From high power terahertz quantum cascade lasers to terahertz light amplifiers

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 201-208).The terahertz (THz) frequency range (300 GHz to 10 THz, wavelength 30-1000 [mu]m), despite having many potential applications, is technologically relatively underdeveloped mainly because of the lack of suitable coherent radiation sources when compared 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 solidstate fundamental oscillator at the frequency range from 1.2 to 5.1 THz. Due to the subwavelength 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 this thesis 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, such as frequency agile sources for THz swept-source optical tomography and local oscillators for THz heterodyne receivers with precise frequency control, 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. The origin of the deterioration in far-field beam patterns and power outputs in long third-order DFB lasers is then identified. This finding leads to a modified third-order DFB laser structure which can achieve perfect phase-matching (PM) condition, resulting in scalable power output and even lower beam divergence when compared with that of a conventional third-order DFB laser. Radiations from up to 151 laser sectors are phase-locked to form a single-lobe beam pattern with divergence ~ 6 x 11° and ~13 mW pulsed power at the end-fire direction. This approach substantially increases the usable length of a third-order DFB laser while keeping a high slope efficiency (140 mW/A). Later development applies the concept of microstrip antenna-a structure commonly used in microwave engineering-to THz photonics devices. By coupling the microstrip 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 DFB laser achieves > 8 mW pulsed power (10% duty-cycle) at 12 K with beam divergence as low as 12.5 x 12.5' and maximum lasing temperature Tmax = 109 K (pulsed) and 77 K (c.w.) with the highest slope efficiency (~450 mW/A) and wall-plug efficiency (0.57%) of all THz DFB laser sources. The second part of the thesis then focuses on the development of the first light amplifier in THz frequency under Fabry-Perot amplifier (FPA) scheme. Although amplification at terahertz frequency in quantum cascade structures has been demonstrated under the transient state or in a integrated platform, none of them is suitable for amplifying continuous-wave free-space THz radiations. The proposed amplifier is consisted of an array of short-cavity surface-emitting second-order distributed feedback lasers arranged in a two-dimensional grid which are operated marginally beneath their lasing thresholds. A overall system power gain of ~5.6x = 7.5 dB at ~3 THz is obtained with ~1 GHz bandwidth. 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. A new locking mechanism for two-dimensional phase-locked laser arrays based on antenna mutual-coupling is also proposed and then successfully demonstrated in the THz frequency using short-cavity DFB THz lasers. Up to 37 lasers are phase-locked to deliver 6.5 mW single-mode pulsed power (4% duty-cycle) at 3 THz with symmetric beam pattern ( 1:10) anisotropic reactive-ion etch on GaAs which is compatible with the metal-metal waveguide platform and the procedure to create airbridge structures by selectively removing the dielectric materials beneath the metal contacts.by TsungYu Kao.Ph. D