Simona Liukaityte

Depolarization and enpolarization DOP histograms measured for surface and bulk speckle patterns

dop histograms are measured in the off-specular far field speckle of disordered media under polarized and unpolarized illumination. Three surface samples with increasing roughnesses, and three bulk samples with different absorption levels, are investigated. Results show that both rough surfaces and absorbing bulks hold the incident polarization, while transparent bulks allow to depolarize or to enpolarize the incident light. Hence we provide a first experimental evidence of such effects.

Ultra-wide-range measurements of thin-film filter optical density over the visible and near-infrared spectrum

We present the improved structure and operating principle of a spectrophotometric mean that allows us for the recording of the transmittance of a thin-film filter over an ultra-wide range of optical densities (from 0 to 11) between 400 and 1000 nm. The operation of this apparatus is based on the combined use of a high power supercontinuum laser source, a tunable volume hologram filter, a standard monochromator and a scientific grade CCD camera. The experimentally recorded noise floor is in good accordance with the optical density values given by the theoretical approach. A demonstration of the sensitivity gain provided by this new set-up with respect to standard spectrophotometric means is performed via the characterization of various types of filters (band-pass, long-pass, short-pass, and notch).

Light scattered by optical coatings: numerical predictions and comparison to experiment for a global analysis

Complex optical coatings may present highly disturbed scattering patterns, both spectrally and angularly. We show in this paper how the development of an accurate dedicated metrology allowed the optimization of numerical models. Our prediction is compared to our measurement.

Measurements of angular and spectral resolved scattering on complex optical coatings

Due to market demand and technical progresses, a new generation of optical components requires much more sophisticated structures with a great number of layers. These complex structures enable to achieve severe optical performances but, at the same time, enhance light scattering processes. For these reasons, it is essential to develop a metrological tool which provides an accurate quantification of the spectral and angular scattering losses behavior with sufficient angular and spectral resolutions. In order to face this issue, new investigations were performed by our group at Institut Fresnel and led to the development of the new scatterometer SALSA (Spectral and Angular Light Scattering characterization Apparatus). The use of both a broad-band light source and a tunable filter allows to accurately select the illumination wavelength and the spectral bandwidth on the whole spectral range of CCD detectivity. In this paper we will present the performances of the setup and some experimental results.

Metal-dielectric absorbers with magnetron sputtering technique

In this paper, we show how thin film absorbing coatings can be designed with multilayers using both metallic and dielectric materials involving some thicknesses that are very low down to some nanometers. Such multilayers enable to reach a very low level of reflectance and a zero value of transmittance, which can be a solution for stray light reduction in optical systems. After a description of the design steps, we will present the manufacturing of such multilayer stacks using magnetron sputtering technique and we will see how such coating technique is very well suited for production due to its high process reproducibility even for very thin layers required in metal-dielectric absorbers. Monitoring of such coatings is also presented with the help of a powerful in situ optical system developed in collaboration with Institut Fresnel that allows characterization of in-situ refractive indices of deposited materials and broadband monitoring of the multilayer stack. Many results will be given on qualification samples, such as environmental tests and spectral characterizations that show the stability of the performances in severe environmental conditions. At last, we will focus on the spectral and angular scattering behavior of such absorbing coatings and we will present several measurements performed on glass or metallic substrates with different roughnesses.

Instantaneous one-angle white-light scatterometer

We present a white light scatterometer operating at a unique scattering direction. Mechanical motions and wavelength scans are removed. The technique provides an immediate flexible characterization of roughness with no loss of resolution.

New challenges in light scattering analysis of complex optical components (Conference Presentation)

Theory and experiment in the field of light scattering from optical coatings have been extensively studied and controlled since the 90’s. Indeed surface and bulk theories were developed for substrates and optical coatings, and have revealed great agreement with experiment. Furthermore, angle-resolved apparatuses were built with detection limits close to scattering from the air particles. All these tools have allowed to characterize roughnesses lower than 0.1 nm; also, the microstructure of thin film layers was investigated versus the deposition technologies. Nevertheless, in the last few years, new challenges for light scattering have merged. Actually, modern deposition technologies with their sophisticated monitoring systems today enable the deposition of large numbers of layers, hence providing complex filters which must be characterized at their working wavelengths or in a wide spectral region. Moreover an increasing demand for micro-structured filters has merged and requires new procedures to discriminate scattering from all micro-devices. In this context, we have developed in our group at Institut FRESNEL new numerical and metrological tools to satisfy these demands. All scattering facilities were rebuilt and upgraded, sometimes with strongly different principles. In this paper, we will present a rapid overview of these developments, with a focus on broad band scattering metrology (400nm-100nm) with no loss of performance, separation of intrinsic (surface profile) and extrinsic (local defects) roughness, and the control of large-angle scattering in ultra-narrow band filters. Examples and applications will be given to emphasize all improvements.

Absorbing Coating in Magnetron Sputtering for Parasitic Light Reduction

Stray light is an important issue in optical systems and may be responsible for huge limitation of final performances. Use of black coated surfaces is known to be an efficient means to reduce such parasitic light sources and various solutions exist that can be applied to mechanical surfaces such as black paints or black anodization; these coatings are relatively thick and to produce thin, black baffle edges, a thin layer technology is thus needed. In this paper, we show how thin film multilayer coatings can be a solution to answer this problematic as it is possible to design accurate spectral response that present a very low level of reflectance with a zero value of transmittance.

Instantaneous One-Angle White-Light Scatterometer

We present a white light scatterometer operating at a unique scattering direction. Mechanical motions and wavelength scans are removed. The technique provides an immediate flexible characterization of roughness with no loss of resolution.

Ultra-wide range system for the spectral transmission measurement of complex optical filters

This paper is devoted to the presentation of a new spectrophotometric system that allows us for the accurate measurement of the transmission of thin-film filters over an ultra-wide optical density range (from 0 to 11) between 400 nm and 1000 nm. After a detailed description of its structure, we define the transmission range reachable to this set-up and we quantify the influence of the spectral profile of the source on this critical feature. A qualification of this bench, including a determination of its noise floor, is performed on a dedicated band- pass filter manufactured by Institut Fresnel, and we conclude by the presentation of results of ultra-wide range transmission measurements achieved on commercially available filters (long-pass, short-pass, and notch).