Terahertz Quantum Cascade Laser Based Frequency Combs PDF Download
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Author: Markus Rösch
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Markus Rösch
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 99
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In this thesis, terahertz (THz) laser frequency combs are improved and THz dual-comb spectroscopy method is demonstrated. To achieve better performance of THz quantum cascade laser (QCL) frequency combs, several broadband homogeneous and heterogeneous gain media are characterized, and corresponding dispersion compensators are designed. All THz QCL frequency combs are fabricated in Microsystems Technology Laboratories at MIT using our group's standard process. By utilizing the broad spectral coverage of THz QCL frequency combs and the multiheterodyne detection method, a prototype of the THz dual-comb spectrometer is demonstrated. This thesis work provides an important step towards realizing laser-based broadband THz spectroscopy system for chemical identification and explosive detection.
Author: Dan Botez
Publisher: Cambridge University Press
ISBN: 1108570607
Category : Technology & Engineering
Languages : en
Pages : 552
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Book Description
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: Chao Xu
Publisher:
ISBN:
Category : Lasers
Languages : en
Pages : 128
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Book Description
Recent advances in Terahertz Quantum Cascade Laser (THz QCL) development are pushing this technology ever closer to practical application, particularly within the spectroscopic field. For this reason, optimizing the operation of THz QCL frequency combs, which can potentially provide unprecedented accuracy and stability to the optical spectra in a broad frequency band, is of particular interest to the research community. The THz QCLs frequency comb was only recently realized using two separate techniques: either a broad-gain active region or a group velocity dispersion controlled waveguide. However, due to residual optical dispersion from both the gain medium and the cold waveguide, comb formation in these reported THz QCLs can only sustain a limited current injection region and the observed comb frequency range is much narrower than the bandwidth of the designed gain medium. To overcome these limitations, this thesis targets a new THz QCL frequency comb device design that simultaneously exploits the broadband gain active region and a group velocity dispersion (GVD)-compensated waveguide over an octave frequency band of 2-4 THz. In designing a broadband gain active region, two heterogeneous structures are proposed and simulated, with one combining three different bound-to-continuum (BTC) active regions operating at a temperature of 25 K, and another one consisting of four different resonant-phonon (RP) active regions operating at the liquid nitrogen temperature (77 K) or higher. The simulation results show that both active region designs can provide a broadband and 'flat-top' gain profile covering the frequency range from 2 to 4 THz. To design a group velocity dispersion-compensated waveguide, strategies are explored for simulating chirped Distributed Bragg Reflectors (DBRs) that can serve as THz QCL metal-metal waveguides, and one-dimensional (1D) and three-dimensional (3D) modeling approaches are established and verified. A novel two-section chirped DBR is proposed, which provides substantially-improved group delay compensation over a broadband octave frequency range from 2 to 4 THz. Two THz QCL structures are grown using in-house molecular beam epitaxy and THz QCL devices equipped with a metal-metal waveguides are fabricated in the University of Waterloo Quantum-Nano-Centre clean-room fabrication lab. The experimental results demonstrate that the new THz QCL active region design can operate up to a maximum lasing temperature of 111 K, and with a broad lasing spectrum covering frequencies from 2.36 to 2.86 THz under pulse mode, at temperature of 13 K. The combined theoretical and experimental work would ultimately lead to the demonstration of improved THz QCL frequency comb operation over the broadband range from 2 to 4 THz.
Author: Yu Wu
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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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.
Author: Tianyi Zeng
Publisher:
ISBN:
Category :
Languages : en
Pages : 105
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Ever since the invention of quantum cascade laser (QCL), the performance and the flexibility in design has made it a desirable source for a wide range of applications, such as trace-chemical sensing, health monitoring, frequency metrology, noninvasive imgaing and infrared countermeasures. The LWIR region (or mid-infrared region), roughly ranging from 2-20 [mu]m, is of particular importance to spectroscopy applications, since many molecular species have their strongest rotational-vibrational absorption bands in that area. Infrared laser spectroscopy began about 40 years ago and has been using a variety of different tunable laser-based sources, particularly lead salt diodes, color center lasers, difference frequency generation and optical parametric oscillators. The large tunabilitiy in the design (lasing frequency, tunability, power, material system, etc.) and the compactness in fabrication and packaging has made QCL an ideal source for laser-based spectroscopy. Traditional spectroscopy systems suffer from problems like large physical dimensions, long data-processing times and spectral resolution restrictions. Therefore the development of a simple, robust, compact and inexpensive optical source/system like QCL frequency combs can largely benefit spectroscopy systems. In the past few years, QCLs have proven to be able to form comb radiation in both LWIR and THz regions. And dual comb spectroscopy has been demonstrated using QCL frequency combs with very short acquisition time ([mu]s). The development of a broadband, high power, narrow linewidth and stable LWIR frequency comb based on quantum cascade laser is the key to realizing such broadband ultrafast spectrometer in the mid-infrared range. This thesis explores the design, fabrication and characterization techniques towards the development of LWIR QCL frequency comb devices for spectroscopic purposes. A complete wet etch epi-up fabrication process is reported, with preliminary results on the dry-etch technique to incorporate dispersion compensation strucutre and epi-down fabricaiton for high power CW mode QCL device. Formation of comb(-like) regime has been observed in two devices, with the Gires-Tournois Interferometer (GTI) mirror providing dispersion from the rear facet. In order to improve the comb performance of these devices, dispersion of the device is measured to provide essential information for the design of chirped top cladding for dispersion compensation. This thesis provides an important step towards the realization of a room temperature, broadband, CW mode LWIR QCL frequency comb device for spectroscopic purposes.
Author:
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Category :
Languages : en
Pages : 4
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The terahertz region is of great importance for spectroscopy since many molecules have absorption fingerprints there. Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multiheterodyne spectroscopy using two terahertz quantum cascade laser combs. Over a spectral range of 250 GHz, we achieve average signal-to-noise ratios of 34 dB using cryogenic detectors and 24 dB using room-temperature detectors, all in just 100 [mu]s. As a proof of principle, we use these combs to measure the broadband transmission spectrum of etalon samples and show that, with proper signal processing, it is possible to extend the multiheterodyne spectroscopy to quantum cascade laser combs operating in pulsed mode. As a result, this greatly expands the range of quantum cascade lasers that could be suitable for these techniques and allows for the creation of completely solid-state terahertz laser spectrometers.
Author: David Patrick Burghoff
Publisher:
ISBN:
Category :
Languages : en
Pages : 190
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In recent years, quantum cascade lasers have emerged as mature semiconductor sources of light in the terahertz range, the frequency range spanning 1 to 10 THz. Though technological development has pushed their operating temperatures up to 200 Kelvin and their power levels up to Watt-level, they have remained unsuitable for many applications as a result of their narrow spectral coverage. In particular, spectroscopic and tomographic applications require sources that are both powerful and broadband. Having said that, there is no fundamental reason why quantum cascade lasers should be restricted to narrowband outputs. In fact, they possess gain spectra that are intrinsically broad, and beyond that can even be tailored to cover an octave-spanning range. This thesis explores the development of broadband sources of terahertz radiation based on quantum cascade lasers (QCLs). The chief way this is done is through the development of compact frequency combs based on THz QCLs, which are able to continuously generate milliwatt levels of terahertz power covering a fractional bandwidth of 14% of their center frequency. These devices operate on principles similar to microresonator-based frequency combs, and make use of the quantum cascade laser's fundamentally large nonlinearity to phase-lock the cavity modes. These devices will enable the development of ultra-compact dual comb spectrometers based on QCLs, and will potentially even act as complete terahertz spectrometers on a chip. This thesis also uses broadband terahertz time-domain spectroscopy to analyze the behavior of THz QCLs. By using QCLs as photoconductive switches, the usual limitations imposed by optical coupling are circumvented, and properties of the laser previously inaccessible can be directly observed. These properties include the gain and absorption of the laser gain medium, the populations of the laser's subbands, and properties of the waveguide like its loss and dispersion. Knowledge of these properties were used to guide frequency comb design, and were also used to inform simulations for designing better lasers.
Author: Yang Yang (Ph. D.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 162
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Book Description
In recent years, there has been growing interest in chip-scale frequency combs, such as micro-resonator combs and semiconductor mode-locked sources. From the mid-infrared to the terahertz regime, it has been shown that quantum cascade lasers (QCLs) are capable of forming a frequency comb state where dispersed cavity modes of the Fabry-Perot cavity are synchronized by third order nonlinearity. With proper dispersion engineering, we have shown that it is possible to create QCL frequency combs at terahertz wavelengths, which possess broadband coverage in a compact package. These QCL combs are particularly attractive as sources for high-sensitivity laser spectroscopy: by using a dual-comb technique, it is possible to perform broadband spectroscopy only using chip-scale components, making it an intriguing candidate for spectroscopic applications in the open field. Moreover, due to the semi-continuous nature of the temporal output from such combs, tracking the instantaneous phase and timing signals of the dual-comb waveform in the time domain becomes feasible. This enables a computational coherent averaging scheme of the dual-comb signal even without external reference. The first part of this thesis describes the development for better THz laser frequency combs. To realize all expectations in the spectroscopy applications using such devices, three main aspects of improvement are highly desired. First, the laser device should have a robust comb state that ideally can operate from device's threshold current, I[subscript th], to its maximum current, I[subscript m]. In addition, the comb states should have a broad spectral coverage: its bandwidth should cover at least an octave span to stabilize its carrier offset. Furthermore, the comb state should have a flexible tunability that allows tuning across the entire free spectral range for gapless sensing. All listed aspects are investigated during the course of this thesis and as a result, a THz QCL device featuring comb state performance over the entire lasing bias range has been demonstrated. Meanwhile, we show that, by compensating cavity dispersion up to higher orders, the comb bandwidth from the full dispersion compensated devices can reach 80 % of its gain bandwidth. One common method to achieve very broadband coverage relies on using the heterogeneous gain media. This comes at the cost of reduced peak gain and hampered temperature performance. Also, engineering the dispersion of such a broadband gain medium becomes extremely challenging, which might not lead to a broader comb coverage albeit its broader gain. However, a unique feature of the metal-metal waveguide is that it is completely agnostic about its bonded gain media. Therefore, it is possible to bond multiple gain media together onto the same chip. The lateral heterogeneous integration scheme is investigated as an alternative method to expand the comb's spectral coverage. We show that using this strategy we can couple the output of combs at vastly different wavelengths without the trade-off with its temperature performance, yet maintain a compact package. Dual-comb spectroscopy allows for high-resolution spectra to be measured over broad bandwidth, but in order to achieve high resolution and acquire low-uncertainty spectroscopic information, the capability for coherent averaging is of the most importance. An essential requirement for coherent averaging is the availability of a phase reference. Usually, this means that the combs' phase and timing errors must be measured and either minimized by stabilization or removed by correction. These hardware-based solutions often require extra electronic or optical components, thus complicates the overall system and further limits the technique's applicability. We demonstrate that it is possible to extract the phase and timing signals of a multiheterodyne spectrum in a completely computational fashion without any extra measurements, which can potentially simplify any dual-comb system. Other works in this thesis include the first proof-of-principle demonstration of THz dual-comb spectroscopy using laser frequency combs, THz hyper-spectral imaging for pharmaceutical compound identification, and the exploratory work on the development of the germanium-on-gallium arsenide platform, a passive on-chip platform showing potential to bridge the THz and mid-infrared regime.
Author: Dimitris Pavlidis
Publisher: John Wiley & Sons
ISBN: 1119460719
Category : Technology & Engineering
Languages : en
Pages : 580
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Book Description
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.
