Elliott Brown (Advisor), Jason Deibel (Committee Member), Julie Jackson (Committee Member), Daniel Lemaster (Committee Member), Doug Petkie (Committee Member)
Doctor of Philosophy (PhD)
Since the first demonstration of the generation of terahertz (THz) pulses from photoconductive (PC) antennas, research has pushed toward the development of smaller, cost efficient, and faster THz systems. This dissertation presents the work accomplished in order to realize these more practical terahertz (THz) photoconductive (PC) systems.
First, this work will present a novel ErAs:GaAs photoconductive switch used to make a THz source excited by 1550 nm laser pulses. It will be shown that the excitation process taking place in the material relies on extrinsic (rather than intrinsic) photoconductivity. Then, several experiments will be presented that aim to improve the efficiency of the device and further the understanding of the underlying physical mechanisms. The erbium composition of the photoconductive layer will be varied and the effects of these variations on THz generation will be investigated. Then the wavelength of the drive laser used to excite the extrinsic photoconductive mechanism will be varied, while recording the photocurrent responsivity. This wavelength study will be used to find the optimal drive wavelength for maximum THz power. In conclusion, the results of these experiments will show that extrinsic PC THz generation is practical, cost effective, and capable of producing an average THz power of more than 100 μ W. Coinciding with this high power level, the bandwidth of this new source was found to be ~350 GHz, corresponding to a photocarrier recombination time of 450 fs. The work presented in this section will provide a path to develop superior THz PC sources that have a higher THz-power-to-cost ratio than the current state of the art. Photoconductive antennas are mostly used to conduct spectroscopy measurements, either in time domain systems (TDS) or in frequency domain systems (FDS). Currently, both techniques can reach high-frequencies (>1 THz) but struggle to do so while making fast, high-resolution measurements (<2 GHz). In addition, both methods can be time consuming to set up and perform. A superior spectrum analysis technique would greatly facilitate THz application development by making results easier and less expensive to obtain. Therefore, the second part of this dissertation addresses the need for quicker and more precise THz spectrum analysis by demonstrating a new type of THz spectrum analyzer based on a high-speed, tunable, Fabry-Perot interferometer. This new and unique spectrum analyzer reduces the time required to obtain a THz spectrum (a few seconds), while producing a more precise result (<2 GHz resolution). After the presentation of this concept, the various experimental design iterations will be shown, while explaining the improvements gained from each. Then experimental demonstrations of the new spectrum analyzer will be presented, and possible future improvements will be discussed. While the Fabry-Perot based spectrum analyzer is an improvement for THz spectroscopy, it can suffer from two issues: mirror reflectivity that changes with frequency, and the inability to easily tune the mirror reflectivity to optimize the system for different applications. These issues make it challenging to obtain an accurate and useful THz spectrum. Therefore the third part of this dissertation is motivated by these problems and presents a solution; the use of structured-surface-plasmon (SSP) enhanced polarizers as Fabry-Perot mirrors. The SSP polarizers used in this work are composed of metal wire-grids with sub-wavelength feature sizes and high metal fill-factors. It will be shown that high fill-factor SSP polarizers can achieve superior THz performance, compared to traditional THz polarizers, with an extinction ratio exceeding 60 dB. With the use of these polarizers as mirrors, the Fabry-Perot can achieve variable mirror reflectivity by changing the polarizer orientation angle. This will allow t...
Department or Program
Ph.D. in Engineering
Year Degree Awarded
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