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Beam Pattern Engineering of Metamaterial Terahertz Quantum-Cascade Devices

  • Author(s): Hon, Philip Wing-Chun
  • Advisor(s): Itoh, Tatsuo
  • Williams, Benjamin S
  • et al.
Abstract

Generation and detection of microwave radiation is done with electronic systems

where 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 via

quantum transitions with emission wavelengths that are dictated by the material.

Solutions moving up towards the THz regime using microwave based solutions are

limited by carrier transit time and RC time-constant limitations. Techniques and

solutions moving down toward the THz regime using photonic techniques have

emission wavelengths naturally limited by the band gap of the material. However,

THz quantum-cascade (QC) lasers, which are an extension of photonic concepts

to lower energies, have artificially engineered energy levels and hence emission

wavelengths. THz QC-lasers have been demonstrated to operate at frequencies

between 1.2 and 5.0 THz and the best high-temperature operation is based upon

the 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-

Pérot (FP) ridge cavity produced a highly divergent beam pattern, characterized

by concentric rings in the far field.

This thesis presents work on a new approach to tailor the beam pattern of THz

MM waveguide QC-devices. Namely, dispersion engineering using metamaterials

based on the composite right/left-handed (CRLH) transmission line formalism is

adapted to the MM waveguide configuration to realize an entirely new class of

devices. Dispersion, radiative loss, and radiation patterns are presented for many

newly designed 1-D and 2-D THz QC transmission line metamaterial designs. The

first ever active 1-D THz QC transmisison line metamaterial is experimentally

characterized and its radiation pattern and polarization closely match theoretical

and full-wave finite element method (FEM) simulated predictions.

Proven microwave techniques such as circuit, antenna cavity modeling and

array factor theory are used to understand the radiative properties of conventional

THz QC-lasers. We predict far-field beam patterns and polarizations, approximate

cavity quality factors, and associate these properties with individual surfaces or

structures of the device. The analysis technique is also applied to the project's 1-D

and 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 a

surface impedance model is developed to characterize the metasurface. The sur-

face impedance model reveals waveguide mode dependent radiative coupling with

the light line and capacitve/inductive surface impedance. Polarization dependent

angle-resolved Fourier transform infrared reflection spectroscopy measurements

match 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, the

feasibility of a 2-D metamaterial inspired QC-laser and an active reflectarray is

considered. Finally, preliminary work on a technology enabling active metasurface

reflector for a QC vertical external cavity surface emitting laser is discussed.

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