Date of Graduation

12-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Magda El-Shenawee

Committee Member

Shui-Qing Yu

Second Committee Member

Morgan Ware

Third Committee Member

Joseph Herzog

Fourth Committee Member

Rick Wise

Keywords

Pure sciences; Applied sciences; Antenna; Optoelectronics; Terahertz

Abstract

Generation of broadband terahertz (THz) pulses from ultrafast photoconductive antennas (PCAs) is an attractive method for THz spectroscopy and imaging. This provides a wide frequency bandwidth (0.1-4 THz) as well as the straightforward recovery of both the magnitude and phase of the transmitted and/or reflected signals. The achieved output THz power is low, approximately a few microwatts. This is due to the poor conversion of the femtosecond laser used as the optical pump to useable current inside the antenna semiconducting material. The majority of THz power comes from the photocarriers generated within ~ 100 nm distance from the antenna electrodes. However, the optical beam covers larger spot size, therefore much of the absorbed optical photons do not contribute to the THz power.

The goal of this work is to advance the design, fabrication, and measurement of THz-PCAs to generate significantly improved output power. This work proposed a plasmonic enhanced thin-film photoconductive antenna to enhance optical carrier generation in the PCA. The electromagnetic wave equations were solved in order to compute the enhanced plasmonic field in the semiconductor. The Poisson’s and the drift-diffusion equations were solved in order to compute the carrier dynamics inside of the semiconductor. A parametric optimization was implemented in order to design the plasmonic nanodisks and the thickness of the ultrathin photoconductive layer. These solutions and optimizations were achieved using the commercial package COMSOL® Multiphysics model. The PCAs’ fabrication was accomplished using the electron beam lithography for patterning the plasmonic nanostructures, the molecular beam epitaxy for the sample growth, the lapping/selective etching for the epitaxial liftoff, and standard microfabrication practices for patterning the antenna and device packaging. The PCA was characterized utilizing a tunable pulsed laser system with a 100 fs pulse width for the optical excitation and a Gentec-EO pyroelectric power detector for measurement of the output THz power. Also, the spectral characterization of the PCA was conducted, in collaboration with Teraview LTD in their site at UK, using a THz time-domain spectroscopy experimental set-up. The results demonstrate the enhancement in the output THz power of the plasmonic thin-film PCAs in comparison with conventional THz-PCAs.

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