Design and Simulation of Terahertz Surface Emitting Quantum Cascade Lasers PDF Download

Author: Martin F. Schubert
Publisher:
ISBN:
Category :
Languages : en
Pages : 94

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Author: Dan Botez
Publisher: Cambridge University Press
ISBN: 1108570607
Category : Technology & Engineering
Languages : en
Pages : 552

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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.

Author: Parastou Mortazavian
Publisher:
ISBN:
Category :
Languages : en
Pages : 105

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Quantum-cascade vertical-external-cavity surface-emitting-laser (QC-VECSEL) is a recently developed compact and coherent source of THz radiation which has demonstrated excellent beam quality and scalable high-power. The key component of the QC-VECSEL is an amplifying reflectarray metasurface made up of an array of sub-wavelength metal-metal waveguides loaded with quantum-cascade (GaAs/AlGaAs) laser gain material. To further the usefulness of this technology for many applications, including spectroscopy, heterodyne detection, and multispectral imaging, broadband and tunable THz QCLs are required. In this work, I investigate the feasibility of two techniques for tuning THz metasurface-based QCLs. First, A Littrow metasurface external cavity laser (ECL) is modeled and studied. We also propose and evaluate a novel method to implement Littrow ECL based upon blazed metasurface gratings. Electromagnetic simulations show that these metasurfaces can provide up to 15% fractional tunability around the center frequency of the laser at 3.3 THz. Preliminary results on several active resonant-phonon quantum-cascade materials are also obtained. Current progress on actual fabrication and device testing is reported.

Author: Benjamin Adams Burnett
Publisher:
ISBN:
Category :
Languages : en
Pages : 223

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The portion of the electromagnetic spectrum between roughly 300 GHz and 10 THz is nicknamed the "THz Gap" because of the enormous difficulty encountered by researchers to devise practical sources covering it. Still, the quantum cascade laser (QCL) has emerged over recent years as the most promising approach to a practical source in the 1-5 THz range. First developed in the higher-frequency mid-IR, where they are now widely available, QCLs were later extended to the THz where a host of greater design challenges awaited. Lasing in QCLs is based on intersubband optical transitions in semiconductor quantum wells, the energy of which can be chosen by design ("bandstructure engineering"). However, simply building a THz optical transition is insufficient; a good design must also produce significant population inversion by the applied cascading electron current, and this requires deep understanding of the transport physics. So far, no THz QCL has operated above the temperature of 200 K, even though the reasons prohibiting high temperature operation are well known. The goal of this Thesis is to put novel ideas for high-temperature operation of THz QCL active regions through rigorous theoretical testing. The central enabling development is a density-matrix-based model of transport and optical properties tailored for use in QCLs, which is general enough that widely varying design concepts can be tested using the same core principles. Importantly, by simulating QCLs more generally, fewer a priori assumptions are required on part of the researcher, allowing for the true physics to emerge on its own. It will be shown that this gives rise to new and useful insights that will help to guide the experimental efforts towards realization of these devices. One specific application is a quantum dot cascade laser (QDCL), a highly ambitious approach in which the electrons cascade through a series of quantum dots rather than wells. Benefits are expected due to the suppression of nonradiative scattering, brought about by the discrete spectrum of electronic states. However, this in turn leads to a highly different physics of transport and effects that are not well understood, even in the case of perfect materials. This work will show that while the benefits are clear, naive scaling of existing QCL designs to the quantum dot limit will not work. An alternative strategy is given based on a revised understanding of the nature of transport, and is put to a test of practicality in which the effects of quantum dot size inhomogeneity are estimated. Another application is to the already existing method of THz difference frequency generation in mid-IR QCLs, which occurs via a difference-frequency susceptibility $\chi^{(2)}$ in the active region itself. For this purpose, the model is extended to enable a coherent and nonperturbative calculation of optical nonlinearities. First, the generality of the method is displayed through the emergence of exotic nonlinear effects, including electromagnetically-induced transparency, in mock quantum-well systems. Then, the modeling concepts are applied to the real devices, where two new and important mechanisms contributing to $\chi^{(2)}$ are identified. Most importantly, it is predicted that the QCL acts as an extremely fast photodetector of itself, giving rise to a current response to the mid-IR beatnote that provides a better path forward to the generation of frequencies below ~2 THz. Finally, the fundamentals of density matrix transport theory for QCLs are revisited to develop a model for conventional THz QCL designs eliminating the usual phenomenological treatment of scattering. The new theory is fully developed from first principles, and in particular sheds light on the effects of scattering-induced electron localization. The versatility of the model is demonstrated by successful simulation of varying active region designs.

Author: Tsung-Yu Kao
Publisher:
ISBN:
Category :
Languages : en
Pages : 114

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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.

Author: Dimitris Pavlidis
Publisher: John Wiley & Sons
ISBN: 1119460719
Category : Technology & Engineering
Languages : en
Pages : 580

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An authoritative and comprehensive guide to the devices and applications of Terahertz technology Terahertz (THz) technology relates to applications that span in frequency from a few hundred GHz to more than 1000 GHz. Fundamentals of Terahertz Devices and Applications offers a comprehensive review of the devices and applications of Terahertz technology. With contributions from a range of experts on the topic, this book contains in a single volume an inclusive review of THz devices for signal generation, detection and treatment. Fundamentals of Terahertz Devices and Applications offers an exploration and addresses key categories and aspects of Terahertz Technology such as: sources, detectors, transmission, electronic considerations and applications, optical (photonic) considerations and applications. Worked examplesbased on the contributors extensive experience highlight the chapter material presented. The text is designed for use by novices and professionals who want a better understanding of device operation and use, and is suitable for instructional purposes This important book: Offers the most relevant up-to-date research information and insight into the future developments in the technology Addresses a wide-range of categories and aspects of Terahertz technology Includes material to support courses on Terahertz Technology and more Contains illustrative worked examples Written for researchers, students, and professional engineers, Fundamentals of Terahertz Devices and Applications offers an in-depth exploration of the topic that is designed for both novices and professionals and can be adopted for instructional purposes.

Author: Jan-Philipp Koester
Publisher: Cuvillier Verlag
ISBN: 3736968825
Category :
Languages : en
Pages : 171

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Edge-emitting quantum-well diode lasers based on GaAs combine a high conversion efficiency, a wide range of emission wavelengths covering a span from 630 nm to 1180 nm, and the ability to achieve high output powers. The often used longitudinal-invariant Fabry-Pérot-type resonators are easy to design but often lead to functionality or performance limitations. In this work, the application of laterally-longitudinally non-uniform resonator configurations is explored as a way to reduce unwanted and performance-limiting effects. The investigations are carried out on existing and entirely newly developed laser designs using dedicated simulation tools. These include a sophisticated time-dependent laser simulator based on a traveling-wave model of the optical fields in the lateral-longitudinal plane and a Maxwell solver based on the eigenmode expansion method for the simulation of passive waveguides. Whenever possible, the simulation results are compared with experimental data. Based on this approach, three fundamentally different laser types are investigated: • Dual-wavelength lasers emitting two slightly detuned wavelengths around 784 nm out of a single aperture • Ridge-waveguide lasers with tapered waveguide and contact layouts that emit light of a wavelength of around 970 nm • Broad-area lasers with slightly tapered contact layouts emitting at 910 nm The results of this thesis underline the potential of lateral-longitudinal non-uniform laser designs to increase selected aspects of device performance, including beam quality, spectral stability, and output power.

Author: Sushil Kumar (Ph. D.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 340

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The terahertz or the far-infrared frequency range of the electromagnetic spectrum (...) has historically been technologically underdeveloped despite having many potential applications, primarily due to lack of suitable sources of coherent radiation. Following on the remarkable development of mid-infrared (...) quantum-cascade lasers (QCLs) in the past decade, this thesis describes the development of electrically-pumped terahertz quantum-cascade lasers in GaAs/AlsGal_. As heterostructures that span a spectral range of 1.59 - 5.0 THz (...). A quantum-cascade laser (QCL) emits photons due to electronic intersubband transitions in the quantum-wells of a semiconductor heterostructure. The operation of terahertz QCLs at frequencies below the Reststrahlen band in the semiconductor (...), is significantly more challenging as compared to that of the mid-infrared QCLs. Firstly, due to small energy separation between the laser levels various intersubband scattering mechanisms are activated, which make it difficult to selectively depopulate the lower laser level. Additionally, as electrons gain enough kinetic energy in the upper laser level thermally activated longitudinal-optical (LO) phonon scattering reduces the level lifetime and makes it difficult to sustain population inversion at higher temperatures. Secondly, waveguide design for terahertz mode confinement is also more challenging due to higher free-carrier losses in the semiconducting doped regions at the terahertz frequencies. For successful designs reported in this work, the lower radiative state depopulation is achieved by a combination of resonant-tunneling and fast LO phonon scattering, which allow robust operation even at relatively high temperatures. An equally important enabling mechanism for these lasers is the development of metal-metal waveguides, which provide low waveguides losses, and strong mode confinement due to subwavelength mode localization in the vertical dimension. With these techniques some record performances for terahertz QCLs are demonstrated including the highest pulsed operating temperature of 169 K, the highest continuous-wave (cw) operating temperature of 117 K, and the highest optical power output (248 mW in pulsed and 138 mW in cw at 5 K) for any terahertz QCL. Towards the bigger goal of realizing a 1-THz solid-state laser to ultimately bridge the gap between electronic and optical sources of electromagnetic radiation, QCLs with a unique one-well injection scheme, which minimizes intersubband absorption losses that occur at longer wavelengths, are developed. Based on this scheme a QCL operating at 1.59 THz (A - 189 ym) is realized, which is amongst the lowest frequency solid-state lasers that operate without the assistance of a magnetic field. This thesis also reports on the development of distributed-feedback lasers in metal-metal waveguides to obtain single-mode operation, with greater output power and better beam quality. The subwavelength vertical dimension in these waveguides leads to a strongly coupled DFB action and a large reflection from the end-facets, and thus conventional coupled-mode theory is not directly applicable to the DFB design. A design technique with precise control of phase of reflection at the end-facets is developed with the aid of finite-element analysis, and with some additional unique design and fabrication methods, robust DFB operation has been obtained. Single-mode surface-emitting terahertz QCLs operating up to - 150 K are demonstrated, with different grating devices spanning a range of approximately 0.35 THz around v - 3 THz using the same gain medium. A single-lobed far-field radiation pattern, higher output power due to surface-emission, and a relatively small degradation in temperature performance compared to the Fabry-Perot ridge lasers makes these DFB lasers well suited for practical applications that are being targeted by the terahertz quantum-cascade lasers.

Author: Chun Wang Ivan Chan
Publisher:
ISBN:
Category :
Languages : en
Pages : 251

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Terahertz Quantum Cascade Lasers (THz QCLs) are arguably the most promising technology today for the compact, efficient generation of THz radiation. Their main limitation is that they require cryogenic cooling, which dominates their ownership cost. Therefore, achieving room-temperature operation is essential for the widespread adoption of THz QCLs. This thesis analyzes the limitations of THz QCL maximum lasing temperature (Tmax) and proposes solutions. THz QCL Tmax is hypothesized to be limited by a fundamental trade-off between gain oscillator strength ful and upper-level lifetime [Tau]. This so-called "ful[Tau] tradeoff" is shown to explain the failure of designs which target [Tau] alone. A solution is proposed in the form of highly diagonal (low ful) active region design coupled with increased doping. Experimental results indicate the strategy to be promising, but heavily doped designs are shown to suffer band-bending effects which may deteriorate performance. In order to treat these band-bending effects, which are typically neglected in previous THz QCL designs, a fast transport simulation tool is developed. Scattering integrals are simplified using the assumption of thermalized sub bands. Results comparable to ensemble Monte Carlo are achieved at a fraction of the computational expense. Carrier leakages to continuum states are also investigated, although they are found to have little effect. Other work in this thesis includes the optimization of double-metal THz waveguides to enable Tmax ~ 200 K, a current world record. Furthermore, laser designs to investigate the leakages of carriers to high-energy subbands and continuum states were fabricated and tested; such parasitic leakages are suggested to be small. Finally, the design of gain media for applications is examined, notably the development of 4.7 THz gain media for OI line detection in astrophysics, and the development of broadband heterogeneous gain media for THz comb generation.

Author: Christopher Curwen
Publisher:
ISBN:
Category :
Languages : en
Pages : 184

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Book Description
Terahertz (THz) quantum-cascade lasers (QCLs) are an emerging semiconductor source of compact, high-power THz radiation. Though first realized more than 15 years ago, THz QCLs continue to suffer from poor beam quality and outcoupling efficiency due to the subwavelength nature of the semiconductor ridge-waveguides typically used. In this thesis, a new technique is discussed for obtaining high power and good beam quality from THz QCLs, the THz quantum-cascade external cavity surface emitting laser (QC-VECSEL). The concept of the QC-VECSEL is to use THz QC-gain material to design a millimeter-scale reflective amplifying surface, or metasurface, for free space THz waves and incorporate it into a free-space THz resonant cavity to provide feedback to the amplification and form a laser. In this manner, the beam shape is determined by the external cavity, which supports fundamental Gaussian solutions. Further, the metasurface itself is composed of a subwavelength array (to prevent diffraction) of surface-coupled QC-elements whose properties, such as phase and polarization response, can be engineered on a unit cell basis allowing for a variety of unique experiments. The power output power of the QC-VECSEL can be scaled by either increasing the size of the metasurface, or increasing the density (or fill factor) of QC-elements across the surface. In this work, large area metasurfaces with high fill-factor have been studied and demonstrated up to 1.35 W of peak output power for a QC-VECSEL operating at 3.4 THz at a heat sink temperature of 4 K. A peak wall-plug efficiency of ~2% is demonstrated, but observation of self lasing from the metasurface at high bias (when no external cavity is provided) in combination with a simultaneous roll-off in VECSEL output power suggests even higher efficiency can be achieved with improved suppression of self-lasing modes. The output beam is well fit to a Gaussian distribution with a 4 degree full-width half-maximum divergence angle. In addition to power and beam quality, the QC-VECSEL opens the door to many interesting and unique studies via engineering of the metasurface properties and external cavity. Much of this thesis describes frequency tuning of QC-VECSELs based on broadband metasurfaces by varying the length of the external cavity. By making the external cavity extremely short (comparable to the operating wavelength), we are able to push all other external cavity modes outside of the gain bandwidth of the metasurface and demonstrate more than 20% fractional single-mode tuning around a center operating frequency of 3.5 THz. Because there are almost no diffraction losses at such a short cavity, the size of the metasurface could be reduced, allowing for continuous wave lasing with up to 20 milliwatts of output power at a heatsink temperature of 77 K, though the output power is highly variable as the reflectance of the output coupler has a strong frequency dependence. At the time of writing this, these are record performances in both frequency tuning and high-temperature continuous wave operation for lasers based on THz QC-gain material. The amount of tuning that be achieved with this approach is limited by the phase response of the metasurface, which squeezes the external cavity modes closer together in the spectral domain. Development of metasurfaces with lower electrical power consumption and higher conversion efficiency for the purpose of improving continuous wave performance. A sparse, patch-based metasurface with reduced power consumption is demonstrated, though the design was not optimal and only showed a 20% reduction in current draw compared to the previously demonstrated metasurfaces. Routes towards improving the performance are discussed. The last subject discussed is the design of a mid-infrared (IR) QC-VECSEL. Due to the large metal losses at mid-IR frequencies compared to THz, the technique used to develop THz QC-VECSELs cannot be directly extended to the mid-IR. We propose a scheme based on a diffraction grating to provide surface coupling of the QC-gain material. Progress on experimental realization is discussed, but lasing has not yet been observed.