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Riley Shurvinton

at Institut Fresnel UMR7249 CNRS

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Proceedings Article | 24 May 2022 Paper

Tunable colorimetric thin-film structures based on phase change materials

Riley Shurvinton, Fabien Lemarchand, Antonin Moreau, Julien Lumeau

Proceedings Volume 12151, 121510E (2022) https://doi.org/10.1117/12.2620977

KEYWORDS: Coating, Refractive index, Reflectivity, Phase shifts, Switching, Opacity, Thin films, Absorption

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Phase change materials (PCMs) are materials whose resistive and/or optical properties can be tuned via a phase shift triggered by an external excitation. Within this class of materials, Ge₂Sb₂Te₅ (GST) is the most well-known and widely used PCM, and is currently employed in applications including read/write phase-change memories. Recently, it and similar materials have also drawn interest for use in photonics applications, due to the high optical contrast upon the phase change. However, for applications in the visible range (380-780 nm), GST and many related materials (such as GeTe and GeSnSbTe) exhibit large absorbance, restricting their use to layers of a few nm thick and limiting the achievable phase shift. One promising alternative PCM for photonics applications is Sb₂S₃. Despite its previous characterization as a “write once/read many times” material, recent work has shown it well-suited to repeated switching, with an activation energy comparable to that of GST. In addition, it exhibits much lower absorbance in the visible range than GST, with strong optical contrast between its crystalline and amorphous phases; and may be thermally, electrically or optically switched, with a switching time on the nanosecond scale. In this paper we present a comparison of tunable color coating designs which exploit the phase changes of Sb₂S₃ and GST. We show that Sb₂S₃ offers superior color contrast between its phases, can be used in thicknesses of up to several hundred nm while still realising saturated color, and that a large color shift on switching of up to ΔE=122.4 is obtained.

Proceedings Article | 12 September 2021 Presentation

A spectrophotometric method for the characterisation of thin metallic layers and validation in the example case of titanium

Riley Shurvinton, Julien Lumeau, Fabien Lemarchand, Antonin Moreau

Proceedings Volume 11872, 118720A (2021) https://doi.org/10.1117/12.2596831

KEYWORDS: Titanium, Dielectrics, Thin films, Coating, Refractive index, Thin film coatings, Silicon films, Silicon, Silica, Multilayers

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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.

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