The density of the material in a deposited film determines many important film properties, for example hardness and abrasions resistance, adherence to the substrate, refractive index, film stress,
flatness and film permeation. Chemical compound films like metal oxides, some nitrides and oxynitrides are mainly produced by reactive PVD processes. The energy input into the growing film strongly influence the density of the resulting film. High energetic coating conditions result e.g. in a high refractive index, but often also in relatively high residual optical absorption and high compressive film stress. In order to obtain films with improved properties immediately after deposition without time consuming post-deposition heat treatments, depositions of RLVIP-Ta2O5 films were carried out under relatively high oxygen pressures and under special rate conditions. The achieved reproducible film properties can practically be accepted for many low loss optical film
Over the past years, DNA-chip technology has exploded. Yet
scientists using such devices have to face many problems. One of
them, due to the very low concentration of biological species to
be detected, is the weakness of fluorescence signal collected
through the reading system (microscope or scanner). To solve this
problem, we proposed to use optical thin films technology. We
studied the potentialities of this method step by step. The first
step was to be able to understand, explain and forecast the
fluorescence emitted by a DNA-chip in terms of fluorescence
angular patterns. A theoretical and experimental study enabled us
to master this issue even in the case of multi-layers substrates.
Using this knowledge we were then able to explain, through
simulations, the potentialities of this new type of substrates in
terms of fluorescence enhancement. Thus we showed that a
theoretical enhancement of twenty-fold (compare to a glass
substrate) was achievable.
Metal island films show exceptional optical properties, which are interesting for linear and non-linear optics. One of the perspective applications of metal islands in optics utilizes the surface plasmon excitation for the design of spectrally selective absorbers. The absorption behaviour of metal islands can be influenced by the dielectric function of the metal, the size and the shape of the clusters and the properties of the embedding medium. It is possible to affect these factors and to prepare materials with tailored optical absorption properties.
We describe the optical properties of silver island films embedded in SiO2 and Al2O3. Despite of the presentation of spectrophotometric data and the film morphology as investigated by Transmission Electron Microscopy (TEM), we demonstrate the correlation between silver island geometry and optical response as well as their dependence on the deposition parameters. The results confirm that the silver cluster surface plasmon absorption line position and width depend on the deposition temperature and the cluster ambient. The optical data could be theoretically reproduced by means of the Rigorous Coupled Wave Approximation (RCWA).
We further demonstrate that the absorptance of the silver clusters may be enhanced up to nearly 100% when the clusters are embedded into a suitable designed multilayer stack.
The fabrication and the studies of sol gel-microcavities strongly doped with CdSe nanocristals are presented. The Fabry-Perot microcavities are fabricated by a Distributed Bragg Reflectors which is covered with an active layer and a silver mirror. The matrix of the doped layer is the ZrO2 material. These microcavities are characterized by reflectometry. The resonant peak is enlarged when his spectral position corresponds to their first absorption line. This means a strong interaction between the narrow peak of the cavity mode and the large first absorption line of the nanocrystals.
Optical interference coatings usually offer large possibilities for the spectral control of specular reflected or transmitted light. In this aim, multilayer structures are calculated and realized according to each specific application. These kinds of components are deposited over plane substrates, or on substrates with large curvature radius. In this paper we show how multilayer components can also be deposited on micro spheres to reach other applications connected with the spectral control of light scattering.
Laterally heterogenous thin solid films represent new design tools that may find applications in reflectors and absorbers from the near infrared up to the ultraviolet spectral regions. Particular examples are reflectors and absorbers based on resonant grating waveguide structures and metal island films.
Because one-dimensional dielectric grating waveguide structures are optically anisotropic, these systems combine lateral periodicity with optical anisotropy and may therefore show high reflectivity and polarizing properties at normal light incidence. In combination with metals, the excitation of surface plasmon polariton modes embodies an effective absorption mechanism which may be utilized for the design of selective absorber coatings. Generally, in these systems, high absorption or reflection may be achieved even in single layer designs by a proper combination of geometrical parameters and optical materials.
The purpose of this contribution is to present the results of analytical and numerical calculations as well as first experimental results for simple grating waveguide structures. Here we deal with all-dielectric systems as well as metal-based absorber designs. Analytical estimations are provided that relate the film thickness to the reflection or absorption wave-length in GWS-structures. Concerning the numerical treatment of the systems, we emphasize the use of Rigorous Coupled Wave Approximation (RCWA) - calculations in thin film optics.
We have developed a novel approach to design ultra-hydrophobic surfaces with optical quality. The nanostructure necessary for the functional effect is realized through enhanced nanoroughness of optical thin films. At the same time, the optical appearance must not be disturbed by scattering from the roughness. We have found that through wide-scale roughness analysis, applying white light interferometry, AFM and STM, and subsequent data reduction the roughness characteristics can be directly related to the wetting properties. As, on the other hand, vector scattering theories connect the roughness properties with scatter losses, a formalism has been established, where both the wetting properties and scattering behavior can be expressed within the same "language". Using this tool, the optical thin film design and the surface nanoroughness can be tailored to fulfill demands on wetting properties as well as on sufficient low scatter levels. For the deposition of high index single layers on Borofloat 33 substrates, qualified substrate-film-combinations are predicted by "virtual" coating simulations. Experiments with single oxide layer as test coatings yielded surfaces with a high water contact angle and light scatter losses below defined scatter thresholds.
We have designed and fabricated a silicon grating which shows antireflection properties in the [4μm ; 6 μm] spectral region. It is shown both theoretically and experimentally that, even if the refractive index and the grating period are in the extend that a simple homogenization theory can not be used, a substantial broadband antireflection effect can be obtained. The grating was made using a wet anisotropic etching technique. The reflectance was calculated with a modal method and compared successfully with the experimental results. It is shown that the grating reduces the silicon substrate
reflectance in the whole [4 μm ; 6 μm] spectral domain by a factor greater than 10.
Author(s): Franeois Royer; Hubert Roux; Damien Jamon; Jean Jacques Rousseau; Daniel Zins; Valerie Cabuil; Charles Bovier
This paper describes how composite material, made of maghemite (γFe2O3) nanoparticles embedded in a silica/titania matrix, can be used to developp new phase matched magnetooptic planar waveguides. Thin film samples are coated on pyrex substrates from a magnetic particles doped sol-gel preparation and using the dip-coating technic. Ellipsometry and Mlines spectroscopy measurements performed on these samples show that a magnetic field applied during the gelation induces an optical anisotropy in the film. Depending on the orientation of the field (in plane or out of plane), the TE0-TM0 planar waveguide phase mismatch is increased or reduced. Interpretations of these results are given and leads of further works using this property are proposed.
Author(s): Marwan A. Ahmed; Jean Claude Pommier; Florent Pigeon; Stephanie Reynaud; Youcef Ouerdane
Polarization filtering in a laser mirror is achieved by means of a corrugation grating defined in the last high index layer of multilayer mirror based on a metal or metallized substrate. The grating couples the undesired TE polarization to a high order propagation mode of the metal based multilayer stack. The resulting dip in the TE spectral reflection curve is wide enough to cover the gain bandwidth of a Nd:YAG active medium. A technology trimming scheme is designed and demonstrated to shift the TE reflection dip to 1064 nm.
In the field of microelectronics, Extreme Ultraviolet (EUV) lithography operating at λ=13,5 nm, appears, today, as the
most promising future technology. The viability of this next generation technology has to face, however, several technical issues. Among them, the realisation of defect-free lithography masks is certainly one of the most serious issue. These masks, which work as reflective components, are composed of an EUV reflective Mo/Si multi-layer coating deposited on a glass substrate and covered, on the top, by an absorbing stack. Up to now, as far as defect specification is
concerned, Ion Beam Sputtering (IBS) deposition has been proved to be the most appropriate technology. Nevertheless, the necessity to meet other specifications like, for instance, maximum EUV reflectance, requires further studies to better understand the way such IBS multi-layer Mo/Si structures are growing. In this paper, an experimental investigation on the deposition of IBS Mo/Si multi-layers is presented. We focused our interest on a comparison between argon and xenon sputtering conditions. After a brief description of deposition process, the mechanical and optical properties of the films are first reported. Then, the microstructures and chemical profiles of Mo/Si multi-layers, deduced from investigations by high-resolution transmission electron microscopy (HRTEM) and energy filtering TEM, are presented. Finally, the Mo/Si mirror characteristics are discussed on the basis of TEM observations and process considerations related to sputtering mechanisms taking place in the IBS deposition tools.
Imaging of the solar corona by selecting Fe IX (λ=17.1nm,), Fe XII (λ=19.5nm), Fe XV (λ=28.4nm) and He II (λ=30.4nm) emission lines with a Ritchey-Chretien telescope requires to coat the optics with multilayers having a high accuracy in their layer thicknesses, a high reflectivity and an optimal bandpass. Multilayers were simulated in order to determine the most adequate formula for each wavelength channel. Mo/Si coatings were deposited by using the ion beam sputtering technique in a high vacuum chamber equipped with a micro balance and an in-situ reflectometer. The multilayers were studied by grazing angle reflectometry at 0.1541nm, and their reflectances around the operating wavelengths were measured on the SA62 IAS/LURE beam line of the SuperACO
synchrotron facility located at Orsay. In addition, aging versus time and behavior of the multilayers under a rapid thermal annealing were investigated.
Performances of the ion-beam deposited multilayers have been improved compared to the Mo/Si coatings obtained in the past by the e-beam evaporation technique for the SOHO mission Extreme UV Imaging Telescope (EIT). The EUVI telescopes for the STEREO mission are being proceduced by depositing these new generation of multilayers onto
primary and secondary mirrors. The reflectivity measurements on a telescope are presented.
The demand to enhance the optical resolution, to structure and observe ever smaller details, has pushed the way towards the EUV and soft X-rays. Induced mainly by the production of more powerful electronic circuits with the aid of projection lithography, optics developments in recent years can be characterized by the use of electromagnetic radiation with smaller wavelength. The good prospects of the EUV and soft X-rays for next generation lithography systems (λ = 13.5 nm), microscopy in the "water window" (λ = 2.3 - 4.4 nm), astronomy (λ = 5 - 31 nm), spectroscopy, plasma diagnostics and EUV/soft X-ray laser research have led to considerable progress in the development of different mulilayer optics. Since optical systems in the EUV/soft X-ray spectral region consist of several mirror elements a maximum reflectivity of each multilayer is essential for a high throughput. This paper covers recent results of the enhanced spectral behavior of Mo/Si, Cr/Sc and Sc/Si multilayer optics.
CaF2 has become the most important substrate for vacuum ultraviolet (VUV) optical coatings. The popularity of CaF2 relies on its transparency range, which extends down to 120 nm. A major drawback of the material is, however, its mechanical softness. Hence careful assessment of the surface quality is necessary in order to obtain low loss components. By combining roughness data from Nomarski Microscopy and Atomic Force Microscopy (AFM) measurements with total and angle resolved scattering measurements at 157 nm scatter losses due to interface roughness as well as bulk inhomogeneities are being studied. Measured losses are furthermore compared with theoretically predicted results. Results are also presented on VUV coatings. The question of the coupling between the substrate surface finishing quality and the optical properties of the coatings is specially addressed.
Since excimer laser applications extend to deep and vacuum UV wavelengths at 193 nm and 157 nm, renewed research interest has recently arisen on fluoride thin films due to their unrivaled position as wide-band-gap material for the vacuum UV (VUV). In order to evaluate the development of mechanical stress in all dielectric fluoride mirrors which causes difficulties to grow the layer stacks on fused silica substrates with a sufficient large number of quarter-wave pairs of a low (L) and of a high index (H) fluoride material, a systematic study was performed on evaporated quarter-wave stacks of LaF3/MgF2 and LaF3/AlF3 with a growing number of LH-pairs. The samples deposited onto fused silica and silicon substrates by a low-loss evaporation technology in a BAK 640 coating plant were investigated by means of complex ex - situ mechanical stress analysis including temperature dependence of stress, optical measurements, infrared measurements, evaluation of structural and morphological parameters by AFM and XRD. When deposited at high substrate temperature of about 300°C, the LaF3/MgF2 tends show high tensile stress due to the thermal stress component arise from the large thermal expansion coefficient difference between the substrate and the film materials resulting in micro crack formation already starting after deposition of about 10 layer pairs. LaF3/AlF3 appear to have a larger crack resistance due to lower stress which can be correlated to the higher water content in these kind of stacks. By adjusting the deposition temperature, mirror stacks with high reflectance at 193nm can be grown.
We report on an arrangement that measures angle resolved scattering (ARS), total scatter (TS), transmittance (T), and reflectance (R) at 157 nm and 193 nm. The ARS set-up is based on a high precision double goniometer arrangement, which can be inserted into the measurement chamber without removing the TS set-up. The TS set-up for detection of forward scatter and backscatter with extremely low background scatter levels of 1 ppm consists of an excimer laser, a Coblentz sphere with detection system, and a beam preparation path. The sphere and preparation path are housed in vacuum chambers allowing operation in vacuum or purge gas. The measurement options altogether constitute a multifunctional system: VULSTAR (VUVLight Scatter, Transmittance, and Reflectance). We present total scatter measurements on deep ultraviolet (DUV) and vacuum ultraviolet (VUV) substrates and optical components with antireflective (AR) and highly reflective (HR) coatings, angle resolved measurements on optical components and R and T measurements on substrates for VUV optical coatings.
Optical coatings for the use in free electron laser systems have to withstand high power laser radiation and the intense energetic background radiation of the synchrotron radiation source. In general, the bombardment with high energetic photons leads to irreversible changes and a discoloration of the specimen. For the development of appropriate optical coatings, the degradation mechanisms of available optical materials have to be characterized. In this contribution the
degradation mechanisms of single layer coatings (fluoride and oxide materials) and multilayer systems will be presented. Fluoride and oxide single layers were produced by thermal evaporation and high energetic ion beam sputter deposition. The same methods were employed for the deposition of multilayer systems. High reflecting coatings for the wavelength region around 180 nm were chosen for the irradiation tests. All samples were characterized after production by
spectrophotometry covering the VUV , VIS, and MIR spectral range. Mechanical coating stress was evaluated with interferometric methods. Synchrotron irradiation tests were performed at ELETTRA, using a standardized irradiation cycle for all tests. Ambient pressure and possible contamination in the vacuum environment were monitored by mass spectrometry. For comparison, the optical coatings were investigated again in the VUV, VIS, and MIR spectral range after irradiation. On selected samples XRD measurements were performed. The observed degradation mechanisms comprise severe damages like coating and substrate surface ablation. Color centre formation in the VIS spectral range and an increase of VUV absorption were found as a major origin for a severe degradation of VUV transmittance On the
basis of the performed investigations, a selection of coating materials and coating systems is possible in respect to the damage effects caused by synchrotron radiation.
Laser-induced damage has long been widely acknowledged as a localized phenomenon associated with the presence of defects such as nodules, scratches, fractures, polishing or cleaning residues, impurities, contaminants, metal or dielectric inclusions, etc. Destructive investigations in ultra pure fused silica have led to the conclusion that defects, typically a few nanometers in size, were responsible for laser damage initiation. The understanding of damage phenomena requires the development of more sophisticated, non destructive tools with both high spatial resolution and high sensitivity, to detect defects as small as possible. Photothermal microscopy has been widely employed to characterize optical absorption, thermal properties of optical materials and for mapping defects. This technique has been coupled and compared with scattering mapping for studying laser damage processes before and after irradiation. Furthermore the
capability of collinear photothermal deflection to reach sub-micrometric resolution by reduction of the pump beam diameter has been theoretically explored and experimentally demonstrated on specially prepared absorbing targets. A photothermal microscope based on photothermal deflection of the transmitted beam and well-suited for multi-scale studies of absorbing defects in thin films has been coupled with an experimental set-up allowing damage threshold
measurement at the same wavelength. We present an overview of these developments in the field of photothermal microscopy and scattering mapping in connection with laser damage.
In the field of thin film coatings, sol-gel (SG) process is an alternative to the conventional Physical Vapor Deposition (PVD) techniques. Sol-gel process is particularly competitive on large-area or fragile substates by taking advantage of various liquid phase deposition techniques performed at room temperature and atmospheric pressure, coupled with the versatility of organo-metallic chemistry. Developed by the French Commission for Atomic Energy (CEA) since 1985 for its former high-power lasers generation, optimized sol-gel coatings proved also very resistant to laser energy.
In 1998, THALES Angenieux (TAGX) was selected by CEA to provide all the sol-gel coatings dedicated to the French Laser MegaJoule (LMJ) prototype, named Ligne d'Integration Laser (LIL).
In cooperation with Saint-Etienne Pole Optique et Vision (POV), TAGX initiated the building of a sol-gel technological platform (SGPF) aimed at demonstrating the feasibility of production of optical and functional coatings on large area substrates. A technology transfer was performed by CEA (Le Ripault) to TAGX focusing on the manufacture mainly of single-layer antireflective coatings (SLAR), but also of multi-layer AR-coatings and of multi-layered highly reflective (HR)-coatings.
Since beginning of 2001 and using SGPF equipments, TAGX successfully coated within specificaitons and schedule most of the 300 optics required for LIL activation. After this 2 years 1/2 production campaign in pre-industrial conditions, we can now analyse the advantages of each deposition technique used, the repeatability of the several processes, and the performance of the various coatings.
Cumulative laser irradiations are performed on silica at 1064nm and 355nm with a 7ns pulsed laser. The experiments are made in bulk and surface of materials thanks to a focused beam (12μm and 8μm respectively for each wavelength). The small beam size combined to a reliable statistical measurement of laser damage, allow us to plot accurate laser damage probability curves. Moreover the use of an adapted statistic model permit to deduce from these curves the laser
damage precursor centers densities. These nano-sized precursors are established by different works to be responsive of the breakdown initiation.
In a previous work realized at 1064nm, we have observed in our specific conditions, that multiple irradiation leads clearly to a decrease of the laser damage threshold. Furthermore we have highlighted that at this wavelength, the precursor densities were invariant with the number of shot. These results had given novel information about the damage initiation process. Indeed the same precursor centers seems to be involved in the initiation process in spite of the decrease of they laser damage threshold whatever the number of shot.
In this paper we will shown new results obtained at 355nm in silica with the same measurement process. The case of silica surface is also examined for the two wavelengths. The experimental data exhibit a strong different behavior, regarding the precursor densities evolution versus the number of shot.
Laser damage in optical components is caused mainly by the presence of sub-micronic defects, inherent to the manufacturing process (metal or dielectric inclusions, fractures, bubbles). An improvement of the laser damage threshold requires an analysis of damage process and an identification of the laser damage precursors. However, the assumed nanometer size of such precursors makes their identification difficult by the usual optical methods.
In this paper, we present a method to obtain the size and complex index of laser damage precursors in thin films or substrates. This method is based on the knowledge of three parameters accessible to measurements, which are the precursor density, the laser damage threshold and the precursor absorption. Density and threshold are
extracted from the fit of laser damage probability curves with the use of a statistic model and absorption is obtained with photothermal measurements. From these measurements, an electromagnetic and thermal model permits to obtain an estimation of both complex index and size of the laser damage precursors. The different experimental and theoretical tools are described in this paper : laser damage testing apparatus, photothermal bench, stochastic model for the interpretation of laser damage probability curves, electromagnetic and thermal model. An application example is given : we present our first results in silica thins films where sub-micronic laser damage initiators at 1064nm have been highlighted and identified with our method.
For fifty years, a considerable effort has been and is still being directed to the production of optical coatings using liquid deposition route. Sol-Gel is a chemical process widely used for oxide material preparation. Based on smooth chemistry (low temperature conditions), sol-gel allows nanoparticle and polymeric material synthesis dispersed in appropriate liquid medium. The process investigated at CEA (French Commission for Atomic Energy) is strongly developed to afford coatings onto mineral or metallic substrates using colloidal oxide-based and/or inorganic-organic hybrid materials. Such a chemical process is sufficiently adjustable to develop purpose-built materials and coatings for high power laser optical components, taking into account the high laser damage threshold requirement. Because the CEA megajoule-class pulsed laser is needing 7,000-m2 of coated area onto 10,000 large-sized optical components, we have developed to date, several optical coating procedures, each optical thin film being prepared from a specific material and deposition process. First need to fulfil was the antireflective (AR) coating required for transparent optics and used to increase laser light transmission and to suppress damaging residual reflection. The as-developed AR-coatings were made of nanosized particle-containing fragile single layer or abrasion-resistant polymeric-based broadband layer stack. For used on highly-reflective (HR) component, a specific unstressed multilayer coating has been developed and deposited onto deformable adaptative end-cavity mirror substrate. This HR-coating is made of quaterwave stack of colloidal-based low index and hybrid high index thin films.
Using such materials, first high ratio polarizing sol-gel coatings have been also produced. Apart optical coating preparation, sol-gel chemistry has been used to develop an hybrid dense protective thin film to enhance durability of oxidation-sensitive silver cavity reflectors. Each coating material preparation and room-temperature deposition process will be described. Because the sol-gel technology offers outstanding technical and economical advantages over the conventional vacuum techniques, this process has been transferred to one of THALES production plant for megajoule-class laser prototype supplying.
Optical Components improvements generally call specific improvements of optical coatings. SESO introduce here, some examples of new coatings, developed for applications in its activity fields such as, space, defense, synchrotrons and high power laser programs, astronomy.
To answer to a need of coating, in a very wide wavelength range, from X rays to FAR IR:
- Metallic reflective coating for X Rays.
- Specific Aluminium coating for EUV up to 190 nm
- Dielectric coatings for wavelength broadband, double or triple bands (VIS + IR)
To answer to a need of High Level Laser damage:
- Hf/Metal coating (R and AR) to face energies up to 40J/cm2
To answer to a need of long lifetime for coatings submitted to stringent environmental conditions:
- enhanced silver coating for astronomical mirrors
- Hard Gold for Space applications
- Protected aluminium for solar simulators
To answer to a need of coating a large variety of substrates:
- R and AR coatings on Silicium, ZnS, ZnSe, Ge, Saphyr...
At least to answer to a need of high uniformity coatings and to realise coating on very large optical components, SESO recently installed a large coating machine able to work with optical components up to Φ 1100mm.
The LIDT and damage morphology of the AR coatings on quartz and sapphire are investigated. A very interesting phenomena is found in the measurement: In the case of single pulse laser, the LIDT of the AR coatings on quartz is higher than that of sapphire; on the contrary, for free pulse laser, the LIDT of the AR coatings on sapphire is higher than that of quartz.
The operation of a polarizing grating laser mirror relies upon the coupling of the undesired polarization to one mode of the multilayer mirror. This is accompanied by an increased electric field in the mirror, thus to a decrease of the damage threshold by a factor 50. The damage threshold of the grating multilayer for the uncoupled lasing polarization is as much as 80% of that of the gratingless multilayer. The laser damage threshold tests have been made on a grating multilayer of SiO2/HfO2 deposited by sputtering.
Oblique-incidence spectrophotometric measurements are considered for reliable determination of the refractive index and thickness of thin-film coatings. By using a model of the spectral transmittance of thin film samples, the effect of systematic factors on the determined thin-film parameters is analyzed. The optical parameters of a silicon-dioxide sample determined from the experimental results obtained with the HUT spectrophotometer are consistent for the incidence angles between 0 and 56.4 degrees, demonstrating high accuracy of film-parameter determination.
Evaporated CeO2 films are known to show a pronounced optical inhomogeneity in dependence on the chosen deposition parameters, especially in dependence on the substrate temperature during deposition. Samples were manufactured by reactive electron beam evaporation and by ion assisted deposition onto substrates held at different temperatures. Optical analysis of spectral transmittance and reflectance data supplemented by x-ray diffraction and infrared measurements were carried out on the basis of an inhomogeneous refractive index layer model. Using the Wemple - DiDomenico single oscillator dispersion formula together with an effective medium approach for porous materials the inner part of the films at the substrate interface was found to show "void-like" behaviour, while most of the film show "crystallite - like" behaviour in accordance with the columnar growth observed with evaporated CeO2 - films. By IAD the optical as well as the crystalline properties of the deposited films were modified.
Thin film multilayer dielectric coatings are widely used for the fabrication of various optical components. The precise knowledg of the optical constants and the thickness of the individual layers is one of the most important factors for the successful design and production of optical interference coatings with optimal performance. As thin film materials within a multilayer stack often have different optical constants compared to single layers deposited at the same conditions a disagreement between measured and predicted optical response of a multilayer system is observed. A better agreement can be achieved if the optical constants of the layer materials are determined from measurements of multilayer stacks. In the present work such an approach is applied for the optical characterization of popular optical coating materials. The optical constants of TiO2, Ta2O5, Al2O3 and HfO2 are determined in the spectral region 200 nm up to 800 nm using the following measurement techniques: spectroscopic ellipsometry, intensity transmission and X-ray grazing incidence reflectometry. The measured samples are periodic stacks consisting of 12 layers made of one of these materials in combination with SiO2. The ellipsometric and intensity transmission data are fitted simultaneously using the Tauc-Lorentz parameterization for the optical constants of the layers. The results are compared with the thickness of the layers obtained from X-ray grazing incidence reflectometry. The comparison of the predicted and measured optical response of a 3 material multilayer stack demonstrates the accuracy of the extracted optical constants.
Author(s): Toufik Boudiar; Beatrice Payet-Gervy; Marie-Francoise Blanc-Mignon; Jean-Jacques Rousseau; Martine Le Berre; H. Joisten; Bruno Canut
Thin films of Yttrium Iron Garnet (YIG) are grown by radio frequency magnetron non reactive sputtering system. Thin films are crystallised by heat-treatment to obtain magneto-optical properties. On quartz substrate, the network of cracks observed on the annealed samples can be explained by the difference between the thermal expansion coefficient of substrate and YIG.
The Faraday rotation of thin films is measured with a classical ellipsometric system based in transmission which allows us to obtained an accuracy of 0.01°. We studied the variation of Faraday rotation versus the applied magnetic field. The variation of the Faraday rotation is the same that this obtained by VSM (vibrating sample magnetometer) analysis.
With a quartz substrate, maximum Faraday rotation is observed to be 1900°/cm at the wavelength of 594 nm for an annealing of 740°C. The variation of the Faraday rotation versus the wavelength is studied. The obtained values are
comparable to those of the literature for the bulk material.
In order to eliminate the stress due to the heat-treatment, we made some films on Gadolinium Gallium Garnet (GGG) which thermal expansion coefficient is near than the YIG one. The material crystallises with no cracks and the Faraday effect is equivalent.
The artificial reproduction of some coloration effects (for instance nacre aspect, iridescence) which appear spontaneously in the nature needs a right description of the different mechanisms which are involved in these phenomena and especially, a detailed analysis of the spectral behavior of the light scattered by such surfaces. With this aim in sight, we have developed a dedicated set-up for the recording of the reflectance distribution function of solid samples in the whole visible spectral range. We have also analyzed the different methods which are able to describe the color information for
scattered light. First experimental results obtained on some organic glass windows are given in conclusion.
Sol-gel ZrO2 thin films deposited by dip-coating on various substrates are characterized. The film thickness, the refractive index and the thermo-optic coefficient (δn/δT) are measured using the prism coupler technique. The thickness of the uniform and good quality sol-gel films ranges between 100nm and 130nm, while the refractive index ranges between 1.9 and 2.1 at 25°C, depending on the substrate material and surface quality, and depending on some process parameters. The δn/δT measurements of the sol-gel ZrO2 thin films deposited on several substrates show that the TM refractive index always exhibits much higher dependence with respect to temperature than the TE refractive index. Such variations had not previously been reported and will require much attention and future study due to the importance of δn/δT for advanced optical telecommunications devices.
Sol-gel route has been successfully achieved to obtain pure Y3Al5O12 and Y3Al5O12:Eu3+ powders and films. Advanced YAG coatings were achieved by two techniques, the spray and dip-coating methods, giving rise to different characteristics and properties. The materials structure and microstructure were analyzed by means of X-ray diffraction and scanning electron microscopy. It has been shown that sol-gel processed YAG powders crystallize around 900°C whereas a temperature much higher is necessary to obtain this compound by solid state synthesis (≈ 1500°C). Additionally the surface of the films was observed by atomic force microscopy. Eventually, laser induced luminescence spectra, as well as luminescence decays of Eu3+ ions show undoubtedly the spectral features of a Eu3+ emitting centre embedded in a unique site of D2 symmetry.
Lithium niobate (LiNbO3) thin films for optical waveguide purpose have been grown on (0001) sapphire substrates using the pulsed laser deposition technique at the wavelength of 355 nm. To improve surface smoothness of the films, an off-axis deposition geometry with substrates parallel to the plasma plume has been used. Optical properties have been studied based on excitation of guided modes in the film with a prism coupler. M-lines observations associated to extraction of guided light in the films have shown that planar LiNbO3 optical waveguide have been elaborated. The
propagation losses at 633 nm have been estimated by measurement of light streak intensity along the propagation direction with a CCD camera. Values of 4.8 and 5.8 dB cm-1 have been attained for TM0 and TE0 guided modes, respectively. The light scattering is mainly due to the presence of residual droplets on the film surface.
The development of tunable filters is a key point for the ability of optical telecommunication networks to comply with the continuous evolution of the service providers needs. Among the numerous technologies which can be used to achieve narrow bandpass filtering devices, we concentrate our analysis on Thin-Film Interference Coatings and compare the principles, performances and use constraints of the various schemes allowing to tune such a thin-film structure : tilt of a standard bandpass filter, translation of a linear variable filter, deformation of the substrate of a DWDM filter, implementation of active spacers with controllable optical thickness (thin-films or solid spaced etalons).
A key role of the design total optical thickness in the synthesis process is demonstrated by practical examples and confirmed by rigorous theoretical results. A special development of the needle optimization procedure called the synthesis by gradual evolution is proposed. This synthesis process does not require any starting design. It automatically constructs a set of designs with various combinations of three major design parameters: merit function value, number of design layers, design total optical thickness. This provides the optical coating engineer with additional opportunities in choosing the most practical design.
The performance of the high temperature resistant polymers Pleximid, APEC and Ultrason as substrate materials in plasma-assisted physical vapor deposition processes was studied and compared with well-known thermoplastics for optical applications. Different effects of UV irradiation and plasma exposure on the polymers' optical features, surface energy and adhesion properties for oxide layers, typically used for interference multilayer coatings, are shown.
Improving materials and devices reliability is a major concern to the spatial industry.
Results are reported for satellite mirrors-like specimens consisting in oxide-protected metal systems. Optical coatings were deposited by electron beam evaporation.
Mechanical stress fields in multi-layered materials play an important role. The stress state can have far-reaching implications both in kinetics and thermodynamics. Therefore an integrated apparatus with four-point bending equipment was designed. The technique allowed us to exert stress into a film or a system of films on a substrate concurrently with thermal treatment.
In order to achieve the first tests performed with the help of the apparatus, various preliminary characterizations were required. The article reports the preliminary micro-mechanical testing of the materials (ultra micro-indentation to evaluate the elastic modulus of the samples materials and wafer curvature technique to determine the specimen residual stress) and the first ageing experiment. Experimental evidence of accelerated ageing under stress is successfully reported.
Metal-dielectric coatings are able to give high rejection in a wide wavelength range due to the reflection effect of metal layers. At the same time it is possible, according to the theory of induced transmission filters, to reach a quite high transmission peak at a single wavelength, starting from a metal layer and adding appropriate dielectric matching layers. Following this theoretical approach, narrow band-pass (10nm) filters having a peak transmission of 70% and a rejection out-band with extreme wavelengths in the ratio 2.5:1, are designed for the visible-near infrared spectrum. The low number of layers makes them interesting when a spatial variable filter has to be realized in a small dimension. Examples of their optical behavior at different wavelengths are reported.
Direct detection and characterization of Earth-like planets orbiting nearby stars are possible with nulling interferometry and coronagraphy in the thermal infrared. With these techniques, the star is extinguished by interference, with a ratio greater than 105 to allow planet detection. Generally, these techniques require a π phase shifter. In this paper, we investigate how thin film coatings can be used to design π achromatic phase shifters. The design, which provides a π phase shift upon reflection, is basically formed with a high reflectance mirror and an antireflection structure that can be designed without any constraint on their own phase properties. The principle of the design is detailed and numerical examples illustrates this concept.
A theoretical approach to design multicycle AR coatings with predetermined residual reflectance, bandwidth, and cycle number is presented. The approach uses a novel step-index concept involving symmetric or asymmetric quarter-wave layer sequences to substitute refractive indices which are less than a given low refractive index. These substitutions result in a chosen step-down index profile matching the refractive index of the substrate to air. Each step of the index profile is the origin of a cycle of the synthesized AR design. Multicycle AR designs for the visible region with bandwidths of 1.6, 2.0, and 3.0 are presented as examples.
The aim of the study was to investigate spatial distribution of electric field in multilayer mirrors, formed by both homogeneous and inhomogeneous layers. Spatial distribution of amplitude and intensity of electric field in different systems of practical importance was investigated. The idea of using inhomogeneous layers to decrease negative effects of high amplitude electric fields on the boundaries between the layers is considered. The results of numeric modeling of properties of such systems are presented.
Narrow bandpass optical interference filters that precisely meet arbitrary Chebychev (equiripple) or maximally flat specifications are designed by refractive index refinement of standard multicavity filters, combined with straightforward manipulations of the solutions. The filters are composed mostly of quarter-wave (QW) layers of two materials. A few non-QW tuning layers are introduced in regions of the multilayer stacks that have a low sensitivity to deposition errors in order to control the passband ripple and halfwidth. Multiple solutions with exactly the same passbands are easily generated, from which those that have the best chance of being successfully fabricated can be selected.
We report in this manuscript the study of solid-spaced Fabry-Perot filters. The use of high quality wafers as thick spacers and broadband dielectric mirrors with only few layers provides filters which have almost the same specifications as classical WDM interference filters. Multiple cavity filters, composed of single cavities of equal or different thick spacers are easy to manufacture and exhibit very low absorption and scattering losses. Experimental
results concerning simple and double cavity filters with thick spacers centered at 1.56 μm with a maximum transmission more than 98 % and a full-width at half-maximum (FWHM) of about 0.5 μm are exposed. We then propose different solutions for the extension to triple cavity filters with improved spectral properties.
A decade ago we introduced empirically derived formulas which allowed one to estimate the performance which can be achieved in the design of broad-band antireflection (AR) coatings based upon the indices of refraction of the materials to used, the bandwidth required, and the overall thickness of the coating. This has proved to be a useful tool to avoid attempting impossible designs and to guide the designer toward an optical result. It can also be helpful to non-designers who need to know what can and cannot be done before specifying a system AR coating requirement. In the new work reported here, forumlas with additional accuracy have been developed by further data generation and the application of modern statistical analysis tools. The overall thickness parameter used in the equations has also been better defined and understood, and the tendency of overall thicknesses to have quantization has been studied further. These findings are discussed in conventional thin film design terms and also from the Fourier synthesis/analysis viewpoint.
The final corrections which might be made in the last two antireflection (AR) layers in the deposition of narrow bandpass filter designs such as might be used for Dense Wavelength Division Multiplexing (DWDM) in the fiber optics communications field were discussed in a previous report. A broader range of techniques and simulations of those final layer adjustments are described here, how they can be done, and the benefits which might be obtained. A surprisingly simple new technique is given which should yield improved results.
We present algorithms that can be used for the on-line characterization and reoptimization of optical coatings in various production environments, even in situations when coating layers are not bulk-like and their parameters may change after the exposition to air. The questions of accuracy and practical implementation of these
algorithms are discussed.
Author(s): Matthias List; Christian Melde; Johannes Struempfel; Claus Illgen
Reactive magnetron sputtering using pulsed technique in combination with dual magnetron design has become a relevant factor for the production of optical coatings. This pulsed reactive magnetron sputtering process enables the deposition of highly insulating layers, like Al2O3, SiO2, Nb2O5, with high deposition rates, sophisticated optical properties and excellent homogeneity on large substrates.
In this paper a Vertical Inline Sputtering System (VISS) for optical coatings is presented. The goal for this new type of production inline coater is a fully automated thickness control for high and low index materials. This shortened the time from design to a coated product.
Fundamental for this approach is the knowledge of the behaviour of reactive magnetron sputtering, the relationship between process parameters and optical layer properties. Due to the influence of process parameters (magnetron power, argon pressure, set point of plasma emission) on layer properties (refractive index, absorption, stress, etc.) an adjustment of layer properties is necessary. Typical correlations for the deposition of SiO2 as a low index material and Nb2O5 as a high index material will be presented. Beside the knowledge about reactive magnetron sputtering a precondition to come to a fully automated production is a reproducible and accurate optical measurement. The set up of optical measurement and the steps to come to a high accuracy measurement are described. With the presentation of different examples of optical coatings the versatility of this production coater will be shown.
The performance of many solid-state devices including emissive displays, optical sensors, integrated optical circuits, and light-emitting diodes can be improved by applying a transparent high refractive index coating (≥ 1.65) onto the light-emitting or light-sensing portion of the device. Ideally, the coating should combine the excellent durability and easy deposition of a spin-applied polymer coating with the high refractive index and optical clarity of a vacuum deposited metal oxide coating such as titanium dioxide or zirconium oxide. While some success has been achieved in combining these very dissimilar materials to form transparent hybrid coating systems, for example, using sol-gel or nanoparticle dispersion techniques, the resulting coating systems often require complicated manufacturing schemes and have limited storage stability and reliability.
We have demonstrated two new approaches to development of high refractive index polymer coatings. In the first approach, an organometallic polymer and a conventional organic polymer are combined to form a compatible coating. When cured at elevated temperatures, the organometallic polymer decomposes to form a highly dispersed metal oxide phase that imparts high index properties to the final hybrid coating. The new coatings are transparent and have
refractive indices ranging from 1.6 to as high as 1.9 depending on the metal oxide content.
The second approach utilizes our discovery that polyimide materials possess naturally high refractive indices in comparison to most polymer materials. Through careful molecular design, we have developed a new class of polyimide materials having refractive indices ranging from 1.60 to 1.78 at visible wavelengths and exhibiting excellent optical clarity. The new polyimides can be spin-applied to a layer thickness of more than 10 microns in a single coating step and form thermally stable films with good mechanical strength and adhesion to device substrates.
The Four Quadrant Phase Mask is a key component for the design of advanced coronagraphs that may be used to search exo-planets. The validity of this concept has been demonstrated in the visible and need now to be demonstrated in the mid infrared. For this purpose, two components are manufactured for wavelengths 4.75 and 16.25 μm. This manufacturing requires the deposition of ZnSe layers using Ion Assisted Deposition, followed by a lift off process.
Luminescent films of Pr3+-doped CaTiO3 have been prepared by dip-coating method from stabilized sols. The structure, the surface and the microstructure of these films have been characterized by X-ray diffraction, atomic force microscopy and electron microscopy, respectively. The luminescence properties of the films (emission and excitation profiles, fluorescence dynamics) have been evaluated and compared to the properties of powder samples. The films exhibit the typical single red photoluminescence of CaTiO3 :Pr3+ after a thermal treatment of one hour at 500 °C and the intensity of the emission increases almost quadratically with the temperature of preparation up to 1000°C.
The feasibility of micrometer scale morphologically controlled 1D stripe arrays by selective hydride vapour phase epitaxy (HVPE) single step was assessed. HVPE is a near-equilibrium growth process which offers perfect selectivity whatever the pattern design thus giving rise to a great flexibility. The HVPE growth being mainly governed by the surface kinetics intrinsic anisotropy of the crystal, we have demonstrated that various growth morphologies could be stabilised at a mesoscopic scale by controlling the hierarchy of the growth rates of the low index faces of III-V crystals via the growth temperature and the composition of the vapour phase. Micrometer scale dielectric periodic structures constituted of 1μm wide GaAs beams alternately stacked with air were then grown by selective HVPE on GaAs substrates. Potential of the HVPE growth technic for the making of submicrometer scale structures is finally discussed.
A technique for bonding semiconductor optics is described. A thin film of chalcogenide glass is sputtered onto each surface to be bonded. The sputtered films are then placed in close contact and heated at low temperature under pressure to cause them to fuse. With careful choice of materials the resulting interface is virtually invisible. The technique has been demonstrated with gallium arsenide plates. A quaternary chalcogenide glass has been developed with a refractive index within 3% of that of gallium arsenide (3.34 @ 2.07μm). The glass sputters with no change in composition onto the surfaces of the plates to be bonded. Heat treatment at less than 200°C results in an interface with an optical absorption of less than 0.1% measured using a laser calorimeter operating at 2.07μm. The absorption of the structure was similar to that of an equivalent single piece of gallium arsenide.
This paper reports on the development of dielectric and amorphous silicon-based thin film stacks for applications at room and liquid nitrogen (LN) cryogenic temperatures. The deposition process was performed using assisted energetic evaporation processes such as Reactive Low Voltage Ion Plating (RLVIP) and Ion Assisted Deposition (IAD). Dielectric coatings produced by RLVIP exhibited fairly good stability at LN temperature. Indeed, as compared to room
temperature, a spectral shift of ∇λ0.5 = -2.9 nm at half maximum was obtained for a 3-cavity bandpass filter centered at 1570 nm. Amorphous silicon and silicon dioxide stacks deposited by IAD processes were tested for optical applications. Amorphous silicon thin films showed good mechanical stability at LN temperature. The influence of deposition parameters and operating temperature on the electrical, mechanical and optical properties of amorphous Si
films was investigated.
Author(s): F. Beauville; D. Buskulic; R. Flaminio; F. Marion; A. Masserot; L. Massonnet; B. Mours; F. Moreau; J. Ramonet; E. Tournefier; D. Verkindt; O. Veziant; M. Yvert; Regis Barille; Vincenzino Dattilo; Daniel Enard; Franco Frasconi; Alberto Gennai; Paolo La Penna; Magali Loupias; Federico Paoletti; L. Bracci; Giovanni Calamai; E. Campagna; G. Conforto; E. Cuoco; I. Fiori; G. Guidi; G. Losurdo; Fabrizio Martelli; Massimo Mazzoni; B. Perniola; Ruagero N. Stanga; Flavio Vetrano; A. Vicere; D. Babusci; Gianfranco Giordano; Jean-Marie Mackowski; Nazario Morgado; Laurent Pinard; Alban Remillieux; Fausto Acernese; Fabrizio Barone; Enrico Calloni; Rosario De Rosa; Luciano Di Fiore; Antonio Eleuteri; Leopoldo Milano; Ketevan Qipiani; Iolanda Ricciardi; G. Russo; Salvatore Solimeno; M. Varvella; Francois Bondu; Alain Brillet; Eric Chassande-Mottin; Frederic Cleva; T. Cokelaer; J.-P. Coulon; B. Dujardin; J.-D. Fournier; H. Heitmann; Catherine N. Man; F. Mornet; J. Pacheco; A. Pai; Herve Trinquet; J.-Y. Vinet; N. Arnaud; M. Barsuglia; M. A. Bizouard; V. Brisson; F. Cavalier; M. Davier; P. Hello; P. Heusse; S. Kreckelberg; A. Claude Boccara; Vincent Loriette; J. Moreau; V. Reita; P. Amico; L. Bosi; Luca Gammaitoni; M. Punturo; F. Travasso; H. Vocca; L. Barsotti; S. Braccini; C. Bradaschia; G. Cella; C. Corda; A. Di Virgilio; I. Ferrante; F. Fidecaro; Adalberto Giazotto; E. Majorana; L. Holloway; R. Passaquieti; D. Passuello; R. Poggiani; A. Toncelli; M. Tonelli; L. Brocco; S. Frasca; C. Palomba; P. Puppo; P. Rapagnani; F. Ricci
The goal of the VIRGO program is to build a giant Michelson type interferometer (3 kilometer long arms) to detect gravitational waves. Large optical components (350 mm in diameter), having extremely low loss at 1064 nm, are needed. Today, the Ion beam Sputtering is the only deposition technique able to produce optical components with such performances.
Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm in diameter. The performances of this coater are described in term of layer uniformity on large scale and optical losses (absorption and scattering characterization).
The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and its installation is in progress on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the R.M.S. wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performance, especially the absorption level (lower than 0.1 ppm) and the mechanical quality factor Q of the mirrors (thermal noise reduction).
Plasma- or ion-assisted coating processes represent the state of the art for the production of high quality interference coatings. To meet the special filter demands of the telecom industry, special types of coating equipment have been developed with outstanding capabilities for producing complex layer systems. However, their limited productivity makes them unsuitable for cost-effective filter production on traditional sized substrates. Therefore, the challenge for future coating equipment is to achieve the same order of accuracy in process control on large substrate areas together with short production cycles and superb layer properties. The existing plasma and ion sources are one limitation for achieving short production cycles. It is their limited performance that restricts the obtainable coating rate for dense, shift-free interference coatings. The direct optical monitoring is, together with the applied coating technology, the key for the production of complex layer systems. This has been shown in many applications, but is still a challenge on large scale.
Optical inhomogeneous gradient films have been produced by reactive pulse magnetron sputtering of a silicon target in a variable mixture of the reactive gases oxygen and nitrogen. The presented coating system allows the automatic deposition of the gradient films with predefined, freely varying refractive index at a deposition rate of 1 nm/sec uniformly across 8” diameter.
This system has been used for deposition of antireflective coatings, rugate filters, edge filters and dichroic filters composed of SixOyNz films with both gradient and sharp transitions between oxide and nitride. The deposited antireflective coatings consist of one gradient and one homogeneous layer and have a reflectivity of less than 0.5% in the wavelength range between 440 and 620 nm. Thermal substrate load during deposition of these coating is considerably lower than for deposition of conventional multilayer of silicon and titanium oxide. An example of a deposited simple narrow band rugate filter has 45 cycles of sinusoidal refractive index modulations between 1.67 and 1.95, resulting in a reflectivity of > 99.9% at 550 nm with a half-power bandwidth of 42.5nm and an average transmission of 95% in the remaining visual spectral range. The produced edge and dichroic filters consist of quarter-wave oxide and nitride layers. All films have been deposited at one sputtering station without interruption of the plasma thus guaranteeing a very stable, reproducible and highly efficient process.
In order to improve their mechanic and climatic properties, we studied the manufacturing of metal-dielectric light absorbers using an Ion Plating Process. The choice of the materials, Hf for the metal and SiO2 for the dielectric, allows us of to use the deposition plant for either metal dielectric or all dielectric HfO2-SiO2 coatings without any change.
After an index characterization of metallic films, we manufactured monochromatic and broad-band coatings. These coatings have been qualified for space environment.
As a consequence of the ever increasing application field of modern optical technologies, new demands for the optimization of deposition processes for high quality optical coatings with increased environmental stability and power handling capability are imposed on thin film manufacturers. Starting from this challenge, the presented work is focused on the development of an ion assisted deposition (IAD) process using a cold cathode ion source. Especially in the mid
infrared wavelength region (MIR) with its water absorption bands, the ion assisted deposition process leads to many practical advantages, e.g. for medical laser applications.
In the present study, a cold cathode ion source was operated with pure oxygen for the deposition of different oxide materials. Besides the determination of the optical properties, the characterization of the thin films included the first application of an in situ optical broadband monitoring system during the IAD process.
The produced single layers and MIR coatings are thermally stable, shift-free, and exhibit lower absorption compared to conventionally deposited coatings. In contrast to the conventional coatings, also no vacuum-to-air shift is observed for the realized MIR coatings. Therefore, the stable and reproducible IAD process in combination with the new process control strategies using the broadband transmittance measurements on the moving substrates allows an advanced process control and a precise determination of the layer thickness.
Author(s): Karin Scherer; Pascale Lacan; Aline Brunet-Bruneau; Herve Cado; Gerhard J. Keller; Richard Bosmans; Josette Rivory
Recently we reported on low refractive index fluorinated silica (SiOF) films obtained by a Molecular Ion Beam Assisted Deposition (MIBAD) process, using a fluorocarbon precursor in an End Hall ion source during evaporation of silica grains. These films were unstable due to moisture absorption and a subsequent chemical reaction, leading to an irreversible change in composition and increase of refractive index.
In this paper we investigate an inert gas enhanced MIBAD process, involving the simultaneous use of a fluorocarbon precursor and an inert gas in the ion gun. Ion beam composition was analyzed by energy selective ion mass spectrometry. Addition of inert gas to the fluorocarbon precursor not only increases the total ion current, but also enhances the dissociation of the fluorocarbon precursor into smaller fragments.
The films were characterized by ellipsometry, IR transmission spectroscopy and in situ and ex situ stress measurements.
Time evolution of film properties was followed for several weeks. For a given fluorocarbon precursor flow rate, refractive index measured directly after deposition increases with increasing inert gas flow rate, while refractive index change with time is reduced. Fluorine content in the films is maximum for low inert gas flow rates, but decreases for higher flow rates to reach approximately the same level as for pure fluorocarbon ion beam. The observed refractive index
stabilization for high inert gas flow rates can be explained by a change in film structure rather than film composition, i.e.
by reduced film porosity due to high inert gas ion bombardment.
With carefully chosen deposition conditions, stable films with low refractive index can be obtained.
This paper is a work-in progress report on the development of sol-gel coatings for high power laser systems in the near-UV, infrared region. Silica, titania and titania-silica acid catalysed sols were prepared by using tetraethoxysilane and titanium isoproxide as precursors. Single and multi-layer coatings were generated by dipping on fused silica substrates. The single films were heated at 500°C and 900°C after deposition in order to investigate the role of the sintering temperature either on the optical properties and on the film laser-induced damage threshold at 1064 nm (Nd:YAG c.w. laser) and 351 nm (XeF excimer laser). The ageing effects due to the exposure to humidity was investigated by testing the damp heat resistance of the coatings in agreement with the ISO environmental test for optical coatings. The silica coatings have been assessed before and after the damp heat test with regard to their laser-induced-damage resistance, reflectance and transmittance properties. The optical parameters (refractive index and extinction coefficient) have been determined by UV-VIS-NIR spectrometry. A global fit procedure based on the simultaneous characterisation of several samples was used for the evaluation of the optical properties of the materials both as single films and inside multi-layer stacks.
During the last few years, refractive index profiles are being studied more intensively. Several papers have been written about the use of optical methods, spectrophotometry, ellipsometry, together with reverse engineering, these have yielded interesting results. Here we study the differences of a ZrO2 film grown on an amorphous substrate and that of a pre-evaporated layer of another material, Y2O3. In the first run, two glass substrates have been coated with an Y2O3 layer. In the second run a bare glass substrate and the pre-coated Y2O3 have been coated with a ZrO2 layer. Each of the materials used had exactly the same growth conditions for all layers. The only difference was the nature of the substrates. The spectra of R and T of both samples have been used for the optical characterisation. Multisample analysis with gradual introduction of new parameters has been carried out. It was found that for the appropriate modelling of the layer, grown on the pre-coated substrate, introduction of an interface layer between the two materials was necessary. The refractive index profiles of both ZrO2 layers have been determined and discussed.
The transport of light through the different materials media is a rich and fascinating topic of research. The problems of the light transmission theory and the Law of Light Transmittance were presented in . This theory is universal for all electromagnetic ranges. The Ewald-Oseen extinction theorem corresponds to the Law of Light Transmittance, which was shown. Here we discuss several aspects of light transport omitted in article  as wave packets, scattering, a local periodicity, and electromagnetically induced transparency.
Due to high conductivity, silver coating is often used as high reflectivity mirror in optical and in microwave spectrum area. The thickness of silver film in optical application is usually less than the one used in microwave range. Thin silver coating has been successfully deposited by thermal evaporation. Meanwhile thick silver film of tens micrometer is often made by electrochemical process.
DC magnetron is well known as good method for depositing metallic coating. Here we present an attempt to apply DC magnetron sputtering for depositing thick silver mirror. Results show that coating made by this method meet well the requirements in microwave range.
Author(s): Gloria Perez; Ines Genova; Concepcion G. Alvarado; Jose Maria Sanz
High performance narrow bandpass filters for the mid-infrared range of the spectrum have been designed, prepared and characterized. The filters must assure an optimal rejection with a mean transmission lower than 0.02%T and a maximum transmission lower than 0.1%T out of the pass band from 1 to 6 microns. The pass band must be centered in 4.52±0.04 microns, have a full width at half maximum of 0.09±0.01 microns and a maximum transmittance higher than 75% when measured at normal incidence. A 75-layer theoretical design meeting all the requirements is obtained by computer optimization using experimental values for the optical constants of the layers; SiO and Ge are the low and high index materials respectively. A tolerance analysis of the design indicates that the rejection band performance is very stable, but the pass band is very sensitive to thickness deviations in a group of seven layers, defined as the critical part of the coating. The filters are prepared by evaporation in high vacuum; the deposition rate and final thickness of each layer are controlled by a quartz-crystal monitor. Filters with the required performance are obtained applying a method based on the previous deposition and optical characterisation of the critical part of the coating. Finally, the parameters of the transmittance measurements have been optimized to assure the necessary accuracy of the spectra, including the blocking of the pass band to reliably define the rejection parameters.
In the context of high power laser applications like the French "Laser Megajoule" or the "National Ignition Facility" in the United States, laser-induced-damage in fused silica appears to be a limitation. Although it has been established that nano-sized defects are suspected to be responsible of the damage initiation in optical
components, the induced-breakdown process is still unknown. The specific apparatus developed at the Institut Fresnel permits to study in real time the laser interaction on an isolated absorbing defect (around 100 nm): Indeed the coupling of a nanosecond focused laser (beam diameter about 10μm) and a high resolution photothermal
microscope (beam diameter 1μm) in a unique facility permits to highlight the evolution of the local absorption of the defect versus irradiation energy density. Furthermore, the morphology of this modification is followed thanks to a "refraction mapping" (photothermal deflection measured at frequency 0). An in-situ
dark field microscope and an ex-situ AFM complete the observations. In order to simulate the presence of the nano-defects, engineered sub-micronic gold particles (100-600 nm) were embedded in silica samples. The whole observations contribute in the understanding of the different stages of the damage initiation and more particularly, a determination of a "pre-damage" threshold can be performed.
YAG optical planar waveguide was elaborated through sol-gel method. Acetate-alkoxide (YAG-I) and 2-alkoxides (YAG-II) methods were used for the sol preparations. The as-deposited layers were characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), m-lines, spectroscopy (MLS) Rutherford Backscattering Spectroscopy (RBS) and waveguide Raman spectroscopy (WRS). No intermediate phases were observed for both synthesis routes. In comparison, (YAG-II) layers presented a better adherence than YAG-I to SiO2 substrate, a higher refractive index and allowed to form a pure phase of YAG at lower temperature and short heat duration (900°C for 10min). However, the interdiffusion of Al and Si ions were detected to influence the refractive index improvement by heat treatments. The propagation loss of the fabricated YAG thin films in amorphous phase can be as low as 1.5±0.3 dB/cm.
The sol gel process is a rather simple route for the elaboration of pure or doped well known oxides such as TiO2, ZrO2, ... or less usual oxides such as Lu2O3, Gd2O3... in thin film form. A proof of the high optical quality of the layers is their ability to propagate light even if their application is not in the waveguide field. Moreover waveguiding properties of the layers allow their studies in waveguiding configuration. Among these methods, the most attractive for the sol gel films is the very low frequency waveguide Raman which provides the nanocrystals sizes and phases according to the annealing temperatures as illustrated by ZrO2 sol gel layers studies. For such measurements the thin films must be very carefully elaborated as described in this presentation. As example of application, Gd2O3 sol gel scintillating films are presented, when studied in waveguiding configuration and used in transmission configuration. The sol elaboration is described. The thickness and the refractive index of the films are analysed as a function of the annealing treatment. Results of the europium doped gadolinium oxide layer under X rays excitation are presented which underlined the attention paid on all the steps of the elaboration procedure.
A photothermal facility implemented for optical characterization at 244nm is presented. This apparatus allows simultaneous absorption, partial scattering and fluorescence mappings of substrates and thin films, using a collinear photothermal deflection technique. A CW 100mW - 244 nm pump laser beam is used, focused with 20 to 5 microns diameter. The technical data, calibration procedure as well as performances in terms of spatial resolution, detectivity and sensitivity are described in this paper. Then we give an application example with the case of an extrinsic absorbing/scattering/fluorescent defect on fused silica.
Prism and grating coupling techniques can be used to retrieve the refractive index and thickness of a thin film or a stack of layers from the measurement of the effective index of the guided modes they propagate. These techniques are discussed as possible means to assess the wafer scale index and thickness uniformity in the prespective of the batch manufacturing of resonant gratings. The application example considered is a grating polarizer used as a microlaser polarizing mirror.
An analytical derivation is provided giving the normalized waveguide thickness at which a corrugation grating defined at the surface of a graded index slab waveguide has a maximum radiation coefficient for TE and TM modes. This condition is also that for a maximum sensing sensitivity for biochemical species placed on top of the slab waveguide.
The optical behavior of multilayer coatings for the VUV region is determined by dispersion and absorption of the used materials, non-ideal interfaces between layers and thickness of each component layer. Therefore, an accurate characterization of a multilayer stack encounters the inherent difficulties related to dealing with a high number of defining parameters. The use of on-line data (data acquired during the deposition process) allows a more precise characterization of the coatings, in comparison with the use of standard spectrophotometric measurements obtained after the deposition of the layers. Here we present the results of the characterization of a high reflecting coating where the component materials are fluorides (LaF3 and MgF2). Several models with an increasing degree of complexity and assuming different multilayer growing concepts have been applied for the description of the analyzed sample.
The observation of hot plasmas in the interstellar medium requests efficient mirrors in the 80-120 nm wavelength range. Contrary to that of most metals, the high reflectivity of pure aluminum is maintained close to 80% in this range. Unfortunately, it is drastically reduced to values lower than 10% by the strongly absorbing thin alumina layer which spontaneously forms itself upon air contact.
Usually the deposition of thin fluorides films on aluminum (MgF2 or LiF) is used to prevent the alumina formation and consequently extend the high reflectivity range from visible towards shorter wavelengths. But this approach works only from the alumina to the fluoride bandgap (λ ≈ 100 nm). Other materials were studied by Larruquert who reported a measured reflectivity as high as 34% at l = 90 nm by using a Al/MgF2/SiC thin films stack.
The aim of the investigation reported here is to define and test different and original aluminum based thin films stacks which optimize the mirror reflectivity in the 80-120 nm range. We present the results of our simulations from a large number of materials and the first experimental tests of the predicted best solutions.
Since first publications on optical broadband monitoring 25 years ago, the technological progress in computational speed and hardware made it possible to expand this basic approach to a versatile tool for thin film thickness control. During the last few years, essential progress could be achieved for optical monitoring systems operating in the visible and near infrared spectral range. In conjunction with powerful computer algorithms these systems allow a precise determination of the thickness of a growing layer during deposition and the reliable production of very complex nonquarterwave interference filters. It was shown earlier that the application of such a monitoring system in an IBS-(ion beam sputtering)-process enables a fully automated rapid prototyping of optical coatings with very complicated designs. Using the ion beam sputter process with its immanent reproducibility of the optical coating properties, even the precise control of layers with a thickness of around a few nanometers is possible. Offering the capability of producing designs containing such delicate thin layers, this process concept opens new prospects for a new class of thin film systems. Recently, the high potentiality of this monitoring concept in applications to ion-assisted coating processes (IAD) could be demonstrated. This paper is concentrated on an examination of error sources, which brought a further improvement in reliability of the monitoring system.
The presented work is embedded in the research network “Integrative Ion Processes for Modern Optics”, called IntIon, consisting of 12 partners from the German optics industry and two research institutes. The main target of the IntIon network is the development of new process concepts on the basis of ion assisted deposition (IAD) for the industrial production of optical thin film components. Besides an improvement in efficiency, a major aim is concentrated on the optical characteristics for selected application fields with high economical potential. In this network, different ion and plasma sources are compared with regard to their qualification for ion assisted deposition processes. This work includes the characterization of the ion energy and ion current using Faraday-cup measurements. The selection of investigated coating materials includes a broad variety of standard and non-standard oxides. First results of the network will be presented for adapted deposition materials and different operation characteristics of ion sources.
Mirrors with a graded reflectance profile have been used for many years in unstable cavities for improving the optical quality of the laser output beams. All the variable reflectivity mirrors are realized with multilayer-coatings containing one or more profiled layers inside the stack. They generally exhibit high reflectance in the central area and very low reflectance in the external area. In particular, phase-unifying (PU) mirrors are graded mirrors properly designed in order to obtain a low wave-front distortion in the transmitted laser beam. In this paper, the laser damage resistance properties of a PU mirror designed for XeF excimer lasers (351 nm) have been studied. The laser-induced damage threshold has been measured by a XeF laser on the high and low reflectivity areas. A correlation between the damage threshold values and the standing wave electric field profile, which settles inside the two coating structures during laser irradiation, has been found.
Arrays of longpass filter coatings for high order suppression in miniature spectrometers were produced by plasma-ion assisted deposition and photolithogaphy. The filter edges were imaged by optical microscopy, scanning electron microscopy, and scanning force microscopy. Whereas a positioning accuracy of about 2 microns was achieved, the width of the filter edges varied between 2 and 10 microns.
A new method is proposed for measurement of the film optical parameters. A p-polarized laser beam falls on the glass surface at θi, and two reflected beams with intensities Ia and Ib from the front and the rear surface of the glass are
detected. The characteristic of the angular modulation of reflectance ratio γ ( γ =Ia/Ib) is closely related to the film optical parameters, namely refractive index nf, extinction coefficient kf and thickness df. By means of measuring the angle distribution γ(θi) and fitting the results with theoretical data, the optical parameters of the thin films can be obtained easily. Experimentally, we measured the exact optical parameters of tetra-neopentoxy phthalocyanine Zinc(TNPPcZn) LB films. In addition, the changes of optical parameters and absorption curves of TNPPcZn LB films in visible range at different annealing temperature were investigated. The results indicate that the changing tendency of the extinction
coefficient of TNPPcZn LB films obtained from these two methods were coincident. When the annealing temperature increased to 150°C, the monomer of TNPPcZn in LB films transformed to aggregate, nf and kf of the films increased. Further, nf and kf decreased as aggregate changed back to monomer again at 300°C.
Author(s): K. C. Mohite; C. Nouveau; S. T. Pawar; B. N. Pawar; S. R. Jadkar; M. G. Takwale
Electron Beam Physical Vapor Deposited (EB-PVD) silicon oxynitride (SiOxNy) films of various compositions between SiO2 and Si3N4 were grown by changing the substrate temperature and deposition time The SiOxNy films were deposited at various temperatures ranging from 100°C to 400°C on single crystal wafer ( single side polish)and soda lime glass substrates. Films were characterized by using XPS, EDS, Photoluminescence, UV-Visible spectroscopy and Ellipsometry.
The ellipsometric measurements shows that the values of refractive indices , n and k are in the range of 1.60 to 1.98 and 0.03 to 0.08 respectively. The reflectivity of 1.64% was observed for the SiOxNy films deposited at T=350°C ant t=1.5 min. The XPS and EDS analysis shows the incorporation of nitrogen in the films increases with increase in substrate temperature and deposition time. The incorporation of nitrogen in the films is further confirmed by photoluminescence spectra. The photoluminescence spectroscopy measurements were done at room temperature. The energy of the PL peak for 2.54eV (~490 nm) excitation is 2.30 eV (~560 nm). From the above results we feel that the EBPVD deposited SiOxNy films has tremendous potential in antireflective applications.
Thin film filters used in fibre optics communications are strictly defined, which requires a totally uniform thickness distribution over the coated substrate. In this paper we firstly analyse the thickness distribution related to chamber geometry and source emission function. Chamber layout and source function are optimised for large area uniformity.
The improvement in production process of high quality optics needs routinary accurate characterization of the optical properties of the sample. Whereas some commercial instruments allow accurate measurements of the transmitted and reflected light, the measurement of the scattered light very often falls below the detection level when standard attachments, like the Ulbricht integrating sphere, are used. In order to characterize scattering levels below 10-3 dedicated set ups are usually used which are quite complicate to run and often do not fit the need of a production department. In this work we present the design and test of scatterometers based on the Coblentz sphere that fit inside a Perkin-Elmer λ-900 spectrophotometer. They are aimed to the measurement of light scattering in both the transmittion and the reflection hemisphere with a detection limit in the range of 50 ppm. The Coblentz sphere is a mirror like collector of the scattered light and does not suffer of the typical limitations of the Ulbricht sphere in terms of low collecting efficiency and spectral limitation in the UV edge. As an example, LaF3 films on silica samples were first characterized with both conventional photometric techniques (specular reflectance and direct transmittance) and ellipsometry and the film thicknesses and refractive indices, as well as inhomogeneity and interface layers, were determined. The scattering data were also independently analysed by considering the vector theory of scattering and the effect on the simulated spectral TS of several power spectrum densities obtained with different hypothesis on the correlation between interfaces.