J Sol-Gel Sci Technol (2011) 60:408–425 DOI 10.1007/s10971-011-2556-y

ORIGINAL PAPER

Sol–gel-derived photonic structures: fabrication, assessment, and application Andrea Chiappini • Alessandro Chiasera • Simone Berneschi • Cristina Armellini Alessandro Carpentiero • Maurizio Mazzola • Enrico Moser • Stefano Varas • Giancarlo C. Righini • Maurizio Ferrari

•

Received: 31 March 2011 / Accepted: 28 July 2011 / Published online: 6 August 2011 Ó Springer Science+Business Media, LLC 2011

Abstract Sol–gel is a handy, very flexible, and cheap method to fabricate, study, and apply innovative photonic structures. The possibility of starting from molecular precursors and elementary building blocks permits to tailor structures at the molecular level and to create new materials with enhanced performances. Of specific interest for the study of important physical effects as well as for application in light management are confined structures on the nanomicro scale as photonic crystal and planar waveguides. Activation by luminescent species and in particular by rare earth ions allows results in the integrated optics area covering application in sensing, biomedical diagnostic, telecommunication, lightning, and photon management. The present review is focused on some recent results obtained by the authors in Sol–gel photonics. The first part presents colloidal structures including single nano-micro spheres and photonic crystal structures. The second part of the

A. Chiappini  A. Chiasera  C. Armellini  A. Carpentiero  M. Mazzola  S. Varas  M. Ferrari (&) CNR-IFN, CSMFO Laboratory, Via alla Cascata 56/c, Povo, 38123 Trento, Italy e-mail: [email protected] S. Berneschi Museo Storico della Fisica e Centro di Studi e Ricerche ‘‘Enrico Fermi’’, Piazza del Viminale 2, 00184 Rome, Italy S. Berneschi  G. C. Righini MDF Laboratory, IFAC-CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy C. Armellini FBK, via Sommarive 18, Povo, 38123 Trento, Italy E. Moser Department of Physics, University of Trento, via Sommarive 14, Povo, 38123 Trento, Italy

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review deals with amorphous and transparent glass–ceramic employed for the fabrication of confined structures in planar format. Some specific application are also reported to highlight the role of sol gel photonics in the development of high performance optical sensors, waveguide lasers, and nanostructured materials. Keywords Colloidal crystals  Opals  Strain sensors  Waveguides  SERS  Rare earth  Glass–ceramics  Silica  Hafnia  Tin oxide  Energy transfer  Photoluminescence  Raman  Nanocrystals  Metallic nanoparticles

1 Introduction Silica-based glasses prepared by sol–gel route take up a consolidated place as materials for photonic. In fact, in respect to other technologies which are employed to develop photonic materials, sol–gel processing exhibits several advantages in terms of rare earth solubility, composition, design, tailoring of optical properties as well as fabrication of films, waveguides, photonic crystals, coating, and bulk glasses [1]. In particular, rare-earth doped glasses, prepared by the sol–gel route, are used in a large number of optical applications, because of the multiple absorption and emission bands available using the various rare-earth elements, and are becoming one of the cheapest and most versatile methods for the fabrication of integrated optics components [2–5]. The fruitful exploitation of silicate glasses 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. Even if several of these devices are actually available on the

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market, at the state of the art the strength of the research on silica-based glasses is focused on optimising chemical composition and developing innovative fabrication processes, in order to reduce the costs and increase the performances of the devices so obtained. In this paper we discuss the most significant results obtained by our research group in colloidal structures and planar waveguides, in order to put in evidence its reliability and versatility of sol–gel technology for photonics application.

2 Photonic colloidal structures Photonic self-assembled structures essentially comprise PCs and glasses. [2, 6, 7] These structures are composed of assemblies of identical shape and size elements crystallized under different lattices. The first requirement, to make good self-assembled photonic structures, is the monodispersity in size of the building blocks. Many materials can be synthesized as particles uniform in size and shape [8], but probably the monodisperse particles most extensively used, when an orderly assembly is required, are the spherical ones. Since different materials require different synthesis techniques and offer different functionalities and processing possibilities, a large number of them have been investigated. The aim of this section is to highlight the mechanism of growth and report some of the main results obtained about the synthesis of monodisperse particles of different materials as polystyrene, silica, and silica activated with rare earth ions as well as for specific structures as core–shell spheres and photonic crystals. Finally two particular applications in the area of sensing will be given. 2.1 Polystyrene spheres Among the materials used for the fabrication of colloidal crystals called also opal the widest spread are polymers, mainly polystyrene (PS) and poly(methyl methacrylate) (PMMA). In particular, latex or PS spheres can be easily synthesized by means of single-stage polymerization process, that is based on the dispersion of monomer in water [9–11]. There occurs the formation of long polymeric chains due to the reaction with the initiator. In order to improve the dispersion of monomer in water, a surfactant is added to the emulsion. The presence of the surfactant in aqueous dispersion results in the formation of micelles, which act as reservoirs for monomers. Thus, the reaction between the monomers dissolved in water and free radicals from the initiator generates nuclei-oligomers in the form of tiny particles. The nucleus grows into large chains until all monomers encapsulated into the micelles have been

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eliminated. It appears evident that modifying the quantity of surfactant in the emulsion it’s possible to control the dimensions of the particles obtaining build blocks with a size distribution that goes from 100 nm to 1 lm [12]; furthermore the surface charge of latex spheres depends on the nature of the initiator used in the synthesis. In Fig. 1 is reported a typical TEM of polystyrene particles synthesized using the protocol described in [11]. 2.2 Silica spheres Complementary to the synthesis of organic spheres, the development of inorganic materials highly inert both chemically and thermally is necessary in several applications. Typically inorganic compounds withstand higher temperatures and have chemical properties very different from those of organic ones. Among them SiO2 has acquired supremacy, in fact silica particles are considered important materials because they are widely used in different fields such as pharmaceutical, clinical, electronic packaging and integrated optics. The best known method to synthesize monodisperse SiO2 spheres was originally developed by Sto¨ber et al. [13] and relies on the hydrolysis (1) of a silicon alkoxide and successive condensation (2) of alcohol and water to form siloxane groups. Si(OC2 H5 Þ4 + 4H2 O ! Si(OH)4 + 4C2 H5 OH

ð1Þ

Si(OC)4 ! SiO2 + 2H2 O

ð2Þ

In fact, under appropriate conditions of temperature, pH, and concentrations this synthesis process yields spheres with diameters ranging from 100 to 600 nm with a few percent standard deviation in diameter [2, 14], while for larger size a strategy based on seeded growth technique has been developed [14]. From expression (1) and (2) appear evident that modifying the quantity of catalyst (ammonia) as well as the water content in the reaction, it’s possible to vary the dimensions of the SiO2 particles keeping low the polidispersity of the nanoparticles (Nps). Moreover it is possible to foretell the final dimensions of these spheres using an experimental expression (3) proposed by Bogush et al. that carry out the connection between d (average diameter of the spheres) and the reagent concentrations expressed in mol/l.   d ¼ A  ½H2 O2  exp B  ½H2 O1=2 ð3Þ with   A ¼ ½TEOS1=2  82  151½NH3  þ 1200½NH3 2 366½NH3 3 and

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Fig. 1 HRTEM characterization of polystyrene spheres synthesized by microemulsion, where an average dimension of 236 nm with a polidispersity of about 3% is determined [11]

B ¼ 1:05 þ 0:523½NH3   0:128½NH3 2 In Fig. 2 is reported a typical SEM image of silica particles synthesized using the protocol described in [2]. 2.3 Core shell spheres activated with Er3? ions Synthesis of core–shell particles is a successful method to fabricate novel materials with different compositions and morphologies. In particular there has been much recent interest in the synthesis of nanoscale particles doped with lanthanide ions, because the optical properties of these materials may be modified as the dimensions of the host

Fig. 2 Electron micrograph of SiO2 particles of average size 250 nm from a solution containing 0.22 M TEOS, 1 M NH3 and 15 M DW, synthesized at room temperature

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material are reduced. In fact silica colloids of predictable size and shape doped with a controllable concentration of lanthanide ions have significant potential for use in optical devices such as microlasers, thin-film device structures, active photonic band gap materials and luminescent markers or nanosensors. In this contest, one of main request is the synthesis of silica monosize nanoparticles activated with RE ions; generally these colloidal SiO2 nanoparticles can be formed by either an acid-catalyzed reaction [15–17] or by basecatalyzed (Stober) reaction, as report in Sect. 2.2. However the incorporation of the RE ions in the silica matrix using a base-catalyzed reaction fails because the RE ions immediately forms an insoluble RE hydroxide [16]; besides the acid-catalyzed hydrolysis of TEOS results in the formation of large particles with sizes and size distributions that are difficult to control [18]. Recently it has been shown by Moran et al. that it is possible to incorporate lanthanide ions during the growth of silica particles, using an acid catalyzed reaction with a polidispersity of about 10%. An alternative approach for the synthesis of monosize silica nanoparticles activated with Er3? ions is based on the core– shell structures. These systems are constituted by a core of silica sphere, synthesized by a base catalyzed process, and a shell of Er2O3–SiO2 obtained using a seeded growth method by an acid catalyst. In Fig. 3 is reported a SEM image of the silica-Er3? activated core–shell structures, where it’s evident the absence of clusters of particles indicating that the seeded growth occurs on individual particles [2]. Moreover photoluminescence measurements, reported in Fig. 4, confirm the incorporation of Er3? ions in the silica shell; in fact, the

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shape of the 4I13/2 ? 4I15/2 emission band is typical of Er3?-activated silica glasses with a main emission peak at 1,533 nm.

Fig. 3 SEM image of particles after seeded growth using the acidbased reaction

Fig. 4 Room temperature photoluminescence spectrum of the 4 I13/2 ? 4I15/2 transition of Er3? ions for the silica core–shell structure, (a) upon excitation at 514.5 nm and (b) upon excitation at 980 nm

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2.4 Colloidal crystals by self-assembly approach The process of self organization of colloidal particles has been extensively studied for the last two decades, the goal is to optimize the condition to obtain a high quality crystal. Typically a large single crystal size is pursued, with controlled orientation, reachable in short time and with good mechanical properties. The aim of this section is to give an overview of main structural and optical properties of these systems. Nowadays the most widely used method for the formation of colloidal crystal is the vertical deposition or convective deposition method, which is based on the evaporation of the liquid (generally ethanol or water) forcing the spheres (building blocks) to arrange in the meniscus formed between a vertical substrate, the suspension and air. This method, as demonstrated by Jang et al. [19], provides precise control over the thickness with a higher crystalline quality respect to others deposition techniques [20]. From a structural point of view, Woodcock theoretically demonstrated that when hard spheres self-assemble in thermodynamical equilibrium, the fcc arrangement is more stable than the hexagonal close packed one (hcp) [21]. This assumption was also confirmed by experimental studies as highlighted in Fig. 5 [2, 22]. Furthermore the periodic arrangement in fcc structure produce a band gap where the light can not propagate in the structure, as is evident from diagram of bands, reported in Fig. 6; in this case there is the formation of a pseudo gap in the C-L direction that can be associated with the plane (111) of the structures. Modifying the structure (i.e. diamond arrangement) or using materials that present high refractive index (i.e. silicon), is possible to obtain photonic crystals with full photonic band gap. In fact, considering a fcc arrangement, the main condition to achieve full photonic band gap is that the refractive

Fig. 5 a SEM image of top surface of an opal structure, where an hexagonal alignment is present corresponding to the plane (111) of a fcc structure b SEM image from a cleaved edge where a quadratic alignment can be observed attributed to the plane (100)

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1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1

U

L

Γ

X

W

1.0

Normalized Reflectance

a/λ

412

20 degree 25 degree 30 degree 35 degree 40 degree

0.8

0.6

0.4

0.2

Wavevector 0.0

Fig. 6 Diagram of band for a direct opal structure constituted by polystyrene NPs of the dimension of 236 nm, assembled in a face cubic centred system

indices ratio of the materials that form the crystals has to be higher than 2.8 [23]. The central wavelength position of the band gap can be determined by a modified Bragg’ Law (4)  1=2 k ¼ 2  0:816  d111  n2eff  sin2 h ð4Þ

460

480

500

520

540

560

580

600

620

640

Wavelength [nm] Fig. 7 Reflectance spectra at different incidence angles performed on the opal formed by the vertical deposition method of 236 nm diameter silica spheres

where d111 corresponds to the interplanar spacing of fcc (111) planes and neff is the effective refractive index in the opal structure. The effective refractive index can be calculated from the following equation for fcc structure: n2neff ¼ f  n2spheres þ ð1  f Þ  n2m

ð5Þ

Here, nspheres and nm are the refractive indices of the nanospheres and the surrounding medium, respectively; f corresponds to the filling fraction of the spheres and it is very closed to the fcc ideal value 74%. Moreover for the fcc lattice with unit cell parameter a, the interplanar spacing dhkl is given by the equation sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 2 dhkl ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ D ð6Þ 2 2 2 2 ðh þ k2 þ l2 Þ ðh þ k þ l Þ where the relationship between unit cell parameter and pffiffiffi diameter of the nanospheres is given by a ¼ 2  D. pffiffiffiffiffiffiffiffi Therefore, d111 is defined as 2=3  D. Instead the wavelength gap Dk (with implied hkl indices) can be determined by the contrast between the refractive index of the spheres (nspheres) and the medium (nm). This wavelength gap can be expressed as follows:   Dk nspheres  nm  ¼ ð7Þ k neff Analyzing Eq. 4 is evident that it’s possible to tune the position of the band (1) modifying the dimension (D) of the NPs, (2) varying the incident angle (h), but also (3) infiltrating the voids of the opal structures with wanted

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Fig. 8 Reflectance spectra at 20° before (a) and after (b) the infiltration of the direct opal

materials. That has been confirmed by experimental measurements as those reported in Figs. 7 and 8. Moreover from the data, as those showed in Fig. 7, a direct value of the interplanar spacing and effective refractive index can be easily determined plotting the wavelength position of the stop band as a function of the incident angle (h) [24]. Furthermore, these colloidal crystals can be also used as template for 3D replicas, known as inverse opals. In fact structures as those reported in Fig. 9 can be produced infiltrating the template with different materials (dielectrics, metals and semiconductors) and subsequently removing the colloidal spheres by either chemical or thermal process. Figure 9 shows two typical SEM images of inverse silica structures obtained using, as starting point, different dimension of PS NPs. From a structural point of view it appears evident that inverse structure is just a negative

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Fig. 9 SEM images of the top surface of inverse silica opal obtained a using NPs of the dimension of 260 nm b and NPs of the dimension of 236 nm

replica of the direct one. Analysing these structures is possible to notice the presence of some typical defects such as point and/or line defects that can be obtained growing the colloidal crystal by mean of convective approach. The first type can be attributed to an absence of an attractive interparticle potential as described in [25], while the line defects are related to the low stacking fault energy as reported in [26]. In any case, the filling factor of these systems, determined by optical measurements [27], is very close to theoretical one (26%); indicating the high quality of the structures as well as the presence of a high surface area. 2.5 Applications Colloidal crystals have been studied in the past decade because of their attractive properties involving photonic band gap (see Sect. 2.4). In particular, several attention has been devoted to the formation of PCs derived from self assembled colloids because of their relative easiness of preparation and the low cost associated with their manufacture. Recently, it has been demonstrated that starting from these structures it is possible to obtain composite films that can be used for optical sensing exploiting the properties of

changing the structural color under an external stimulus as demonstrated in the papers published by Ozin et al. [28] and Fudouzi et al. [29]. Moreover, colloidal crystals can be used as scaffolds in order to create metallo-dielectric systems (MDCS); in this field one of the most important structures is represented by metallic nanoparticles infiltrated in PCs. These new materials have attracted considerable attention because of their potential applications in photonics [30], sensing [31] and as SERS substrates [32]. In this section we present and propose two different colloidal crystal structures: (1) closely packed colloidal polystyrene particles (CPCP) embedded in a poly-dimethylsiloxane (PDMS) elastomeric matrix and (2) metallodielectric systems, based on the attachment of gold nanoparticles on the network of the inverse opal. These can be used as strain sensor and SERS substrate respectively. 2.5.1 Colloidal crystal exhibiting mechanochromism (CPCP) Figure 10a illustrates the idea of tuning the structural colour of the PC and (b) shows a typical SEM image of the surface of a PS opal embedded in elastomeric polymer (PDMS).

Fig. 10 a dependence of the inter-planar spacing on the applied strain. Comparison between the initial (upper) and strained (lower) configurations b SEM image of the surface of the PS spheres arranged with a cubic close packing in a PDMS elastomer

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414

70 60 50

(b) initial

Wavelength position [nm]

(a) Intensity [arb. units]

Fig. 11 Relationship between the peak positions and elongation of the silicone rubber sheet owing to stretching. a Reflectance of the photonic crystal. b Peak reflectance wavelength as a function of the elongation

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0.5 mm elongation 1.0 mm elongation 1.5 mm elongation 2.0 mm elongation 2.5 mm elongation 3.0 mm elongation

40 30 20 10 0 450

500

550

600

650

Wavelength [nm]

700

585 580 575 570 565 560 555 550 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Elongation [mm]

Fig. 12 Changes in the structural colour of the colloidal crystal film deposited on a Viton substrate (1.6 9 1.6 cm2); a photographic image of the initial sheet (L), b photographic image of the stretched sheet (L ? DL)

As reported in Sect. 2.4 the reflecting behaviour of the colloidal crystal can be expressed by the modified form of the Bragg’ law (see Eq. 4). In this case when an axial strain is applied to the PC, the inter-planar spacing is modified due to the transversal contraction, as illustrated in Fig. 10a [29]. A prototype sample of CPCP, has been tested to identify the relationship between the applied strain and the wavelength of the reflected peak. In Fig. 11a is reported the reflectance spectrum due to the Braggs diffraction from the fcc (111) planes of the PC crystal as a function of elongation DL. We can notice that the peak position blue shifts from 583 (initial point) to 550 nm, while the reflectance intensity gradually decreases [33]. Furthermore Fig. 11b shows the shift of the reflectance peak as a function of elongation; one can notice, in particular, that a linear relationship exists for elongations as large as 2 mm. In this region, the lattice distance of the ccp (111) planes, d, also decreases at the same rate. For higher elongation values, the position of the peak does not change significantly, due to the fact that interplanar distance remains constant for the applied mechanical strain since the PS spheres are in contact with each other. Figure 12 shows the changes in the structural colour of the colloidal crystal film from yellow to green by the

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elongation DL of the rubber sheet due to mechanical strain, indicating that for some strain values it is possible to detect the variation by necked eyes. 2.5.2 Metallo-dielectric structures Metallo-dielectric structures (MDCS) were realized by immobilization of gold (Au) NPs, about 15 nm in dimension, on the network of an inverse silica opal (ISO). The details are reported elsewhere [34]. Figure 13a reports a typical SEM image of MDCS structure, where we can notice that Au Nps are immobilized on the network of the ISO system. Optical properties of the MDCS structures were investigated by means of absorption measurements. In Fig. 14, the absorbance spectra, collected at normal incidence, of a silica inverse opal (reference) and of metallo-dielectric structures obtained after immersion of ISO into the Au Nps colloidal solution for different times are reported. Analyzing Fig. 14 we can notice that all the MDCS structures present two characteristic peaks: one centered at *600 nm and the second one at *520 nm. The peak centred at *600 nm is attributed to the stop-band of the inverse structure which is originated from the diffraction of

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Fig. 13 a SEM image of a metallo-dielectric colloidal system, where Au nanoparticles are immobilized on the network of the inverse silica opal structure; b SEM image of the Au NPs used for the formation of MDCS Fig. 14 Absorption spectra related to: inverse silica opal (black line), MDCS_1 h (dash line), MDCS_2 h (solid line) and MDCS_4 h (dot line), In the inset is reported the absorption spectrum of the gold colloidal solution realized, where the LSPR is centered at 520 nm

the 3D ordered system. Moreover we can notice that the position of the diffraction peak remains fixed at *600 nm for all the spectra reported, suggesting that the Au Nps are predominantly immobilized on the surface of the structures or infiltrated just in the first layers of the PCs. The peak centered at *520 nm can be assigned to the localized surface plasmon resonance (LSPR) of Au Nps immobilized on the silica network of the inverse structure. In fact this peak is almost in the same position as that obtained for the synthesized gold colloidal nanoparticles in aqueous solution. In order to prove that this type of systems can be used as SERS substrates we have performed Raman measurements using benzenethiol (BT) as a probe molecule. In Fig. 15 are reported the Raman signal obtained on different structures: (a) MDCS, (b) golf film (GF) and (c) inverse silica opal (ISO). The details are reported in ref [34].

Fig. 15 (a) Raman spectra of benzenethiol adsorbed on the metallodielectric colloidal structure (MDCS_2h), (b) on sputtered gold film, (c) on a ISO after spotting on this a 5 ll drop of pure BT and left to dry out

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Analyzing Fig. 15, we can notice that only for the MDCS structure we can very easily identify the typical Raman bands of BT and we can clearly notice that the spectrum (a) appears significantly enhanced if compared to Raman spectra obtained on ISO_BT and GF structure suggesting that metallo-dielectric colloidal structures can be used for the detection of biomolecules at low concentrations. In this section some recent results concerning ordered photonic structures have been presented. Now let us move to the second part of the review devoted to sol–gel-derived photonic structures in planar format.

3 Sol–gel derived planar structures Sol–gel-derived optical waveguides and planar lightwave circuits are a well consolidated research area. In this context a significant role is played by the binary systems and in this section we will discuss silica-hafnia and silica-tin oxide. Hafnium, like Ti, belongs to group 4 in the periodic table. Its oxide has a wide bandgap of about 5.5 eV allowing transparency from the ultraviolet to the midinfrared spectral region, exhibits high refractive index, and a phonon energy cutoff lower than that of SiO2 (700 vs. 1,100 cm-1). This binary system has demonstrated to be suitable also for fabrication of amorphous planar waveguides [35], glass–ceramic waveguides [36], and tapered rib waveguide laser [37]. Tin oxide is a wide-band gap semiconductor (Eg = 3.6 eV) which is studied for many applications such as gas sensors, solar energy cells and transparent conductors. In addition, it has been found to have interesting properties suitable for photonics such as a refractive index of 1.89 at 632 nm and maximum phonon energy below 630 cm-1. SnO2 nanocrystals embedded in SiO2 glass matrices constitute an interesting system for the development of glass– ceramic waveguides where the SnO2 nanocrystals play an efficient role as rare earth ion sensitizers [38]. The top-down and bottom-up methods have been used as two approaches for preparing nanocomposite systems. The top-down approaches seek to create nanocomposite devices by using larger, externally-controlled ones to create a new system while the bottom-up approaches seek to have smaller components and arrange them into a more complex system [36]. Here we focus our attention on the top-down approaches where an optimized thermal annealing protocol is used to nucleate nanocrystals in an amorphous matrix. In this section we discuss the more significant results obtained for the rare-earth-activated amorphous and nanostructured SiO2–HfO2 and nanostructured SiO2–SnO2 systems in order to put in evidence their reliability and

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versatility for photonics application. Moreover, fabrication and assessment of a monolithic Nd-doped Silica-Hafnia sol– gel, tapered rib waveguide laser are reported. 3.1 Silica-hafnia amorphous planar waveguides The first requirement to obtain effective waveguiding structures is to optimize the sol–gel fabrication protocol and in particular the composition. Two aspects have been considered: (1) the role of hafnia concentration on the structural and optical properties; (2) the role of Er3? content on the luminescence efficiency. The silica-hafnia planar waveguides were prepared by dip-coating deposition on v-SiO2 and silica-on-silicon substrates. The starting solution, obtained by mixing tetraethylorthosilicate (TEOS), ethanol, deionized water and hydrochloric acid as a catalyst, was pre-hydrolyzed for 1 h at 65 °C. The molar ratio of TEOS:HCl:H2O was 1:0.01:2. The quantity of EtOH was chosen in order to keep constant the 20 cc volume of the solution. Ethanolic colloidal suspension was prepared using as a precursor HfOCl2 [39] and then added to the TEOS solutions, in order to obtain solutions with different Si/Hf molar ratios. The quantity of ethanol was adjusted for each solution in order to obtain a final total concentration of 0.448 mol/l. Erbium was introduced as Er(NO3)35H2O with different Er/(Si ? Hf) molar concentration. The final mixture was let to react under stirring for 16 h at room temperature. Using the solutions described above, several sets of waveguides were prepared using a dipping rate of 40 mm/min. Before further coating, a 10-dip cycle, the film was heated for 2 min at 900 °C. Finally, the waveguides were submitted to a further annealing at 900 °C, whose duration was different for each waveguide. Table 1 give a survey of the fabrication protocol, optical and spectroscopic parameters for SiO2–HfO2 waveguides activated by 0.3 mol% Er3? ions, as a function of the hafnia content. All waveguides in Table 1 support at least one TE and one TM mode at 543.5 nm and at 632.8 nm, one mode at 1.5 lm, the values of the effective refractive index depending on the Si/Hf ratio. There is not negligible birefringence which we assigned to viscosity effects. Comparing the different heat treatments of the waveguides it appears evident that the introduction of hafnium dioxide favors the densification of the glasses, decreasing the annealing time necessary to eliminate the residual OH- group. Figure 16 shows the Raman spectra of (a) 90SiO2– 10HfO2, (b) 90SiO2–40HfO2 planar waveguides, collected in VV polarization with excitation of the TE0 mode. The Raman spectrum of a 80SiO2–20TiO2 planar waveguide processed with an intermediate annealing at 700 °C for 2 min, followed by a final annealing at 900 °C for 2 min

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Table 1 Fabrication protocol, refractive index (TE and TM polarization), thickness, 4I13/2 lifetime, and 4I13/2 ? 4I15/2 bandwidth for SiO2–HfO2 planar waveguides activated by 0.3 mol% Er3? ions SiO2/HfO2 molar ratio

Ref. index @ 543.5 ± 0.005

Ref. index @ 632.8 ± 0.005

Thicknes ± 0.05 (lm)

Number of dips

Final heat treatment @ 900 °C

4

I13/2 lifetime ± 0.5 (ms)

4

90:10

1.474 TE

1.470 TE

2.05

50

30 h

7.1

50

1.472 TM

1.468 TM 1.27

30

210 min

6.6

50

1.10

30

5 min

5.9

51

0.96

25

5 min

5.5

51

80:20 70:30 60:40

1.547 TE

1.543 TE

1.541 TM

1.537 TM

1.605 TE

1.600 TE

1.593 TM

1.588 TM

1.669 TE

1.663 TE

1.652 TM

1.646 TM

Fig. 16 Raman spectra of (a) 90SiO2–10HfO2, (b) 90SiO2–40HfO2 planar waveguides, collected in VV polarization with excitation of the TE0 mode. The Raman spectrum of a 80SiO2–20TiO2 planar waveguides (c), is also reported for comparison

(c), is also reported for comparison [40, 41]. What is important here is that complete densification of the silicahafnia system can be achieved without any evident crystallization process, known to be highly detrimental for optical applications. The bands characteristic for the OHgroups in silicate glasses, generally observed at 3,670 and 3,750 cm-1, are absent in the Raman spectra. However, the Raman spectrum of the silica-titania planar waveguide, Fig. 16c exhibits Raman structures in the region between 150 and 350 cm-1, attributed to devitrification of the silica-titania film [40]. In contrast, the Raman spectra of the silica-hafnia waveguides are typical of an amorphous system and do not show evidence for hafnia crystallization, that would display several sharp peaks in the region between 100 and 800 cm-1 [42]. As a consequence of these observations we can conclude that the more efficient composition is 70SiO2–30HfO2.

I13/2 ? 4I15/2 bandwidth ± 2 (nm)

As far as regards the role of hafnia concentration on the spectroscopic properties we note that the bandwidth of the 4I13/2 ? 4I15/2 emission is practically independent on the composition of the waveguide. This is due to the fact that HfO2 promotes a disruption of the silica network as we proved by Raman measurements [35, 39, 43]. As a consequence, Hf4? increases the number of Si–O non-bridging groups accounting for a general network flexibility, which may accommodate Er3? contents without appreciable matrix strains [35, 39, 43]. The 4I13/2 lifetime decreases with the increasing of the HfO2 molar concentration. This is an interesting point related to the role of hafnia. EXAFS measurements performed on the SiO2–HfO2 waveguides reported in Table 1 have demonstrated that Er3? ions are preferentially dispersed in HfO2-rich regions of the glassy waveguide, even at the lowest HfO2 concentration [44]. The 4I13/2 lifetime shortening can be explained by considering that the incorporation of HfO2 in silica strongly modifies the next-nearest shell environment around Er3? ion, inducing an increase of the electric dipole component of the 4I13/2 ? 4I15/2 transition probability. Higher is the hafnia content higher is the probability for Er3? ion to find Hf in its second coordination shell, then enhancing the radiative relaxation rate. Numerical simulations by the molecular dynamics method add further information concerning the structure of rare-earth-activated SiO2–HfO2 system and strongly confirm the conclusions above reported [45]. Monteil et al. simulated two sets of Eu3? doped SiO2–HfO2 samples of various compositions. yEu2O3 - xHfO2 - (2048 y - x)SiO2 with x corresponding to the ratio Hf/ (Si ? Hf) = 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 95% and y corresponding to the concentration in europium of 2.4 and 4.9 mol%. The obtained coordination number for the second coordination shell of silicon and hafnium demonstrated that that the final structure shows a phase separation between Si-rich domains and Hf-rich domains. Even for a low silicon concentration, the second coordination shell of

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Fig. 17 Snapshots of two different samples. Small grey atoms are silicon, small white ones are hafnium and large black atoms are europium. Oxygen are not drawn for the sake of clarity. a Hf/(Hf ? Si) = 20%, Eu = 2.4 mol%; and b Hf/(Hf ? Si) = 95%, Eu = 4.9 mol% [45]

Si is mainly constituted of Si and for a low concentration of Hf, the second coordination shell of Hf is mainly constituted of Hf. Analysing radial distribution functions, as well as snapshots reported in Fig. 17, it appears that Eu3? ions prefer Hf-rich domain where they can find a larger amount of non-bridging oxygen to satisfy their high coordination number requirement. For instance, at a concentration of ˚ , Eu3? is sur50% of hafnium, and at a distance of 4.5 A rounded in average by 6.6 Hf and 0.2 Si [45]. Table 1 indicates that the more efficient compositions are 70:30 and 60:40. In fact, for both the composition we obtain very similar spectroscopic properties with a reasonable annealing time. However, the two compositions are not equivalent. In fact, X-ray photoelectron spectroscopy (XPS) measurements performed on the amorphous waveguides as a function of hafnia content allowed us to identify both structural changes and the critical HfO2 abundance at which the phase separation occurs [46]. In particular we have demonstrated that for HfO2 content higher than 30 mol% phase separation takes place [46]. The situation is well pictured in Fig. 18 where the oxygen abundance of the SiO2, Si–O–Hf and HfO2 components as obtained from XPS analysis are plotted as a function of the HfO2 molar concentration. It appears that the HfO2 component emerges at expenses of the intermediate Si–O–Hf phase for HfO2 content higher than 30 mol%. This limit for phase separation is in agreement with Raman analysis of the silica-hafnia waveguides reported in [43]. In this work the 40HfO2–60SiO2 waveguide shows characteristic optical vibrations of HfO2 crystals. The same Raman bands are not present at Hf concentrations B30 mol% indicating the absence of phase separation. As a consequence of these observations we can conclude that the more efficient composition is 70SiO2–30HfO2.

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Fig. 18 Oxygen atomic abundances relative to SiO2, Si–O–Hf bond, and HfO2 obtained from the XPS measurements plotted versus HfO2 molar concentration [46]

Now let us consider the second request, i.e. the role played by Er3? content. The active ion concentration is a critical parameter to design planar waveguides for photonics applications. In fact, when the erbium concentration increases the average distance between neighboring Er3? ions simultaneously decreases and electric dipole–dipole interactions between the different Er3? ions become more significant. Under this condition, processes which include energy migration and upconversion can take place, lowering the fraction of excited Er3? ions at a given pump power. As a consequence, a decrease of the luminescence lifetime of the metastable 4I13/2 state as a function of increasing Er3? concentration occurs, as described by the following empirical formula [47, 48]:

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sobs ¼ 1þ

419

s0  p

ð8Þ

qEr Q

where sobs is the observed luminescence lifetime, s0 the ideal luminescence lifetime in the limit of zero rare earth concentration, qEr the Er3? ion concentration, Q the quenching concentration and p a phenomenological parameter characterizing the steepness of the corresponding quenching curve. In order to avoid the difficulty with the physical meaning of p, usually only Q is estimated from the fitting curve, at which concentration the lifetime becomes half of s0 , because sobs is equal to half of s0 , when qEr is equal to Q. In order to determine Q in our waveguides we prepared a set of 70SiO2–30HfO2 planar waveguides activated by different Er3? content. The 4I13/2 decay curves exhibited a single exponential profile and the lifetime values are reported in Fig. 19 as a function of Er3? content. Two important result are obtained for the 70SiO2–30HfO2 system: (1) the 4I13/2 lifetime at very low erbium content is 6.7 ms, and can be considered as the radiative lifetime of Er3? ion in this matrix; (2) the quenching concentration is around 1 mol%. On the basis of these results optimized planar waveguides for integrated optics were prepared. The Figure of Merit is given in Table 2. 3.2 SiO2–HfO2 glass ceramic waveguides fabricated by top-down technique On the basis of the results reported in Sect. 3.1, glass ceramic waveguides activate by 0.3 mol% Er3? were

8

4

I13/2 lifetime [ms]

6

4

2

0 0.01

0.1

1

3+

Er concentration [mol %] Fig. 19 Luminescence lifetime of the 4I13/2 metastable state of the Er3? ions as a function of Er3?molar concentration for Er3?-activated 70SiO2–30HfO2 planar waveguides fabricated following the protocol given in Table 1. The red curve represents the result of the fit of the data to Eq. 8

fabricated by top-down technique starting from parent (100 - x)SiO2–xHfO2 amorphous waveguides with x B 30, and performing a suitable thermal annealing. In order to nucleate nanocrystals inside the planar waveguide, additional heat treatments were performed in air at a temperature of 1,000 °C and 1,100 °C for 30 min (see Table 3). The samples were introduced in the furnace at the temperature of 800 °C with an heating rate of 15 °C/min in order to avoid surface cracking. Figure 20 shows a high resolution transmission electron microscopy image of the W1000 glass ceramic waveguide with HfO2 nanocrystals of about 2–3 nm in diameter. Details concerning fabrication as well as spectroscopic and optical assessments are reported in [50, 51]. Here we evidence only the main significant properties of these glass ceramic waveguides. The presence of HfO2 nanocrystals, directly linked to the hafnia content, is reflected by a change in the shape and bandwidth of the photoluminescence spectra as well as in the lifetime, with respect to those usually observed in the amorphous silica hafnia waveguides. The spectroscopic features of erbium doped SiO2–HfO2 planar waveguides in function of the heat treatment, obtained using a Ar? laser as excitation source are reported in Fig. 21. It is evident as: (1) glass ceramics photoluminescence spectra show well resolved Stark components typical of Er3? emission in a crystalline like environment; (2) the 4 I13/2 ? 4I15/2 spectral bandwidth measured at 3 dB from the maximum of the intensity are 28 ± 1, 19 ± 1, 17 ± 1 nm for x = 10, 20, 30 mol% HfO2, respectively; (3) the 4I13/2 lifetime decreases from 9.0 to 8.7 and to 7.2 ms, as the HfO2 content increases from 10 to 20% and to 30% [50]. Note that the bandwidth of the parent planar amorphous waveguides is 50 ± 2 nm for all the parent samples, and that the 4I13/2 radiative lifetime is 7.1 ± 0.5, 6.5 ± 0.5, and 5.9 ± 0.5 ms for x = 10, 20, 30 mol% HfO2, respectively. The glass ceramic waveguides were all single mode at 1.5 lm with losses of about 1 dB cm1 @ 1,542 nm. Looking at the main differences between amorphous and glass ceramic waveguides, the spectroscopic behavior is consistent with a change in the local environment around the erbium ions due to the formation of HfO2 nanocrystals. As the Er3? local environment becomes ordered, it limits the inhomogeneous broadening typical of glassy environment and, therefore, the bandwidth narrows. The increasing of the lifetime in glass ceramic in respect to the parent amorphous waveguides is explained by the fact that the crystalline environment induces the shortening of the nonradiative processes as well as a reduction of the luminescence quenching as discussed in [49–52].

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Table 2 Figure of Merit of optimized silica-hafnia planar waveguides for integrated optics Er3? (mol%)

Confinement coefficient @ 1,542 nm

Propagating mode @ 1,542 nm

Losses @1542 nm (±0.3 dB/cm)

Quantum efficiency

4

I13/2 lifetime ± 0.5 (ms)

4

70:30

0.3

0.81

1

\0.3

88%

5.9

50

Intensity [arb.units]

SiO2/HfO2 molar ratio

Waveguide labeling

Crystallites size (±1 nm)

Losses @1542 nm (±0.3 dB/cm)

Lifetime @1532 nm (ms)

900

0

\0.3

2

W1000

1,000

3

1

5.1

W1100

1,100

5

[2

6.1

W900

Thermal treatment (°C)

Intensity [arb.units]

Table 3 Annealing temperature, size of nanocrystals, attenuation coefficient, and 4I13/2 lifetime of the Er3?-activated SiO2-HfO2 planar waveguides

I13/2 ? 4I15/2 bandwidth ± 2 (nm)

1

0,1

(c) (a)

(b)

0,01 0,00

0,01

0,02

0,03

Time [s]

(a) (b) (c)

1400

1450

1500

1550

1600

1650

1700

Wavelenght [nm] Fig. 21 Room temperature luminescence spectra of the 4I13/2 ? 4I15/2 transition of Er3? ion for the for the (a) W900, (b) W1000, (c) W1100 planar waveguides, obtained by exciting the TE0 mode at 514.5 nm. In the insert are shoved the decay curves of the luminescence from the 4 I13/2 ? 4I15/2 metastable state of Er3? ions for the (a) W900, (b) W1000, (c) W1100 waveguides, upon 514.5 nm excitation

Fig. 20 HRTEM image of the 70SiO2–30HfO2 glass ceramic waveguide activated by 0.3 mol% Er3?, W1000 sample, annealed at 1,000 °C for 30 min, showing HfO2 nanocrystals homogeneously dispersed in the amorphous matrix. Single domain nanocrystals are clearly evidenced [49, 50]

3.3 SiO2–SnO2 glass–ceramic waveguides From a technological point of view we note that patterning of photonic structures on the silica-hafnia film requires photolithography followed by CHF3 reactive ion etching (RIE) for rib waveguides and electron beam lithography followed by the same RIE process in order to etch the gratings into the rib waveguide [37]. A more simple approach consists in the exploitation of photorefractive properties. Highly efficient Bragg gratings and channel waveguides were demonstrated by UV exposure of silicagermania film, thanks to the fact that the amount of the photo-induced positive index change was sufficient for the

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fabrication of channel waveguides [53]. The inclusion of rare-earth elements, by allowing to combine photorefractive and active properties, may provide an effective route to the simple fabrication of integrated optics amplifiers and lasers. In this context we recently fabricated SiO2–SnO2 glass–ceramic waveguides doped with 1 mol% of Eu3? with SnO2 content as high as 25 mol% [38]. (100 - x) SiO2–xSnO2 (x = 8, 16 and 25 mol%) glass–ceramic thinfilm waveguides doped with 1 mol% Eu3? were fabricated by the sol–gel technique and dip coating process. The starting solution was obtained by mixing tetraethylorthosilicate (TEOS), ethanol, deionized water and hydrochloric acid as a catalyst, and was prehydrolyzed for 1 h at 65 °C. The molar ratio of TEOS:HCl:EtOH:H2O was 1:0.01: 37.9:2. An ethanolic colloidal suspension, prepared using SnCl22H2O and Eu(NO3)35H2O as precursors was added to the solution containing TEOS. The final mixture was left at room temperature under stirring for 16 h, and then the resulting sol was filtered with a 0.2 lm Millipore filter. Eu3?-activated SiO2–SnO2 films were deposited by dipcoating on pure vitreous-SiO2 and silicon substrates. Each layer was annealed in air for 3 min at 800 °C prior to the next dip. Finally, the films resulting from 20 coatings were stabilized by a treatment for 10 min in air at 800 °C. As a

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result of this procedure, crack-free waveguides were obtained. Formation and growth of nanocrystals in the parent amorphous waveguides was induced by heat treatment in air at temperatures ranging from 900 to 1,100 °C. grid. Figure 22a and b show the TEM images at different magnifications for the x = 25 mol%, 1,100 °C, 5 h heattreated glass–ceramic waveguide. A narrow size distribution of uniformly dispersed nanocrystals in the silica matrix is seen in Fig. 22a. Figure 22b shows the lattice fringes of SnO2 nanocrystals, with dimensions of about 4 nm. Furthermore, the formation of tetragonal rutile SnO2 crystals in the films was confirmed by Raman spectra, which exhibited the characteristic peaks at 475 and 631 cm-1. Luminescence spectra exhibit sharp peaks typical of Eu3? ion in a crystalline like environment, indicating that most part of Eu3? ions are embedded in SnO2 nanocrystals. Figure 23a shows the excitation spectra of the x = 25 mol% sample as a function of the thermal annealing obtained monitoring the 5D0 ? 7F2 Eu3? emission. The broad band centred at about 310–330 nm (3.8–4.0 eV), depending on the heat treatment, corresponds to the interband electronic transition of the SnO2 nanocrystals. The blue shifted position of this band in comparison with that of the bulk SnO2 (3.6 eV, 345 nm) attests for the presence of nanocrystals in the silica matrix, in agreement with TEM image of Fig. 22b. The intensity of the UV band increases with the increase in the heat treatment, and an enhancement of more than seven times is observed for the glass ceramic heat treated at 1,100 °C, 5 h compared to the as prepared sample treated at 800 °C. Figure 23b shows the excitation spectra of the 1 mol% Eu3? doped (100 - x)SiO2–xSnO2 glass ceramics treated at 1,100 °C for 30 min as a function

421

of SnO2 content. The intensity of the 320 nm SnO2 absorption band increases by about 15 times, moving from x = 8 to x = 25 mol%. The excitation spectra clearly evidence the role of interband electronic transition of SnO2 nanocrystals on the luminescence of Eu3? ions. An attenuation coefficient a low as 0.8 ± 0.2 dB/cm at 632.8 nm was measured in the 75SiO2–25SnO2 glass– ceramic waveguide and preliminary measurements on the effect of exposure to the UV excimer laser irradiation have shown a high negative refractive index change (-10-3) in the precursor 75SiO2–25SnO2 amorphous thin film. This is more than enough to realize optical Bragg gratings in these materials by the UV irradiation and phase mask technique. 3.4 Applications The sol–gel technique is playing an increasing role in the development of optical materials for application in Planar Integrated Circuit. In particular, SiO2 based planar waveguides produced by sol–gel route, could be promising materials for Erdoped waveguide amplifiers (EDWA) to be used in metropolitan area networks [39]. The fruitful exploitation of sol–gel derived silicate glasses is not restricted only to the area of Information and Communication Technology. Many other photonic devices, with a large spectrum of application covering Health and Biology, Structural Engineering, and Environment Monitoring Systems, have been developed during the last years. In this section we present the result on fabrication and assessment of a monolithic Nd-doped Silica-Hafnia sol–gel, tapered rib waveguide laser that can be employed in laboratory-onchip applications.

Fig. 22 TEM images of the 1 mol% Eu3? doped 75SiO2–25SnO2 glass ceramic waveguide treated at 1,100 °C for 5 h (a), showing SnO2 nanocrystals dispersed in the amorphous matrix (b)

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Intensity [arb. units]

10

1100°C - 5h 1100°C - 30' 10 1000°C - 30' 900°C - 30' 800°C - 30'

(a)

8

8

6

6

4

4

2

2

0 280

x = 25 x = 16 x=8

(b)

320

360

0 400 280

320

360

400

Wavelength [nm] Fig. 23 Room temperature excitation spectra of 5D0 ? 7F2 emission at 613 nm of Eu3? in the 1 mol% Eu3? doped (100 - x)SiO2–xSnO2 samples: (a) x = 25 mol% sample after various heat treatments in air; (b) x = 8,16,25 mol% samples heat treated at 1,100 °C for 30 min

3.4.1 Monolithic Nd-doped Silica-Hafnia sol–gel, tapered rib waveguide laser The main motivation to develop a monolithic waveguide laser was its potential application in a laboratory-on-chip system: the integration of a laser source monolithically to optical based biosensors could simplify the supporting equipment needed for their operation and, therefore, their cost, as well as the complexity of their operating procedure. [37]. In the case of the waveguide laser it was used the silicahafnia sol–gel composition, but doped with Nd instead of Er, looking at targeted application. The process started with pre-hydrolyzing tetraethylorthosilicate (TEOS), ethanol, deionized water and hydrochloric acid, as a catalyst, for 1 h at 65 °C. The molar ratio of TEOS:EtOH:H2O:HCl was 1:37.9:2:0.01. An ethanolic colloidal suspension was prepared using as a precursor HfOCl28H2O and then added to the TEOS solution, with a Si/Hf molar ratio of 70/30. Neodymium was added as Nd(NO3)3.6H2O dissolved in ethanol with a Nd/(Si ? Hf) molar concentration of 1 mol%. The final mixture was stirred at room temperature, for 16 h. The obtained sol was filtered with a 0.2 mm filter. The sol–gel guiding layer was fabricated by 18 cycles of spin-coating at 2,500 rpm for 40 s on a 200 SOS wafer (with 2 mm thermal oxide). Each cycle the layer was annealed in air for 50 s at 900 °C. After 10 spincoatings, the film was cured at 900 °C for 2 min. The final film was cured at 900 °C in air for 5 min. The channel waveguide shown in Fig. 24 has a rib crosssection. The rib width is defined by the lateral distance between two 35 nm deep trenches etched into the guiding layer. The rib waveguide starts with a 50 mm wide section that contains an input/output grating. This width eases the focusing and positioning requirements of the pump beam.

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Fig. 24 Nd doped sol–gel tapered rib WG laser (schematic view), not drawn to scale for clarity. (a) IO grating coupler, (b) partial reflection grating, and (c) total reflection grating. The resonant coupling angles via the IO-grating coupler are shown as -10° (input) for the pump and as -48° (output) for the signal. Adapted from [37]

The waveguide tapers linearly within 7 mm to a width of 4 mm in order to enlarge the pump intensity in the waveguide cross section to enhance gain, as well as to maintain a single lateral mode [37]. The sol–gel layer was characterized, giving a refraction index of 1.6176 at 806 nm and of 1.6136 at 1,064 nm, respectively. Its thickness was 604 nm. The measured attenuation from the signal attenuation measurement set was 1.65 dB/cm, which indicates a scattering attenuation coefficient of 38 m-1. The rib waveguide laser was pumped by a tunable Ti:Sa laser at 790 nm. Lasing action started at a pump power of about 21 mW, at around k = 1,062.4 nm. As pump power increased, other bands were observed, first at around k = 1,063.9 nm at a pump power of 26 mW and latter at around k = 1,060.85 nm at a pump power of 30 mW. A fourth band was observed at around 1,059.3 nm at a pump power of 223 mW. More technical and scientific detail are reported in [37]. Here we want stress that starting from silica-hafnia system a Nd doped silica-hafnia sol–gel tapered rib waveguide laser was fabricated. CW pumping was coupled in via an input/ output grating which has also coupled out the lasing signal, while reflection gratings supported the feedback. A lasing threshold of 20 mW and an output power of up to 2.45 mW were measured from a 3 cm long device. The rib waveguide laser was pumped by a tunable Ti:Sa laser at 790 nm. Lasing action started at a pump power of about 21 mW, at around k = 1,062.4 nm. As pump power increased, other bands were observed, first at around k = 1,063.9 nm at a pump power of 26 mW and latter at around k = 1,060.85 nm at a pump power of 30 mW. A fourth band was observed at around 1,059.3 nm at a pump power of 223 mW. More technical and scientific detail are reported in [37]. Here we want stress that starting from silica-hafnia system a Nd doped silica-hafnia sol–gel tapered rib waveguide laser was fabricated. CW pumping was coupled in via an

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input/output grating which has also coupled out the lasing signal, while reflection gratings supported the feedback. A lasing threshold of 20 mW and an output power of up to 2.45 mW were measured from a 3 cm long device.

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represents a breakthrough to higher performance and low cost products as well as to innovative physics. Acknowledgments Part of this work was performed in the framework of the research projects PAT-FaStFal, ITPAR Phase II area Nanophotonics, NAOMI, and COST MP0702: Towards Functional Sub-Wavelength Photonic Structures.

4 Conclusions 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. The specific properties of the sol–gel technique make this approach very effective to succeed in the above mentioned topics covering material science and photonics and some examples are reported in this review. In the first part different colloidal systems have been discussed. Silica and polystyrene spheres are presented as well as core–shell sphere activated by rare earth ions. Starting from the nano sphere 3D photonic crystals can be fabricated. In this paper some examples of direct opals are given and in particular it is demonstrated that it is possible to tune the position of the stop band infiltrating the voids of the opal structures with wanted materials. The section is closed by the presentation of a novel structural coloured material exploited as strain sensor and of a SERS biosensor constituted by a metallo-dielectric structure realized by immobilization of gold nanoparticles on the network of an inverse silica opal. In the case of planar waveguides the more significant results obtained for the rare-earth-activated amorphous and nanostructured SiO2–HfO2 and nanostructured SiO2–SnO2 systems are reported. In particular the results on the fabrication and assessment of a monolithic Nd-doped SilicaHafnia sol–gel, tapered rib waveguide laser are reported. Of particular interest appears the demonstration of the role of SnO2 nanocrystals as rare earth sensitizers and the possibility to develop suitable fabrication protocols allowing the fabrication of rare earth activated glass ceramic waveguides. Finally, let us recall that also microresonators, not reviewed in this paper, can profit from the improvement of sol–gel technique. The coated microspheres have exhibited a whispering gallery mode fluorescence behaviour quasi independent on the thermal effects related to the injected power. It is also confirmed that sol–gel coating is compatible with functionalization or chemical modification of the microsphere surface without significant de-crease of the resonator’s quality factor: this characteristic is important for the use of microspheres as potentially high-sensitivity biosensors [54]. In conclusion, we believe that the possibility to develop appropriate protocols for fabrication of optimized confined structures in sol–gel photonic glasses on nano-scale domain

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16.

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28. 29. 30.

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