In this chapter, we overview recent developments of a simulation framework capable of capturing the highly nonequilibrium physics of the strongly coupled electron and phonon systems in quantum cascade lasers (QCLs). In midinfrared (mid-IR) devices, both electronic and optical phonon systems are largely semiclassical and described by coupled Boltzmann transport equations, which we solve using an efficient stochastic technique known as ensemble Monte Carlo. The optical phonon system is strongly coupled to acoustic phonons, the dominant carriers of heat, whose dynamics and thermal transport throughout the whole device are described via a global heat-diffusion solver. We discuss the roles of nonequilibrium optical phonons in QCLs at the level of a single stage , anisotropic thermal transport of acoustic phonons in QCLs, outline the algorithm for multiscale electrothermal simulation, and present data for a mid-IR QCL based on this framework.

After more than 25 years of continuous and increasing investment in research and development, the quantum cascade laser (QCL) is now revolutionizing a multitude of applications which aim to address major global challenges, from climate change to the cost of health care, to the pollution of our atmosphere and oceans, to the protection of our soldiers and first-responders. This chapter provides an insider’s perspective and historical context to the present and rapid trajectory in widespread QCL deployment across multiple applications and markets which has been building over the past two decades and will likely continue to benefit our quality of life and standard of living over the next hundred years.

Quantum cascade lasers (QCL) can be powerful testing grounds of the fundamental physical parameters determined by their quantum nature. In this chapter we describe a set of experimental techniques to explore the linewidth, frequency and phase stability of far-infrared QCLs. By performing noise measurements with unprecedented sensitivity levels, we highlight the key role of gain medium engineering and demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth. We discuss phase-locking of THz QCL to a free-space comb generated in a LiNbO3 waveguide, and present phase and frequency control of miniaturized QCL frequency combs. This work paves the way to novel metrological-grade THz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and quantum technologies. The physical processes and dynamics presented here open groundbreaking perspectives for the development of quantum sensors, quantum imaging devices and q-bits made by entangled teeth for photonic-based quantum computation.

Quantum cascade lasers provide a variety of challenges to theory, which are outlined in this review: (i) The choice of basis states is discussed, where energy eigenstates are badly defined if the full periodic structure is considered. (ii) The tunneling through barriers requires a treatment of quantum coherences, which sets particular demands on the formulation of the quantum kinetic equations. Here the advantages and disadvantages of different approaches such as rate equations, Monte-Carlo simulations, different density matrix approaches, and Green’s functions methods are addressed. (iii) The evaluation of gain is detailed, where broadening is of utmost importance. (iv) An overview regarding the electrical instabilities of the extended structure due to domain formation is given, which strongly affect the overall performance

Terahertz quantum cascade laser sources based on intra-cavity difference frequency generation are currently the only electrically-pumped monolithic semiconductor light sources providing broadly-tunable terahertz output at frequencies up to 6 THz at room temperature. Relying on the active regions with the giant second-order nonlinear susceptibility and the Cherenkov phase matching scheme, these devices demonstrated drastic improvements in performance in the past several years and can now produce narrow-linewidth single-mode terahertz emission that is tunable from below 1 THz to almost 6 THz with power output sufficient for imaging and spectroscopic applications. This chapter provides a comprehensive overview of this device technology

This chapter reviews the applications of terahertz (THz) quantum cascade lasers (QCLs). THz QCLs have come a long way since their first demonstration in 2002. Although still operating at or close to cryogenic temperatures, their applications have been multiplying steadily over the last decade, helped by the availability of compact commercial THz QCL systems and the growing adoption of the THz QCL technology in the THz scientific community. Currently, the key fields of THz QCL applications are imaging, spectroscopy and sensing.

Laser feedback interferometry, based on the self-mixing (SM) effect in quantum cascade lasers (QCLs), is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and SM is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. The SM phenomenon in QCLs has been exploited for a wide range of applications, including the measurement of internal laser characteristics, two- and three-dimensional imaging, materials analysis and near-field imaging. This chapter provides an overview of the SM effect in QCLs, and reviews the state of the art in sensing using this technique.

High-power terahertz quantum-cascade lasers (QCLs) are desired for a variety of applications in imaging and spectroscopy. The best performance at practical operating temperatures for single-mode terahertz QCLs is realized with metallic cavities due to a strong plasmonic mode confinement of the optical mode within the cavity. However, such plasmonic lasers suffer from poor beam shapes, low output power, and multi-mode spectral behavior. Development of distributed-feedback (DFB) techniques to improve spectral as well as modal properties becomes indispensable for terahertz QCLs to address targeted applications that typically require single-mode operation, frequency stability and specificity, and optimal far-field beam quality with single-lobed profile and low angular divergence. This chapter describes the theory, design methodologies, and key results from a sampling of a wide variety of DFB techniques that have been implemented in literature for monolithic terahertz QCLs with metallic cavities in both edge-emitting and surface-emitting configurations, either of which have their specific application areas and advantages much-like that for infrared diode lasers.

This chapter reviews typical waveguide and active region designs for quantum cascade lasing in the terahertz (THz) frequency range. Operating principles are analyzed in details with special attention paid to the most recent developments with the state-of-the-art device performance. The maximum operation temperature of THz QCL is still the main obstacle for its wide employment in applications, although it has been lifted to 250 K, allowing cryogenic-free THz coherent radiation for potentially portable applications. Optimization of various limiting factors in the most advanced resonant-phonon designs or the combined designs with scattering-assisted injection scheme could be promising for further breakthroughs in achieving higher temperature operations. The discussions in this chapter mainly focus on the matured GaAs/AlGaAs material system, but the design strategies can be applied to THz QCLs utilizing other material systems, which may overcome the main challenges of the GaAs/AlGaAs material system and achieve better performance in the future.

Optical gas sensing is a promising alternative to analytical, electrochemical and semiconductor sensors that can offer fast responses times, minimal drift, high gas specificity, with zero cross-response to other gases. Quantum cascade lasers represent the optimal choice as mid-IR sources due to their high output power, compactness, narrow spectral linewidth and broad wavelength tunability. Among optical techniques, Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) has been demonstrated to be a leading-edge technology for real-world gas detection applications, thanks to its modularity, ruggedness, portability and real-time operation capability. QEPAS sensors typically achieve gas detection limits of few parts-per billion level. The basic principles of PAS are provided with a discussion on optoacoustic waves generation and detection. Quartz tuning forks physics is presented in detail, covering aspects like flexural modes resonance, including overtone, quality factor and microresonator tubes configuration. Finally, an overview of QCL-based QEPAS gas sensors for real-world applications, like environmental monitoring, breath sensing, leak detection and multi-gas detection is provided.

The advent of optical frequency combs revolutionized many research fields from metrology to high precision spectroscopy. It was recently demonstrated that broadband quantum cascade lasers can operate as frequency combs. As such, they operate under direct electrical pumping at both mid-infrared and terahertz frequencies, reaching powers in the watt range with multi-terahertz bandwidths. As their key application field, they unlock the advantages in speed and accuracy of the dual-comb spectroscopy technique in a frequency range where molecules have their fundamental vibrational and rotational bands. In this Chapter we review the design and basic functioning principles of these devices, the characterization of their coherence properties as well as few example applications.

Quantum cascade lasers are based on Intersubband transitions between quantum confined states in semiconductor heterostructures. The origin of these states is briefly described in this chapter starting with linear combination of atomic orbitals and then proceeding to the k.P theory. The relations between the interband and Intersubband transitions including their oscillator strength and selection rules are established. It is shown that “giant” Intersubband dipole owes its existence to the confinement induced band mixing. Aside from the radiative Intersubband transitions investigated in this chapter, nonradiative transitions also play important roles in QCL operation, hence most relevant of these processes: electron phonon, electron-electron, interface roughness and alloy disorder are also described in detail.

This chapter provides an overview of a class terahertz quantum cascade lasers based upon amplifying electromagnetic metasurfaces. The metasurface comprises two-dimensional arrays of sub-wavelength surface radiating antenna elements, in which the antennas are loaded with the quantum cascade laser gain material. Several types devices are described: (a) vertical-external-cavity surface-emitting-lasers (VECSELs) in which the amplifying metasurface is paired with external optics to form a laser cavity; (b) monolithic metasurface lasers in which the metasurface array self-oscillates in a coherent supermode; and (c) metasurfaces which operate below threshold as free-space terahertz amplifiers. The metasurface approach allows the realization of large-area radiating apertures while preserving the sub-wavelength sized of the individual metallic waveguide antenna elements. This has resulted in significantly improved performance and functionality in many categories, including lasers with high-quality beam patterns, high-efficiency lasers with scalable output powers, broadband spectral tunability of single-mode emission, and free-space amplification of terahertz beams.