Download or read book Modelocking of THz Quantum Cascade Lasers written by Valentino Pistore and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: THz QCLs are nowadays considered a promising platform for the generation of intense and ultrashort THz pulses. Owing to their fast gain recovery time, passive modelocking of THz QCLs has so far proved to be difficult. On the contrary, active modelocking with a microwave modulation has been successfully applied. The pulse duration, however, has been arduous to reduce despite years of research. In 2017, THz pulses as short as 4ps have been generated by our group with the application of an integrated structure (a GTI) aiming to reduce the chromatic dispersion. The research in this thesis starts from this point.In particular, I present dispersion engineering in THz QCLs in order to obtain very short pulses even from relatively narrow-band devices. This is achieved using proven active modulation methods that can tune the QCL emission from high to low dispersion regimes. I also show that THz QCLs can present a strong amplitude modulation of their emission profile and that they can spontaneously emit pulses as a result of a self-locking mechanism, contrary to the expected frequency modulated response. As a consequence, this indicates that the fast gain recovery time is not a limiting factor for the generation of pulses. I also show this passive self-locking scheme for passive pulse generation in the framework of the first demonstrations of harmonic modelocking of THz QCLs. Finally, a new phenomenon is presented where the modes of a free running THz QCL can beat together to generate free space microwave emission.

Download or read book Mid Infrared and Terahertz Quantum Cascade Lasers written by Dan Botez and published by Cambridge University Press. This book was released on 2023-06-30 with total page 551 pages. Available in PDF, EPUB and Kindle. Book excerpt: A state-of-the-art overview of this rapidly expanding field, featuring fundamental theory, practical applications, and real-life examples.

Download or read book Towards Actively Mode locked Terahertz Quantum cascade VECSELs written by Yu Wu and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Spectroscopic Applications of Terahertz Quantum Cascade Lasers written by Tasmim Alam and published by Cuvillier Verlag. This book was released on 2020-10-29 with total page 132 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Frequency Stabilization of a Single Mode Terahertz Quantum Cascade Laser to the Kilohertz Level written by and published by . This book was released on 2009 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH2DOH THz CO2 optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t ~ sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers.

Download or read book Development of Terahertz QCLs written by Sushil Kumar (Ph. D.) and published by . This book was released on 2007 with total page 340 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Surface emitting Distributed Feedback Terahertz Quantum cascade Phase locked Laser Arrays written by Tsung-Yu Kao and published by . This book was released on 2009 with total page 114 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Frequency Stabilization Tuning and Spatial Mode Control of Terahertz Quantum Cascade Lasers for Coherent Transceiver Applications written by Andriy Danylov and published by . This book was released on 2010 with total page 266 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Design and Simulation of Terahertz Surface Emitting Quantum Cascade Lasers written by Martin F. Schubert and published by . This book was released on 2005 with total page 94 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Photonics for THz Quantum Cascade Lasers written by Maria Ines Amanti and published by . This book was released on 2010 with total page 146 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Concepts and Applications of Nonlinear Terahertz Spectroscopy written by Thomas Elsaesser and published by Morgan & Claypool Publishers. This book was released on 2019-02-22 with total page 159 pages. Available in PDF, EPUB and Kindle. Book excerpt: Terahertz (THz) radiation with frequencies between 100 GHz and 30 THz has developed into an important tool of science and technology, with numerous applications in materials characterization, imaging, sensor technologies, and telecommunications. Recent progress in THz generation has provided ultrashort THz pulses with electric field amplitudes of up to several megavolts/cm. This development opens the new research field of nonlinear THz spectroscopy in which strong light-matter interactions are exploited to induce quantum excitations and/or charge transport and follow their nonequilibrium dynamics in time-resolved experiments. This book introduces methods of THz generation and nonlinear THz spectroscopy in a tutorial way, discusses the relevant theoretical concepts, and presents prototypical, experimental, and theoretical results in condensed matter physics. The potential of nonlinear THz spectroscopy is illustrated by recent research, including an overview of the relevant literature.

Download or read book From High Power Terahertz Quantum Cascade Lasers to Terahertz Light Amplifiers written by Tsung-Yu Kao and published by . This book was released on 2014 with total page 208 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Mode Locking of Quantum Cascade Lasers written by and published by . This book was released on 2007 with total page 42 pages. Available in PDF, EPUB and Kindle. Book excerpt: A theoretical and experimental study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes in QCLs: One is spatial hole burning (SHB), and the other one is related to the Risken-Nummedal-Graham-Haken (RNGH) instability predicted in the sixties. A model that can account for coherent phenomena, a saturable absorber and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with narrow active region and/or with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have weaker saturable absorber, do not exhibit a Rabi splitting, and their multimode regime is governed by SHB. This experimental phenomenology is well explained by our theoretical model.

Download or read book Quantum Cascade Lasers written by and published by . This book was released on 19?? with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Multimode Dynamics in Quantum Cascade Lasers written by Christine Yi-Ting Wang and published by . This book was released on 2008 with total page 244 pages. Available in PDF, EPUB and Kindle. Book excerpt: The main experimental signature of RNGH instability is a splitting corresponding to twice the Rabi frequency in optical spectrum. In QCLs this coherent instability is enhanced due to the large Rabi frequency compared to the relaxation rates. We have also shown that spatial hole burning, which is not so readily observable in diode lasers, also plays an important role in QCLs. Both experimental data and simulations based on Maxwell-Bloch equations are presented. In the second part of this thesis, we demonstrate active mode-locking in QCLs. The stable mode-locked pulse train was generated by actively modulating the pumping current of a small section on a QCL. Stable mode locking was confirmed by second-order interferometric autocorrelation measurements, and a FWHM of 3 ps and about 0.5 pJ per pulse were deduced from the autocorrelation traces. The system is also simulated using Maxwell-Bloch equations incorporated with a modulation term. We anticipate our results to be a big step toward a compact, electrically-pumped source generating ultrashort pulses in the mid-infrared and terahertz ranges.

Download or read book Design Analysis and Characterization of Indirectly pumped Terahertz Quantum Cascade Lasers written by Seyed Ghasem Razavipour and published by . This book was released on 2013 with total page 129 pages. Available in PDF, EPUB and Kindle. Book excerpt: Quantum cascade laser (QCL), as a unipolar semiconductor laser based on intersubband transitions in quantum wells, covers a large portion of the Mid and Far Infrared electromagnetic spectrum. The frequency of the optical transition can be determined by engineering the layer sequence of the heterostructure. The focus of this work is on Terahertz (THz) frequency range (frequency of 1 - 10 THz and photon energy of ~ 4 - 40 meV), which is lacking of high power, coherent, and efficient narrowband radiation sources. THz QCL, demonstrated in 2002, as a perfect candidate of coherent THz source, is still suffering from the empirical operating temperature limiting factor of T [ap] h̳[omega]/kB, which allows this source to work only under a cryogenic system. Most of high performance THz QCLs, including the world record design which lased up to ~ 200 K, are based on a resonant phonon (RP) scheme, whose population inversion is always less than 50%. The indirectly-pumped (IDP) QCL, nicely implemented in MIR frequency, starts to be a good candidate to overcome the aforementioned limiting factor of RP-QCL. A rate equation (RE) formalism, which includes both coherent and incoherent transport process, will be introduced to model the carrier transport of all presented structures in this thesis. The second order tunneling which employed the intrasubband roughness and impurity scattering, was implemented in our model to nicely predict the behavior of the QCL designs. This model, which is easy to implement and fast to calculate, could help us to engineer the electron wavefunctions of the structure with optimization tools. We developed a new design scheme which employs the phonon scattering mechanism for both injecting carrier to the upper lasing state and extracting carrier from lower lasing state. Since there is no injection/extraction state to be in resonance with lasing states, this simple design scheme does not suffer from broadening due to the tunneling. Finally, three different THz IDP-QCLs, based on phonon-photon-phonon (3P) scheme were designed, grown, fabricated, and characterized. The performance of those structures in terms of operating temperature, threshold current density, maximum current density, output optical power, lasing frequency, differential resistance at threshold, intermediate resonant current before threshold, and kBT/h̳[omega] factor will be compared. We could improve the kBT/h̳[omega] factor of the 3P-QCL design from 0.9 in first iteration to 1.3 and the output optical power of the structure from 0.9 mW in first design to 3.4 mW. The performance of the structure in terms of intermediate resonant current and the change in differential resistance at threshold was improved.

Download or read book High Power and High Efficiency Operation of Terahertz Quantum Cascade Lasers at 3 3 THz Supported by the National Basic Research Program of China Under Grant Nos 2014CB339803 and 2013CB632801 and the National Natural Science Foundation of China Under Grant written by and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract : A high-power and high-efficiency GaAs/AlGaAs-based terahertz (THz) quantum cascade laser structure emitting at 3.3 THz is presented. The structure is based on a hybrid bound-to-continuum transition and resonant-phonon extraction active region combined with a semi-insulating surface-plasmon waveguide. By optimizing material structure and device processing, the peak optical output power of 758 mW with a threshold current density of 120 A/cm 2 and a wall-plug efficiency of 0.92% at 10K and 404mW at 77K are obtained in pulsed operation. The maximum operating temperature is as high as 115 K. In the cw mode, a record optical output power of 160 mW with a threshold current density of 178 A/cm 2 and a wall-plug efficiency of 1.32% is achieved at 10 K.