The terahertz (THz) frequency range (300 GHz to 10 THz, wavelength 30-1000 μm), despite having many potential applications, is technologically relatively under-developed mainly because of the lack of suitable coherent radiation sources when com- pared with nearby electromagnetic radiation spectrum. The invention of the THz quantum cascade laser, a electronically-pumped semiconductor heterostructure which emits photons from electronic intersubband transitions, provides the first solid state fundamental oscillator at the frequency range from 1.2 to 5.1 THz. Due to the sub-wavelength confinement nature of the metal-metal waveguide used in most of the THz QC lasers, far-field beam patterns from lasers with simple Fabry-Perot waveguides are divergent and far from ideal Gaussian beams.
The first part of the talk describes the development of single-mode THz QC lasers on metal-metal waveguides. Starting with the corrugated third-order DFB laser—a clever laser structure which utilizes end-fire array effect to achieve low divergence beam patterns—several applications using densely-packed third-order DFB laser arrays have been investigated. With the improved design rules and fabrication techniques, 830 GHz single-mode frequency coverage on a monolithic multicolor DFB laser array has been achieved.
Later development applies the concept of microstrip antenna—a structure commonly used in microwave engineering—to THz photonics devices. By coupling the antenna to each grating aperture of a perfectly phase-matched DFB laser, the radiation impedance of the laser can now be tuned to enhance the overall emission efficiency. This novel genre of Microstrip Antenna Coupled DFB laser (MAC-DFB laser) achieves the highest slope efficiency (∼450 mW/A) and wall-plug efficiency (0.57%) of all THz DFB laser sources.
The second part of the talk then focuses on the development of the first light amplifier in THz frequency using active meta-surface. The proposed amplifier is consisted of an array of short-cavity surface-emitting second-order distributed feed-back lasers arranged in a two-dimensional grid with sub-wavelength spacing. A overall system power gain of 7.5 dB is achieved at ∼3 THz . The free-space THz light amplifier can be used as the pre-amplifier for a THz heterodyne receiver system to reduce the receiver system noise, or be placed on the focal plane of a THz imaging system to enhance the signal-to-noise ratio of the image and reduce the acquisition time.
Advisor: Prof. Qing Hu
Committee: Prof. Tomas Palacios, Prof. Michael Watts