the initial red coordinates of the pristine glass and the white light emission of the silver clusters, as show between points B2 and B3 from Fig. 7(b). Such measurements results from micro-luminescence in confocal microscopy, so that there are measured at the silver cluster positions. By adjusting the probed volume of the material of the one hand, and by fixing the filling rate of silver cluster (typically with the inter-distance between successive laser passes in a given plane, or by producing thick multi-plane patterns) on the other hand, one can fully control the continuous evolution of the measured tri-chromatic coordinate between red to white emission. This suggest thus the ability to perform the localized 3D inscription of emitting areas with tunable tri-chromatic coordinates, which goes beyond the spatially homogeneous control of the tri-chromatic coordinates means of selecting different fluorescence excitations wavelengths as shown in Fig. 2(b) (from the pristine Eu 3+ -based red emission to a pinkish emission resulting from the red Eu 3+ ion emission and the blue Ag+ ion emission in the pristine glass). Laser inscription allows thus an additional experimental parameter to tune the 3D localized tri-chromatic coordinates. the PZ:Ag:Eu glass, compared to the 2.44 ns lifetime for the PZ:Ag glass. This supports the hypothesis of a quenching by energy transfer from these clusters to the neighboring Eu 3+ ions which directly affect the emission rate of the cluster emission. Figure 8(c) also reveals a lower emission intensity of silver clusters in the presence of Eu 3+ ions, such partial emission quenching being also consistent with nonradiative energy transfer by resonant energy transfer from silver clusters to Eu 3+ ions. By properly taking into account the normalization with the incident pump intensity at 435 nm for both histograms of Fig. 8(c), the corresponding timeintegrated intensity of both PZ:Ag and PZ:Ag:Eu glasses led to the intensity quenching

𝑄 ! !",$% &'%( / ! !" &'%( = 1 -𝐼 $%,'( )*(+ 𝐼 $% )*(+ & ~ 41%
of the silver cluster emission. Note that such global intensity quenching includes any sources of quenching, namely the energy transfers at play towards the Eu 3+ ions but also others possible phenomena whose contributions will be discussed below. As the fluorescence emission of the silver clusters corresponds to a broadband multi-band emission that covers the whole visible range and depends on laser irradiation parameters, fluorescence lifetime was also evaluated with a multiple exponential decay model in the sub-ns and ns range. As detailed in the experimental section, curve fitting with a multi-exponential decay model was performed on decays with better statistics, corresponding to the binning of pixels of larger regions of interest. As shown in Fig. 8(d), optimal time decay fitting was achieved with a four-exponential model with the adjusted amplitudes and time constants being reported in Table 1. The used model demonstrates that the complexity of lifetime decay of the system under investigation requires at least four typical decay times, but reality may even be more complex with additional much shorter decay times.

Table 1: Fitting parameters with a four-exponential decay time model.

Based on the fitting results from Table 1, the intensity-weighted average emission lifetime 𝜏 ,-.. = (∑ 𝐴 , 𝜏 , 0 ,12.. 3)/(∑ 𝐴 , 𝜏 , ,

) is of 𝜏 $% ,-.. = 2.44 ns for the PZ:Ag glass and it decreases down to 𝜏 $%,'( ,-.. = 1.94 ns in the Eu 3+ -containing glass, in agreement with the lifetimes denoted by the peak positions of the histograms of Fig. 8(c). This result corroborates the occurrence of energy transfer from Agm n+ species levels to Eu 3+ ions. Accordingly to the expected lifetime of europium Eu 3+ in such glass, which is expected to be around the millisecond for the most intense emission around 612 nm, the collected decay measurement strictly corresponds to the silver cluster contribution.

To go further in the interpretation of energy transfers, based on the fitting results from Fig. )*+'. / 7 !"

)*+'. ~27% with the intensity quenching rate 𝑄 ! !",$% &'%( / ! !" &'%( ~ 41% helps sorting the contributions of the emission quenching of the silver clusters. Indeed, the ratio 𝑄 7 !",$% )*+'. / 7 !"

)*+'. 𝑄 ! !",$% &'%( / ! !" &'%( & = 27% 41% ⁄ ~66%
indicates that 66% of the emission quenching of the silver clusters in presence of Eu 3+ ions is actually due to energy transfer. In addition, the estimation 1 -𝑄 7 !",$% )*+'. / 7 !"

)*+'. 𝑄 ! !",$% &'%( / ! !" &'%( & ~33%
indicates that 33% of the emission quenching of the silver clusters in presence of Eu 3+ ions stands either for a lower concentrations of Silver emitters, affecting the fluorescence intensities but not the lifetimes. This can reasonably be interpreted by a slightly lower efficiency of silver cluster production during the femtosecond laser writing process. Indeed, in the laser inscription stage, we observed that Eu 3+ ions undergo multi-photon absorption and subsequent reddish fluorescence emission, which partially reduces the effective femtosecond laser intensity and thus competes the activation of the silver photochemistry and the associated creation and growth of silver clusters.

Regarding the Europium emission intensity, the 612 nm Eu 3+ emission increases up to 5-fold and to 3.5-fold for excitations at 325 nm and 405 nm, respectively (Figs. 9(a) and 9(b)), indicating an energy transfer from silver species to Eu 3+ ions [( 17),( 23), [START_REF] Jiao | Effect of Silver Ions and Clusters on the Luminescence Properties of Eu-Doped Borate Glasses[END_REF]]. The Eu 3+ emission enhancement is clearly dependent with the excitation wavelength as a larger efficiency is observed for an excitation at 325 nm. Such wavelength excitation dependence may be directly interpreted from Fig. 5, with a more efficient excitation of silver clusters at 325 nm compared to 405 nm, leading thus to a more intense fluorescence emission and consequently to a larger amount of excited silver clusters compatible with resonant non-radiative energy transfers to Eu 3+ ions. Still, the existence of various populations of distinct silver clusters created during DLW has been yet reported [( 12)], showing a partially-selective excitation with radiations at 325 nm or 405 nm which may also play a selective role if the observed energy transfer. Indeed, these distinct types of silver clusters may show distinct energy transfer efficiencies to the Eu 3+ ions. The present spectroscopy cannot directly identify the nuclearities and associated stabilized Redox charges of these silver species, nor their respective populations [( 12)]. According to the singlet-triplet energy level model of Agm n+ species proposed by Velazquez et al. [(27)] and to the results obtained from the photoluminescent excitation and emission spectra of the Eu 3+ and Agm n+ species in the co-doped glass before and after laser/matter interaction, the energy transfer pathway was tentatively described in Fig. 10. Under UV-visible, Ag + ions and Agm n+ species can be excited to the S1 and T2 energy levels, and then the S1→S0 and T2→S0 transitions may transfer the excitation energy to Eu 3+ by relaxations, populating thus the 5 H3 level and by non-radiatively relaxing from the 5 D4 and 5 L6 levels down to the 5 D0 energy level. The subsequent radiative transitions from the 5 D0 energy upper level leads to the visible Eu 3+ emissions observed in Figs. 2 and6. Besides, it is worth to mention that the spin-forbidden transition of T2→S0 with relatively longer lifetime may mainly contribute to the above energy transfer processes. Still, at this level of description, the present spectroscopy cannot directly identify the mechanisms at play during the non-radiative resonant energy transfers, namely one cannot directly discriminate between Dexter resonant energy transfers (DET) related to electron exchange operating at very short scale (requirement of the wave function overlap) or Förster resonant energy transfers (FRET) related to dipole-dipole interactions spanning over longer scales possibly up to several nanometers typically (but highly sensitive to the solicited oscillation strength, which is known to rather weak for the lanthanide ions) [( 28)]. A balance between DET and FRET both occurring simultaneously may even exist. Such discrimination in the origin of the involved resonant energy transfer could further be addressed thanks to a set of glasses with various rare earth doping concentrations, which is beyond the scope of the present article. Following the laser/matter interaction a subsequent heat treatment done at 400 ⁰C (Tg+20°C) for 30 min in the glass revealed the crystallization of plasmonic silver nanoparticles. Such plasmonic structure precipitation will not be described in detail the present paper, as they were investigated earlier by Marquestaut et al. [(12)]. These plasmonic structures led to a quenching of the energy transfer between Ag species and Eu 3+ ions, so that the enhancement of the Eu 3+ emission mostly cancelled after thermal treatment, as shown in Figure 11 for an excitation at 325 nm. Indeed, the Eu 3+ emission enhancement ratio drops from 2.5 to 5-fold down to 1.2-to 1.8-fold (with an enhancement drop from 5-fold down to 1.2-fold for the highest laser-deposited dose from pattern C6). The precipitation of silver nanoparticles decreases the active population of silver clusters that can be excited and further transfer energy to Eu 3+ ions, reducing thus the energy transfer ability. Additionally, such silver nanoparticles can also contribute to quench the fluorescence emission of the remaining silver clusters, reducing furthermore the probability of energy transfer to Eu 3+ ions.

Figure 11.

Normalized amplitude of the micro-emission of the Eu 3+ fluorescence with respect to the pristine glass at 612.2nm, for a 325 nm excitation before (dashed line, empty dots) and after thermal heat treatment (TT) at 400 °C (Tg+20°C) for 30 minutes, showing both the decrease of the Eu 3+ fluorescence enhancement for high enough deposited laser doses (partial growth and subsequent destruction of populations of initially large silver clusters and correlative growth of metallic plasmonic silver NPs) also well as the minor but non-zero increase of the Eu 3+ fluorescence enhancement for lower deposited laser doses (partial growth from laser-induced defects or from populations of initially small silver clusters towards larger silver clusters that show a better resonant energy transfer toward Eu 3+ electronic levels.

CONCLUSION

Femtosecond laser inscription was achieved in a Eu 3+ -doped Silver-containing Zinc phosphate glass, demonstrating that the insertion of rare earth ions does not inhibit the photo-activation of silver chemistry. By means of micro-spectroscopy of fluorescence emission, tri-chromatic coordinates in the CIE chromaticity diagram were estimated showing that such coordinates can be continuously controlled from the red light aspect dominated by the Eu 3+ emission towards the white light aspect dominated by the silver cluster emission. This demonstrated the ability to perform a 3D-localized controlled modification of the local tri-coordinates thanks to the laser irradiation parameters. Compared to the pristine, Eu 3+ emission in the structured region showed an increase up to 5-fold and to 3.5-fold for excitations at 325 nm and 405 nm, respectively. Such an enhancement is co-localized with the fluorescent laser-inscribed silver clusters, which suggests a resonant non-radiative energy transfer from excited silver clusters to Eu 3+ allowing then the fluorescence of the rare earth ion. FAST-FLIM imaging the efficient resonant energy transfer from the silver clusters and to Eu 3+ ions. The precipitation of metallic silver plasmonic nanoparticles in the silver cluster location, obtained following a heat treatment above glass transition temperature, vanishes the enhancement of the Eu 3+ emission. Femtosecond laser inscription in rare earth doped silver-containing glasses clearly demonstrates the effect of the silver cluster on the enhancement of europium ions and allows writing 3D localized intense tunable emission patterns from red to white for a broad excitation spectrum in inorganic materials. This approach could be extended to other rare earth doped photosensitive glasses, including photo-thermo-refractive glasses [(29)], to promote 3D high contrast luminescent pattern in various spectral ranges.

Figure 2 .

 2 Figure 2. (a) Fluorescence emission spectroscopy of the pristine PZ:Ag-Eu sample for three distinct excitation wavelengths, showing two broad emission bands of two distinct environments of the silvers ions, labeled site A (exc. 235 nm and em. 290 nm, lower amplitude) and site B (exc. 260 nm and em. 380 nm, larger amplitude), as well as the emission of the 4f-4f transitions from the 5 D0 level of the Eu 3+ ions (left). Note that the excitation at 365 nm is selective of the Eu 3+ excitation, compared to the excitation of the Ag + ions. (b) CIE chromaticity diagram of the pristine PZ:Ag-Eu sample for three distinct excitation wavelengths.

!

  !"#. = 235 '( )*+ , ! !"#. = 260 '( )*+ / ! !"# . = 365 '( )*+ 0

Figure 3 .Figure 4 .

 34 Figure 3. Fluorescence excitation spectroscopy of the pristine PZ:Ag-Eu sample for three distinct emission wavelengths, showing two broad excitation bands of two distinct environments of the silvers ions, labeled site A (exc. 235 nm and em. 290 nm) and site B (exc. 260 nm and em. 380 nm), as well as the excitation of the 4f-4f transitions of the Eu 3+ ions.Direct Laser Writing of Fluorescence Structures. Figure4(a) shows a wide-field fluorescence microscopy image of coil patterns obtained by DLW for PZ:Ag-Eu glass sample motion with controlled speed from 10 to 100 µm/s (corresponding to C, B and A lines), and for different irradiances from 4.0 to 6.7 TW cm -2 (labeled columns 3, 4, 5 and 6). Those pattern allow investigating the influence of the laser parameters in the silver clusters growth and their interaction with the Eu 3+ ions. A high-resolution confocal image for excitation 435 nm (obj. 100x NA1.4) is given in Fig.4(b), showing the detailed distribution of the fluorescent doubleline structure produced by several laser passes. These double lines correspond to the formation of parallel planes containing the silver clusters at the border of the voxel of interaction. This phenomenon has been described in details elsewhere [(4)].

Figure 5 .

 5 Figure5. Micro-absorption spectroscopy of the laser/matter interaction matrix for laserinduced squares, observed with a 50´50 μm 2 field of view, showing the spatially-averaged differential linear absorption coefficient with respect to the pristine glass, revealing the creation and growth of two mains absorption bands (typically centered at 3.7 eV and 4.35 eV nm, depending on irradiation conditions) with respect to the laser irradiance at constant speed translations of (a) 10 μm.s -1 and (b) 100 μm.s -1 .

2 6. 1 TW/cm 2 5. 2 TW/cm 2 6. 6 TW/cm 2

 2122262

Figure 7 .

 7 Figure 7. CIE chromaticity diagram of the laser/matter interaction matrix for some laserinduced squares with excitation at 325 nm.

Figure 8 :

 8 Figure 8: (a) Composite image of the laser-induced pattern C3 (10 µm/s, 4 TW/cm 2 ) in the PZn:Ag glass depicting a color image of the laser that provides the pixel-by-pixel mean lifetime retrieved by fast-FLIM algorithm (see color bar from 0 to 7 ns), as well as a grey-scale inset image (see color bar from 0 to 255 counts) that provides the fluorescence intensity; (b) same composite image FLIM and fluorescence intensity for the PZn:Ag:Eu glass; (c) Intensityweighted lifetime histograms for the PZn:Ag and PZn:Ag:Eu glasses obtained from the FLIM images depicted in 8(a) and 8(b), respectively; and (d) normalized fluorescence decays of both the PZn:Ag and PZn:Ag:Eu glasses and their ideal respective fittings with a four independent exponential decay model. All data was obtained with a 435 nm pulse excitation of 6 ps at 4.75 MHz, and a collection over the 470-550 nm range.

Figure 9 :

 9 Figure 9: Normalized amplitude of the micro-emission of the Eu 3+ fluorescence with respect to the pristine glass at 612.2nm, (a) for a 325 nm excitation and (b) for a 405 nm excitation, respectively showing up to 5-fold and to 3.5 emission enhancements.

Figure 10 .

 10 Figure10. Energy level diagram showing schematically the possible energy transfer processes from Agm n+ to Eu 3+ (energy transfer labelled ET1), and from Ag + to Eu 3+ (energy transfer labelled ET2), upon excitation in the ultraviolet or visible range, leading to silver species emissions but also the indirect excitation of Eu 3+ and its emission in the visible.

Table 1 ,

 1 the amplitude-weighted average emission lifetime 𝜏 456*. = (∑ 𝐴 , 𝜏 ,

	,12..3	)/(∑	,12..3	𝐴 ,	)