Beam Pattern Engineering of Metamaterial Terahertz Quantum-Cascade Devices

Author(s): Hon, Philip Wing-Chun | Advisor(s): Itoh, Tatsuo; Williams, Benjamin S | Abstract: Generation and detection of microwave radiation is done with electronic systemswhere the underyling processes involve oscillating free charges (such as on an an-tenna or within a transistor or diode). On the higher energy side of the spectrum,generation and detection of near infrared and visible radiation is achieved viaquantum transitions with emission wavelengths that are dictated by the material.Solutions moving up towards the THz regime using microwave based solutions arelimited by carrier transit time and RC time-constant limitations. Techniques andsolutions moving down toward the THz regime using photonic techniques haveemission wavelengths naturally limited by the band gap of the material. However,THz quantum-cascade (QC) lasers, which are an extension of photonic conceptsto lower energies, have artificially engineered energy levels and hence emissionwavelengths. THz QC-lasers have been demonstrated to operate at frequenciesbetween 1.2 and 5.0 THz and the best high-temperature operation is based uponthe metal-metal (MM) waveguide configuration, in which the multiple-quantum-well active region is sandwiched between two metal cladding layers, typically sep-arated by 2-10 μm. Soon after the demonstration of MM waveguide QC-lasers,it was recognized that the beam pattern from a conventional cleaved-facet Fabry-Perot (FP) ridge cavity produced a highly divergent beam pattern, characterizedby concentric rings in the far field.This thesis presents work on a new approach to tailor the beam pattern of THzMM waveguide QC-devices. Namely, dispersion engineering using metamaterialsbased on the composite right/left-handed (CRLH) transmission line formalism isadapted to the MM waveguide configuration to realize an entirely new class ofdevices. Dispersion, radiative loss, and radiation patterns are presented for manynewly designed 1-D and 2-D THz QC transmission line metamaterial designs. Thefirst ever active 1-D THz QC transmisison line metamaterial is experimentallycharacterized and its radiation pattern and polarization closely match theoreticaland full-wave finite element method (FEM) simulated predictions.Proven microwave techniques such as circuit, antenna cavity modeling andarray factor theory are used to understand the radiative properties of conventionalTHz QC-lasers. We predict far-field beam patterns and polarizations, approximatecavity quality factors, and associate these properties with individual surfaces orstructures of the device. The analysis technique is also applied to the project's 1-Dand 2-D THz CRLH QC-devices yielding qualitative agreement with experiments.The first THz design, analysis and experimental verification of a metasur-face comprised of an array of passive THz QC transmission lines is presented.By using the cavity model, array factor, circuit and electromagnetic theory asurface impedance model is developed to characterize the metasurface. The sur-face impedance model reveals waveguide mode dependent radiative coupling withthe light line and capacitve/inductive surface impedance. Polarization dependentangle-resolved Fourier transform infrared reflection spectroscopy measurementsmatch the model and full-wave FEM predictions, further assisting the under-standing of such devices.To address the broader goal of a directive and scalable THz QC-device, thefeasibility of a 2-D metamaterial inspired QC-laser and an active reflectarray isconsidered. Finally, preliminary work on a technology enabling active metasurfacereflector for a QC vertical external cavity surface emitting laser is discussed.