Nature is a great source of inspiration for scientists and engineers to design and fabricate functional devices. Many animals and plants present a structural coloration, which is caused by the interaction of light with periodic structure, usually used in camouflage or to transmit information. Several natural surfaces show superwettability properties that allow self-cleaning abilities and water harvesting.
Here, we show two examples of biomimetic coatings inspired by the cuticle of the Hoplia cerulea beetle and Stenocara beetle. The coatings were fabricated by using a simple and scalable approach based on layer-by-layer deposition of sol-gel solutions by spin coating.
We developed a multilayer film with structural color by alternating a low refractive index material (silk fibroin) with a high refractive index layer (titania nanosheets).
Inspired to the back of Namib desert beetle, we fabricated micropatterned surfaces through a controlled dewetting of bilayer films.
Due to the weak thermal and chemical stability of organic resins which are used for conventional white LEDs to embed phosphors, inorganic color converters such as phosphor ceramics and phosphor-in-glasses are currently being used to replace conventional color converters based on organic materials, especially for high power and high brightness applications. In this paper we report on the study of sintered glass ceramics based on low melting glass in which commercial YAG:Ce3+ phosphors are embedded. A low Tg is necessary to avoid high temperature sintering which can damage the optical properties of the embedded phosphors. Two different types of glass have been studied: borosilicate and tellurite. The compositions have been optimized in terms of stability, sintering efficiency and thermal conductivity. Selected samples were optical charterized using a GaN high power multimode 450 nm Laser Diode, with a maximum output power of 1.6 W at 1.5 A.
We investigated both the effect of high irradiation density and high operating temperature, as well as their color-rendering index. The sintered glass ceramic based on borosilicate glass showed better high power stability because of its higher thermal conductivity.
Rare earth ions (RE3+) have typical photoluminescence emissions due to internal 4f orbital transitions. These emissions are narrow, with long excited state lifetimes and have the capability of spectral manipulation like wavelength shifting, down-conversion or up-conversion processes. Therefore, RE-doped materials are widely used for optical applications. However, the narrow absorption bandwidths and the small excitation cross sections for their optical transitions are major limiting factors for the full exploitation of their potentials. In this work, we show that the addition of metal nanoaggregates as broadband and efficient sensitizers can be a viable strategy to overcome these limits. Silica-zirconia (70% SiO2 – 30% ZrO2) glass-ceramic films doped by Tb3+/Yb3+ ions and an additional 5 mol.% of Na2O were prepared by sol-gel synthesis followed by a thermal annealing at 1000°C. Ag introduction was then obtained by ion-exchange in a molten salt bath and the samples were subsequently annealed in air at 380°C or 430°C to induce the migration and aggregation of the metal. The structural, compositional and optical properties of the materials were investigated, providing evidence for efficient broadband sensitization of the rare earth ions by energy transfer from Ag-dimers or multimers, which could have applications for increasing the efficiency of silicon solar cells.
This work focuses on the fabrication processes and photonic assessment of SiO2-SnO2:Er3+ monoliths. To obtain the crack-free and densified system, the sol-gel derived synthesis protocols and heat-treatment processes were optimized. The absorption measurements were employed to assess the effect of the heat-treatment on the samples and specially to estimate the –OH content. The XRD patterns were used to investigate the crystallization as well as the structure of the monoliths. The emission spectra, performed at different excitation wavelengths, evidence the presence of Er3+ in the SnO2 nanocrystals and the energy transfer from SnO2 to the rare earth ions. In addition, the efficient role of SnO2 nanocrystals as Er3+ sensitizers are also experimentally confirmed in this system.
KEYWORDS: Optical coatings, Ions, Aerospace engineering, Particles, Astronomical imaging, Monte Carlo methods, Multilayers, Statistical analysis, Space operations, Photonics systems
The scientific goals required to the next-generation space missions lead the development of innovative instrumentation, conceived to operate in increasingly harsh environments. Optical coatings are among the sub-systems which can highly suffer the agents in such environments. In particular, as recently demonstrated, the accelerated ions and particles can potentially jeopardize the coatings optical performances, with a consequent degradation of the overall functionality of an instrument. Despite its importance, this issue is still poorly investigated. In fact, the fragmentary knowledge of the space environments and the low number of previous ground testing experiments complicates the definition of clear procedures to investigate the behavior of the optical coatings in space. A systematic approach devoted to identify a methodology for the validation of optical coatings under ions irradiation is presented. Monte Carlo simulations are used to evaluate the effects induced by different ion species and energies on both layers and multilayers of different materials, getting an accurate overview of the main criticalities. Such results are then used to plan representative irradiation experiments and the subsequent analysis procedures needed for a proper characterization of the exposed samples. In this paper, a summary of the experiments performed so far is presented. Thanks to these studies we have identified three main damage mechanisms which can be used to explain most of the degradation effects observed when an optical coating is irradiated with low energy particles. A brief discussion of such mechanisms is reported.
Future space missions will operate in very harsh and extreme environments. Optical and electronics components need to be optimized and qualified in view of such operational challenges. This work focuses on the effect of low alpha particles irradiation on coatings. Low energy He+ (4 keV and 16 keV) ions have been considered in order to simulate in laboratory the irradiation of solar wind (slow and fast components) alpha particles. Mono- and proper bi-layers coatings have been investigated. The experimental tests have been carried out changing doses as well as fluxes during the irradiation sessions. Optical characterization in the UV-VIS spectral range and superficial morphological analysis have performed prior and after irradiation.
Metallic films of palladium (Pd) and palladium-tin (Pd-Sn) have been deposited by evaporation technique. They were used as sensitive material for optical sensor by measuring the variation of absorbance. All samples were then oxidized by annealing at 500°C in low vacuum atmosphere. All the films were investigated by X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) to observe the influence of the structure and morphology on the optical properties of the films, carrying useful information for the sensing properties of the different sensing materials. Furthermore, the sensing performances were tested by monitoring the variation on the optical absorbance induced during the absorption / desorption of hydrogen gas. While the use of Pd for gas sensing has been widely covered for electrical and SPR sensors, this work aims to extend our comprehension of the optical sensing behavior, especially in absorbance-mode, of the thin films of PdO, Pd-Sn and PdO-SnO2.
When Palladium film is exposed to hydrogen, it becomes palladium hydride. A change in the complex permittivity of the metal film results in a change of the optical properties that depends on hydrogen concentration. Ellipsometry is the technique of choice to measure the optical constants prior and during hydrogenation. Sensors are then usually designed and optimized to measure changes in transmittance or reflectance of the palladium films. Films of different thicknesses have been realized and tested to verify potential applications in hydrogen sensing by studying the optical response prior, during and after hydrogenation, to assess in particular the reversibility of the process. Within this work a deep analysis carried out by x–ray reflectance (XRR) shows that during hydrogenation the films change also their thickness, and the amount has been assessed for a specific hydrogen concentration. Ellipsometric measurements have been therefore corrected taking into account such variation to determine the optical constants. Such structural property of the palladium hydride may be exploited in surface plasmon resonance transducers, which are sensitive also to the change of the sensing film thickness during detection.
KEYWORDS: Space operations, Optical components, Ions, Particles, Electrons, Particles, Coating, Gold, Platinum, Monte Carlo methods, Information technology
Space exploration is linked to the development of increasingly innovative instrumentation, able to withstand the operation environment, rich in ion particles and characterized by high temperatures. Future space missions such as JUICE and SOLAR ORBITER will operate in a very harsh and extreme environment-. Electrons and ions are considered among the causes of potential damage of the optical instrumentation and components. Development of hard coatings capable to preserve their optical properties is pivotal. Different coating materials have been exposed to ion irradiation in particle accelerators. Change in optical performances has been observed in the extreme ultraviolet and visible spectral region and structural properties have been analyzed by different techniques. The knowledge of the damage mechanisms and thresholds allows the selection of more promising candidate materials to realize the optical components for the new frontiers space missions.
We obtained Gallium-doped and Aluminum-doped Zinc Oxide nanocrystals by non aqueous colloidal heat-up synthesis. These nanocrystals are transparent in the visible range but exhibit localized surface plasmon resonances (LSPRs) in the near IR range, tunable and shiftable with dopant concentration (up to 20% mol nominal). GZO and AZO inks can be deposited by spin coating, dip coating or spray coating on glass or silicon, leading to uniform and high optical quality thin films. To enhance absorbtion in the infrared region, samples can be annealed in inert or reductant atmosphere (N2/Argon or H2 in Argon) resulting in plasmon intensity enhancement due to oxygen vacancies and conduction band electrons density increment. Then IR plasmon has been exploited for gas sensing application, according to the plasmon shifting for carrier density variations, due to electrons injection or removal by the target gas/sample chemical interactions. To obtain a functional sensor at low temperature, another treatment was investigated, involving surfanctant removal by dipping deposited films in a solution of organic acid, tipically oxalic acid in acetonitrile; such process could pave the way to obtain similar sensors deposited on plastics. Finally, GZO and AZO thin films proved sensibility to H2 and NOx, and in particular circumstances also to CO, from room temperature to 200°C. Sensibility behavior for different dopant concentration and temperture was investigated both in IR plasmon wavelengths (~2400 nm) and zinc oxide band gap (~370 nm). An enhancement in sensitivity to H2 is obtained by adding Pt nanoparticles, exploiting catalytic properties of Platinum for hydrogen splitting.
The direct optical monitoring of electron exchange on single plasmonic nanoparticles, involved in chemical reactions with gas molecules, is one of the main challenges in the heterogeneous catalysis and gas sensing fields.
Catalysts are substances that speed up reactions by providing an alternative pathway with lower activation energy than that required for the uncatalysed reaction. A lot of research, both fundamental and applied, has been carried out to investigate how catalysts work and to increase their efficiency.
The present work shows how the use of Dark Field Microscopy (DFM) coupled with surface plasmon spectroscopy, enables the direct observation of the kinetics of H2 gas interaction with single gold nanorods (NR) coupled with Pt nanoparticles (NPs) and/or with metal oxide matrices. The plasmonic particles, gold NRs, act as optical probes, and enable the monitoring of the electron exchange through the measurement of their surface plasmon resonance (SPR) band shift. To improve the redox reaction kinetics, the Au NRs have been coupled with Pt NPs and embedded also into a TiO2 or ZnO low scattering matrix. The Au NRs, the Pt, TiO2 and ZnO NPs have been synthetized by colloid chemistry. Several samples made of bare Au NRs, or Au NRs coupled with only Pt NPs or with Pt and TiO2 NPs or with Pt and TiO2 have been deposited by spin coating on silica substrates.
The longitudinal Au SPR band shift has been monitored by DFM looking at the variation of the scattering spectrum of a single Au NRs in the presence of H2. Time-resolved measurements have been also conducted at fixed wavelength in order to monitor the kinetics of the H2 reaction. With such measurements it was possible to elucidate the importance of the adsorbed oxygen and the TiO2 matrix on the H2 reaction with the Pt NPs.
Large attention has been directed toward carbon nanotubes as material for chemical sensors. However, little attention was paid toward the different behavior of the metallic and semiconductive carbon nanotubes as optical sensing materials.
Semiconductive or metallic Single Wall Carbon Nanotubes (SWCNTs) have been deposited on gold nanoparticles (NPs) monolayer and used as plasmonic based gas sensor.
The coupling between SWCNTs and Au NPs has the aim of combining the reactivity of the nanotubes towards hazardous gases, such as H2, CO, NO2, with the Localized Surface Plasmon Resonance (LSPR) of gold NPs. The LSPR is known to be extremely sensitive to the changes in the dielectric properties of the surrounding medium, a characteristic that has been widely exploited for the preparation of sensing devices. While the use of SWCNTs for gas sensing has been covered in multiple reports, to the best of our knowledge this is the first time that SWCNTs are used as sensing material in an optical sensor for the detection of reducing and oxidizing gases.
Two different techniques, ink-jet printer and dropcasting, were used for depositing the transparent CNTs film on the plasmonic layer. Both the deposition techniques proved to be effective for the development of transparent optical sensing films.
Metallic SWCNTs showed high sensitivity toward H2 at low temperature and an enhancement of performance at 300°C with the detection of low concentration of H2 and NO2. On the contrary, the semiconductive SWCNTs displayed very poor gas sensing properties, especially for the thinner film.
Nanoporous TiO2 anatase film has been investigated as sensitive layer in Surface Plasmon Resonance sensors for the
detection of hydrogen and Volatile Organic Compounds, specifically methanol and isopropanol. The sensors consist of a
TiO2 nanoporous matrix deposited above a metallic plasmonic grating, which can support propagating Surface Plasmon
Polaritons. The spectral position of the plasmonic resonance dip in the reflectance spectra was monitored and correlated
to the interaction with the target gases.
Reversible blue-shifts of the resonance frequency, up to more than 2 THz, were recorded in response to the exposure to
10000 ppm of H2 in N2 at 300°C. This shift cannot be explained by the mere refractive index variation due to the target
gas filling the pores, that is negligible.
Reversible red-shifts were instead recorded in response to the exposure to 3000 ppm of methanol or isopropanol at room
temperature, of magnitudes up to 14 THz and 9 THz, respectively. In contrast, if the only sensing mechanism was the
mere pores filling, the shifts should have been larger during the isopropanol detection.
We therefore suggest that other mechanisms intervene in the analyte/matrix interaction, capable to produce an injection
of electrons into the sensitive matrix, which in turn induces a decrease of the refractive index.
For the fabrication of photonic devices the sol-gel technique is a potentially lucrative alternative to methods such as
physical vapor or chemical vapor deposition because of its solution-processability, low cost and relative ease of
production. In this work we harness this potential by developing based photonic devices which incorporate highly
luminescent CdSe@ZnS core-shell semiconductor quantum dots (QDs) doped within inorganic (TiO2, ZrO2) or hybrid
organic-inorganic sol-gel films. As a pre-requisite to the formation of such devices, luminescent waveguides emitting
between green and red have been obtained and their optical properties have been characterized. The photochemical
stability of these waveguides was found to highly dependent on the exact sol-gel material used. QDs:Titania based
composites were found to be inherently photo-unstable due to photoelectron injection into the bulk matrix and
subsequent nanocrystal oxidation. In comparison, zirconia composites were significantly more robust with high
photoluminescence retained up to annealing temperatures of 300 °C. Despite this difference in photo-chemical stability,
both titania and zirconia composite waveguides exhibited amplified stimulated emission (ASE) with one-photon and
two-photon optical pumping, however only zirconia based waveguides exhibited long term photostability. This Zirconia
based films have been used for the realization of distributed feedback lasers and Bragg micro-cavities.
The successive ion layer deposition reaction (SILAR) technique has been applied to CdSe based systems to develop
Type 1 heterostructures . In such structures, the CdSe core is covered by wider band gap semicondutors to improve the
emission properties. Cores of different dimensions has been synthesised and two different shell structures have been
addressed.
The obtained particles have been characterised by TEM technique, while UV-Vis absorption and photoemission
spectroscopy were used to characterise the optical properties of the particles in the colloidal solution.
The obtained particles were also introduced in a ZrO2 sol-gel matrix to fabricate photoluminescent waveguides, which
were characterised also by spectroscopic ellipsometry.
A stable and narrowly distributed dispersion of Mn-doped ZnS (sphalerite) nanoparticles with an average diameter of 1-2
nm, has been synthesized via chemical precipitation without using any surfactant. The surface of the particles has been
functionalized with acrylic acid for compatibilization with polymethylmetacrylate (PMMA). A thorough morphological
and optical characterization is proposed. A high nonlinearity in the response of the material is evidenced, with the
maximum of luminescence obtained with 5%mol Mn doping.
Au and Pt nanoparticles are prepared with colloidal techniques in order to achieve high morphological quality, capped
with a polymer and then embedded inside a TiO2 sol-gel matrix, resulting in a homogeneous dispersion of both metal
colloids, confirmed by TEM analyses. Refractive index values measured with ellipsometry increase with the annealing
temperature, with quite a linear trend, and at the same time the Au surface plasmon resonance peak undergoes a red shift:
the refractive index evaluated from the Au plasmon band is slightly lower than the measured value, indicating that the
refractive index just around metal particles is different from the average of the matrix, likely because of the polymeric
capping agent. Optical gas sensing tests towards CO and H2 are presented as one of the possible applications of these
nanocomposites.
In the last years inorganic semiconductor (particularly CdSe and CdS) quantum dots (QDs) have received great attention
for their important optical properties. The possibility to tune the emission wavelength, together with their high
fluorescence quantum efficiency and photostability, can be exploited in photonic and optoelectronic technological
applications. The design of DFB devices, based on QDs as active optical material, leads to the realization of compact
laser systems. In this work we explore the use of an inorganic/organic hybrid material composed of CdSe-ZnS
semiconductor quantum dots doped into a zirconia sol-gel matrix for optical gain applications. Through the use of soft
lithography on a sol-gel germania-silica hybrid, large scale distributed feedback gratings can be created. Used in
conjunction with the CdSe-ZnS/ZrO2 hybrids, these gratings can act as microcavities and allow for the realization of true
lasing action. The lasing properties within these devices are characterized in the femtosecond regime by both one- and
two-photon excitation. From experimental data the value of the optical gain of the core-shell quantum dot samples has
been estimated. Moreover, one- and two-photon lasing threshold and stability are reported.
In this work we present a theoretical and experimental work due to develop a performing configuration for gas-sensing
through the employ of Surface Plasmon Resonance effect.
Different sensing layers have been studied and tested on our optical bench assembly. Metallic (Au) and bimetallic
(Ag/Au) layers have been properly designed through simulations and then have been realized through electron-beam
evaporation. TiO2 and TiO2 - Au doped layers have been deposited on the top of some of the metallic samples. These
layers were prepared by the sol gel route. This kind of material is expected to be suitable as a gas sensor for its
nanosized structure and its stability.
The optical bench configuration for the experimental exploitation of SPR is presented. It is based on a collimated beam, a
rotational stage with a triangular prism and a single photodiode.
Finally the sensing properties of the different sensing layers prepared was tested to some organic vapours. Preliminary
results are presented.
We present degenerate and nondegenerate two-photon absorption spectra in a series of CdSe and CdTe quantum dots. The measurements show that the two-photon absorption (2PA) spectrum is strongly dependent on the quantum dot size and that the 2PA coefficient decreases as the quantum dot size decreases, and it is larger for the frequency nondegenerate process. Previously we had shown a theoretical analysis of these results based on a simple model using the effective mass approximation. Although this model works well for larger quantum dots, it fails for the smaller ones. Here we use the more (formula available in manuscript) model for the band structure and consider the hole band mixing in quantum dots to describe our data. This theory better describes the spectral structures for smaller quantum dots and also predicts the decrease of the 2PA coefficient with the decrease of quantum dot size. This is due to the reduction of the number of possible transitions and the blue shift of the optical bandgap from quantum confinement. This theory predicts the reduction of the 2PA coefficient with size, although our experimental results show an even stronger reduction.
Nanostructured TiO2 films and Au-TiO2 nanocomposite thin films prepared by sol-gel method have been deposited onto gold covered glass substrates and glass substrates in order to study their optical properties using Surface Plasmon Resonance and Optical Absorption measurements. Both techniques have been used to study the sensing features of both kind of films to different vapour organic compounds. A comparative study of the two techniques has allowed us to know the possible benefits than can be found when the sensing material is a nanocomposite thin film.
Paolo Mazzoldi, S. Padovani, F. Enrichi, G. Mattei, Cinzia Sada, E. Trave, Massimo Guglielmi, Alessandro Martucci, Giancarlo Battaglin, E. Cattaruzza, Francesco Gonella, C. Maurizio
A set of samples co-doped with Er and Ag were prepared with a combined sol-gel and ion-exchange route. This multistep process sytnthesizes samples in which the silver atoms are dispersed in the matrix and/or aggregated in Ag multimers or nanoclusters. The samples exhibit a different photo-luminescence response depending on the sensitizing effect due to silver atoms. The spectroscopic properties were correlated with the structural properties investigated by extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray diffraction. The Er3+ excitation via energy transfer, obtained in a wide range of wavelengths, has been clearly related to the presence in the sample of Ag multimers.
Hybrid organic-inorganic thin films doped with lead sulfide nanocrystallites were synthesized by a combination of colloidal chemistry and sol-gel processing. In order to study the influence on the spectroscopic properties of the crystallite surface, and hence the related defect states, PbS-doped films with different sulfur to lead ratios and different surface capping agent concentrations were fabricated. X-ray diffraction measurements showed the presence of nanoparticles with a mean diameter raging from 3 to 5 nm. The absorption spectra showed a large blue shift of the absorption edge to shorter wavelength, indicating strong quantum confinement. Strong photoluminescence emission in the near infrared was found by pumping at 514 or 532 nm. The emission intensity and its position were found to be dependent on the elaboration parameters. The film fabrication process influences only slightly the good particle size distribution of the colloidal PbS solutions. Best results were obtained for films with low sulfur to lead ratio or with high capping agent concentration.
Sol-gel films doped with MeS (Me=Cd or Pb) nanocrystals or with MxOy (M=Ni or Co) nanocrystals have been fabricated. Two different strategies were used: for the sulfides a colloidal doping sol was prepared at room temperature and then mixed with the matrix sol. For the oxides a bifunctional ligand was used, bearing either an amine group capable of coordinating the M ions and hydrolysable siloxane groups for anchoring the metal complex moiety to the silicate matrix. In this case the oxide nanocrystals precipitate at 500°C. Films containing MeS nanocrystals showed both non-linear and active optical properties. Nanocomposite films doped with MxOy showed optical gas sensing properties.
Sol-gel is one of the possible production techniques of silica-on-silicon integrated optical devices, combining low cost with a great flexibility. In the frame of a European project, we have investigated the application of the sol-gel technique for the realization of an erbium-doped optical amplifier. In particular we developed GeO2-SiO2-Al2O3 sol-gel waveguides doped with Er. The material was optimized in terms of heat treatments, Al2O3 co- doping content and Er doping level. RBS measurements showed that well densified films could be obtained heating the coatings at 700 degrees C in O2 flux. Waveguide properties were tested measuring the propagation losses at 840 nm. In the planar waveguide pumped at 980 nm, erbium showed fluorescence around 1530 nm with up to 6.5 ms. In order to achieve lateral confinement, the planar waveguides were co-doped with Na2O and channel waveguides were fabricated by ion exchange. The feasibility of sodium-silver and sodium-potassium ion exchange was demonstrated. The influence of the process parameters on passive and active optical properties of the films was studied and straight channel waveguides with Gaussian mode profiles were obtained by the sodium-silver route.
Among the various possible production techniques of silica- on-silicon integrated optical devices, sol-gel is the one which combines low cost with a great flexibility and the ease of doping the silica matrix with nonlinear and active compounds. In the frame of an European project, we have investigated the application of the sol-gel technique to the realization of an erbium-doped optical amplifier, operating in the third telecommunication window. Here, in particular, we refer to the development of an optimum fabrication strategy for the guiding structure. A strip-loaded configuration was chosen. Design optimization was carried out by means of a MATLAB software code, mainly based on the Effective Index Method. For what concerns the technical side, two different routes were followed: that of the Low Index Load and that of the High Index Load. Pros and cons of both structures were carefully evaluated through numerical simulations as well as experimental analysis, in order to choose the best performing one. Results of the design procedure and the characterization of the fabricated waveguides are described here.
Thin silica-titania planar waveguides doped with different concentrations of lead sulfide (10 - 25 mol%) have been prepared by a sol-gel process. It consist of three steps: (a) preparation of a colloidal sol of semiconductor particles; (b) preparation of an alkoxide solution, precursor of the glass- like matrix; (c) mixing of the colloidal sol and the alkoxide solution. Films were deposited on fused silica by dipping and densified for 1 h at 300 degrees Celsius in nitrogen. The mean particle size is 3 nm and the optical absorption edge is situated around 1100 nm. The nonlinear properties have been investigated using degenerated four wave mixing (DFWM) an a nonlinear m-line technique. Different Nd:YAG lasers with pico- and nanosecond pulses at 1064 nm have been used. Depending on the PbS concentration we measured a high negative nonlinear refractive index of n2 equals -3 to -9 10-8 cm2kW for nanosecond pulses (m-lines) and -1 to -10 10-10 cm2/kW for picosecond pulses (m-lines and DFWM). The response time of the nonlinearity is below 30 ps. All observed nonlinear effects are fully reversible and we did not observe any photodarkening. Straight, monomode channel waveguides have been fabricated on theses films.
Sol-gel-based photonic components' technology is gaining an increasing interest. This is due in part to the overall properties of the sol-gel process, and in part to the specific characteristics which concern the inclusion into a glass matrix of nonlinear and/or active compounds, thus paving the way to the development of all-optical devices. Here a brief review is presented of the progress under way in the design and development of different sol-gel glass matrices where neodymium and erbium ions have been dispersed. So far, at our knowledge, a measurement of optical gain in this class of materials is still lacking; however, optical propagation and spectroscopic properties of the newly developed materials are quite satisfactory and perspectives of realizing lasers and amplifiers before too long are good.
The realization of low-loss optical waveguides by the sol-gel method and the subsequent production of strip waveguides by direct laser densification are described. These waveguides have been characterized by both the optical and structural point of view, and the results are discussed.
Silica-titania optical waveguides doped with CdS and PbS were prepared by the sol-gel method. Thiourea was used as an internal source for sulfur. Cadmium and lead were introduced as acetates. Using tetraethoxysilane (TEOS) and Ti(OBu)4 as precursors for silica and titania, films with typical thickness of 0.15 - 0.2 micrometers were obtained. In order to obtain thicker layers TEOS was substituted with methyltriethoxysilane (MTES). The effect of this precursor on solution and waveguide properties is discussed.
Laser heating can be used to fully densify selected areas of a partially densified sol-gel film deposited onto a glass substrate, thus substituting the conventional heat treatment in an oven. Here the topographic and optical characteristics of strip waveguides produced in a SiO2-TiO2 film by direct writing with a focused CO2 laser are presented, and the critical parameters of the process discussed.
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