Development of intersubband terahertz lasers using multiple quantum well structures

Abstract

This thesis describes an experimental and theoretical effort in developing intersubband THz lasers using multiple-quantum-well structures. Scarcity of compact solid state sources in this frequency range, and to demonstrate a novel unipolar laser technology, motivated this research. Transport studies for realizing THz intersubband population inversion, new methods for long-wavelength mode confinement, and farinfrared spectral measurement techniques are critical steps in achieving this goal. Conduction-band three-level subband systems in triple-quantum-well structures using GaAs/Alo.3Gao.7As heterostructures were proposed, designed, and simulated by a numerical method. The numerical simulation is a self-consistent solution among the Schrödinger equation, Poisson equation, and rate equations. Electrons are injected by resonant tunneling to populate the upper subband; the lower subband is depopulated by fast longitudinal optical (LO) phonon scattering. THz emission devices consist of many modules of such triple-quantum-structures with the three-level systems cascade connected to each other. Dynamic charge of electron is provided by the 6-doping per module. Temperature-dependent intersubband scattering plays a key role in transport modeling and therefore the degree of population inversion. Systematic calculations were performed to address issues of hot electron effect, lattice heating, and non-equilibrium optical phonons. Guidelines for device design and optimization were provided. The measured dc I-V at cryogenic temperature confirmed the design expectations. Plasma confinement is used for making THz laser cavities. The minimum cavity loss can only be achieved by using metallic waveguides. The first metallic waveguide, which incorporates non-alloyed ohmic contact, was successfully fabricated by combining wafer bonding and selective etching techniques. Schemes for THz emission couplings were investigated by quantifying coupling loss, including surface coupling by gratings and edging coupling by facets. The first free-space THz spectral measurement system was developed using a Fourier Transform Infrared (FTIR) spectrometer. This experimental set-up was successfully demonstrated in resolving THz emission by using step-scan and lock-in techniques, and a fast Ge:Ga photon detector. Spontaneous intersubband THz emission was observed with linewidth narrower than 0.65 THz, and center frequency at the designed value of 3.8 THz. Different triple-quantum-well structures were designed, grown, and tested. The measured emission power levels were one order of magnitude lower than calculated values, and possible extra cavity loss mechanisms were discussed. To verify the triple-quantum-well structure design, a mid-infrared absorption measurement was performed on a sample grown on semi-insulating substrate. Information such as subband energy separations, dipole moments, and linewidth broadening, was extracted from the absorption spectrum and gave a good confirmation on numerical simulations and MBE growth quality of the MQW structures.