Multiple Birefringence Effects in the Diluted Magnetic Semiconductor Cadmium Manganese Telluride: Applications from Microwave to Terahertz.

Abstract

Nondestructive evaluation is commonly used in science and industry to evaluate the properties of a material, component or system without causing damage. In this thesis, optical nondestructive evaluation tools are developed using a diluted magnetic semiconductor, CMT. A single <110> CMT crystal probe is employed as a sensor in both the microwave and THz electromagnetic wave region. Both theory and experimental results demonstrate that a diluted-magnetic-semiconductor CMT crystal exhibits both Faraday rotation and electric-field-induced linear birefringence. Utilizing this characteristic, a single probe that is capable of sensing both electric and magnetic fields independently is presented here. In addition, the linear electro-optic coefficient, r41 for CMT has been calculated from electric-field measurements to be 3.5 ± 0.2 pm/V. The ability to measure the electric or magnetic field components is exploited to develop a Poynting vector sensor that requires no further transformational calculation or physical information about the device. Maps of the microwave Poynting vector along a 50-Ω microstrip with different terminations were experimentally determined. An open termination microstrip shows no energy flow, whereas a matched-load microstrip shows consistent energy flow along the microstrip transmission line. These results demonstrate that the Poynting vector can be extracted from the components of the electric and magnetic field by utilizing a single <110> CMT crystal. CMT is also employed as an EO sensing crystal in a THz time-domain reflectometry system that is used to detect both surface and subsurface defects of thermal barrier coatings. A 50 μm surface defect of a thermal barrier coating is identified even though its width is beyond the diffraction limit associated with a 1-THz frequency component. Moreover, when a history of pulsed reflectometry measurements is maintained, this THz system can also monitor the evolution of thermally-induced oxide layers and voids – embedded at a ceramic/metal interface – that are on the order of a single micrometer in thickness. This was accomplished through the observation of THz pulse time delays and changes in the width and shape of the THz pulses. These features can be used to predict the lifetime of thermal-barrier coatings.