Spectral Tuning of Polarization Selective Reflections Bands in GLAD deposited HfAlN chiral sculptured thin films
arxiv(2024)
摘要
We present the first report on fabrication of Hafnium aluminum nitride chiral
sculptured thin films (CSTFs) using reactive magnetron sputtering in a glancing
angle deposition configuration, and the analysis of its optical polarization
properties. The resulting CSTFs were designed to give interference extrema or
so-called circular Bragg (CB) resonances at desired wavelengths in the region
from 370 to 690 nm. This was achieved by tailoring the growth of the chiral
thin films to obtain a dielectric pitch between 87 and 260.9 nm. The spectral
positions of the obtained CB resonances were compared to values from analytical
expressions. Contrary to the common case where the dielectric pitch is half of
the growth-related rotational pitch due to a 180 symmetry, this pitch was
shown to be the same as the rotational pitch. It is concluded that this is due
to the c-axis of the CSTF being tilted about 45 from the substrate
normal. The morphology and crystallographic characterizations were done using
scanning electron microscopy and X-ray diffraction, respectively, while the
tilt of the crystal lattice was corroborated using X-ray diffraction pole
figures. The optical response from the CSTFs was analyzed using Mueller matrix
spectroscopic ellipsometry from which the degree of circular polarization at
the CB resonances was obtained. In addition, a strong non-reciprocal reflection
was observed which could be attributed to the helicoidal morphology and the
intrinsic crystal tilt. An optical layered model of the chiral structure
including azimuthal twist and using the Cauchy dispersion relations was used to
simulate the Mueller matrix elements and compare with the ellipsometry
measurements. The correlation between the simulated and experimental data gave
information of the morphological parameters of the CSTF and its optical
properties.
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