Surface Emitting Distributed Feedback Terahertz Quantum Cascade Phase Locked Laser Arrays

A new approach to achieve high-power, symmetric beam-pattern, single-mode THz emission from metal-metal waveguide quantum-cascade laser is proposed and implemented. Several surface-emitting distributed feedback terahertz lasers are coupled through the connection phase sectors between them. Through carefully choosing the length of phase sectors, each laser will be in-phase locked with each other and thus create a tighter beam-pattern along the phased-array direction. A clear proof of phase-locking phenomenon has been observed and the array can be operated in either in-phase or out-of-phase mode at different phase sector length. The phase sector can also be individually biased to provide another frequency tuning mechanism through gain-induced optical index change. A frequency tuning range of 1:5 GHz out of 3:9 THz was measured. Moreover, an electronically controlled "beam steering" device is also proposed based on the result of this work. This thesis focuses on the design, fabrication and measurement of the surface-emitting distributed feedback terahertz quantum-cascade phase-locked laser arrays.

A state-of-the-art overview of this rapidly expanding field, featuring fundamental theory, practical applications, and real-life examples.

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 ( 10 x 10°). This new coupling scheme can be extended to other electromagnetic systems with sub-wavelength confined elements such as plasmonic lasers and nanolasers. This thesis also reports the development of fabrication techniques required to bring the aforementioned novel THz cavity designs from concepts to reality which include a high aspect ratio ( 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.

Learn how the rapidly expanding area of mid-infrared and terahertz photonics has been revolutionized in this comprehensive overview. State-of-the-art practical applications are supported by real-life examples and expert guidance. Also featuring fundamental theory enabling you to improve performance of both existing and future devices.

Vertical External Cavity Surface Emitting Lasers Provides comprehensive coverage of the advancement of vertical-external-cavity surface-emitting lasers Vertical-external-cavity surface-emitting lasers (VECSELs) emit coherent light from the infrared to the visible spectral range with high power output. Recent years have seen new device developments – such as the mode-locked integrated (MIXSEL) and the membrane external-cavity surface emitting laser (MECSEL) – expand the application of VECSELs to include laser cooling, spectroscopy, telecommunications, biophotonics, and laser-based displays and projectors. In Vertical External Cavity Surface Emitting Lasers: VECSEL Technology and Applications, leading international research groups provide a comprehensive, fully up-to-date account of all fundamental and technological aspects of vertical external cavity surface emitting lasers. This unique book reviews the physics and technology of optically-pumped disk lasers and discusses the latest developments of VECSEL devices in different wavelength ranges. Topics include OP-VECSEL physics, continuous wave (CW) lasers, frequency doubling, carrier dynamics in SESAMs, and characterization of nonlinear lensing in VECSEL gain samples. This authoritative volume: Summarizes new concepts of DBR-free and MECSEL lasers for the first time Covers the mode-locking concept and its application Provides an overview of the emerging concept of self-mode locking Describes the development of next-generation OPS laser products Vertical External Cavity Surface Emitting Lasers: VECSEL Technology and Applications is an invaluable resource for laser specialists, semiconductor physicists, optical industry professionals, spectroscopists, telecommunications engineers and industrial physicists.

Scaling the continuous-wave (CW) power of quantum cascade lasers (QCLs) beyond ~5 W has proven difficult, and beam-quality degradation is common when scaling the device volume for high power. The primary objective of this work was to develop methods for spatially-coherent power scaling of mid-infrared-emitting QCLs to high CW powers. Two approaches were investigated: 1) resonant leaky-wave-coupled antiguided phase-locked laser arrays; and 2) grating-coupled surface-emitting lasers (GCSELs). These two approaches can be combined to realize high surface-emitted powers in a spatially and temporally coherent beam pattern. Optical and thermal models of planarized leaky-wave-coupled phase-locked QCL arrays were coupled together to investigate the influence of thermal lensing on modal behavior. Self-focusing under thermally-induced index variations across the array were found to impact the field profile and promote multi-moding due to gain spatial hole burning. Two techniques were found to mitigate this effect: 1) employing anti-resonant reflective-optical waveguide terminations outside the array; and 2) chirping the element width across the array to obtain identical optically-equivalent widths under CW operation, eliminating thermal lensing at a particular operating condition. Five-element phase-locked arrays of 4.7 μm-emitting QCLs were demonstrated which operate in a near-diffraction-limited beam (primarily in the in-phase array mode) to 5.1 W peak pulsed power, in agreement with simulations. Spectrally resolved near- and far-field measurements indicate that the wide spectral bandwidth of the QCL core promotes multi-mode operation at high drive levels. An optimized array design was identified to allow sole in-phase mode operation to high drive levels above threshold, indicating that full spatial coherence to high output powers does not require full temporal coherence for phase-locked laser arrays. Lastly, a novel method for obtaining a single-lobed beam pattern from transverse magnetic (TM)-polarized GCSELs is proposed: resonant coupling of the optical mode of a QCL to the antisymmetric surface plasmon mode of a 2nd-order distributed feedback metal/semiconductor grating results in strong antisymmetric-mode absorption. Lasing in the symmetric mode, resulting in a single-lobed far-field beam pattern from the substrate emission, is strongly favored around resonance. For infinite-length devices, the symmetric mode has negligible absorption loss while still being efficiently outcoupled by the grating.

Coherent power scaling of quantum cascade lasers (QCLs) for high-power, single-mode continuous-wave (CW) operation has proven to be quite a difficult task - while the device volume could be scaled for higher output power, many other factors such as beam quality and thermal resistance are negatively impacted if the device design is not carefully considered. The main objective of this work has been to develop methods for realizing high continuous-wave (CW) output power QCLs with high beam quality and minimal beam steering. One attractive approach for tackling this problem is the use of resonant leaky-wave-coupled antiguided phase-locked laser arrays. This dissertation focusses on two approaches to achieve high coherent power: 1) one alternate to the resonant leaky-wave-coupled antiguided phase-locked array concept, so called 'reverse-taper'laser; and 2) one combining grating-coupled surface-emitting lasers (GCSELs) with resonant leaky-wave-coupled antiguided phase-locked arrays, where each array element is coupled in both the lateral and longitudinal direction; thus, has a potential for multi-watt -CW surface-emitted output powers with good beam quality and narrow spectral linewidth. The novel geometry reverse-taper QCL device can scale the output power while maintaining good beam quality and beam stability - the tapered region scales the output power, while the emitting facet is located at the narrow-end taper section, which provides mode filtering by suppressing high-order spatial modes. A small degree of collimated-beam centroid movement (

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Quantum cascade lasers (QCLs) are attractive for high-resolution spectroscopy because they can provide high power and a narrow linewidth. They are particularly promising in the terahertz (THz) range since they can be used as local oscillators for heterodyne detection as well as transmitters for direct detection. However, THz QCL-based technologies are still under development and are limited by the lack of frequency tunability as well as the frequency and output power stability for free-running operation. In this dissertation, frequency tuning and linewidth of THz QCLs are studied in detail by using rotational spectroscopic features of molecular species. In molecular spectroscopy, the Doppler eff ect broadens the spectral lines of molecules in the gas phase at thermal equilibrium. Saturated absorption spectroscopy has been performed that allows for sub-Doppler resolution of the spectral features. One possible application is QCL frequency stabilization based on the Lamb dip. Since the tunability of the emission frequency is an essential requirement to use THz QCL for high-resolution spectroscopy, a new method has been developed that relies on near-infrared (NIR) optical excitation of the QCL rear-facet. A wide tuning range has been achieved by using this approach. The scheme is straightforward to implement, and the approach can be readily applied to a large class of THz QCLs. The frequency and output stability of the local oscillator has a direct impact on the performance and consistency of the heterodyne spectroscopy. A technique has been developed for a simultaneous stabilization of the frequency and output power by taking advantage of the frequency and power regulation by NIR excitation. The results presented in this thesis will enable the routine use of THz QCLs for spectroscopic applications in the near future.

The invention of optical frequency combs generated by mode-locked lasers revolutionized time and frequency metrology in the late 1990s. This concept has been explored in several laser systems; the quantum cascade laser (QCL) is one such system that operates in the terahertz (THz) frequency range.THz QCL was first invented in 2001 as a reliable semiconductor source for compact, high-power THz radiation. The inherently strong third-order nonlinearity in its QC-gain medium allows for spontaneous frequency comb formation as a result of spatial hole burning induced by Fabry-Perot cavities and four-wave mixing, which synchronizes the dispersed cavity modes. It was noticed that the self-generated combs are naturally frequency-modulated with quasi-continuous power output, whereas amplitude-modulated combs, i.e., mode-locking, are considered challenging in THz QCLs because of the inherent fast gain recovery time. One effective method to trigger active mode-locking is RF injection locking. It involves injecting RF current modulation into the QC-device at a frequency that is close to the cavity round-trip frequency. This locks the spacing between adjacent lasing modes, and pulses with a duration of 4-5 ps have been reported. In recent years, the study of frequency comb/mode-locking in THz QCLs has raised increasing interest because of its potential for a number of applications, including astronomy, biomedicine, fast spectroscopy, non-invasive imaging, and non-destructive evaluation. So far, research has concentrated on ridge-waveguide and ring QCLs. On the other hand, THz quantum-cascade vertical-external-cavity surface-emitting-laser (QC-VECSEL) was introduced in 2015 as a novel external cavity configuration of THz QCLs.The key concept of THz QC-VECSEL is to engineer its gain chip into a millimeter-scale reflectarray metasurface for free-space THz radiation and further incorporation into a resonant laser cavity as an active reflector. This enables watt-level output power with near-Gaussian distributed beam quality; versatile functionality may be incorporated into the amplifying metasurface; and broadband frequency tunability is provided by the VECSEL architecture. Despite the fact that VECSELs are widely used for mode-locking at near-infrared and optical frequencies, THz QC-VECSELs have not yet been exploited in frequency comb and mode-locking applications. In this thesis, we report for the first time the techniques utilized to achieve frequency comb/mode-locking operations in THz QC-VECSELs. Both the metasurface design and VECSEL cavity geometry are optimized for this purpose. The double-patch metasurface design is considered optimal for broadband frequency response and low dispersion, and a well-designed RF package is needed for efficient RF signal injection and extraction. On the other hand, an off-axis parabolic (OAP) mirror is introduced to build a V-shaped intra-cryostat focusing VECSEL cavity. This OAP-focusing cavity design eliminates most of the intra-cavity diffraction losses and, therefore, enables lasing in an ultra-long external cavity using a small-sized metasurface that supports continuous wave (CW) biasing. It is highly suited for frequency comb/mode-locking applications as the cavity round-trip frequency is lowered to a typical value of 3-5 GHz. In contrast to ridge-waveguide or ring QCLs, self-generated frequency combs have not been observed in THz QC-VECSELs --- in fact, they prefer to lase in a single-mode regime primarily due to a lack of spatial hole burning.To promote multimode operation in THz QC-VECSELs, we present a technique based on a specific combination of output coupler thickness and external cavity length. Through Vernier selection and reflectance compensation in a cascaded Fabry-Perot cavity, we are able to perform simultaneous nine modes lasing with a free-spectral range (FSR) of ~21 GHz. The number of lasing modes that can be generated using this method is limited by the maximum available output coupler thickness. A more effective way to promote multimoding, as well as possible frequency comb or even mode-locking operations, is through RF injection locking.The successful demonstration of RF injection locking in THz QC-VECSELs for the first time is the main focus of this thesis. Lasing spectral broadening has been observed under strong RF modulation, with a maximum bandwidth of around 100-300 GHz. An intermodal beat-note is produced as a result of beating between each of the two lasing modes. It is locked to the RF injection signal as the injection frequency is tuned around the cavity round-trip frequency. This suggests that the lasing modes are equally spaced, which is a prerequisite of frequency comb/mode-locking. Several impacting factors, including metasurface design, external cavity length, and optical feedback, are experimentally investigated in the RF-injection locked QC-VECSELs, which may help control and tune the laser states. THz QC-VECSEL is consequently considered to be a superior platform that enables a more thorough investigation of the fundamental physics of mode-locking/frequency comb operation in QCL systems. Our research on mode-locked THz QC-VECSELs opens the way for future development of semiconductor lasers operating in the 2-5 THz region that produce picosecond-scale pulses.