Beam Pattern Engineering of Metamaterial Terahertz Quantum-Cascade Devices
- Author(s): Hon, Philip Wing-Chun
- Advisor(s): Itoh, Tatsuo
- Williams, Benjamin S
- et al.
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.