Terahertz Quantum-Cascade Transmission-Line Metamaterials
May 02, 2012
from 03:00 PM to 05:00 PM
|Where||ENGR. IV Bldg. Maxwell Room 57-124|
|Contact Name||Amirali Tavallaee|
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Amir Ali Tavallaee
Advisor: Benjamin S. Williams
Terahertz quantum-cascade (QC) lasers operating at 0.6-5 THz (n»60-500 µm) are poised to become the dominant solid-state sources of continuous-wave (cw) far-infrared radiation enabling applications in terahertz spectroscopy, imaging, and sensing. QC-lasers are the longest wavelength semiconductor laser sources in which terahertz gain is obtained from electronic intersubband radiative transitions in GaAs/AlGaAs heterostructure quantum wells. Since their invention in 2001, rapid development has enabled demonstration of cw powers greater than 100 mW. However, challenges still remain in the areas of operating temperature, laser efficiency and power, and beam quality to name a few. The highest-temperature operation of terahertz quantum-cascade lasers (200 K pulsed, 117 K cw) depends on the use of a low-loss ``metal-metal'' waveguide where the active gain material is sandwiched between two metal cladding layers; a technique similar, in concept, to microstrip transmission line technology at microwave frequencies. Due to the subwavelength transverse dimensions of the metal-metal waveguide, however, obtaining a directive beam pattern and efficient out-coupling of THz power is non-trivial.
This talk presents the demonstration of a one-dimensional waveguide for terahertz quantum-cascade lasers that acts as a leaky-wave antenna and tailors laser radiation in one dimension to a directional beam. This scheme adapts microwave transmission-line metamaterial concepts to a planar structure realized in terahertz metal-metal waveguide technology. The leaky-wave metamaterial antenna operates based on a propagating mode with an effective phase index smaller than unity such that it radiates in the surface direction via a leaky-wave mechanism. Surface emission (40° from broadside) with a single directive beam (FWHM=15°) at 2.74 THz was demonstrated from terahertz QC-lasers with leaky-wave coupler antennas which exhibited slope efficiencies four times greater than conventional Fabry-Perot metal-metal waveguides. Using this technique the first demonstration of beam scanning for a terahertz QC-laser was reported (from 35-60°) as the frequency of operation varied from 2.65-2.81 THz.
Towards the bigger goal of realizing an active terahertz metamaterial to ultimately develop ``zero-index'' terahertz quantum-cascade lasers immune to spatial hole burning, or ``negative-index'' metamaterials for superresolution terahertz imaging, a composite right-/left-handed transmission-line metamaterial based upon subwavelength metal-metal waveguide loaded with terahertz QC material was demonstrated. Due to the addition of distributed series capacitors (realized by introducing gaps in top metallization) and shunt inductors (realized by operating in the higher-order lateral mode of the waveguide), the transmission-line metamaterial exhibits left-handed (backward waves or negative index) leaky-wave propagation from 2.3-2.45 THz, in addition to the conventional right-handed leaky-wave behavior (from 2.6-3.0 THz).
Amir Tavallaee is a PhD candidate at UCLA in the electrical engineering department specializing in terahertz quantum-cascade lasers, active terahertz metamaterials, and sub-wavelength plasmonics.