arXiv subjects

Daniel Franta

Publications and source records attributed to Daniel Franta.

3 recordsLinked to original sources

arXiv2026

Dispersion models describing coupled systems in doped crystals. II. Infrared phonon excitations in GaAs single crystals and phonon-plasmon coupling in Zn-doped GaAs

We present a temperature-dependent infrared study of intrinsic and Zn-doped GaAs within a generalized dispersion framework describing coupled phonon and free-carrier excitations. Reflectance, transmittance, and ellipsometric data measured from 300 to 440 K are fitted simultaneously using Gaussian-broadened distributions for single-phonon modes and triangle-delta-triangle representations for multiphonon absorption bands. The formalism provides a self-consistent description of lattice and carrier contributions to the dielectric response and reproduces the evolution of phonon-plasmon coupling with doping and temperature. In heavily doped samples, a pronounced Fano-type asymmetry of the TO-phonon resonance is observed and attributed to interference between phonon excitations and the Drude background of light and heavy holes. The resulting optical constants remain physically consistent across the far- and mid-infrared range and demonstrate the applicability of the framework to coupled bound and free excitations in doped semiconductors. The approach provides a quantitative tool for modeling infrared optical properties of GaAs and related photonic materials.

physics.optics↗

arXiv2026

Effect of oxygen content on optical, structural, and dielectric properties of Al$_x$Ta$_y$O$_z$$ thin films

This study reports on the optical, structural, and dielectric properties of aluminum tantalum oxide (Al$_x$Ta$_y$O$_z$) thin films deposited at low temperature on silicon and steel substrates by pulsed direct current reactive magnetron sputtering of a target containing 80 at.% aluminum and 20 at.% tantalum in Ar/O$_2$ atmosphere. Oxygen flow rates ranging from 5.0 to 20 sccm corresponded to O content changes from 57.7 to 69.6 at.% and resulted in large differences in dielectric behavior, from films with no measurable dielectric strength to a dielectric strength of 231 V$\mu$m$^{-1}$, respectively. Ab initio calculations were employed to explain the large property changes, and we show that a decrease in the dielectric strength can be linked to the formation of metal-metal bonds in the material, when the O content is less than what would correspond to a stoichiometric Ta$_2$O$_5$ and Al$_2$O$_3$ mixture. The electronic states corresponding to the metal--metal bonds are located in the band gap close to the top of the valence band, leading to an effective band gap reduction, which is directly supported by X-ray photoelectron spectroscopy valence band measurements and by a broad optical absorption in the visible region.

cond-mat.mtrl-sci↗

arXiv2016

Materials Pushing the Application Limits of Wire Grid Polarizers further into the Deep Ultraviolet Spectral Range

Wire grid polarizers (WGPs), periodic nano-optical meta-surfaces, are convenient polarizing elements for many optical applications. However, they are still inadequate in the deep ultraviolet spectral range. We show that to achieve high performance ultraviolet WGPs a material with large absolute value of the complex permittivity and extinction coefficient at the wavelength of interest has to be utilized. This requirement is compared to refractive index models considering intraband and interband absorption processes. We elucidate why the extinction ratio of metallic WGPs intrinsically humble in the deep ultraviolet, whereas wide bandgap semiconductors are superior material candidates in this spectral range. To demonstrate this, we present the design, fabrication and optical characterization of a titanium dioxide WGP. At a wavelength of 193 nm an unprecedented extinction ratio of 384 and a transmittance of 10 % is achieved.

physics.optics↗