Claude Martin

Pre

, the concept of 'solidarité' is frequently employed today to evoke practices of reciprocity and mutual assistance at family and kinship level. This double usage, "micro" and "macro", is not however without its problems. Should we somewhat unconcernedly employ the term 'solidarity' both for what occurs in a family network on the occasion of an exchange of services or the donation of an amount in cash and what occurs when two persons are mutually associated through their affiliation to one and the same pension fund? Can it be considered quite the same form of solidarity when it originates in the private or public sphere and when it originates in the collective or family environment? What are the components of a solidarity behaviour that brings together different dimensions and may cause us to adopt a similar employment of the concept of solidarity that is at one and the same time both familial and national?

One way of answering these questions may consist, as Michel Messu suggests, in simply rejecting the expression "familial solidarity" and taking this to mean nothing more than a delusion ii . But it may also be held that recourse to the concept of solidarity both at the level of society, of corporate entities, the family or kinship is freely adopted today to establish a bond between these different levels. If today one speaks so much about the role of familial solidarities, it is undoubtedly because the practices of mutual assistance are held to be one of the very numerous manifestations of contemporary social ties together with forms of solidarity that are said to be collective and/or statesponsored, capable of responding to contemporary social problems. Use of the expression then becomes a symptom, the manifestation of a contemporary manner of conceiving our societies and the problems with which they are confronted.

In this way, one comes for example to consider, more or less explicitly, that these various forms of solidarity can compensate one another mutually. Consequently, to hold that when one of this form is intensified, the other declines and vice-versa, is but a

single step, often taken particularly in political rhetoric. For this reason, it is sometimes felt that the development of the État-providence has contributed to a weakening of the "strong ties" of family together with the obligations and norms of mutual aid which it assumed in conventional societies. This in turn called for a strengthening of familial solidarities to compensate for the limitations of collective solidarity. The profile of a "minimum State" or a "weak State" would render it necessary to develop a "welfare family" or "family providence" and vice-versa, that is to say, the development of an État-providence or Welfare State would contribute to the undoing of fundamental social ties and the consolidation of individualism and utilitarianism in social relationships. These are some of the implications of familial solidarities.

The contribution of sociologists to this use of the concept of familial solidarity is manifestly central. Many of them have sought to define this concept better, particularly by distinguishing between what appertains to material, instrumental mutual aid (from monetary exchanges to the provision of services capable of being themselves transformed into a monetary equivalent) and what appertains to social ties and inclusion in relational networks [START_REF] Déchaux | Les échanges dans la parenté accentuent-ils les inégalités ?[END_REF], the former being susceptible to guaranteeing protection and the latter social insertion. This triple economic, domestic and social function of familial solidarities is in itself witness of its complexity. Whereas for some sociologists accessibility is conditional upon the achievement of contacts and social ties of the 'ego' with a third party, for others it is necessary to avail oneself of the materiality of reciprocities, the importance and intensity of exchanges and flows.

It is not our intention in this chapter to propose a "substantial" definition of familial solidarity but rather it will be our endeavour to retrace the conditions that have allowed this concept to impose itself little by little whether on the work of experts, on political discourses or in everyday speech, not without giving rise to a number of amalgams, sources of confusion and misunderstandings. We propose to give pride of place to academic usage of the concept of familial solidarity. This entails both an understanding of the context in which this conception has taken root and the discussions or controversies that such academic usage has engendered, constantly interwoven as it is with political and media usages. We are therefore proposing an assessment, necessarily partial, of sociological writings on familial ties together with their consequences in terms of conception of what is entailed in familial solidarities giving rise to the belief that this concept causes us essentially to create a certain number of contemporary public problems. Its inaccuracy then becomes secondary in respect of this function of creating the public problems and solution scenarios which it evokes.

Emergence and apogee of the concept

The issue of familial solidarities in French contemporary sociological research is to be found in the course of two episodes relatively distinct from one another: the first of these goes from the early 60's to the middle of the 70's during which the theme of familial solidarity is linked with analysis of ties of kinship, the primary sociality; the second which is more recent covers the period from 1985 to 1995, when the concept of familial solidarity clearly imposed itself not only in terms of sociological research but also in public debate. This rediscovery of familial solidarities now comes in a very different context from the preceding one: that of the withdrawal or crisis of the "protective" State with its consequences in terms of the risk of exclusion. Sensitivity to the concept of familial solidarity is observable in both its positive and negative effects, so to speak, in the sense that one will examine the protective role of the family as closely as the devastating effects of the absence of familial social ties on isolated or "disaffiliated' individuals, that is to say, "those lacking in social ties". Concern for the victims of social exclusion or for those without available familial resources to cope with the difficulties of their existence will serve to centralise to an even greater extent the protective virtues of the family and the assistance that it provides. By means of this, familial solidarity becomes ancillary to the shortcomings of public solidarities, an outer protective circle for the potential benefit of all social entities involved in socio-familial relations.

The kinship network: sociability and mutual assistance

From the early 60's and up to the end of the 70's, a certain number of anthropological and sociological researches were devoted to the study of kinship networks or blood ties as a relational and mutual assistance network. The aim therefore was to assess the density of social bonds within the system of kinships and to test the hypotheses of the conceptive contraction of kinship to the nuclear family or household. These studies also purported to ascertain the effects produced through organisation of the kinship system in terms of socialisation and normative production and also in terms of mutual protection.

Anthropologists and ethnologists have never ceased to affirm the pertinence of the "network" concept to comprehend the familial environment or village community relationships. Rather than "family", they preferred to speak of "kinship" to designate the grouping of individuals united by ties of blood and alliances or "parentèle" so as to include certain "acquaintances" with whom one "feels in close relationship". The anthropological approach is not restricted to the study of traditional societies. In his work Sociologie de la famille, M. [START_REF] Segalen | Sociologie de la famille[END_REF] takes this important trend of research into account for the analysis of kinship networks in contemporary urban societies (chapter 4).

Most of these works on the study of kinship networks contest the thesis formulated by T. [START_REF] Parsons | The Kinship System of the Contemporary United States[END_REF] according to which the industrialisation process has contributed in isolating the conjugal domestic group from its kinship network and in promoting the "nuclear family" model, where the nucleus consists of a conjugal household based upon marriage and composed of the father whose "instrumental" role consists in assuring relations with the outside world, the mother who is entrusted with an "expressive" role within the family nucleus and a small number of children iii . Without entering here into a systematic analysis of these diverse contributions, one could just take a few significant examples to highlight certain essential points.

As early as 1957, two British researchers put T. Parsons' hypotheses to the test. In effect, the study of M. [START_REF] Young | Family and Kinship in East London[END_REF] developed on Bethnal Green, a working quarter of London, describes the density of familial relations at the level of the urban area in question, the residential proximity, the frequency of encounters and exchanges and the role assumed by relatives in the creation of a friendly network. But these writers stress above all the importance of the mother-daughter relationship, of particular importance in the choice of residence, qualified as "matrilocale". Most of the couples actually live with/near the wife's relatives so as to enable the maintaining of ties, advice, mutual assistance etc., supported by the mother-daughter relationship. On the occasion of a second study, carried out some years later, these same writers found themselves in a position to evaluate some determinate effects consequent upon renovation of the urban district in question, by examining a section of the population of Bethnal Green relocated in Greenleigh [START_REF] Young | Family and Class in a London Suburb[END_REF]. Thanks to this it was possible to note that the physical removal of blood-relationships undermined the mother-daughter relations which was so important in the first study whereas this physical removal of blood-relations had consolidated the conjugal relationship thereby permitting a reduction in the logic of the clear-cut distribution of the roles assumed by the couple (thereby coinciding with the hypothesis of Elizabeth Bott mentioned hereunder). In the urban and more anonymous communities of this type "the status substitutes the relation network" (Segalen, 1993:107) and the nature of the sociability network is profoundly modified, freeing itself of kinship logics or the pressure of "strong ties".

But on the issue of familial relationships, the theory of family-linked social networks is principally associated with the name of E. [START_REF] Bott | Family and Social Network[END_REF] iv . The aim of this researcher is to understand to what extent conjugal relations and distribution of the roles assumed by the couple are connected with or independent of the household's openness to, or exclusion of, the outside world and wider kinship network. Thus, she has proposed a hypothesis according to which the degree of division of roles assumed by the couple is linked to the density of the network of relations of the married couple outside the household. The more a sociability network has tightened around a familial nucleus and blood-relations with strong interconnections have been intensified by spatial proximity, the more one has to deal with families in which the division and hierarchisation of sexual roles predominatea circumstance that corresponds rather to the working class model. In such cases a strong mutual social pressure is exercised. On the contrary, a more open, more flexible network, less localised and more diversified, permits a certain normative flexibility and corresponds to more egalitarian roles to be assumed by the couple, a circumstance more characteristic of the middle classes being less centred on kinship. The network will therefore include friends and colleagues. This is however in this work less a question of mutual assistance and solidarity practices than a question of the norm and distance to the roles prescribed by tradition and communitarian adherence.

In France, some studies in the mid 70's were devoted to this perspective of analysis of the kinship network. They confirmed the permanence of the familial network beyond the strictly conjugal ties and the reduction in the size of households. L. [START_REF] Roussel | La famille après le mariage des enfants[END_REF][START_REF] Roussel | La famille après le mariage des enfants[END_REF][START_REF] Gokalp | Le réseau familial[END_REF], described the structure of this network specifically restricting themselves to relations with the more immediate next of kin (grand-parents, parents, children, brothers and sisters, brothers-in-law and sisters-inlaw). These studies assessed not only the proximity of residences but also the frequency of encounters notwithstanding the limited number of cases of married children coresiding with their parents, that is to say the density of relations within the bloodrelations network. C. Gokalp thus showed that 63% of the persons interviewed during her inquiry were living at a distance of less than 20 kilometres from their parents. In the light of these studies, the French family would appear to be heavily concentrated in very limited areas, with the children seeking to maintain proximity with their parents and, later in life, with their own children, thereby prolonging a sort of "distant closeness" with the remainder of their kin network.

But in France the concept of familial solidarity is primarily linked with the work of Agnès Pitrou. In her research work, A. [START_REF] Pitrou | Le soutien familial dans la société urbaine[END_REF]1978) went beyond a description of "familial geography" so to speak to broach not only the subject of the differential sociability of social classes in an urban environment but also that of the level of reciprocation and familial solidarity. Her study has once again clearly demonstrated the strength of kinship ties, their intensity, frequency etc. but above all the fact that they are vectors of a great number of exchanges of services, goods, counselling and information v . Reciprocations however appear to be structured differently according to the social class in which they are performed (between the "subsistence aid" in the more popular environments and the "promotional aid" among the middle classes), but also according to stages in the life cycle or even the size of communes.

In the light of these results, A. Pitrou stresses the quasi-assurential role played by relatives in the event of socio-economic difficulties and this all the more in popular environments where there appears to be greater dependence on assistance of this kind. The family plays a role both of daily assistance and protection in cases of "great blows". Although in the more affluent environments solidarity of this kind may also be operative, it is achieved in a different way. On the one hand, families have a greater number of assets to exchange and, on the other, the aim is to enable the younger generations to accede as quickly as possible to a social condition which is at least equivalent to that of their parents. As against this, mutual assistance would appear to be more diffused, more concealed, less explicit.

In the course of this initial phase which introduces the subject of mutual assistance within the kinship network, the essential points discussed would seem rather to appertain to a theoretical struggle between those who give priority to the kinship and network aspect and those who hold with a micro-sociological conjugal approach, between the defenders of a socio-anthropological reading of kinship relationships in a somewhat holist perspective and those who subscribe to a more individualist perspective of the analysis of strategies and interests within the environment of the family and the couple. There is rarely any question, except in the work of A. Pitrou, of the protective capacity of kinship in respect of the limits of collective protection.

It is noteworthy that, in the 70's and 80's, studies on the blood-relationship network were relatively rare and awakened only a feeble echo in France by comparison with the flood of studies and debates on the subject of familial ties from the standpoint of the crisis and weakening of conjugal relationships. Following this brief parenthesis of interest, the subject of familial or kinship solidarities gave place little by little to concern for the crisis or even total collapse of the family and the focusing of attention on conjugal aspects.

Explosion of the subject: the new "kinship regime" and recognition of familial solidarities.

It was however during the end of the 80's and even more in the course of the 90's that the subject of familial solidarities really imposed itself both in public and political debate and in the forums of experts and decision-taking assemblies vi .

The familyvector of solidarity: such would clearly appear to be the conclusion of the very numerous research studies which, during the period mentioned, undertook an assessment of the ties of kinship and stressed the role of familial solidarities in respect of economic support, the transfer of goods and symbols, access to accommodation, to ownership or employment, aid in the form of assistance with domestic chores and caring. This trend of research includes the work of J.-H. [START_REF] Déchaux | Relations et solidarités de parenté en France[END_REF]Déchaux ( , 1990aDéchaux ( and b, 1994) ) on "solidarités ou l'économie cachée de la parenté" (solidarities or the concealed economy of kinship), those of M. [START_REF] Marpsat | Les échanges au sein de la famille[END_REF], R. Dandurand and R. Ouellette (1992), B. Bawin-Legros and J. Kellerhals (1991) on reciprocations and intergenerational solidarities, those of M. [START_REF] Anselme | Les réseaux familiaux dans le parc HLM. Premier constat[END_REF], P. [START_REF] Cuturello | L'entraide familiale dans la société urbaine[END_REF]), A. Gotman (1988), T. Blöss and F. Godard (1990), C. [START_REF] Bonvalet | La famille et le marché du logement : une logique cachée[END_REF], Y. [START_REF] Grafmeyer | Héritage et production du statut résidentiel : éléments pour l'analyse de milieux locaux[END_REF], Bonvalet & Gotman, (1993), on mutual assistance and familial mobilisation in respect of access to accommodation, those of C. [START_REF] Martin | Support et affection. Logiques d'échange et solidarités familiales après la désunion[END_REF]1994a;[START_REF] Martin | Lien familial et vulnérabilité[END_REF] on sociability and social support following separation or family break-up, or again works that analyse the exchange of services in kinship relations in respect of gifts, indebtedness or other reciprocities [START_REF] Bloch | Du don à la dette : la construction du lien social familial[END_REF][START_REF] Godbout | L'esprit du don[END_REF][START_REF] Godbout | La dette positive dans le lien familial[END_REF].

Similarly it may be noted that the study of reciprocity in kinship relations became, more or less simultaneously, the subject matter of important statistical enquiries both in France, in Belgium, Switzerland and Canada, for the purpose of assessing the intensities and flows. A mobilisation of this kind gives reason to believe that the social requirement of an understanding on the part of the public authorities had by then become more pressing. For France mention should be made of the enquiry carried out by C. Bonvalet, H. Le Bras, D. Maison and L. Charles of INED (national institute of demographic studies) on "relatives and parents" (1993); that of [START_REF] Fougeyrollas-Schwebel | Groupe familial et parenté : les circuits d'échanges de travaux domestiques[END_REF] on familial assistance and relations, in the enquiry entitled "Modes de vie" (Lifestyles) undertaken by INSEE and the CNRS, the enquiry by Claudine Attias-Donfut (1995) on familial solidarities among three successive generations. In Switzerland and Belgium similar enquiries were implemented, one by a team from the University of Geneva under the direction of J. Kellerhals [START_REF] Coenen-Huther | Les réseaux de solidarité dans la famille[END_REF], and another by a team from the University of Liège under the direction of B. Bawin-Legros. In Quebec, two research studies were undertaken for a similar purpose, one guided by C. Lebourdais on the familial network and the other by J. Godbout and J. Charbonneau on gifts, exchanges and solidarities in kinship relations. Similar quantitative enquiries are undertaken at the moment in Spain and Portugal.

Clearly, the subject of networks and kinship solidarity again became topical. Thus for example mention may be made of the commentaries of H. [START_REF] Mendras | La seconde révolution française[END_REF] vii or J.-H. Déchaux (1990a) both of whom stressed the need to renew the examination of kinship in the contemporary period viii . Both advance, each in his own way, the hypothesis of the emergence of a new "regime of kinship". In the chapter devoted to this subject, H. Mendras insists on the relationship established by ethnologists between the size of the domestic group and the importance of the network; the more the domestic group is reduced, the more extensive the network becomes. This in turn implies a reorganisation of kinship relationship: "a strengthening of the 'parentèle' and restriction of the household go together inasmuch as separated dwelling places do not necessarily entail the weakening of family ties"(op. cit.: 243).

This new organisation of relationships in the kinship derives from a phenomenon termed "intimité à distance" (distant-closeness) without weakening the intensity or frequency of reciprocations. On the contrary, the author envisages the emergence of a new form of relationship that prioritises solidarity free of daily obligations. Such "distant-closeness" provides an important margin of manoeuvrability for the parties involved, children in particular, who can engage in complex strategies so as to utilise more advantageously these interconnected networks of kinship. This approach however entails a determinate risk: that of reducing the reality of the effective exercise of the relationship to the emerging practices of certain social categories: the educated middle class.

J.-H. Déchaux for his part insists on the logics of reciprocation in these new patterns of kinship. Allying himself with the viewpoints of A. Pitrou, he again distinguishes between, on the one hand the concentration of less affluent families on blood-relations representing a sort of "counter-society" that the parties will always prioritise as against institutional solidarity and, on the other, the "logics of dissimulation of intentions" that becomes, even outside the middle classes where it prevails, the new model associated with the wish for autonomy. This dissimulation of material interests in reciprocation is based on an important norm whereby parents are solidary with their children and provide them with assistance without any need on the part of these latter to request it at any time. According to the author, this dissimulation is linked to the contradiction between aspiration to a condition of autonomy and the reality of dependence which entails a rationalised discourse on the question of gratuitous reciprocity.

Finally, this same author stresses the importance of ties of filiation when confronted with the vulnerability of couples which is itself linked to the autonomisation of women through the taking up of employment. "Is family lineage becoming the only fixed point of a kinship system in process of segmentation owing to the multiplication of divorces?" (Déchaux, 1990b: 102). Having said this, he is of the opinion that novelty resides here in the fact that the solidarity of blood relations is today freely granted and that it is in process of substituting that of the couple in respect of the provision of protection. The functioning of the couple on the other hand will be increasingly comparable with other spheres of social and professional life: that of permanent negotiation, where nothing is definitively acquired. By comparison with the precarious nature of the conjugal relationship, the place of negotiation, kinship would become the place of permanence. From the traditional concept of the family tree, kinship would be on the way to becoming a network where "it is by now structured more by affinity and personal choice than by genealogy" (ibidem, 1990; 105). But this return to the theme of the solidarity of kinship goes far beyond the narrow field of the sociology of the family. In effect, on the subject of research into the gerontology and sociology of health, a vast range of literary works also evokes this form of familial solidarity in terms of "social support" ix adopting concepts to be found in Anglo-Saxon literature. What is at stake is the role expected of and/or assumed by families in promoting a condition of health and in regard to care of the elderly. Following a period of pronounced institutionalisation of care, especially for elderly persons, there developed in the course of this period recognition of "familial production of care" [START_REF] Bungener | La production familiale de santé : le cas de l'hospitalisation à domicile[END_REF] or of "non-professional health work" generated within the family circle, more frequently by women [START_REF] Cresson | Le travail sanitaire profane dans la famille : Analyse sociologique[END_REF], what is known in the Anglo-Saxon world as caring practices.

From the standpoint of social and political demand, the aim of such valorisation of familial solidarities is twofold: on the one hand, it is a matter of reducing the cost of institutional care and on the other, the persons concerned are expected to find in such valorisation of familial solidarity the benefit of maintaining their environment and network. The studies undertaken nevertheless sought to show that the family has probably never ceased to fulfil its central role, supplemented by available public and private services [START_REF] Lesemann | Famille-Providence. La part de l'État. Recherche sur le maintien à domicile[END_REF]Lesemann & Martin, 1993). Every appeal to the restitution of familial solidarities thus overlooks the requirement that these should be more stable and permanent than public services and/or formal services for families.

In this way one finds the effective establishing of the connection between the problematisation of kinship as a mutual assistance network and the matter of the withdrawal of State solidarities (Martin, 1995b). This has been underscored by M. Segalen in this way: "The renewal of sociological interest in the ties of kinship derives from the reversal of ideological positions and profound social and demographic changes that have produced an impact on the social body, and notably on its ageing process. Owing to the crisis that has arisen in the État-providence, other forms of solidarity, in particular familial solidarities between the generations have been recognised" (Segalen, 1993: 233).

Private and public solidarity balance to face inequalities and social exclusion

The come-back of the familial solidarity issue in the late 80's was therefore linked to a new context, that of the crisis occurring in the social protection system. Thus, on the occasion of the 1983 Symposium entitled "Recherches et familles" (Researches and families), under the auspices of the French Presidency of the Republic, which was undoubtedly an indication of renewed interest on the part of political circles in the matter of the family x , Giovanni Sgritta, greatly in advance of others, foresaw this change of perspective. He therefore wrote: "In the immediate aftermath of the War and up to the end of the 60's, social research tended to reflect the crisis occurring in the institution of the family whereas at present it reflects rather the crisis in a certain social pattern, it is the crisis of the État-providence, highlighting at one and the same time the maintaining and the function of the family in the implementation of important social responsibilities" (1983, p. 169). Giovanni Sgritta's hypothesis of an analytical shift from the crisis in the institution of the family towards that of public solidarities has subsequently been largely verified in the sense that this context of the État-providence crisis has imposed new categories of reflection on those responsible for the implementation of social policies. Sociability, mutual assistance, relational support and concern on the part of relatives have thus become instrumental in the formulation of public policies.

The example of the provision of care for elderly dependants is clearly one of the sectors where the matters outlined above have become more evident, quite often for the purpose of stressing the extent to which the family, and above all women, continue to be the cornerstone of the support provided xi . Hence one is witnessing a recognition of this "deposit" of "natural" and gratuitous solidarity that is now being flourished as an alternative to the shortcomings and limits of collective solidarities.

The public authorities have since then become aware of the irreplaceable character of this non-monetary economy, this protective capacity on the part of "parents and relatives". With the crisis of the Welfare State, the policy makers have acknowledged the impossibility of substituting the responsibilities inherent in domestic duties required of women in certain periods of their life-cycle with the development of public and institutional services whether at the level of the care of very young children or again, and to an even greater extent, of elderly dependants. Since the cost for the society would seem less and less bearable, the policy makers recognized and promoted these gratuitous services conceived as a moral obligation whereby certain tasks are assigned to private providers. In consequence of this, "what was until then considered as a residual of past times, destined to disappear gradually thanks to the development of institutional relationships, has reappeared under a new form as a network of basic social relationships that enables individual members of society to remain united and keep for themselves an area of social activities preserved from the market" (Insel, 1993, p. 221). This function of "natural" and gratuitous support is nevertheless an undertaking that is very unequally distributed among genders. Clearly, this entails a highly important issue concerning the gender division of roles and responsibilities and even the recognition of domestic work as a real work which has to be retributed. A. [START_REF] Chadeau | Que vaut la production non-marchande des ménages ?[END_REF] thus estimated, on the basis of the "time use enquiry" undertaken by INSEE, that the quantity of time devoted to non-remunerated domestic work (preparation of meals, shopping, washing, ironing, caring for babies, children and adults, household chores etc.) was, in 1985, 25% greater than the time devoted by French society as whole to remunerated employments, allowing for the fact that the greater part of this work was performed by women. Nevertheless, for "anti-utilitarist" academics, this gratuitous work does not cover all the goods exchanged within the household environment and which to a considerable extent are not substitutable (the gift of life, affection, cultural enhancement, traditional gift-giving etc.). By adding this second circle of what might be termed "the gift-circuit" and reciprocations between households (monetary gifts between households through the intermediary of associations, foundations, charitable societies, traditional gift-giving etc.) A. Insel proposed early in the 90's an initial, "very limited and very approximate assessment of donations in contemporary French society, that is to say a volume amounting to approximately three quarters of GDP (Insel 1993, p. 234). Since then, familial solidarity raises the evermore-complex question of the boundary between paid employment, remunerated or indemnified activities, the services sector, voluntary work and non-remunerated domestic activities (Evers, Pijl, Ungerson, 1994).

Relational vulnerability or the virtues of "close-up protection"

In evaluating the weight of this domestic economy or "close-up protection", consideration has also been given to what might cause its absence. This is the other side of the rediscovery of familial solidarities; the impact of "life without family", as A. Pitrou puts it, with the risks that the fact of being excluded from this insertion and primary socialisation pattern engenders, the risk of isolation, solitude and dependence in respect of public solidarities. Ever since the introduction in France of the Revenu Minimum d'Insertion (R.M.I. -Income Support) in 1988, the phenomenon of isolation has become a matter of increasing importance. Has it not been stressed on the occasion of the research work and assessments leading up to the R.M.I. that over 70% of beneficiaries were "isolated", taking into consideration that this implied individuals who were single and without family.

The profound ambiguity of this omnium gatherum or cover-all category of "isolated" individuals is evident [START_REF] Martin | Risque solitude. Divorces et vulnérabilité relationnelle[END_REF][START_REF] Kaufmann | Vie hors couple, isolement et lien social : figures de l'inscription relationnelle[END_REF]. In effect, to say that an adult has not created a partnership or had children does not mean that he or she is isolated, without contacts or sociability. As against this, this percentage would seem sufficiently meaningful to support the following considerations: poverty and insecurity go hand in hand with isolation or solitude. Over and above the number of persons to be designated as "poor", it was endeavoured to determine the number of "isolated" persons (those living alone) by attempting to establish what the growth of this phenomenon might signify: independence or an assertion of autonomy or, on the contrary, imposed conditions of isolation and instability [START_REF] Lefaucheur | Personnes seules à Paris : "nouveaux célibataires" ou "veuves solitaires[END_REF][START_REF] Delbes | Croissance du nombre des isolés en France : vers de nouveaux comportements[END_REF][START_REF] Kaufmann | Vie hors couple, isolement et lien social : figures de l'inscription relationnelle[END_REF]. Although one should not allow oneself to be blinded by this phenomenon of isolation in respect of accommodation, since the pattern of mutual assistance is not just a matter of living together xii , it is undeniable that a certain number of persons living alone are solitary, that is to say, living in a situation of vulnerability.

The "risk of isolation" is linked to the issue that, in the mid-80's, was termed the "new poverty", that is to say, a form of poverty affecting persons "ready and fit for employment" but who, following a hitch in their trajectory, find themselves drawn into a situation of precariousness. Among these hitches, one undoubtedly finds first and foremost the loss of employment, but family breakdowns also appear to be a source of "désinsertion" or exclusion. S. Paugam, J-P. Zoyem and J-M Charbonnel have thus highlighted the relationships that clearly exist between unemployment and conjugal breakdown, insofar as one automatically entails the other thereby bringing about a situation of social exclusion (Paugam et alii, 1993). R. Castel (1991a;1995) and C. [START_REF] Martin | Risque solitude. Divorces et vulnérabilité relationnelle[END_REF] even put forward the hypothesis of a "relational vulnerability" by envisaging a "dual form" of the individual's fragility, corresponding on the one hand to non-insertion or professional exclusion and, on the other, brought about by the loss or absence of sociability and the support provided by the family. Castel also proposes a concept designed to qualify this process: "disaffiliation, a particular way of dissociation of the social ties" (p. 139). His hypothesis is as follows: "First of all, in pre-industrial societies, social interventions always had the aim of attempting to master a double shift, in respect of integration through employment and in relation to insertion in a relational pattern; secondly, the specific nature of the present situation can be understood as a new episode in this struggle against the double instability in the organisation of employment and the structuring of sociability" (ibid: 140). In support of these insertion/integration axes, Castel makes a distinction between four modes of existence in contemporary society: the "integration zone" where permanent employment goes side by side with relational supports; the "vulnerability zone" which associates employment instability with the fragility of social relations; the "assistance zone", characterised by the absence of employment frequently linked to the impossibility of working and satisfactory social insertion; and, finally, the "disaffiliation zone" where the absence of employment goes hand in hand with isolation. "Today, the integration zone is breaking up, the vulnerability zone is expanding and continually strengthening the disaffiliation zone. Is the sole remedy to this scenario a reinforcement of the assistance zone? (ibid: 152). R. Castel interprets the fragility of couples, the reduction in the size of families, the relational complexity of new unions, as indicative of the contemporary force of disaffiliation, but also as the result of interventions on the part of the État-providence. "The undermining of the family structure may not be due to shortcomings on the part of the État-providence. Rather, it might be an effect of its relative success. In warding off a certain number of social risks, the State has left the family face to face with its relational fragility… Consequently, what has been left to the management of the modern family is to an ever greater extent its own relational capital." (Castel, 1991b: 31).

J.-H. Déchaux is similarly oriented when he proposes to "show that new forms of poverty appeared as a result of the weakening of the social links, whereas traditional poverty, that takes root and proliferates on the outskirts of society, diminishes" (Déchaux, 1990c: 9). Among these new forms of poverty one finds single-parent families that are especially prevalent among poorer households [START_REF] Martin | Lien familial et vulnérabilité[END_REF]. Although it is evident that disunion (the principal cause of single parenthood today) systematically engenders a more or less drastic economic impoverishment of the guardian parents' household, it nevertheless does not entail conditions of poverty in all cases but merely accelerates the process of destabilisation of already vulnerable individuals xiii . In 1997, little more than 20% of single parent families were considered as living in conditions of poverty (according to OECD) whereas in the United Kingdom nearly 60% of single mothers were classified as poor. In effect, the reason for this condition of poverty resides not so much in the family structure as in whether or not the single mother is in employment, with a full-time job or not. Equally, considerable importance attaches to whether or not the ex-companion contributes to the upkeep of the children and to whether or not assistance is forthcoming from relatives [START_REF] Martin | Lien familial et vulnérabilité[END_REF]. But when these different factors add up negatively the situation of instability becomes extremely delicate.

To sum up, if kinship provides protection against the risks and economic difficulties inherent in contemporary society, it is easy to understand the importance of being integrated in a network of this kind. Whether it is a matter of young persons in search of their first job and who cohabit for an increasingly longer period of time with their parents (Blöss et alii, 1990a and b;[START_REF] Galland | Un nouvel âge de la vie[END_REF][START_REF] Bozon | Les enjeux des relations entre générations à la fin de l'adolescence[END_REF], couples who separate and avail themselves of the support of their relatives to surmount this critical period [START_REF] Martin | Lien familial et vulnérabilité[END_REF] or elderly persons who must count on the presence and support of their daughters to cope with their increased dependence (Lesemann & Martin, 1993), it would seem evident that such family protection may be needed throughout all one's life. Without it, the individual has no alternative but to rely in one way or another on public solidarity.

Solidary practices and social classes

Having imposed itself in scientific debate and as a political objective, the issue of familial solidarities raised a certain number of questions. The first concerns social stratification effects and differences in functioning according to social class. What is at stake is important since it is also a question of determining whether or not such primary forms of solidarity succeed in reducing social inequalities and if so, to what extent. But even more fundamentally the question arises as to the bases of such mutual assistance practices. Have they the same meaning for the protagonists according to social classes, or according to age and generation categories, according to the levels and nature of reciprocity and mutual help (whether monetary or otherwise) etc.?

In the light of available works, it appears likely that the expression of relational support and even the feeling of mutual obligation varies considerably according to social status. In more traditional circles (whether among the less affluent categories or at the other extreme the particularly well-off), the duties of solidarity and mutual help within the familial network will tend to predominate. On the other hand however, this obligation, this unconditional assistance, rules out any contravention of the norms currently in force in these background. This 'should-be-so' behaviour sometimes gives rise not only to a feeling of limitation and social control of one's private life but also, on occasion, to a sense of loneliness. This may perhaps be defined as a communitarian functioning with a degree of rigidity in the roles to be assumed (the role of the relatives, of the father, the mother, the brother etc.).

In other backgounds, culturally more generously endowed, where the networks are more frequently composed of friends and non-family relationships, the forms of solidarity appear to differ distinctly xiv . The rule from which one must not depart is rather that of autonomy. The integration norm is less the unconditional mutual obligation obtaining within the familial network than autonomy within the friendship network. In a situation of this kind, the important thing is to be able to decide for oneself whether or not to assume the obligations of the proposed relationshipan elective involvement, so to speak. One may also take care to ensure the reciprocation of any help and support given, a balance in terms of the exchange of goods and services; if this is not possible, it would inevitably imply dependence on one's network, which is unacceptable. Moreover, autonomy of this kind may be all the more easily advocated in the case of those who benefit from conditions of socio-professional insertion that frequently enables them to look after themselves. Additionally, although the kinship may be less central in the network, it is nevertheless present and very frequently assumes silently a role of unconditional support. One may therefore continue to be partially dependant on one's relatives while still conserving one's autonomy (as regards self-determination). In such case, it is the distance from the roles or position prescribed by one's status that appears primordial, thereby revealing a desire for emancipation in respect of the conventional definitions of the roles in question. This gap in the functioning of mutual help results also in differences in the level of help. The more affluent is a social network, the more important is the mutual help [START_REF] Déchaux | Les échanges dans la parenté accentuent-ils les inégalités ?[END_REF]Martin, 1995b;Paugam et alii, 1997). Those who are better off, with material means, commit themselves more readily than those who have little, whether in respect of material or financial help or for "subsistence" aid, "lending a helping hand" by taking over everyday responsibilities such as caring for the children, etc. [START_REF] Pitrou | Vivre sans famille[END_REF]Degenne et alii, 1997;[START_REF] Paugam | Le soutien financier de la famille : une forme essentielle de la solidarité[END_REF].

But assistance is not only an affair of available means. It relates also to differences in the meaning of mutual help. To take into account these differences in terms of practices of mutual help, we have proposed another explanatory mechanism [START_REF] Martin | Lien familial et vulnérabilité[END_REF]: "One more easily support those who are less in need of assistance". In effect, in the perspective of the triple obligation of donation envisaged by Marcel Mauss (give, receive and return), the need involves the risk of dependence. Essentially, help is all the less easily granted by relatives and friends when the need for assistance is greater; this in turn is conditional upon each of the individual protagonist's perception of the autonomy or, alternatively, the risk of dependence on the part of the beneficiary. When support is synonymous with the development of a state of dependency, it tends to dissuade both giver and beneficiary since, on the one hand, there is a risk of not having any form of reciprocation and, on the other, of undergoing an excessively onerous social and relational disqualification xv .

To appreciate the meanings and basis of mutual help or private solidarity, it would appear opportune to take into account the logics of reciprocal exchanges, inasmuch as these are themselves dependent on social classes and the types of network involved. Doubtless one finds in such cases the distinctions put forward by Agnès [START_REF] Pitrou | Vivre sans famille[END_REF] that oppose on the one hand the centralising on the part of the less affluent classes on blood-relations and, on the other, the friendship networks and the promotional logics of familial solidarities among the middle classes. In recognising an increasingly important role for these informal types of support and in considering them capable of compensating for the limitations of collective solidarity, the public authorities refuse to take into account the very considerable divergences from the norm and make of the family a value in itself.

Conclusion

Although sociology has greatly contributed in popularising usage of the familial solidarity concept, it has not always done so in the same perspective nor has it always had recourse to the same concepts. The kinship network, the primary form of sociability, social support, social care, domestic work, the distribution of the roles between gender, are but some of the concepts entailed in this evaluation. The theoretical stakes have also evolved, that is to say, from the controversy taking place between those academic researchers who favour the anthropological approach of kinship and those centred upon the conjugal dimension, and that which has by now concentrated debate upon the balance to be attained between private and public forms of solidarity. So, we are not facing only a theoretical matter for the discipline that sociology constitutes, in the sense that this issue is becoming to an ever greater extent a political card to be played in the controversy that opposes a number of ideological trends regarding the future and forms of collective protection.

To be convinced of this, it will in effect suffice to examine the public and political debate more closely. The "family question" has today made its appearance both in France and in European public debate under the form of a certain number of issues including solidarities, familial responsibilities and/or obligations in association with questions such as poverty, insecurity, delinquency and incivility. The role of the family in respect of the protection of vulnerable individuals, whether it is a question of children, adolescents, young adults, those who have difficulties in respect of social integration or remaining in employment, chronically ill persons, the handicapped, elderly dependants etc., appears increasingly more centralised in the context of the reduction of public expenditures and the consolidation of social inequalities. Insertion in familial networks appears to an evergreater extent to be a means whereby to ensure "social cohesion" and the avoidance of "social fracture". This central question is accepted as problematic on the basis of the following argumentation: on the one hand, nations would become subject to the need to control their public expenditures because of the elements of international competition and the globalisation of economic exchanges thereby leaving an ever-smaller margin of manoeuvre in the drawing up of policies which are by definition costly. On the other hand they would undergo rapid transformation as regards familial behaviours with a falling off in fecundity, the ageing of the population, the disruption of stages in the life cycle, the weakening of family ties or even obligations and solidarities of which it was once the vector. In consequence, these elements represent what is primarily at stake and the arguments advanced in discussion on the future of social protection systems.

In this way, the "familial question" would appear to be a component of the social question and occupies an increasingly important place in commentary on the crisis and evolution of social protection systems. It is no longer a matter of a sectorial question that could be reduced to an analysis of the policies relating to family or to what the State and social partners conceive for the support and protection of families, but rather of a more far-reaching questionthat of allocations between private and public protections, that of the respective roles of the State, territorial communities, the services sector, the market and the family in respect of the protection of individuals and their trajectories.

For purposes of simplification, one may contrast two currents of opinion on this "familial question" as formulated here. The first considers that demographic and familial disruptions are largely the result of the development of the État-providence itself that, by substituting itself for the familial institution, has deprived it of its functions, thereby giving rise to an unruly and devastating individualism, but also to a de-responsibilising of citizens and their dependence on the State (Murray, 1984). The solution would therefore consist in shifting this trend in an other direction by returning to "initial values" and restoring the obligations and responsibilities within the primary networks of family relationships. This strategy therefore consists in handing back the responsibility for the solution of a certain number of problems to private networks and in not envisaging anything as an alternative solution. The State would thus rid itself in some way of familial solidarities. A strategic, less radical alternative would consist in the observation of familial obligations.

The second current of opinion considers on the contrary that the development of the État-providence and services rendered to families has only completed and restored balance to functions that had been very unequally fulfilled by the primary groups without ever having fully taken over such functions. In short, familial solidarities would continue to perform their role in conjunction with collective protection systems, and this inasmuch as the aim today is to assist the provision of aid on the part of the closer kinship networks so as to enable them to discharge their responsibilities rather than to shoulder them anew with such obligations. In this perspective, such complementarity must be maintained at all costs so as to avoid social regression, the undermining of private solidarities or the disintegration of social ties. Two scenarios are however still possible according to whether one prioritises the development of professional services or, on the contrary, seeks to provide monetary support for the members of a family that finds itself in need of assistance.

As one sees, it is particularly difficult to claim that one has put the concept of familial solidarity to good sociological use in this context. It would be better therefore to admit that it constitutes a pertinent category to analyse public action, a manner in which to name and give meaning to mutual assistance practices within and between households, practices that, it so happens, have never ceased to exist.

i . According to the Littré of 1877, cited by L. Bourgeois, definition of the term "solidarité" in everyday speech implies "mutual responsibility established between two or more persons". For Léon Bourgeois, there exists a universal law of solidarity founded on the interdependence of the human species and causing them to associate on the basis of a quasi-contract: "Each and every living person bears a debt towards every human being because of, and proportionate to, the services rendered him by the efforts of all. This exchange of services is the substance of the quasi-contract of association that links all mankind and the legitimate aim of social law is the equitable assessment of the services thus reciprocally rendered, that is to say the equitable sharing of profits and costs, social assets and liabilities. (Bourgeois 1998: 49). ii . "Certainly, it is not in all these descriptions of the manner in which care, mutual aid, assistance, transmission can be put into practice… in short, in its various implementations at familial level, that the essence of familial solidarity can be found. Here, the concept of solidarity adds nothing to the concept of family"… "Hence the theoretical interest in distinguishing between 'social solidarity' and 'familial solidarities'." (Messu, 2000: 126-127).

iii . For a critical analysis of T. Parsons' theses, reference may be made to the work of Andrée [START_REF] Michel | Sociologie de la famille et du mariage[END_REF].. iv . One can again cite the works of M.B. [START_REF] Sussman | The Help Pattern in Middle-class Family[END_REF] and those of [START_REF] Sussman | Kin-family Network[END_REF].

v . This mutual assistance is not only financial but involves as well the preparation of meals, household chores, DIY, the manufacture of children's clothing and above all child-minding vi . It is noteworthy moreover that the work of A. Pitrou Vivre sans famille (Life without family) was re-edited in 1992 under the title: Les solidarités familiales (Familial solidarities). This is not a mere statement of fact since it is indicative of transition to this second phase. Ten years after the first edition of her work, Agnès Pitrou insisted above all on the reciprocal effects of these familial mutual assistance systems with social policies but also on the need for a better understanding of the impact of the precarious nature of conjugal ties on familial solidarity of this kind. "Is it possible to have solidarity based upon ephemeral and increasingly complex ties in cases where they unite a couple with children born of successive unions?" (Pitrou, 1987: 209).

vii . Particularly Chapter XIII of the fourth part entitled "La force de la parenté" (The force of kinship), pp. 234-252. viii . Research carried out in Quebec at the end of the 80's should be included in this same perspective inasmuch as it makes a comparison between workers sociability at the beginning of the 60's and that of the 80's (Fortin et alii, 1987). This research undertaking once again confirms, irrespective of the period in which they occur, the density of kinship relations that even polarises all sociability in the less affluent classes; geographical proximity which gives rise to numerous forms of reciprocation; the widening gap between the domestic sphere traditionally the competence of the woman and the professional sphere largely the competence of men, the strength of motherdaughter ties and mutual assistance given by mothers in everyday life (the care of children, household chores, domestic assistance etc.). These authors stress certain points already commented upon: the quasi-disappearance of inter-generational cohabitation, substituted by "localised ties of kinship"; the gradual diversification of the social ties as it ascends the social ladder; the materialisation among the middle classes of a "companionship" status which is more widely based on friends and colleagues than on the family (which is more easily abandoned) and which also entails a more egalitarian conception of the relationship in the couple.

ix . Under its gerontological aspect, cf. the works of [START_REF] Favrot | L'activité de soins dans le système d'activité familial. Facteurs d'insertion et de rejet[END_REF]; Frossard & coll. (1988);[START_REF] Grand | La famille dans le support social des personnes âgées[END_REF] or those who studied the impact of social and familial support on the promotion of physical or mental health (Le [START_REF] Disert | La maladie : la part du social[END_REF][START_REF] Berkman | Assessing Social Networks and Social Support in Epidemiologic Studies[END_REF][START_REF] Cresson | Le travail sanitaire profane dans la famille : Analyse sociologique[END_REF]). x This symposium followed the major turn of the French political life, with the election of François Mitterrand, who ask for a global assessment of the social sciences to Maurice Godelier and then express the interest of the left wing government for the family issues by organizing this symposium. xi . For French-language publications see: Grand et alii, 1989;Lesemann & Martin, 1993;Martin, 1995a;[START_REF] Joël | Aider les personnes âgées dépendantes. Arbitrages économiques et familiaux[END_REF] xii . H. Le Bras has criticised the overall picture given by households statistics. "If households are considered in the usual way, they give an impression of isolation. The households of single persons or of couples account for more than half the total and isolated individuals a quarter of this. On the contrary, if one refers to the individual, one gets the impression of a gregarious society, fewer than one person out of ten lives in isolation, while half of the individuals are to be found in households composed of at least four persons" (Le Bras, 1988: 29). What is important is not so much the proportion of persons who live alone as the degree of reciprocation taking place between non-cohabitants: "Very frequently, members of a family in a wider sense live separately but in proximity and meet one another frequently or even share meals or expenditures. There is greater flexibility between family and accommodation" (Le Bras, 1988: 29). xiii . This hypothesis of a form of relational vulnerability must not however dispense us from an evaluation of the terms of solitude and isolation; terms that one rightly associates with single-parent situations (notably in consequence of the creation of the single-parent benefit "allocation de parent isolé"). In effect, for some the fact of living in a single-parent situation does not automatically means social isolation. Such persons may have chosen to remain in this situation without however seeking to be cut off from their families, from their ex-spouse or companion and his or her relatives and/or from a close network of friends and colleagues. For others on the contrary, the same situation may engender a profound feeling of loneliness resulting in failure in the individual's attempts to embark upon a new life. A person surrounded by relatives and friends may nevertheless feel lonely and, reciprocally, someone who is apparently isolated may not be in any way affected by his or her solitude.

Everything depends upon the importance accorded the matter and the idea that each individual has of his or her own network, family life, conjugal condition etc [START_REF] Martin | Lien familial et vulnérabilité[END_REF].

xiv The structure of the sociability and mutual assistance networks is closely linked with the social status [START_REF] Héran | La sociabilité : une pratique culturelle[END_REF][START_REF] Bonvalet | Proches et parents[END_REF]. . As stressed by A. Petitat: "A gift that cannot be reciprocated implies for the recipient that he is not worthy of it and is not qualified to enter with dignity into a relationship of reciprocity" (Petitat, 1991: 55).

  xv


Brigitte Boulard

Tran T T Van

Anna Łukowiak

Adel Bouajaj

Rogéria Rocha Gonçalves

Andrea Chiappini

Alessandro Chiasera

Wilfried Blanc

Alicia Duran

Sylvia Turrell

Francesco Prudenzano

Francesco Scotognella

Roberta Ramponi

Marian Marciniak

Giancarlo C Righini

Maurizio Ferrari
email: maurizio.ferrari@ifn.cnr.it

Photonic glass-ceramics: consolidated outcomes and prospects

Keywords: Transparent glass ceramics, Bottom-up and top-down fabrication techniques, Confined structures, RF-Sputtering, Sol-gel, Oxide nanocrystals, Rare-earth ions, Photoluminescence

come

INTRODUCTION

Glass photonics is pervasive in a huge number of human activities and drive the research in the field of enabling technologies. The fruitful exploitation of glass photonics is not restricted only to the area of Information and Communication Technology. Many other photonic devices, with a large spectrum of applications covering Health and Biology, Structural Engineering, and Environment Monitoring Systems, have been developed during the last years. Glass materials and photonic structures are the cornerstones of scientific and technological building in integrated optics. Photonic glasses, optical glass waveguides, planar light integrated circuits, waveguide gratings and arrays, functionalized waveguides, photonic crystal heterostructures, and hybrid microresonators are some examples of glass-based integrated optical devices that play a significant role in optical communication, sensing, biophotonics, processing, and computing. This cross-disciplinary approach leads to constructed luminescent structures that can perform sensing and functionalized structures to successfully address socioeconomic challenges, such as security, cost-effective healthcare, energy savings, efficient and clean industrial production, environmental protection, and fast and efficient communications. Photonics, with its pervasiveness, has already been identified as an enabling technology, and through advanced research in glassbased integrated optics systems, photonics can contribute to finding new technical solutions to still unsolved problems, and pave the way to applications not yet imagined.

Among the different glass-based systems, transparent glass-ceramics are nanocomposite materials which offer specific characteristics of capital importance in photonics. These two-phase materials are constituted by nanocrystals embedded in a glass matrix and the respective composition and volume fractions of crystalline and amorphous phase determine the properties of the glass-ceramic. Transparency is crucial property, in particular for dielectric optical waveguides and optical fibers. Another important point is the effect of the nanocrystals activated by rare earth ions on the spectroscopic properties. Fabrication, assessment and application of glass-ceramic photonic systems, deserve an appropriate discussion which is the aim of this paper, focused on luminescent glass-ceramics.

A SHORT HISTORY OF GLASS-CERAMICS

The scientific and technological activity involving the development of materials at nano-micro scale and related converging technologies allows progress in the conception, design, and realisation of systems and devices with substantially improved performance and significant scientific results. This activity is coming from far both in terms of time and motivation. Table I presents some milestones concerning a brief history of glass-ceramic materials. René-Antoine Ferchault de Réaumur, the brilliant physicist and naturalist, was also in charge of the manufacture of porcelain. He tried to produce a porcelain which could compete with the porcelain of China, without being able to fully solve the problem but creating that opaque glass that bears the name of "porcelain Reaumur" (1739) [START_REF] Baumé | Chimie expérimentale et raisonnée[END_REF]. We have to wait until the year 1952 to appreciate the invention of glass-ceramics thanks to the experiment performed by the Corning Glass Works scientist Stanley Donald Stookey. In the description of the experiment we read that the furnace containing the glass accidentally overheated. Instead a melted glass Stookey observed a white material with the shape of the original glass. He recognized the obtained material as a specific new material that we know as "glass-ceramic" and was initially described as a polycrystalline ceramic materials, formed through the controlled nucleation and crystallization of glass, where the amount of residual glassy phase is usually less than 50% [START_REF] Holand | Glass Ceramic Technology[END_REF]. The commercial outcome of this material was the immediate exploitation of its unique and amazing thermal and mechanical properties leading to the cookware pieces still now known as Pyroflam and Vision products. These strong and thermal shock resistant glass-ceramics have monopolized the research for a long time until the interest in transparent glass-ceramics has made its way thanks to both fundamental research and the required novel application in optics. Is only in 1968 that Schott AG produced a transparent glass-ceramic called Zerodur employed for a number of very large telescope mirrors and also used as sample reference to test the validity of innovative glass structural models [START_REF] Champagnon | Early stages of nucleation of Zerodur glass: Very low frequency Raman scattering and small angle X-ray scattering investigations[END_REF]. The first application of transparent glass ceramics in the field of photonics appears in 1995 with the paper of Tick et al. concerning the exploitation of transparent glass ceramics for optical amplifiers operating at 1300 nm [START_REF] Tick | Transparent glass ceramics for 1300 nm amplifier applications[END_REF]. In the abstract of the paper authors say:" The properties of an oxyfluoride glass ceramic that possesses high transparency after ceramming are described. Approximate1y 25 vo1% of this material is comprised of cubic, fluoride nanocrystals and the remainder is a predominantly oxide glass. When doped with Pr 3+ , the fluorescence lifetime at 1300 nm is longer than in a fluorozirconate glass, suggesting that a significant fraction of the rare-earth dopant is preferentially partitioned into the fluoride crystal phase. This material has the added advantage of being compatible with ambient air processing". It's worthy to note that this paper contains the more significant topics that will be developed in the next 20 years in the field of photonic glass ceramics, i.e. (i) the fundamental role of oxyfluoride as parent glass; (ii) the role of the nanocrystals in reducing the concentration quenching; (iii) the role of nanocrystals with low cut-off energy in reducing non-radiative transitions [START_REF] Tick | Transparent glass ceramics for 1300 nm amplifier applications[END_REF]. After this work we can find in literature a huge amount of papers regarding rare earth activated oxyfluoride based transparent glass ceramics [START_REF] Tikhomirov | Fabrication and characterization of nano-scale, Er 3+ -doped, ultra-transparent oxy-fluoride glass-ceramics[END_REF][START_REF] Beggiora | Structure of the nanocrystals in oxyfluoride glass ceramics[END_REF][START_REF] Mattarelli | Tm 3+ -activated transparent oxy-fluoride glass ceramics: structural and spectroscopic properties[END_REF][START_REF] De Pablos-Martín | Effects of Tm 3+ addition on the crystallization of LaF 3 nanocrystals in oxyfluoride glasses: optical characterization and up-conversion[END_REF]. However, the main result in photonic glass ceramics has been obtained in waveguiding application. A rare earthactivated glass-ceramic waveguide constitutes a potential significant system to behave as an effective optical medium for light propagation and luminescence enhancement [START_REF] Guddala | Glass-ceramic waveguides: Fabrication and properties[END_REF]. Looking at this specific application the problem of the transparency becomes crucial and in fact n 1998 Tick published a paper with the explicit title "Are low-loss glassceramic optical waveguides possible?" [START_REF] Tick | Are low-loss glass-ceramic optical waveguides possible?[END_REF]. The answer was positive and he concluded the paper suggesting that the minimum transmission loss limit of the investigated effective medium glass ceramics is in the order of tens of decibels per kilometre (dB/km), once all of the impurities are eliminated. In the same paper some general criteria for light propagation are given. These criteria concern nanocrystals size, narrow particle-size distribution, inter-particle spacing, and clustering. In 2000 Tick, Borrelli and Reaney published another significant paper discussing the relationship between structure and transparency in glass-ceramic materials into a near single-mode optical waveguide fiber geometry [START_REF] Tick | The relationship between structure and transparency in glass-ceramic materials[END_REF]. In this significant paper authors investigate the transparency analysing the composition dependence of the lattice parameters and refractive index of the nanocrystals, their size distribution, the particle separation and the actual crystalline volume fraction [START_REF] Tick | The relationship between structure and transparency in glass-ceramic materials[END_REF]. A detailed scattering analysis was performed using two different approaches. One is as a contiguous two-phase system with the scattering characterized by a correlation function. The other is a particle approach, where one uses an effective medium scattering model. The authors conclude that although the latter predicts the correct order of magnitude of the scattering, it cannot distinguish the scattering behaviour as a function of ceramming conditions used. This point is still of crucial interest and several efforts have been done to achieve general rules to correlate scattering attenuation with appropriate ceramming protocols. From the point of view of simulation many techniques have been developed for analyzing scattering problems and several interesting solutions have been discussed [START_REF] Doicu | Light Scattering by Systems of Particles[END_REF]. Moving to fabrication, although a general behaviour is not yet determined excellent results have been obtained for specific compositions [START_REF] Ferrari | Rare-earth-doped glasses for integrated optical amplifiers[END_REF][START_REF] Péron | Erbium doped fluoride glass-ceramics waveguides fabricated by PVD[END_REF][START_REF] Alombert-Goget | Silica-hafnia-based photonic systems[END_REF][START_REF] Bhaktha | Spatially localized UV-induced crystallization of SnO 2 in photorefractive SiO 2 -SnO 2 thin film[END_REF][START_REF] Chiasera | R"are earth activated glass-ceramic in planar format[END_REF][START_REF] Dieudonné | Upand downconversion in Yb 3+ -Pr 3+ co-doped fluoride glasses and glass ceramics[END_REF] and we will present some examples in this paper.

GEO 2 TRANSPARENT GLASS CERAMIC PLANAR WAVEGUIDES

Recently effort is devoted on growth, characterization on GeO 2 based system for his employment for applications on various modern fields such as electronics and photonics. GeO 2 exhibits, in fact, many interesting properties that make this material suitable for applications in optical, electronic and optoelectronic devices [START_REF] Chiasera | GeO 2 glass ceramic planar waveguides fabricated by RF-sputtering[END_REF][START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF][START_REF] Prakapenk | High pressure induced phase transformation of SiO 2 and GeO 2 : difference and similarity[END_REF]. It is of strategic importance to develop fabrication protocols allowing obtain GeO 2 transparent glass ceramic planar waveguides exhibiting low attenuation coefficients and simultaneously embed GeO 2 nanocrystals with a specific phase [START_REF] Prakapenk | High pressure induced phase transformation of SiO 2 and GeO 2 : difference and similarity[END_REF]. Among the various techniques can be used to fabricate these particular kinds of nanostructured systems such as sol-gel techniques with top-down and bottom-up approaches, and physical vapor deposition, we have demonstrate that RFsputtering (RFS) is a suitable route to fabricate optical planar waveguides and photonic microcavities operating in the visible and NIR regions [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF][START_REF] Valligatla | High quality factor 1-D Er 3+ -activated dielectric microcavity fabricated by rf-sputtering[END_REF]. In order to produce transparent glass-ceramic, a common procedure employ heat treatments of the systems, but lately efforts was performed for the development of alternative treatments such as laser annealing (LA) that presents advantages in terms of temporal and spatial annealing control in respect to the conventional thermal annealing [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF][START_REF] Dutta | CO 2 laser annealing of Si 3 N 4 , Nb 2 O 5 and Ta 2 O 5 thin-film optical waveguides to achieve scattering loss reduction[END_REF]. The CO 2 laser annealing has been successfully used to reduce scattering losses in different kinds of amorphous optical planar waveguides [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF], and for the fabrication of glass-ceramic coating and glass-ceramic waveguides with low attenuation coefficient [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF]. Anyway it is of crucial importance a precise characterization of the structure of the samples to assess the effect of the LA process and tailor the irradiation protocols. Moreover, it is well known that important structural modification, leading to artificial results, can be induced in GeO2 based materials by electron irradiation [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF]. For these reasons, information on the structural properties of the waveguides before and after the laser annealing process are obtained by various techniques and in particular from positron annihilation spectroscopy (PAS), specifically by the Doppler broadening spectroscopy (DBS). PAS is a well-known non-destructive spectroscopy technique employed to investigate materials structure [START_REF] Macchi | Single-crystal silicon coimplanted by helium and hydrogen: evolution of decorated vacancylike defects with thermal treatments[END_REF]. Experimental details could be found in [START_REF] Chiasera | GeO 2 glass ceramic planar waveguides fabricated by RF-sputtering[END_REF].

In Table 2 are reported the optical parameters for the samples as prepared and after CO 2 laser irradiation for 2h. The waveguides have a thickness of about 1µm and support one mode at 1319 nm and 1542 nm. Before and after LA the refractive indices measured in TE and TM polarization modes are equal within the experimental uncertainty, so the birefringence can be considered negligible. Comparing the refractive indices in the GeO 2 waveguides before and after LA we observe an increase of about 0.04 with the irradiation at all the wavelengths. The laser annealing allowed significant reduction of the attenuation coefficients. In fact, we observe an attenuation coefficient at 1542 nm of 0.5 dB/cm for the irradiated system while for the system before LA is 0.9 dB/cm. As shown in Table 2, the reduction of the attenuation coefficient with the CO2 laser irradiation is present at all the considered wavelengths. The decrease in the total attenuation coefficient for the CO2 laser irradiated systems, that take in account of all the contribution to the losses such as volume and surface scattering and absorption, has been attributed by Dutta et al. [START_REF] Dutta | CO 2 laser annealing of Si 3 N 4 , Nb 2 O 5 and Ta 2 O 5 thin-film optical waveguides to achieve scattering loss reduction[END_REF] to the reduction of the surface roughness

The optical parameters measured after 1h of LA are equivalent to those reported for 2h. EDXS analysis was used to monitor the chemical composition of the sample, as deposited and after LA. EDXS confirm that the composition of the sample is not affected by the laser irradiation and the correct stoichiometry between germanium and oxygen is always present.

In figure 1, the Micro Raman spectra measured at room temperature for GeO2 planar waveguides before and after LA at different times are reported. The Raman spectrum of the as prepared GeO2 waveguide before irradiation is typical of a GeO2 amorphous system. The amorphous nature of the GeO2 film is clearly pointed out by the broad peak at 440 cm-1 [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF]. After 1 h of CO2 laser irradiation, the Raman spectra indicate that in the amorphous GeO2 matrix there is the presence of a rutile-like GeO2 crystalline phase [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF]. After 2 h of CO2 laser irradiation, the Raman spectrum shows that in the amorphous system there is also the presence of a trigonal GeO2 crystal phase [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF].

AFM images of representative 1.8 x 1.8 µm2 areas for the as prepared sample and for that CO2 laser irradiated for 2h are reported in figure 2 (a) and 2 (b) respectively. AFM analysis put in evidence a roughness of 1.05 ± 0.05 nm in the as prepared sample and a roughness of 0.80 ± 0.02 nm for the waveguide after 2h of CO2 laser irradiation. This result correlates with the reduction of the attenuation coefficient with the laser annealing and confirms surface morphology as an important source of losses. Moreover, on the surface of the sample, the AFM image of figure 2 (b) shows the presence of nanometric structures with sizes ranging from 10 to 50 nm (see also figure 2 (c)). These structures are assigned to GeO2 nanocrystals. It is worth noting that, although the presence of these scattering points are expected to increase the attenuation coefficient by increasing the scattering losses of the waveguide, the protocol here developed for the CO2 irradiation allows reducing the attenuation coefficient by decreasing the surface scattering losses [START_REF] Dutta | CO 2 laser annealing of Si 3 N 4 , Nb 2 O 5 and Ta 2 O 5 thin-film optical waveguides to achieve scattering loss reduction[END_REF]. In the as prepared film, positrons detect a first thin superficial layer of about 10 nm which is characterized by a very high S n value of 1.006. This thin layer is followed by another GeO 2 layer (labelled layer II) uniform up to the silica substrate and characterized by a S n = 0.952. With the laser irradiation, the superficial layer reduces to a half of its initial thickness, and the uniform second layer observed in the as prepared film splits now into two regions, named layers II and III, respectively. The first region is structurally similar to the second layer of the non-irradiated sample having about the same S n value ( 0.950). In the sample irradiated during 1 h the thickness of the second layer is around 300 nm; and in the case of the sample irradiated 2 h, this thickness decreases up to approximately 170 nm. On the other hand, the second region (i.e., layer III) is characterized by a higher S n value ( 0.960) than the previous one and it extends up to the silica interface. The detection by DBS of a thin superficial layer that decreases its thickness with the laser irradiation treatment, well-correlates with the results obtained by AFM. Probably, this layer is highly defected due to their S n values by which it is characterized (higher than the S n values found in the bulk of the films). Both, the decrease in roughness and thickness of the defected superficial layer could be linked to the decrement of the attenuation coefficient. From the results reported in Table 2, it can be seen that the S n values obtained for layer III are slightly higher than those of layer II, indicating a structural change of the GeO 2 samples. Besides, the thickness of this modified layer is larger in the sample annealed for a longer irradiation time. In the case of layer II, for the laser treated films S n values are almost equal to that not treated sample. Moreover we can note that: a) the bulk of the film interested by the structural change increases by increasing the irradiation time from 1 to 2 h; b) the change starts from the film/substrate interface moving towards the surface of the films. Raman spectroscopy reveals the presence of different GeO 2 phases. The rutile GeO 2 phase present after 1 h of irradiation exhibits a higher refractive index than that of the trigonal phase observed after 2 h of irradiation [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF]. However, DBS results would allow inferring that the lack of substantial changes in the macroscopic refractive index measured after 1 and 2 h of irradiation, respectively, could be attributed to a balance between the contributions of the different phases to the modified matrixes. The behavior described in point b) could be assigned to the heating of the substrate by the CO 2 laser inducing a progressive modification of the film from the substrate to the surface. The SiO 2 matrix, in fact, due to the lattice vibration in the region of 940 cm -1 , presents a higher absorbance in the region of the CO 2 laser line with respect to that of the GeO 2 system [START_REF] Chiasera | CO 2 laser irradiation of GeO 2 planar waveguide fabricated by rf-sputtering[END_REF]. For that reason the CO 2 radiation is mainly absorbed by the SiO 2 substrate that induces the change in the GeO 2 film.

Er 3+ -ACTIVATED SnO 2 SOL-GEL-DERIVED GLASS-CERAMICS: AN ENERGY TRANSFER CASE STUDY

During the last years several work has been performed in order to develop energy transfer based systems for photoluminescence application. In this scientific area we have up-and down-converters for planar waveguides for integrated optics [START_REF] Chiasera | R"are earth activated glass-ceramic in planar format[END_REF], visible laser light sources and solar cells efficiency enhancement [START_REF] Dieudonné | Upand downconversion in Yb 3+ -Pr 3+ co-doped fluoride glasses and glass ceramics[END_REF][START_REF] Alombert-Goget | Rare-earth-activated glasses for solar energy conversion[END_REF], specific nanostructured system used as sensitizers of rare earth luminescence [START_REF] Alombert-Goget | Glass-Based Erbium Activated Micro-Nano Photonic Structures[END_REF]. Recently, a detailed work regarding the effect of the local environment on the spectroscopic properties of rare-earth-activated tin dioxide glass ceramic was presented [START_REF] Van | Environment segregation of Er 3+ -emission in bulk sol-gel-derived SiO 2 -SnO 2 glass ceramics[END_REF][START_REF] Van | Erbium-doped tin-silicate sol-gel-derived glass-ceramic thin films: Effect of environment segregation on the Er 3+ emission[END_REF]. Tin dioxide , which is a high-refractive index semiconductor (E g = 3.6 eV at 300K), and is stable both chemically and mechanically, is an excellent choice for the crystalline component of the system. As it is also transparent through the visible and infrared regions, it covers the emission range for active ions like erbium. Compatibility is enhanced by the fact that tin oxide nano-crystals can be excited by a broad range of UV wavelengths, thus with the use of broad band arc lamps, while Er 3+ ions have very narrow excitation peaks.

Studies on silica-tin bulk glass ceramics have shown that the concentration of SnO 2 has a definite effect on the structure  ex = 514 nm Figure 3: Photoluminescence spectra of a (100-30)SiO 2 -30SnO 2 waveguide, doped with 0.5 mol% Er 3+ and heat-treated at 1200°C, upon excitations at 351 and 514.5 nm of the SiO 2 glass phase [START_REF] Van Tran | Controlled SnO 2 nanocrystal growth in SiO 2 -SnO 2 glass-ceramic monoliths[END_REF][START_REF] Van Tran | Investigations of the effects of the growth of SnO 2 nanoparticles on the structural properties of glass-ceramic planar waveguides using Raman and FTIR spectroscopies[END_REF]. In addition, while rare earth ion concentrations as high as 1 mol% can be doped into the glass ceramic, Er 3+ ions are found mainly in the glass phase, which results in high levels of quenching effects. In order to investigate any possible energy transfer between erbium ions and SnO 2 nanocrystals, the environment of the Er 3+ ions was studied via infrared photoluminescence measurements which were recorded using 351 nm and 514.5 nm excitation wavelengths. These two lines correspond to the excitation of the SnO 2 crystals and to the excitation of the 4 I 15/2 → 2 H 11/2 transition of Er 3+ ions across their bandgap, respectively. Figure 3 shows the photoluminescence spectra of the 4 I 13/2 → 4 I 15/2 transition of Er 3+ ion for the samples containing 30 mol% SnO 2 , doped with 0.5 mol% Er 3+ and heat-treated at 1200°C. Under excitation at 514.5 nm, one obtains an emission spectrum characteristic of Er 3+ ions in an amorphous medium, with a broad band centered around 1535 nm. However, excitation at 351 nm results in a completely different spectrum, in which the presence of narrow bands at 1521, 1531, 1549 and 1571 nm are assigned to the Stark transitions between 4 I 13/2 and 4 I 15/2 levels . When the erbium ion occupies the Sn 4+ sites in the cassiterite structure, the electrostatic field of the surrounding environment of ions removes degeneracy of the free ion J levels and makes them split into doubly degenerated stark level [START_REF] Van | Environment segregation of Er 3+ -emission in bulk sol-gel-derived SiO 2 -SnO 2 glass ceramics[END_REF][START_REF] Van | Erbium-doped tin-silicate sol-gel-derived glass-ceramic thin films: Effect of environment segregation on the Er 3+ emission[END_REF]. This emission results from energy transfer between SnO 2 nanoparticles and the rare-earth ions by Forster's mechanism. Accordingly, this spectrum indicates that most part of Er 3+ ions are embedded in SnO 2 nanocrystals. Consideration of these spectral features indicates that the Er 3+ ions exist in two types of sites: those embedded in tin oxide nanocrystals and those within the amorphous silica matrix. The influence of the incorporation of erbium in the tin dioxide nanoparticles on luminescence were studied by the photoluminescence measurements of films with different Er 3+ concentrations. With increasing erbium concentration, the near-infrared emission intensity at 1540 nm is steadily enhanced. The PL intensity of the 2 mol% Er-doped sample is more than three orders of magnitude greater than that of the 0.5 mol% Er-doped one. This behavior indicates that quenching of luminescence did not occur in the 2 mol% erbium doped systems.

CONCLUSIONS

Transparent glass ceramic photonic systems have demonstrated to be crucial for several applications and especially in integrated optics. The importance of these nanocomposite materials is evident when luminescence efficiency and attenuation losses are the more important requirements. Concerning the role of nanocrystals as rare earth sensitizers, SnO 2 nanocrystals constitutes a textbook example. SnO 2 is a wide band-gap (Eg = 3.6 eV at 300 K) n-type semiconductor and exhibits a maximum phonon energy of about 630 cm -1 . The rare earth ion can be incorporated in the SnO 2 nanocrystal and substitutes for the Sn 4+ ions in the cassiterite-structured SnO 2 nanocrystals. Among the different fabrication techniques we have mentioned the laser annealing used in the case of GeO 2 transparent glass ceramic planar waveguides. The GeO 2 planar waveguide after 2h of CO 2 laser irradiation exhibits an increase of 0.04 in the refractive index, measured at 1542 nm. We demonstrate that the laser annealing significantly reduce propagation loss in GeO 2 planar waveguides that laser annealing decreases the surface roughness. From another side Raman results have shown that laser treatment for 1 h and 2 h results in realization of structures at nanometric scale on the surface of the samples with dimension from 10 to 50 nm attributed to the presence of GeO 2 nanocrystals. These behaviors evidence the role of the laser heat treatment on the glass inducing a progressive modification of the films. We can therefore conclude that the prospects of glass photonic are still very good, with continuously growing applications in the area of integrated optics, lasing, lightning, frequency converters and sensing. Optimization of the synthesis processes of glasses tailored for the specific application and of the confined structures fabrication technologies may guarantee that new record performances of rare earth doped glass based photonic devices will be achieved. In respect to the development of photonic glass ceramics the immediate problems to solve are: (i) reproducible fabrication protocols; (ii) modelling of transparency constraints including disorder; (iii) enhance solubility for the rare earth ions.

Figure 1 MicroFigure 2 .

 12 Figure 1 Micro Raman measurements carried out at room temperature for GeO 2 planar waveguide.

Table 1 .

 1 A brief history of glass-ceramics

	Milestone

Table 2 .

 2 Optical parameters for the samples as prepared and after CO 2 laser irradiation for 2h.

	Laser wavelength	Refractive Index	thickness	Attenuation coefficient
		(nm)	TE	TM	(µm)	(dB/cm)
	632	before irradiation after irradiation	1.614±0.001 1.652±0.001	1.616±0.001 1.653±0.001	1.1±0.1 1.1±0.1	1.9±0.2 1.1±0.2
	1319	before irradiation after irradiation	1.590±0.01 1.631±0.01	1.590±0.01 1.634±0.01	1.0±0.1 1.0±0.1	1.4±0.2 0.7±0.2
	1542	before irradiation after irradiation	1.585±0.01 1.623±0.01	1.585±0.01 1.624±0.01	1.0±0.1 1.0±0.1	0.9±0.2 0.5±0.2

400 500 600 700 800 900 1000 before irradiation Normalized Intensity [arb.units] Raman Shift [cm-1] Amorphous GeO 2 Trigonal GeO 2 after 2h irradiation after 1h irradiation Rutile GeO 2

Table 2 .

 2 S n values characterizing each layer in the as prepared and the two irradiated samples, as obtained by fitting the positron depth profiles. The density value of the GeO 2 film was used as a guess parameter into the frame of the fitting procedure. The boundary depths are measured from the surface of the sample.

			layer I		layer II		layer III	bulk
	Sample	 = 3.15 g/cm 3	 = 3.15 g/cm 3	 = 3.15 g/cm 3	= 2.1 g/cm 3
		S n	depth (nm)	S n	depth (nm)	S n	depth (nm)	S n
	before irradiation	1.006	11 ± 2	0.952	983 ± 20	--	--	1.000
	after 1h irradiation	1.003	2 ± 0.2	0.946	320 ± 25	0.966	983	1.000
	after 2h irradiation	0.980	5 ± 0.1	0.950	175 ± 32	0.960	983	1.000

ACKNOWLEDGEMENTS

This research was performed in the framework of the projects CNR-PAS (2014-2016), CNR-CNRST (2014-2016), COST ACTION MP1401, Premiale Centro Fermi PIHH, MaDEleNA PAT (2013-2016). R.R. Gonçalves and M. Ferrari acknowledge Brazilian Scientific Mobility Program "Ciências sem Fronteiras".

Let us to thank a number of young colleagues who are closely collaborating to this topic. Some of them are working at their PhD thesis: Simone Normani, Anna Piotrowska, Iustyna Vasilchenko, other are highly appreciated for their invaluable technical support: Cristina Armellini, Alessandro Carpentiero, Maurizio Mazzola, Stefano Varas.


H Francois-Saint-Cyr

T J J Larson

C Prosa

Guillermier

I Martin

W Blanc

P Lecoustumer

C Hombourger

D Neuville

D J Larson

T J Prosa

C Guillermier

Correlative Compositional Analysis of Fiber-Optic Nanoparticles

HAL is

Development of new active optical fiber devices requires materials with augmented intrinsic properties, using luminescent ion-doped silica as a host glass. Nanoparticles (NPs) in optical fibers can provide this augmentation as they can combine the sturdiness and low cost of silica with particular spectroscopic behavior that would not appear in a pure silica local environment. Ideally, NPs would fully encapsulate luminescent ions to produce engineered spectroscopic properties. This technology would be of great interest for a large domain of applications: high power fiber lasers, random lasers, light sources with new wavelengths and telecommunications.

As silicate systems have a large phase immiscibility domain when they contain divalent metal oxides (such as Mg), one can take advantage of thermal treatments inherent to the MCVD process to obtain NPs through phase separation. NPs are grown in situ within the material. Although glass-ceramics were discovered in 1950's, there is a great demand on experimental data to understand the early stages of nucleation [1]. Modern glass ceramics are generally obtained through many trial-and-error processing steps, involving variations in composition as well as thermal treatments. Although classical nucleation theory was the first model proposed to explain those phenomena, growth rate mismatches could encompass several order of magnitude. According to this capillary assumption-based model, nuclei properties are the same as those of the bulk (same structure and composition). Recent articles disprove assumption of structure, pointing toward NPs structural changes [2] and transition from amorphous nuclei to crystalline NPs [3]. Compositional changes for small particle sizes (~1-10 nm) have been measured in alloys with Anomalous Small Angle X-Ray Scattering (ASAXS) [4] and in steels with Atom Probe Tomography (APT) [5]. Recent developments in APT allowed the extension of such studies to glass-ceramics, presenting additional challenges in specimen preparation and yield [START_REF] Larson | Local Electrode Atom Probe Tomography[END_REF]. In the current work, we report experimental data disproving the second capillary assumption at the early stage of nucleation-growth process.

Composition of NPs was investigated using APT, NanoSIMS and Transmission Electron Microscopy. The Mg distribution in silica-based glass doped with Mg, P, Ge and Er is reported in Figure 1. When NPs sizes range in the 1-10 nm domain, we can clearly quantify the NPs concentration of Mg, P and Er (Figure 2). These nucleation and growth mechanisms in optical fibers impact the overall material community as experimental results could refine the theories behind these processes.

Figure 1 .

 1 Figure 1. Mg-based dielectric nano-particles (pink) surrounded by silica matrix (blue).

Figure 2 .

 2 Figure 2. Proximity histogram displaying the evolution of Mg, Er, P, and Ge concentrations from the silica matrix toward the center of the Mg-based dielectric nanoparticles.