Sensitive photothermal deflection technique for measuring absorption in optically thin media

Photothermal spectroscopy on-chip sensor for the measurement of a PMMA film using a silicon nitride micro-ring resonator and an external cavity quantum cascade laser

Laser-based mid-infrared (mid-IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC-QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770-1560 cm-1), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin-coated on top of a silicon nitride (Si3N4) micro-ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe-pump dual beam detection scheme, we exploit a Si3N4 transducer coated with PMMA, as a proof-of-principle for an on-chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid-IR beam on top of the ring's area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid-IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on-line/in-line monitoring in several industrial processes.

Spatial Localization of Defects in Halide Perovskites Using Photothermal Deflection Spectroscopy

Photothermal deflection spectroscopy (PDS) emerges as a highly sensitive noncontact technique for measuring absorption spectra and serves for studying defect states within semiconductor thin films. In our study, we applied PDS to methylammonium lead bromide single crystals. By analyzing the frequency dependence of the PDS spectra and the phase difference of the signal, we can differentiate between surface and bulk deep defect absorption states. This methodology allowed us to investigate the effects of bismuth doping and light-induced degradation. The identified absorption states are attributed to MA+ vibrational states and structural defects, and their influence on the nonradiative recombination probability is discussed. This distinction significantly enhances our capability to characterize and analyze perovskite materials at a deeper level.

Direct measurement of the extinction coefficient by differential transmittance

A new procedure to measure the extinction coefficient k of film materials that are relatively transparent is presented. This procedure does not require the use of an optical-constant model or the knowledge of extra physical properties of the material, such as the specific heat capacity. It involves preparing a sample with two areas, at least one of them coated with the film, whereas the other may remain uncoated or may be coated with a different thickness of the same material. The differential transmittance between the two sample areas is shown to be proportional to k of the film material in the following measurement conditions: the incident light is p polarized and it impinges at the film material Brewster angle. The differential transmittance is obtained with a single measurement by making the light beam or the sample to oscillate with respect to one another and by using a lock-in amplifier; for normalization purposes, the transmittance in one of the sample areas is also measured. The proportionality factor between the normalized differential transmittance and k only involves the wavelength, the film thickness, and the Brewster angle. The knowledge of the film Brewster angle requires that the film refractive index (n) is measured beforehand; this can be performed with standard procedures, such as ellipsometry, since such techniques are efficient at measuring n of a transparent material, but are inefficient at measuring a small k. The procedure is exemplified with the calculation of k in the far ultraviolet of AlF3 films deposited by evaporation. The dependence of the uncertainty of k obtained with this procedure is analyzed in terms of the uncertainty of the film n, of wavelength, and of the degree of polarization of the incident beam. The selection of a substrate with similar n to the film material is also discussed. The uncertainties involved with the present procedure were analyzed for a specific example and an uncertainty of 2 × 10-5 in k calculation is considered feasible.

Current Understanding of Band-Edge Properties of Halide Perovskites: Urbach Tail, Rashba Splitting, and Exciton Binding Energy

The band-edge structure of halide perovskites, derived from the hybridization of atomic orbitals, plays a fundamental role in determining their optical and electronic properties. Several important concepts have been frequently discussed to describe the influence of band-edge structure on their optoelectronic properties, including Urbach tail, Rashba splitting, and exciton binding energy. In this Perspective, we provide a fundamental understanding of these concepts, with the focus on their dependence on composition, structure, or dimensionality. Subsequently, the implications for material optimization and device fabrication are discussed. Furthermore, we highlight the Rashba effect on the exciton fine structure in perovskite nanocrystals (PNCs), which explains the unique emissive properties. Finally, we discuss the potential influence of band-edge properties on the light emission process. We hope that this Perspective can inspire the investigation of band-edge properties of halide perovskites for light-emitting diodes, lasers, and spin electronics.

Life on the Urbach Edge

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

Characterization of Chirality in Diffractive Metasurfaces by Photothermal Deflection Technique

Chirality, a lack of mirror symmetry, is present in nature at all scales; at the nanoscale, it governs the biochemical reactions of many molecules, influencing their pharmacology and toxicity. Chiral substances interact with left and right circularly polarized light differently, but this difference is very minor in natural materials. Specially engineered, nanostructured, periodic materials can enhance the chiro-optical effects if the symmetry in their interactions with circular polarization is broken. In the diffraction range of such metasurfaces, the intensity of diffracted orders depends on the chirality of the input beam. In this work, we combine a photothermal deflection experiment with a novel theoretical framework to reconstruct both the thermal and optical behavior of chiro-optical behavior in diffracted beams.

In situ photothermal deflection spectroscopy revealing intermolecular interactions upon self-assembly of dye monolayers

We demonstrate the potential of photothermal deflection spectroscopy (PDS) to study the self-assembly of dye monolayers in situ. Beyond the determination of adsorption kinetics at specific wavelengths, PDS gains its strength from yielding UV-vis absorptance spectra of SAMs in situ, unaffected by scattering, from which supramolecular interactions can be deduced.

Picowatt calorimeter for optical absorption spectroscopy

An optical picowatt calorimeter at 4 K is demonstrated to measure absorption spectra from 330 nm to 1700 nm of optical samples. The minimum detectable absorbed power is 10 pW, resulting in absorption sensitivities of 0.3 ppm for 30 µW of incident power from a tunable source and 0.6 ppb for 15 mW laser excitation. Active temperature stabilization of main components of the cryogen-free cryostat and the use of paramagnetic temperature sensors with superconducting quantum interference device readout provided a low noise environment (700 nK temperature rms) and nK temperature resolution.

Intensity dependent deflection spectroscopy for absorption measurements

We report on a method for the characterization of optical absorption in semiconductors at photon energies below the bandgap energy. We use intensity dependent deflection spectroscopy to measure spatially resolved the optical absorption and to separate the occurring absorption mechanisms. To this end, we take advantage of the different intensity scaling of these mechanisms and extract the material parameters by fitting the intensity dependent absorption to a physical model. Our method enables a simple but sufficient determination of crucial optical loss properties (e.g. impurity related absorption and two-photon absorption) in various semiconductor systems, e.g. substrates for optical components or solar cells.

Carotenoid Nuclear Reorganization and Interplay of Bright and Dark Excited States

We report quantum chemical calculations using multireference perturbation theory (MRPT) with the density matrix renormalization group (DMRG) plus photothermal deflection spectroscopy measurements to investigate the manifold of carotenoid excited states and establish their energies relative to the bright state (S₂) as a function of nuclear reorganization. We conclude that the primary photophysics and function of carotenoids are determined by interplay of only the bright (S₂) and lowest-energy dark (S₁) states. The lowest-lying dark state, far from being energetically distinguishable from the lowest-lying bright state along the entire excited-state nuclear reorganization pathway, is instead computed to be either the second or first excited state depending on what equilibrium geometry is considered. This result suggests that, rather than there being a dark intermediate excited state bridging a non-negligible energy gap from the lowest-lying dark state to the lowest-lying bright state, there is in fact no appreciable energy gap to bridge following photoexcitation. Instead, excited-state nuclear reorganization constitutes the bridge from S₂ to S₁, in the sense that these two states attain energetic degeneracy along this pathway.

A Review of Photothermal Detection Techniques for Gas Sensing Applications

Photothermal spectroscopy (PTS) is a technique used for determining the composition of liquids, solids and gases. In PTS, the sample is illuminated with a radiation source, and the thermal response of the analyte (e.g., refractive index) is analyzed to gain information about its content. Recent advances in this unique method of detecting gaseous samples show that photothermal gas spectroscopy can be an interesting alternative to commonly used absorption techniques. Moreover, if designed properly, sensors using PTS detection technique can not only reach sensitivities comparable with other, more complex techniques, but can significantly simplify the design of the sensor. In this review, recent developments in photothermal spectroscopy of gases will be summarized and discussed.

Intra- and inter-nanocrystal charge transport in nanocrystal films

The exploitation of semiconductor nanocrystal (NC) films in novel electronic and optoelectronic applications requires a better understanding of charge transport in these systems. Here, we develop a model of charge transport in NC films, based on a generalization of the concept of transport energy level ET to nanocrystal assemblies, which considers both intra- and inter-NC charge transfer processes. We conclude that the role played by each of these processes can be probed from temperature-dependent measurements of charge carrier density n and mobility μ in the same films. The model also enables the determination of the position of the Fermi energy level EF with respect to ET, an important parameter of charge transport in semiconductor materials, from the temperature dependence of n. Moreover, we provide support to an essentially temperature-independent intra-NC charge carrier mobility, considered in the transport level concept, and consequently the frequently observed temperature dependence of the overall mobility μ in NC films results from a temperature variation of the inter-NC charge transport processes. Importantly, we also conclude that the temperature dependence of conductivity in NC films should result in general from a combination of temperature variations of both n and μ. By applying the model to solution-processed Si NC films, we conclude that transport within each NC is similar to that in amorphous Si (a-Si), with charges hopping along band tail states located below the conduction band edge. For Si NCs, we obtain values of ET - EF of ∼0.25 eV. The overall mobility μ in Si NC films is significantly further reduced with respect to that typically found in a-Si due to the additional transport constraints imposed by inter-NC transfer processes inherent to a nanoparticulate film. Our model accounting for inter- and intra-NC charge transport processes provides a simple and more general description of charge transport that can be broadly applied to films of semiconductor NCs.

Configuration optimization of photothermal deflection for measurement sensitivity enhancement

An accurate theoretical model based on thermoelasticity theory and Fresnel diffraction integral is developed to describe the photothermal deflection (PTD) signal with a continuous-wave modulated Gaussian beam excitation. A PTD experiment is performed to investigate the dependence of PTD amplitude on the experimental parameters, such as the radius, waist position, and wavelength of the probe beam, and the detection distance. Good agreement between the experimental and theoretical results is obtained. The results reveal that the optimal detection distance highly depends on the probe beam waist position and wavelength, and the PTD amplitude can be enhanced by optimizing the probe beam radius and waist position as well as the detection distance. Moreover, it is demonstrated experimentally that the PTD amplitude is inversely proportional to the probe beam wavelength by using three probe lasers with a wavelength of 375 nm, 543 nm, and 632.8 nm. Therefore, the measurement sensitivity of PTD technique could be enhanced by using a short-wavelength probe beam.

Absorption measurements in optical coatings by lock-in thermography

We evaluate and apply lock-in thermography as a method to quantitatively evaluate absorption losses of optical coatings. The principle of the method consists of applying periodically modulated laser intensity on the coatings and to monitor the periodic surface temperature evolution with an infrared camera. By application of a lock-in correlation procedure and using calibrated absorption samples, it is possible to obtain quantitative absorption values and to obtain absorption mappings with spatial resolution that depends on the optical configuration. Numerical simulations and experiments were performed in the case of 10-60 W laser irradiation at 1060 nm on different single layer coatings and highly reflective mirrors. In the tested conditions, the measurement of absorption down to 1 ppm level could be reached. The advantages, limitations, and potential applications of the technique are discussed.

Non-interferometric photoacoustic remote sensing microscopy

Elasto-optical refractive index modulation due to photoacoustic initial pressure transients produced significant reflection of a probe beam when the absorbing interface had an appreciable refractive index difference. This effect was harnessed in a new form of non-contact optical resolution photoacoustic microscopy called photoacoustic remote sensing microscopy. A non-interferometric system architecture with a low-coherence probe beam precludes detection of surface oscillations and other phase-modulation phenomenon. The probe beam was confocal with a scanned excitation beam to ensure detection of initial pressure-induced intensity reflections at the subsurface origin where pressures are largest. Phantom studies confirmed signal dependence on optical absorption, index contrast and excitation fluence. In vivo imaging of superficial microvasculature and melanoma tumors was demonstrated with ~2.7±0.5 μm lateral resolution.

Growth, characterization, and thin film transistor application of CH3NH3PbI3 perovskite on polymeric gate dielectric layers

Micron-size organolead perovskite crystals grown on insulating polymeric surfaces as gate dielectric materials for high performance thin film transistors.

Infrared Surface Studies of Opaque or Scattering Materials Using Photothermal Beam Deflection Spectroscopy

Infrared (IR) photothermal beam deflection spectroscopy (PBDS) is briefly described and some of its applications to studies of carbons and highly scattering materials are reviewed. PBDS is especially useful for the study of materials which absorb IR radiation very strongly or act as strong IR scatterers, so that conventional IR techniques fail. Application of PBDS to study the thermal degradation of a phenol-formaldehyde resin, the reaction of NH3 and H2O with the surfaces of intermediate-temperature chars, the effect of Fe3+ on the charring of cellulose, the dehydration of titanyl sulphate, and TiO2 pigments, are described.

Analysis of burn-in photo degradation in low bandgap polymer PTB7 using photothermal deflection spectroscopy

Sub-bandgap defect characterization in PTB7 by photothermal deflection spectroscopy (PDS).

Photothermal speckle modulation for noncontact materials characterization

We have developed a noncontact, photothermal materials characterization method based on visible-light speckle imaging. This technique is applied to remotely measure the infrared absorption spectra of materials and to discriminate materials based on their thermal conductivities. A wavelength-tunable (7.5-8.7 μm), intensity-modulated, quantum cascade pump laser and a continuous-wave 532 nm probe laser illuminate a sample surface such that the two laser spots overlap. Surface absorption of the intensity-modulated pump laser induces a time-varying thermoelastic surface deformation, resulting in a time-varying 532 nm scattering speckle field from the surface. The speckle modulation amplitude, derived from a series of visible camera images, is found to correlate with the amplitude of the surface motion. By tuning the pump laser's wavelength over a molecular absorption feature, the amplitude spectrum of the speckle modulation is found to correlate to the IR absorption spectrum. As an example, we demonstrate this technique for spectroscopic identification of thin polymeric films. Furthermore, by adjusting the rate of modulation of the pump beam and measuring the associated modulation transfer to the visible speckle pattern, information about the thermal time constants of surface and sub-surface features can be revealed. Using this approach, we demonstrate the ability to distinguish between different materials (including metals, semiconductors, and insulators) based on differences in their thermal conductivities.

Sensitivity enhancement of surface thermal lens technique with a short-wavelength probe beam: experiment

Surface thermal lens is a highly sensitive photothermal technique to measure low absorption losses of various solid materials. In such applications, the sensitivity of surface thermal lens is a key parameter for measuring extremely low absorption. In this paper, we experimentally investigated the influence of probe beam wavelength on the sensitivity of surface thermal lens for measuring the low absorptance of optical laser components. Three probe lasers with wavelength 375 nm, 633 nm, and 1570 nm were used, respectively, to detect the surface thermal lens amplitude of a highly reflective coating sample excited by a cw modulated Gaussian beam at 1064 nm. The experimental results showed that the maximum amplitude of surface thermal lens signal obtained at corresponding optimized detection distance was inversely proportional to the wavelength of the probe beam, as predicted by previous theoretical model. The sensitivity of surface thermal lens could, therefore, be improved by detecting surface thermal lens signal with a short-wavelength probe beam.

Two-dimensional carrier distribution in top-gate polymer field-effect transistors: correlation between width of density of localized states and Urbach energy

A general semiconductor-independent two-dimensional character of the carrier distribution in top-gate polymer field-effect transistors is revealed by analysing temperature-dependent transfer characteristics and the sub-bandgap absorption tails of the polymer semiconductors. A correlation between the extracted width of the density of states and the Urbach energy is presented, corroborating the 2D accumulation layer and demonstrating an intricate connection between optical measurements concerning disorder and charge transport in transistors.

Probing losses of dielectric multilayers by means of Bloch surface waves

We exploit the excitation of electromagnetic surface waves on high-quality dielectric multilayers to measure the very low extinction coefficient of the structures, with a resolution down to 4·10(-7) and in a simple optical configuration. The effect of exposition to a rhodamine 6G solution in water and ethanol is also reported, including dye adsorption in the layers and bleaching upon resonant excitation.

Surface heating by optical beams and application to mid-infrared imaging

Heating of surfaces by optical beams is investigated theoretically and compared with experimental results in the context of infrared imaging with vanadium dioxide thin films. Using known solutions for the diffusion of point heat sources at the interface between two semi-infinite media, the theory is extended to beams of Gaussian and flat profiles, for steady-state and dynamic regimes. Parameters relevant to imaging, such as spatial resolution and response time, are linked to thermal diffusivity, beam dimensions, and intensity.

Biomedical photoacoustic imaging

Photoacoustic (PA) imaging, also called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade. It is a hybrid modality, combining the high-contrast and spectroscopic-based specificity of optical imaging with the high spatial resolution of ultrasound imaging. In essence, a PA image can be regarded as an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but its optical properties, specifically optical absorption. As a consequence, it offers greater specificity than conventional ultrasound imaging with the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores, but with greater penetration depth than purely optical imaging modalities that rely on ballistic photons. As well as visualizing anatomical structures such as the microvasculature, it can also provide functional information in the form of blood oxygenation, blood flow and temperature. All of this can be achieved over a wide range of length scales from micrometres to centimetres with scalable spatial resolution. These attributes lend PA imaging to a wide variety of applications in clinical medicine, preclinical research and basic biology for studying cancer, cardiovascular disease, abnormalities of the microcirculation and other conditions. With the emergence of a variety of truly compelling in vivo images obtained by a number of groups around the world in the last 2-3 years, the technique has come of age and the promise of PA imaging is now beginning to be realized. Recent highlights include the demonstration of whole-body small-animal imaging, the first demonstrations of molecular imaging, the introduction of new microscopy modes and the first steps towards clinical breast imaging being taken as well as a myriad of in vivo preclinical imaging studies. In this article, the underlying physical principles of the technique, its practical implementation, and a range of clinical and preclinical applications are reviewed.

Accurate temperature model for absorptance determination of optical components with laser calorimetry

In the international standard (International Organization for Standardization 11551) for measuring the absorptance of optical components (i.e., laser calorimetry), the absorptance is obtained by fitting the temporal behavior of laser irradiation-induced temperature rise to a homogeneous temperature model in which the infinite thermal conductivity of the sample is assumed. In this paper, an accurate temperature model, in which both the finite thermal conductivity and size of the sample are taken into account, is developed to fit the experimental temperature data for a more precise determination of the absorptance. The difference and repeatability of the results fitted with the two theoretical models for the same experimental data are compared. The optimum detection position when the homogeneous model is employed in the data-fitting procedure is also analyzed with the accurate temperature model. The results show that the optimum detection location optimized for a wide thermal conductivity range of 0.2-50W/m·K moves toward the center of the sample as the sample thickness increases and deviates from the center as the radius and irradiation time increase. However, if the detection position is optimized for an individual sample with known sample size and thermal conductivity by applying the accurate temperature model, the influence of the finite thermal conductivity and sample size on the absorptance determination can be fully compensated for by fitting the temperature data recorded at the optimum detection position to the homogeneous temperature model.

Development of photoacoustic spectroscopy with a piezofilm

We have developed photoacoustic spectroscopy with a piezofilm. A piezofilm is a piezoelectric element made from plastic polyvinylidene fluoride having piezoelectrical effect. Photoacoustic spectra (375-675 nm) of water, dye aqueous solution, and benzene, are measured with a xenon lamp. The piezofilm is directly immersed in the liquid samples for sensitive detection of the signal. The sensitivity of the method is shown to be as high as for photothermal deflection spectroscopy. Compared with the conventional methods such as photoacoustic spectroscopy with a piezoceramic and photothermal spectroscopy with a double beam configuration, the present method is favorable from its handy and simpler experimental setup.

Combined laser calorimetry and photothermal technique for absorption measurement of optical coatings

To the best of our knowledge, a combined sensitive technique employing both laser calorimetry and a surface thermal lens scheme for measuring absorption values of optical coatings is presented for the first time. Laser calorimetric and pulsed surface thermal lens signals are simultaneously obtained with a highly reflecting UV coating sample irradiated at 193 nm. The advantages and potential applications of the combined technique and the experimental factors limiting the measurement sensitivity are discussed.

Simultaneous absorption, scattering, and luminescence mappings for the characterization of optical coatings and surfaces

An experimental setup, based on the laser-induced deflection technique, is developed to measure simultaneously the 244 nm laser absorption, scattering, and luminescence on optical components. The different techniques and methods that have been specifically developed to obtain both high resolution (micronic) and sensitivity (a few 10(-7) of the incident power for the absorption, 10(-8) for scattering and fluorescence) are presented. Different applications are then explored: the study of losses in deep UV multilayer coatings (HfO2/SiO2 mirrors) and the analysis of contamination defects on bare substrate and structural defects in optical coatings.

Photothermal deflection in multilayer coatings: modeling and experiment

A model of the photothermal deflection signal in multilayer coatings is presented that takes into account optical interference effects and heat flow within the stack. Measurements are then taken of high-reflectivity HfO2/SiO2 ultraviolet mirrors made by plasma ion assisted deposition and compared to calculations. Good agreement is found between the experimental results and the model. Using this model for the calibration and the setup described, one can measure absorption in multilayer coatings accurately down to 10(-7) of the incident power.

Photothermal characterization and stability analysis of polymeric dye lasers

The millisecond heat dissipation of pump energy in polymeric, solid-state dye lasers has been studied with photothermal deflection spectroscopy (PTDS) to determine the contribution of that process to photodegradation of the active material. The samples were solutions of Rhodamine 6G in 2-hydroxyethyl methacrylate copolymerized with various amounts of methyl methacrylate or ethylene glycol dimethylacrylate to change the microstructure properties of the matrix. Values of the thermal diffusivity measured with PTDS were in the range 0.6-1.1 x 10(-3) cm(2) s(-1) for all the compositions studied here. A comparison of these values with previous optical data on lasing efficiency and photostability for the same samples indicates that the macroscopic rate of thermal diffusion is not the key factor that limits the efficiency and stability of these lasers, at least for low pump repetition rates (<1 Hz).

Measuring the absolute absorptance of optical laser components

The precise determination of the absolute absorptance of a laser component is of high scientific and commercial importance. Our intention is to demonstrate that laser calorimetry can be a reliable and sensitive characterization tool for this purpose. Furthermore, the limitations of laser calorimetry are discussed and suggestions for possible revisions of the ISO 11551 (International Organization for Standardization, Geneva, Switzerland) standard are made. Finally, laser calorimetry is compared with photothermal deflection methods with respect to their practicability in different fields of laser optic characterization.

Solarization of glass substrates during thin-film deposition

We demonstrate that solarization occurs in glass substrates during thin-film deposition and that it induces high absorption near the surface of the substrate. Solarization has been observed especially in ion-plating deposition. We show that the solarization of the substrate is caused by electromagnetic radiation emitted from the material to be evaporated. The radiation is due to the energy losses of the heating beam of electrons (bremsstrahlung radiation). Multicomponent glasses such as BK7 are much more sensitive to solarization than fused-silica substrates. The photoinduced high absorption can be partially reversed by thermal annealing.

Synthesis and physicochemical characterization of silicon oxynitride thin films prepared by rf magnetron sputtering

SiO(x)N(y) thin films deposited by rf magnetron sputtering to realize low-loss optical multilayers have been studied. We have analyzed the variations of the optical and physicochemical properties of oxynitride layers according to the deposition parameters: the gas partial pressures, the rf power, and the target composition. A linear variation of the layer refractive index as a function of the oxygen partial pressure was observed as well as a strict substitution of O atoms by N atoms. Thanks to IR spectrophotometric analyses, a model of the oxynitride amorphous structure was proposed and confirmed by Bruggeman and Gained approximation methods. Finally, the absorption level of the oxynitride layers was studied by photothermal deflection spectroscopy.

Development of low-loss sapphire mirrors

We report on the successful development of low-loss sapphire mirrors for use at a 1-mum wavelength. Methods for polishing and coating are described. The analysis of each process shows a roughness of better than 0.1 nm, a coating scattering of 1 ppm, and a surface scattering of 13 ppm. The mirrors have been characterized in a Fabry-Perot cavity, having a finesse of 100, 000. Mode doublets result from the birefringence of the coatings.

Characterization of optical coatings by photothermal deflection

An overview of photothermal deflection principles and applications is given. The modeling of temperature distribution and the calculation of deflection that is due to both the refractive-index gradient and the thermal deformation of the sample are presented. Three configurations usually employed are compared, and their respective advantages are discussed in relation to their application. The calibration for absolute measurement of absorption is detailed, showing that calibration limits the accuracy of measurement. Some examples of specific information obtained by photothermal mapping of absorption are given.

Theoretical and experimental studies of pump-induced probe deflection in a thermal medium

We analyze the deflection of a probe beam because of pump-probe interaction in a high-absorbing thermal medium. We extend the existing theory by accounting for translation of a finite-width probe because of deflection within the nonlinear sample. We also provide expressions for the number of resolvable angles of the probe for possible applications of the setup as a beam deflector and study conditions for the maximization of the deflection angle and the resolution. We present experimental results obtained with a solution of chlorophyll in ethanol as the thermal medium.

Absorption mapping for characterization of glass surfaces

The surface quality of bare substrates and preparation procedures take on an important role in optical coating performances. The most commonly used techniques of characterization generally give information about roughness and local defects. A photothermal deflection technique is used for mapping surface absorption of fused-silica and glass substrates. We show that absorption mapping gives specific information on surface contamination of bare substrates. We present experimental results concerning substrates prepared by different cleaning and polishing techniques. We show that highly polished surfaces lead to the lowest values of residual surface absorption. Moreover the cleaning behavior of surfaces of multicomponent glasses and their optical performance in terms of absorption are proved to be different from those of fused silica.