Broadband IR stokes polarimetry for the electro-optic characterization of cadmium zinc telluride
Date
2017-12-21
Authors
FitzGerald, William
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Abstract
The infrared portion of the electro-magnetic spectrum is a challenging region in which
to perform optical techniques, limited by both device efficiency and availability. In
this dissertation, a new optical technique is introduced to facilitate polarization state
measurement across the mid-IR. In addition, cadmium zinc telluride (CZT) is investigated
as a potential new material suitable for electro-optic devices which function in the mid-IR,
while also being characterized by other optical analysis methods.
Thin film interference is discussed as it relates to optical techniques and electronic
devices. A Stokes polarimeter is used to study the oxide development on the surface of
CZT electronic devices, and the effect of natural thin films on substrates used in optical
techniques is discussed. In particular, the impact of thin film interference on sum-frequency generation spectroscopy measurements of methyl group orientation are assessed.
An FTIR source operated in step-scan mode is used to create a broadband, IR Stokes
polarimeter which measures the polarization state of light from 2.5-11 μm simultaneously.
Its design, involving two photo-elastic modulators and an analyzer, and theory are described in detail. This instrument is demonstrated by measuring linearly polarized light, and is applied to the measurement of the refractive index dispersion of quartz from 2.5-4 μm, which goes beyond the limits of literature values.
Electro-optic crystals of CZT with electrodes of gold and indium are characterized
at each wavelength in the mid-IR in terms of their electro-optic effects and apparent
depolarization using the Stokes polarimeter. The material displays high-resistivity,
allowing it to be operated with up to 5 kV applied DC voltage. The linear electro-optic
effect is observed, but overall properties of the samples are found to be heavily dependent
on the choice of metal for the electrodes. With a high-work function electrode material
in gold, a large depletion region is created when high voltage is applied, which leads to a
gradient in electric field throughout the material. This causes a beam of light transmitted through it to experience a distribution of electro-optic behaviours, which leads to overall
depolarization of the light. Indium’s work function is lower than gold’s, and is closer to
that of CZT. With indium electrodes, the electric field is found to be more consistent, and
behaviour is much closer to ideal.
The electro-optic effect of CZT is also characterized with AC applied voltage in order
to assess its suitability to AC applied voltage applications. The power supply used for
this was limited to 60 Hz, which precludes a complete characterization in this regard, but
unexpected behaviour was seen. A methodology utilizing an oscilloscope and FTIR was
developed in order to more completely understand the material response, and divergent
behaviour with positive and negative voltage was found.
Description
Keywords
applied optics, polarimetry, ellipsometry, electro-optic materials, CZT, cadmium zinc telluride