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We present narrow-band polarization-sensitive reflectance of GaN/AlGaN heterostructures in the mid-infrared range. Experimental measurements performed at 15° angle of incidence show the excitation of a Berreman mode at the interface between GaN and sapphire substrate. A transfer matrix method for anisotropic layers has been used to analyze the obtained results. The contribution of the two-dimensional electron gas at the interfaces of the heterostructures has been included by proper modelization of an effective thin layer.

We performed infrared optical characterization of polycrystalline β-Ga2O3 films, in the 10-18 μm range, deposited by metal organic chemical vapor deposition on sapphire substrates. Our results show that it is possible to obtain a dominant β-phase film, with a well-defined, normal to surface z-axis orientation. These results are confirmed by reflection spectra performed at 45° incidence angle which reveals a z-phonon Reststrahlen band as a function of the incident field linear polarization.

We investigated infrared reflectivity of undoped and Tungsten (W) doped Vanadium dioxide (VO2) films at varying temperatures. Undoped VO2 exhibited a clear phase transition at 100°C, achieving near 0% reflectivity, or perfect light absorption. As W doping concentration increased, the phase-transition temperature decreased, maintaining the zero-reflectivity condition. Only a 0.75% W doping enabled room temperature perfect absorption without heating the film.

The phonon modes of materials contain critical information on the quality of the crystals. Phonon modes also offer a wide range of polarization-dependent resonances in infrared that can be tailored to applications that require large dielectric function contrast in different crystal directions. Here, we investigate the far-field characteristics of MOCVD-grown Ga2O3 thin films. With a combination of cross-polarization FTIR and AFM characterization techniques, we propose an easy and non-invasive route to distinguish κ and β phases of Ga2O3 and study the quality of these crystals. Using numerical methods and cross-polarization spectroscopy, the depolarization characteristics of β-Ga2O3 are examined and depolarization strength values as high as 0.495 and 0.76 are measured, respectively, for 400 and 800 nm-thick β-Ga2O3. The strong birefringence near optical phonon modes of an 800 nm β-Ga2O3 on a sapphire substrate is used to obtain several polarization states for the reflected light in the second atmospheric window 8–14 µm. We anticipate that our findings open a new path for material characterization and wave plate design for the mid-IR range and offer novel possibilities for the future of IR on-chip photonics, thanks to the compatibility of β-Ga2O3 with standard nanofabrication technology. [ABSTRACT FROM AUTHOR]

We performed infrared optical characterization of polycrystalline MoO3 films deposited by pulsed laser deposition on fused silica substrates. Several samples have been fabricated using different parameters such as temperature and oxygen pressure. Our analysis shows that under appropriate fabrication conditions it is possible to obtain a dominant α-phase film, with a well-defined, normal to surface (z-axis) orientation. These results are confirmed by reflection spectra performed at 45° incidence angle revealing a strong modulation of the sharp z-phonon Reststrahlen band as a function of the incident field linear polarization.

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters. [ABSTRACT FROM AUTHOR]