This PDF file contains the front matter associated with SPIE Proceedings Volume 11872 including the Title Page, Copyright information, and Table of Contents.

Modern design tools allow solving a wide set of the most challenging multilayer design problems, but they require a deep knowledge of synthesis methods, and good expertise in applying various design tricks to obtain a good solution. During the last two decades computational performance of modern computers allows one to consider more computationally demanding approaches, including those ones based on deep search methods. Deep search approaches require enumeration of all possibilities on each iteration of a method. The number of different possibilities (layer insertion locations for the needle optimization, layer boundaries for the gradual evolution iteration, layer number for the design cleaner, etc.) is growing with the complexity of the coating. Therefore, the computational time of any deep search method considering all possibilities is also growing dramatically. To mitigate this challenge, we propose to use self-adapting algorithms accumulating information on main computational metrics (the numbers of required iterations, the evolution of merit functions versus iteration, the behavior of the merit function gradient norm). Accumulated information can be efficiently used to complete the computations of promising variants and to interrupt iterations of unfavorable variants very early. Proposed self-adapting and self-learning method serves as a superstructure upon well-known multilayer optimization methods, such as needle optimization, gradual evolution, design cleaner, etc. A huge advantage of the new self-learning method is that it does not require any careful fine tuning during computations. New method can run in completely automatic mode and nevertheless provide high quality solutions of challenging design problems.

Optical thin-film design has been intensely studied over the last decades and there exist methods to synthetize numerous optical functions, usually considering only specular spectral responses. Benefiting from the rise of artificial intelligence, new methods seek to provide other solutions. Light scattering properties have rarely been taken into account in the design of thin-films because of their extreme complexity, even though it might be critical for some demanding applications. We propose a light scattering-based design method of complex thin-films using deep neural networks. Our implementation aims to design components with controlled light scattering levels and acceptable specular properties.

Optical sources within micro-cavities have been extensively studied for years [1]–[3]. It is today well known that over-coating these cavities with multilayer optics allows to modify their emission patterns in free space in the surrounding media (substrate and superstrate). While sources within single overcoats radiate a quasi-lambertian pattern, more elaborated structures allow to confine the angular patterns in narrow angular or spectral regions. Applications concern lightening and bio-photonics, micro and single photon sources, antennas… However, most overcoats in micro-cavities are classical multilayer designs currently used in the field of thin-film optics, such as mirrors and narrow-band filters, so that one may wonder whether these cavities can be optimized with much more (huge) efficiency. Furthermore, until now the presence of guided waves was not considered in this optimization process. The objective of this paper is to introduce specific techniques to design huge enhancement of emission patterns from micro-cavities. We first introduce zero-admittance layers [4]–[6] in the cavity and show that they also strongly increase (by several decades) in free space the emission pattern. Then we are interested in designing a cavity for which the modal light would largely dominate the free-space light; here we use modal light to designate the light trapped in the cavity and which propagates in the form of guided modes without radiation losses. In order to reach this goal, we show how to design specific poles of the reflection factor in the modal range, that are known to be the modal constants of the layered structure. All results allow to reach a total modal energy much higher than the integrated free-space light, or conversely. Applications may concern micro-lasers, guided waves and lightning, and address the extension of design techniques to the situation of multilayer waveguides.

Multilayer dielectric gratings (MLDG) are key optical components of Petawatt-class laser that are used to compress short pulses of high intensities. Laser-induced damage can occur on the top area of the components, typically arising in the pillars periodically etched. This phenomenon limits the power yielded by high power laser facilities such as PETAL (PETwatt Aquitaine Laser) laser facility. PETAL is expected to delivery pulses with a wavelength around 1053 nm, an energy around 3 kJ and a pulse duration between 0.5 and 10 ps. Coupled with LMJ (Laser MegaJoule), PETAL aims to study materials in extreme conditions to reproduce the environment in the heart of stars or planets, fusion by inertial confinement, particularly rapid ignition and shock ignition, and nuclear physics for medical proton therapy. In this study, we present a process to improve the laser-induced damage threshold of PETAL pulse-compression gratings in sub-picosecond regime by reducing the electric field intensity in the pillars. PETAL gratings have specific parameters of operation: Transverse Electric polarization, under vacuum, a period equal to 1780 lines per mm and diffraction efficiency higher than 95% for the -1st order. Theoretical designs are calculated with a code developed at the Fresnel Institute. The code solves Fresnel equations by using the differential method, Fast Fourier Factorization (FFF) and S matrix propagation algorithm. As a result, we obtain the distribution of the electric field and diffraction efficiency of any given diffraction order. First, starting with a given MLD mirror, we calculate an etching profile that maximizes the diffraction efficiency at the -1st order by taking into account the manufacturing constraints of future suppliers. Then, we optimize the mirror stack without changing the etching profile. We modify only the first top layers under the grooves. We obtained theoretical designs with the same etching profile and identical diffraction efficiency, associated with different electric field intensity values and expected different laser induced damage thresholds.

One of the more challenging aspects of optical thin film design has been to design the s- and p-polarizations of a filter to have the same reflectance as each other at a given angle of incidence. There have been various contributions in the literature for over half of a century to the understanding of this behavior. Some of these works are reviewed and their results are illustrated in newer graphic formats with examples. Baumeister, in 1961, reported in Optica Acta on “The Transmission and Degree of Polarization of Quarter-wave Stacks at Non-normal Incidence.” In 1970, Costich described a method of transforming massive media to nonpolarizing effective massive media. He applied two design techniques to the problem of making polarization independent metal-dielectric-metal interference filters and polarization independent beam splitters. The polarization dependence of interference coatings may be reduced by combining two approaches. The first is the use of a polarization insensitive layer combination, and the second is to transform the massive media to effective massive media having little polarization splitting of their effective indices. In 1976, Thelen reported on nonpolarizing interference films inside a glass cube. He described another method using only quarter-wave layers including the equations to accomplish this and he refers to the work of Baumeister reported from 1961. He points out some limitations to the Costich approach. Henderson showed the benefits of using three materials instead of only two in a 1978 paper. Two materials could be made non-polarizing for one wavelength and angle, but three materials would provide non-polarizing performance over a broader wavelength range. At normal incidence, a layer-pair of quarter waves of high and low indices is the basic building block to stack one layer-pair upon another to create a high reflection at a given wavelength band centered at the design wavelength for which the layers are quarter-waves. When plotted on a Fresnel Reflectance Amplitude diagram, each quarter-wave forms a semicircle at the design wavelength, and the layer pair then covers 360°. In the case of non-polarizing designs, the building blocks composed of three materials are formed from four quarter-waves or 720° of phase change. These have the property at the design wavelength and angle to advance the reflectance equally for both the s- and p-polarization. There is an apparent similarity here between the effect of this medium-index layer and the typical halfwave thickness high index layer as an achromatizing layer for a three material broadband four quarter-wave antireflection coating design. There is also a similarity on a reflectance amplitude diagram to the mathematical shape of a Limacon for both the s- and p-polarization. The s- polarization has an obvious internal Limacon loop and the p-loop expands to appear more as a spiral in some cases. The Herpin/Epstein concept might lead one to consider accomplishing the function of the three-material designs with only two materials. The possibilities and limitations of only two-material designs are examined.

Thin film techniques also find applications outside their classical sectors, and among them is the terahertz field1. These waves allow to penetrate a large variety of materials opaque to optics, and find applications in security and defence, automobile and avionics, medicine … THz waves provide new imaging techniques (transverse mode) but can also be used to probe in detail the depth of samples2 in the form of single layers or multilayers, which is the scope of this paper. Actually we take profit from thin film design procedures (usually developed for visible and infrared ranges) in order to address reverse engineering in the THz range. We first emphasize some key differences due to the fact that most broad-band THz sources are pulsed sources (here the THz pulse duration was around 3ps). Hence conversely to optics where optical properties are intensity data issued from spectrophotometric measurements, THz pulses directly allow to record the temporal signals with and without sample interaction, which gives the modulus and phase of the spectrum in the frequency domain. The consequence is that we operate the reverse engineering procedure in the complex plane (in opposition to the real axis of photometry), which involves more data. Here the pulse duration is around 3ps, and the frequency domain with acceptable noise is limited to [0.2 THz – 3,5 THz]. A few classical (inorganic) etalon samples are first analysed and their echoes are exploited to reveal their thicknesses under the assumption of negligible absorption. Then we use reverse engineering to take account of absorption and fit all data in the THz range, which confirms the previous results but with more accuracy. The resulting thicknesses are compared with success to the provider data. In a last step we investigate vegetal tissues (sunflower leaves), which is a much more complex task3. This study falls within the context of the optimization of plant production in regard to global warming and increasing demography, a challenge which requires to analyse and control the hydric stress of plants. Actually there is a growing demand to develop non-contact techniques to analyse leaves microstructure and understand their interaction with the surrounding medium. However the vegetal leaf is highly heterogeneous and cannot be analysed with optics, due to high diffuse reflectance of transmittance (no specular beams). A solution is provided by the THz waves, due to their much larger wavelengths which reduces scattering and the weight of heterogeneities. We show that in this THz regime, the sunflower leaf indeed behaves like a homogeneous multilayer, and this allows to use reverse engineering to extract the leaf design. Results emphasize a 8-layer stack including trichomes, cuticules, epidermis and mesophyll layers4. For each layer we extract the thickness and complex index. To our knowledge this is the first time the leaf multilayer structure is extracted with accuracy with non-contact techniques.

Semi-absorbent metallic layers offer some unique possibilities in thin-film coating design due to their versatile dispersion properties. However, the presence of absorbance and the dependence of the properties on thickness present significant challenges for characterisation of such films. Therefore, as of today, this is no reliable and universal technique to characterize these layers. We propose here an alternative spectrophotometric method to determine the refractive index of a semitransparent metallic thin film. The method involves the preparation of a semi-reflective silicon substrate plus a thick dielectric layer several hundred nanometers thick. A thin, semitransparent metallic film is then deposited over this dielectric layer, creating an asymmetrical Fabry-Perot structure. The resulting spectrum displays oscillatory features from the dielectric layer, which are modulated by the dispersion properties of the thin metallic layer to be determined. A numerical optimization is then used to estimate the refractive index dispersion via use of an appropriate dispersion model. The sensitivity of the spectrum to the dispersion properties of the thin metallic layer allows these properties to be determined with a higher accuracy and robustness. In this paper, we detail a numerical and experimental validation of the method in the case of titanium thin films. To model the dispersion properties of these layers, we use the combined Modified Drude and Forouhi-Bloomer models. The index dispersion was determined for a range of titanium layer thicknesses from 10 nm to 70 nm. We show that the proposed method is accurate and stable and allows determining dispersion properties that can then be used for the design of multilayer structures for purposes such as colorimetry.

From plasmonics with pure coinage metallic materials to the current advances in phase-change materials (PCM), research in nanophotonics has rapidly evolved, pushed by its wide gamut of applications. In this contribution, we will present three phase-change material examples: MoOx, Ga₂S₃ and GaS as promising candidates for reconfigurable plasmonic applications with fast and low-loss response. The first because modification of its oxygen stoichiometry (2< ×<3) induce a phase change in the VIS region, suitable for developing reflective pixels for display applications. The other two, because they show phase transitions due to changes in their lattice configurations, making them attractive for new broadband devices for switching and photodetection applications. They will be characterized and studied by resorting to exact DFT calculations. Also, their plasmonic response as well as their possible plasmonic coupling effects will be analyzed by considering the different envisaged applications.

With the increase of the laser powers and the decrease of the pulse durations, materials with very large optical nonlinearities are of great interest for the laser community. Indeed, depending on the type of the nonlinearities, they can be used for several applications including frequency conversion and self-focusing. In particular, saturable absorbers are widely studied, as they allow mode-locking of laser systems or super-resolved laser writing. In this work, we have studied the saturable absorption efficiency of Sb₂Te₃ thin layers. Layers with thicknesses ranging from 2.5 to 30 nm have been deposited using electron beam deposition (Bühler SYRUSpro 710). These films were then annealed in a temperature-controlled furnace at 250°C during 1h to ensure that the layers are completely crystallized. These layers were then thoroughly inspected with X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Backscattered electron detector (BSD) and Transmission Electron Microscopy (TEM). The nonlinear optical properties under nanosecond and femtosecond pulse duration were also studied for each layer using the Z-Scan technique. These studies allowed the determination of the nonlinear absorption and the nonlinear refraction of the samples under two different wavelengths at each pulse duration regime. In this paper we present a correlation between the structure of the Sb₂Te₃ thin films and the observed nonlinearities.

Optical surfaces are achieving increasingly complex shapes which brings out challenges to functionalize them conformally for improved performance. Besides, plastic optics pose a great challenge on coating technologies due to their large coefficient of thermal expansion and poor adhesion of functional coatings. Here, the potential of plasma enhanced atomic layer deposition (PEALD) technique to develop uniform and 3D-conformal films on polycarbonate (PC) (Makrolon) planar and dome substrates has been explored. It enables to grow conformal Al₂O₃, TiO₂ and SiO₂ films on steeply curved PC substrates. Moreover, we demonstrate an 11-layer antireflection (AR) coating reaching about 0.2% reflection at 905 nm wavelength on the entire outer surface of several PC domes along with a consistent optical performance on the inner surface. The adhesion and environmental stability tests according to ISO-9211-04 resulted in promising adhesive and environmentally durable films on PC dome optics. These results suggest a possible way to grow uniform, dense, conformal, and stable optical coatings on sensitive polymer PC substrates for desired optical applications.

This contribution describes an approach to realize dichroic mirrors for fifth-harmonic separation with a diameter of 12”. The optics will be used at the National Ignition Facility to build a diagnostic based on Optical Thomson Scattering, which requires a high energy, pulsed laser operating at 211nm wavelength. Since the ultra-violet absorption edge of the most commonly used high refractive coating materials is above 211nm, only a few oxide materials as for example alumina are suitable for this wavelength. The applied material combination Al₂O₃/SiO₂ provides a small refractive index contrast of about 0.2, which requires a coating process with a very high precision and uniformity to realize complex thin film designs. To achieve a physical layer thickness uniformity better than 0.5%, the linear motion concept of the MAXIMA ion beam sputtering machine [1] is combined with an additional substrate rotation. The layer thickness is controlled precisely by optical broad band monitoring in the wavelength range from 220nm to 1050nm. To realize a surface figure of λ/10 at 633nm for a clear aperture of 250mm diameter, the multi-antireflection coating on the backside is utilized for stress compensation. Experimental results regarding the spectral performance, the mechanical stress, the surface roughness and the laser damage resistance will be presented and discussed.

Hybrid integrated photonics open up new application perspectives due to compact size and the shift to cost-efficient components. Therefore, integration of optical and electro-optical functionalities into photonic chips has recently attracted great interest. Research has been directed towards miniaturization of demanding spectral transfer properties for individual applications. However, it remains challenging to implement highly complex transmission and reflection characteristics with few additional process steps. In this contribution, we report on our advancement in the field of optical thin-film coating fabrication, which enables a manufacturing process comparable to die assembly in electronics. We have combined a sacrificial-substrate approach with the production of miniaturized optical thin-film coatings by ion-beam sputtering. The concept is applicable to high precision coatings with more than 130 individual layers and adding up to over 26 µm total film thickness. Segmentation down to sizes of 25 μm x 25 μm pieces is realized by laser cutting of the coating. By completely removing the substrate afterwards, we achieve a freestanding thin-film and thus minimized thickness. Our measurements indicate no general performance loss compared to coatings on glass substrates. Additionally, the substrates refractive index and absorption do not have to be considered in the multilayer-coating design. Therefore, the design can be optimized and matched to the refractive index of specific waveguides on the chip. Furthermore, we demonstrate the compatibility to releasable transfer tape. With this, we aim for enabling a high-volume feed of miniaturized thin-film filters to an automated assembly process.

Since its invention in 1974, atomic layer deposition (ALD) has shown tremendous performance in depositing thin film structures for various applications in physical, chemical, biological and medical sciences. The unique layer-by-layer growth mechanism of ALD enables exceptional uniformity, conformality and accurate control of film thickness for a plethora of materials. In physical sciences, especially in optical systems, these properties are of utmost importance as sustaining optical performance not only requires a high degree of uniformity, but also excellent conformality when it comes to complex micro- or macrostructures. To reach a high level of uniformity and conformality on complex macro shapes, such as highly curved lenses or spherical domes, effort needs to be made to specifically twist, turn, rotate or otherwise move the structures. For the traditional physical vapor deposition, this comes with an extensive load of mechanical work and process optimization, ultimately leading to a tight control of the process parameters. In the end, the optimized processes may still not lead to a sufficiently homogeneous film deposition on the most challenging structures. In this work, we demonstrate that we have been able to overcome these constraints by utilizing our P-series ALD batch tools to deposit various optical coatings on complex 3D macrostructures. As example structures, we use a topless cube (length, height, width = 150 mm) and a hemispherical dome (diameter = 155 mm). By depositing various low-loss oxide materials (SiO₂, Al₂O₃, HfO₂, TiO₂ and Ta₂O₅) and by measuring their resulting thickness distributions from various locations throughout the 3D bodies, we have been able to achieve non-uniformities ranging from less than 1 % to 2.5 % over the entire structures. In addition, we have designed and deposited anti-reflective, highly reflective and edge-pass coatings on these structures and verified that excellent spectral responses from 400 to 2500 nm can be obtained. Ultimately, our results demonstrate an exceptional method to realize high-quality optical coatings on the most challenging surfaces, paving the way for the mass production of critical optical components in macroscale for a variety of applications ranging from military to microscopy.

In this work we present reactive sputtered SiOxNy films with a variable refractive index as a convienent solution for contrast improvement of liquid crystal diffuser multi stacks in near-to-eye AR/VR displays. The focus concerns minimization of light reflections between internal structures, in particular ITO, by optimizing internal layers through tailored properties of thin film coatings, as well as subsequent laser patterning of thin film stack. Inorganic thin films have been deposited on glass by physical vapor deposition. Corresponding refractive index, thickness, uniformity and dielectric characteristics and other electro-optical properties have been measured and their impact on the resulting optical performance of the final integrated element stack has been compared against counterparts utilizing traditional polyimide and SiO_x films.

Transparent thermoplastic polymers are widely used as materials for precision optical lenses as well as for sensing and lighting. The advantages of transparent polymers for optical parts are significant weight reduction, high impact strength, molding options and cost saving mass-production. Antireflection (AR) coatings are essential to improve transmission and contrast of lenses, windows and display covers. Polymer-capable coating conditions must be investigated for each type of polymer because of the varying chemical and physical properties of optical polymers. A presently well-established coating technology for plastics is plasma ion-assisted deposition (Plasma-IAD). It enables the coating deposition at low temperature as well as low-energy plasma conditions and ion bombardment to tailor the optical and mechanical properties of oxide layers. A good understanding of complex interactions of polymer surfaces with plasma and high-energetic radiation is a key factor to achieve polymer optics with high-end AR-properties and long-time durability. The Aim of this study is to evaluate and to understand the surface properties of polymers which are relevant for the deposition of optical coatings and for its later application. The investigation is focused primarily on the new polymer types APEL, Iupizita EP and OKP. They are compared with the long-established materials such as polycarbonate (Makrolon) and ZeonexE48R. The optical properties of the polymers are systematically studied including the influence of aging caused by UV-irradiation, humidity and heat. In addition, properties like surface hardness, water absorption and thermal stability are compared and discussed. Different pre-treatments and designs are considered to bond multilayer AR systems to surfaces with high adhesive strength. In addition, plasma-etching technology AR-plas is applied to achieve AR properties for the visible spectral range (VIS).

Anti-reflective (AR) coatings are indispensable for an excellent imaging of optical systems. Common AR coating systems consist of layer stacks of alternating low and high refractive index materials with the residual reflection depending mainly on the low refractive index (LRI) of the last layer relative to air. However, conventional LRI materials are limited to SiO₂ (n = 1,46 at 532 nm) and MgF₂ (n = 1,38 at 532 nm), where MgF₂ is not environmentally stable. Nanostructures with an adjustable effective LRI in the range of 1.07 to 1.25 are an attractive alternative to these materials. Integrated as the last layer in the stack system, these structures significantly improve the optical performance compared to a conventional interference coating system, as a broadband AR coating can be realized, which is less sensitive to high angles of light incidence. Nanostructures can be produced using various methods - e.g. wet chemical or lithographic. However, these methods are expensive and time-consuming, as often more than one manufacturing step is necessary. At the Fraunhofer Institute IOF in Jena, self-assembling AR nanostructures have already been successfully fabricated for several years using a conventional plasma-ion-assisted-deposition (PIAD) technology. Thereby organic material is deposited on the substrate via thermal evaporation and subsequently self-assembling nanostructures are formed by an ion plasma source. Melamine and Uracil are already being used successfully as organic material. The advantage of this method is the generation of nanostructures in one process which is cost- and time-effective. In this talk, we want to introduce Xanthine, another organic material that is also highly promising for the generation of nanostructures exhibiting an LRI. We will demonstrate the formation of these nanostructures during plasma etching and investigate their optical performance for the use as an AR coating.

Protective and enhanced Silver coatings enable broadband high reflectance and durability. Energetic deposition provides densified layers to protect Silver mirrors and enhance optical reflectance in desired spectral bands. We employed variable angle spectroscopic ellipsometry to analysis ion beam assisted SiO₂ and Nb₂O₅ films for optimizing enhanced silver performance in the near UV. We generated a visualized field guide for coating process development based on spectral reflectance measurement. Adjusted ion beam assisted deposition parameters included ion energies, ion currents, gas flow rates and deposition rates. Appropriate ion interaction with the deposited materials led to an increase of both film homogeneity and densification. Accelerated UV ozone exposure tests confirmed that the optimized ion beam assisted deposition yielded stable optical performance.

Ceramic phosphors have been used for non-contact, 2-dimensional temperature sensing in high strain and high temperature environments. The Novel Aerosol Deposition Technique was used to fabricate coatings of Dysprosium doped Yttrium Aluminum Garnet and Mg₄F₂GeO₄:Mn phosphors. Preliminary microstructural/optical characterization has been conducted. Dysprosium doped Yttrium Aluminum Garnet coatings were found to contain both garnet and perovskite phases and had an as-sprayed crystallite size of 25-30 nm. The fast response time and thermal/environmental stability of the doped ceramic phosphors make them excellent materials for temperature measurements on surfaces such as rapidly rotating turbine components. The high density, robust, room temperature deposition of phosphors through the Aerosol Deposition process greatly improves the current state-of-the-art of non-contact temperature sensing

Structural, optical, and electrical properties of ultrathin chromium films manufactured using magnetron sputtering were investigated. The films showed pure metallic chromium phase yet their refractive index and extinction coefficient result very different from previously reported in literature. Structural, electrical and optical properties of ultrathin chromium layers are discussed in detail. The obtained optical constants of ultrathin chromium films show a specific trend with the film thickness increase. Precise knowledge of optical constants of ultrathin chromium films is important for many electro-optical and optical applications.

Various types of optical monitoring systems are established in industry. They range from single wavelength, over monochromatic to broadband monitoring to calculate a monitoring signal, which allows to terminate each layer in a filter at the required thickness. State of the art monitoring systems offer the capability of monochromatic and broadband monitoring in a single device. With these technologies available, the question arises how to combine these monitoring strategies for a specific application in a way, which leads to accurate coating results with the least sensitivity to production errors and thus to the highest yield. To answer this question without the need to perform costly coating runs, we developed a software tool, which mimics all the monitoring features of Evatec’s GSM optical monitoring system. Additionally, the software is able to disturb the simulated ideal monitoring signal with errors such as detector noise, drifts, deviations in shutter delay times, etc. The values of these disturbances are specific to the deposition tool. They were determined based on the broadband spectra of actual coating runs. By starting a virtual coating run with defined disturbances, the thickness deviations expected with a selected strategy can be assessed and the development of thickness deviations during the run, i.e. error compensation and error accumulation can be simulated. Within the software, parameters for the termination of each layer can be varied individually and the effect on the coating result can be observed. In order to demonstrate the capability of this tool, a specific coating design was then selected. For this design various monitoring strategies were tested, broadband strategies with different wavelength ranges, monochromatic strategies varying wavelength assignment per layer but also mixed strategies of broadband and monochromatic monitoring. The most stable monitoring strategy resulting from these simulations was coated as well as some of the less promising candidates and their results were compared.

Optical monitoring is a go to method for complex filters deposition; however, it can be easily shown that filter performance is dependent on monitoring strategy. When a non-quarter wave design needs to be deposited, usually one or minimal number of monitoring wavelengths is selected. This allows to use correction algorithms based on swing that are compatible with level-cut monitoring to a great extent. This approach has a significant drawback that it is very difficult to find one or few wavelengths that can be used for all layers of a complex filter. We present a different approach that relies on the selection of the best monitoring wavelength for each layer using pre-defined criteria that secure minimized thickness errors for each individual layer. Wavelength selection process uses several important criteria, such as monitoring wavelength sensitivity to errors in previous layers, transmittance evolution speed versus layer thickness growth, noise of measurement setup… We show that an additional important criteria is spectral resolution of the optical monitoring system and its impact on filter’s spectral response after each layer. Last, to ensure stable deposition we show that some precautions must be made to avoid false turning point detections. Using a binary approach for each criteria (pass or fail), monitoring wavelengths then can be selected automatically based on criteria defined above. In this work we demonstrate that such an approach can be implemented on stable deposition technique such as plasma assisted reactive magnetron sputtering (Bühler HELIOS machine) for different types of filters with various complexities. We illustrate our results for example on an 8-layer beamsplitter, a 37 layer D65 compensation filter, or a 100 layer custom shape spectral filter that are all very sensitive even to small thickness errors. Similar or better spectral performances are achieved compared with classical optical monitoring approaches but with an automatically determined optical monitoring strategy.

It is shown that the previously proposed geometric approach adequately describes the effect of error correlation based on the results of computational manufacturing experiments with the selected type of optical monitoring. The question of a sufficient number of simulation runs is investigated, and it is shown that a reliable estimate of the correlation coefficient describing the strength of the error correlation effect is obtained with a realistic number of computational manufacturing experiments. To investigate the presence of the error self-compensation effect, the error self-compensation coefficient is introduced. Using sets of correlated error vectors obtained in computational manufacturing experiments, the probability density function of the distributions of this coefficient is calculated. To assess the strength of the error self-compensation effect, the estimate is proposed based on a comparison of the influence of correlated and uncorrelated thickness errors.

Light scattering [1] has been extensively studied in multilayer optics, and it is today possible to predict and measure total losses lower than a few ppm in the whole continuous spectral range (400nm -1700nm) [2]. Such accuracy is crucial for applications in high precision optics, including space applications, mirrors for gyro-lasers and detection of gravitational waves. Despite this relevant state of the art, numerous efforts are still devoted to control and reduce large-angle scattering in complex interferential filters. The same balance is observed for mirrors for which total losses should not exceed a few ppm. Within this context it must be stressed that one effect was not considered until now in the energy balance of an optical multilayer. This missing term is the trapped scattering [1], that is, the amount of light which remains embedded within the stack and cannot merge in free space. This paper is focused on the theory of trapped scattering [1,3,4]. One key question concerns the amount of trapped light in multilayers, and mainly, whether this trapped light is greater or lower than the radiative light which merges outside the stack. It should also be stressed that the trapped scattering intervenes in the absorption. The guided modes are attenuated along propagation and this process creates an additional “modal” absorption. The result is that the modal absorption may dominate the classical absorption, and this information is crucial to avoid any useless optimization of the thin film deposition parameters. Complex filters are analyzed in detail in this paper and allow a better understanding to address the challenge of the “ppm barrier”. To our knowledge, it is the first time that trapped scattering is quantified within multilayer optics, and we expect this work will be useful for the community. References [1] C. Amra, M. Lequime, and M. Zerrad, Electromagnetic Optics of Thin-Film Coatings. Cambridge University Press, 2021. [2] M. Lequime, S Liukaityte, M. Zerrad, C. Amra, “Ultra-wide-range measurements of thin-film filter optical density over the visible and near-infrared spectrum,” Opt. Express 23, 26863-26878 (2015). [3] C. Amra and S. Maure, ‘Electromagnetic power provided by sources within multilayer optics: free-space and modal patterns’, J. Opt. Soc. Am. A, Nov. 1997. [4] C. Amra and S. Maure, ‘Mutual coherence and conical pattern of sources optimally excited within multilayer optics’, J. Opt. Soc. Am. A, Nov. 1997.

Light scattered from optical components is a source of critical noise for interferometric gravitational wave detectors such as LIGO, Virgo, KAGRA, or the future space antenna LISA. Therefore it is important to accurately quantify the amount of light coherently back-reflected or back-scattered by optical components involved in the design of such instruments. As an example, for a good quality plane silica window (1 nm RMS roughness) illuminated by a Gaussian beam with a waist of 1 mm, the amount of light back-scattered by each interface is between -95 dB and -125 dB following the angle of incidence. This defines the stringent sensitivity requirements we have to satisfy for such applications. In our communication, we present the measurements performed on a Silver coated mirror using a low coherence interferometer implemented with balanced detection.

Optical thin-film components have known major developments over the last three decades and find now applications in many scientific fields. But it remains difficult to measure and characterize complex components. Light scattering properties are usually neglected in the characterization even though it is well-known that it can be critical for some very demanding applications where the miniaturization of optical instruments make them more sensitive to crosstalk. We present in this paper an upgraded version of a spectrally and angularly resolved scatterometer operating on a continuous spectral range from 400 to 1700 nm and reaching unprecedented levels of detection and accuracy.

Scattering techniques are today well controlled to characterize roughnesses of high-precision substrates and coatings, which are around a fraction of a nanometer in the optical bandwidth [1, 2, 3]. All these techniques involve a receiver, sample or beam motion so as to record the whole angular scattering pattern by reflection and transmission. However, in some situations these angular motions are penalising. This is the case when fast roughness-measurements are required (on-line measurements), or when the sample cannot be displaced (case of large pieces), or when only one scattering direction is allowed (case where the sample cannot be separated from a system) … For these reasons we recently proposed proposed [4] an alternative which consisted in using white light so as to cancel any mechanical movement in a scattering system. The resulting system is a one-angle white-light scatterometer with a reduced spatial frequency bandwidth. The principle of this white light scatterometer relies on the wavelength-angle equivalence in the spatial frequency. However additional concepts must be introduced to make the white light scattering to be proportional to roughness. Actually we have to shape the wavelength spectrum of illumination in a specific way given by theory. Two experimental techniques are presented to shape the incident spectrum. The first [4] is based on an interferential filter but suffers some disadvantages, such as the absence of tunability or retroaction. The second technique [5] involves micro-mirror or LCD matrices coupled with gratings, and offers several advantages. Fast retroaction allows to take account of a shift in the source power, and the tunability also offers the opportunity to build quasi-arbitrary filters. This last remark allows to extract a series of roughness moments (not only the roughness), so that the autocorrelation function of the topography can be reconstructed. We will discuss advantages and limits of this new technique.

We present a new version of a goniometric light scattering instrument with high resolution imaging. This equipment is able to simultaneously record one million of BRDFs associated with as many 52 μm x 52 μm elementary pixels optically defined at the surface of a plane sample by a telecentric objective and a scientific grade CCD matrix.

In this work we present a new all-silica coating - polarizer, which is also capable to withstand high density of radiation. In order to demonstrate the versatility of presented approach, several coating designs have been modelled and two of them fabricated together with the full-scale measurements and analysis necessary for polarizers implementation into high power microlaser systems. Two polarizing coatings at the wavelength of 355 nm have been formed using two stepper motors based GLAD system. Afterwards optical and structural analysis have been performed including spectrophotometric, atomic-force microscopy (AFM), scanning electron microscopy (SEM) and optical resistivity measurements.

The increase of continuous wave laser power is an important topic in various new industrial and defence applications. One of the important limitation is due to the thin film component absorption (intrinsic and defects-related) that induces beam distortions and eventually laser-induced damages. In order to study this absorption, it is of prime importance to accurately measure low absorption levels and to determine the origin of this absorption. In this work, we present the use of Lock-In Thermography (LIT) to absorption measurement. This technique relies on the use of a pump laser beam at 1 μm that is modulated at low frequency and an infrared camera that images the thin-film sample that is being heated. By applying a lock-in treatment on thermal images, we show that we can obtain an image of the temperature increase over the optical component with low noise level. A LIT setup with a sensitivity of a few ppm and a ten times better accuracy is demonstrated. We also show that this setup can be used to make mappings of local absorption and can easily reveal local defects with absorption that can be one order of magnitude higher that intrinsic one. This setup is finally implemented to make measurements on different single layer thin-films. Layers made with different materials (Nb₂O₅, SiO₂, TiO₂, HfO₂) and deposited by plasma ion assisted deposition or plasma-assisted reactive magnetron sputtering are studied. We explore also the effect of annealing on these dense coatings. Finally, we investigate how these intrinsic absorption levels can be used to investigate the absorption of multilayers structures.

Optical coatings have been extensively characterized these last decades, due to more and more severe requirements. These characterizations include optical properties, uniformity, hardness, adhesion and stress, damage threshold, absorption, scattering and others… Despite this state of the art, photo-induced thermal radiation [1]–[3] in optical coatings was rarely investigated in detail until now, while emissivity plays a key role in numerous sectors related to energy and defence, space optics and MIR imaging... This is the scope of this paper to provide an exact theory of thermal radiation in optical multilayers submitted to an arbitrary illumination (continuous, pulsed, modulated). The spectral range of thermal radiation (TR) is temperature (Tp) dependent. Hence it varies with incident power, and with the imaginary indices of the thin film materials. Index dispersion also plays a key role for short duration beams. As always with thin films, the TR waves follow Maxwell equations and their angular and spectral patterns are shaped by the coating design, which may open an opportunity to control these patterns [4], [5]. Relying on the fluctuation dissipation theorem [1], we introduce in Maxwell equations the adequate currents responsible for thermal radiation. These currents are temperature related, for which reason the depth and time distribution of temperature is also calculated for arbitrary illumination regimes. The Tp calculation relies on the analogy [6], [7] between thermal (diffusion equation) and optics (propagation equation) in metallic media. The resulting currents are bulk currents which vary within the depth of the stack, which creates a similarity with luminescent micro-cavities [8]. Eventually we calculate the waves emitted from these currents and this gives the angular, wavelength and temporal patterns of thermal radiation which merges in free space. The TR patterns are analysed for a series of coatings and a few ideas are presented to reduce, confine, or enhance the thermal waves.

Optical components are the main parts in laser systems, which limit the total generated output power due to laser-induced damage. At nanosecond laser pulses materials experience thermal expansion, therefore optical coatings gain stress leading to breakdown. Moreover, the main resistance to laser radiation is limited by material itself (band gap). Glancing angle deposition method is presented to produce porous nanostructured coatings, which are characterized by low inner stress. Optical resistivity dependance on porosity of several materials such as aluminium and niobium oxides single layers were evaluated. Furthermore, all-silica Bragg mirrors are formed and optical properties investigated in different environments to achieve stable and superior optical resistance.

Modern laser systems have paved the way for spaceborne laser applications such as Earth's surface and atmosphere monitoring. Well known technologies like Nd:YAG lasers are often employed; however, they do not always comply with all the different requirements for space missions. High optical efficiencies and tunable wavelength, which are desirable for many applications, can be reconciled with a simple laser design employing Alexandrite crystals. Horizon 2020 project presented here discuss the results on the development of alexandrite laser crystal treatment prior to coating deposition, as well as future plans on crystal interference coating deposition for LIDT improvement.

Multilayer optical components for ultrafast laser applications operating in the mid-infrared (MIR) spectral range from 3 to 15 μm exhibit optical and physical properties essentially different from those ones of the coatings operating in the visible-near-infrared spectral range, namely, they should (i) comprise layers from special MIR thin-film materials; (ii) contain much thicker layers; (iii) deal with adhesion problems due to high stresses; (iv) deal with regions of O-H absorption in the MIR range; (v) exhibit positive group delay dispersion; (vi) be ultra-broadband in order to support short pulses. The mid-infrared thin film materials Ge, ZnS, YbF3, and LaF3 were carefully characterized. ZnS/YbF3 and Ge/YbF3 dichroic filters specified in the visible, near- and mid-infrared spectral ranged were designed, produced, and characterized. The filters operate at Brewster angle 67.5°. Almost one-octave broadband dispersive mirrors operating in the mid-infrared spectral range were developed for the first time. The mirrors consist of Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse. Using the mirrors, the 86 fs pulses were compressed up to 55 fs.

Total Internal Reflection Fluorescence (TIRF) Microscopy is widely used in cell biology to monitor dynamic biomolecular events occurring at the plasma membrane in living cells such as virus assemblies and particle budding. However, these studies are limited by the weak fluorescence signal and background noise degrading the spatial resolution. We present highly sensitive fluorescent TIRF imaging using classical glass coverslips coated with a resonant multi-dielectric thin film. Such dielectric multilayer is optimized to generate a large field enhancement under TIR illumination at the free interface. However, dielectric materials usually have low imaginary indices that are not experimentally measurable and introducing large discrepancies with the theory. We tackled the k issue by adjusting the oxygen level within the last thin layer to fully control its absorption. We then qualify such dielectric stack resonance in TIRFM configuration for fluorescent viruses imaging.