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The Raman spectrum and particularly the Raman scattering intensities of monoclinic β-Ga2O3 are investigated by experiment and theory. The low symmetry of β-Ga2O3 results in a complex dependence of the Raman intensity for the individual phonon modes on the scattering geometry which is additionally affected by birefringence. We measured the Raman spectra in dependence on the polarization direction for backscattering on three crystallographic planes of β-Ga2O3 and modelled these dependencies using a modified Raman tensor formalism which takes birefringence into account. The spectral position of all 15 Raman active phonon modes and the Raman tensor elements of 13 modes were determined and are compared to results from ab-initio calculations.

Molecular self‐attack: According to mythology, a scorpion may sting itself to death; similarly, 3‐aminopropyltriethoxysilane catalyzes its own hydrolysis in the atomic layer deposition (ALD) of SiO2 thin films and nanostructures. Between 120 and 200 °C, the growth rate is constant at 0.06 nm per ALD cycle. The SiO2 films are chemically and optically pure. SiO2 nanotubes of aspect ratio 500 exhibit smooth walls of accurately controlled thickness.

The insulator-to-metal transition (IMT) in vanadium dioxide (VO 2 ) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO 2 for optics, the wavelength-dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO 2 thin films across the IMT is characterized for free-space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective-medium theory. VO 2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol-gel). While there are differences in the optical properties of VO 2 synthesized under different conditions, these differences are surprisingly small in the 2-11 µm range where the insulating phase of VO 2 also has relatively low optical loss. It is anticipated that the refractive-index datasets from this article will be broadly useful for modeling and design of VO 2 -based optical and optoelectronic components, especially in the mid-wave and long-wave infrared.

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We present a formalism for calculating the Raman scattering intensity dependent on the polarization configuration for optically anisotropic crystals. It can be applied to crystals of arbitrary orientation and crystal symmetry measured in normal incidence backscattering geometry. The classical Raman tensor formalism cannot be used for optically anisotropic materials due to birefringence causing the polarization within the crystal to be depth dependent. We show that in the limit of averaging over a sufficiently large scattering depth, the observed Raman intensities converge and can be described by an effective Raman tensor given here. Full agreement with experimental results for uniaxial and biaxial crystals is demonstrated.

Efficient suppression of reflection is a key requirement for perfect absorption of light. Recently, it has been shown that reflection can be effectively suppressed utilizing a single ultra-thin film deposited on metals or polar materials featuring phonon resonances. The wavelength at which reflection can be fully suppressed is primarily determined by the nature of these substrates, and is pinned to particular values near plasma or phonon resonances-the former typically in the ultraviolet or visible, the latter in the infrared. Here, we explicitly identify the required optical properties of films and substrates for the design of absorbing antireflection coatings based on ultra-thin films. We find that completely suppressed reflection using films with thicknesses much smaller than the wavelength of light occurs within a spectral region where the real part of the refractive index of the substrate is n ≲ 1, which is characteristic of materials with permittivity close to zero. We experimentally verify this condition by using an ultra-thin vanadium dioxide film with dynamically tunable optical properties on several epsilon-near-zero materials, including aluminum-doped zinc oxide. By tailoring the plasma frequency of the aluminum-doped zinc oxide, we were able to tune the epsilon-near-zero point, thus achieving suppressed reflection and near-perfect absorption at wavelengths that continuously span the near-infrared and long-wave mid-infrared ranges.

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