Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Electrical Engineering

First Adviser

Kumar, Sushil

Other advisers/committee members

Kumar, Sushil; Stavola, Michael; Wierer, Jonathan; Zhou, Chao

Abstract

Since the fist demonstration of a terahertz (THz) semiconductor quantum cascade laser (QCL) in 2001, significant progress has been made towards development of THz QCLs. Among all solid-state sources of radiation, THz QCLs are the only type of coherent sources of THz radiation that can provide average optical power output of much greater than 1mW. THz QCLs could be designed to emit in the frequency range of 1.2-5.4 THz, and continuous-wave (cw) operation has been realized above the temperature of liquid-Nitrogen. Despite these developments, challenges still remain towards understanding the fundamental physics of intersubband transitions on which QCLs are based, and also towards further development of THz QCLs so that they could be readily used for various applications in THz chemical sensing and spectroscopy. This thesis describes four distinct technical contributions in the area of THz devices based on intersubband transitions in semiconductor superlattices: (a)~design and experiments towards developmentof hole-based intersubband lasers, (b)~demonstration of first electrically tunable THz QCLs that could operate at temperatures much greater than that of liquid-Helium, (c)~theoretical work towards development of a THz QCL based spectroscopic sensing scheme, and (d)~design and preliminary demonstration of a THz quantum-cascade intersubband photodetector utilizing a fast resonant-phonon depopulation scheme for relaxation of photo-excited electrons for high-temperature operation. The key aspects of each of the above contributions are briefly summarized below. THz intersubband transitions within the conduction band of semiconductor superlattices have been widely studied in the past decade leading to development of THz QCLs. However, considerably few experimental studies have explored the properties of hole-hole intersubband transitions in the valence band. By taking into account the band mixing effect in the valence band, optical transitions between hole-subbands can occur with either the normal-to-plane polarized light or in-plane polarized light. This makes valence band intersubband transitions particularly interesting, becausesurface-emitting QCLs and normal-incidence THz detectors could potentially be developed. This work develops a density-matrix model of hole transport in p-type quantum-wells that is suitable for numerical evaluation of intersubbandgain or loss. Two hole-based intersubband laser designs are proposed and experimentally investigated. Sequential resonant-tunneling transport is demonstrated, which indicates effective quantum-transport for heavy-holes and is promising toward development of the first hole-based intersubband laser.Many molecules have very strong characteristic THz rotational and vibrational transitions, and hence could be ``fingerprinted'' with THz spectroscopy. Such applications ideally require electrically tunable THz sources. For THz QCLs, mode pulling due to Stark-shift of the intersubband gain transition provides electrical tuning at low-temperatures. In this thesis, a new tuning technique based on detuned intersubband absorption is developed, which can efficiently modify electric-susceptibility of the semiconductor medium and thus tune the resonant-frequency of a THz microcavity. Utilizing a coupled microcavity architecture, this approach is then utilized to electrically tune THz QCLs, with a major advantage that such a mechanism works for QCLs operating at much higher temperatures compared to previously demonstrated methods. Based on theory and finite-element electromagnetic simulations, this thesis also proposes a broadband THz spectroscopic sensing scheme that utilizes on-chip arrays of single-mode QCLs. Such a scheme could be utilized for sensing of the THz refractive-index of solids or liquids in a reflection geometry. No THz detectors or spectrometers are required because the active region of the QCL acts as a non-linear detection medium by itself. The sensing scheme is inherently immune to intensity fluctuations of QCLs' output or other interference effects because it relies on measurement of the shift in the resonant-frequency of the QCLs. Better than parts per million refractive-index-unit (RIU) sensitivities are estimated across the broad frequency range for the proposed sensing scheme. In addition to high-power radiation sources, compact, low-cost, and high-sensitivity THz photodetectors are also essential components towards realization of THz sensing and spectroscopy systems. This thesis also proposes a novel type of sensitive THz photodetector, the quantum-cascade detector (QCD), which operates at non-zero electrical bias. The design scheme primarily utilizes a superlattice structure that utilizes ultrafast electron-phonon scattering in GaAs/AlGaAs for relaxation of photoexcited electrons in the conduction band, much like that in resonant-phonon depopulation based THz QCLs. Qualitative analysis about the detector's current-noise, responsivity(R), and detectivity (D*) suggest that the newly proposed intersubband photodetectorcould potentially operate at higher temperatures than existing zero-bias quantum-cascade detectors, and with higher responsivities. Preliminary experimental results are demonstrated, which show strong photo-response at a temperature of 50 K that is already comparable to the maximum operating temperature of conventional QCDs reported inliterature.

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