Volume 14, Issue 21 - Special Issue: Quantum Cascade Laser: 30 Years of Discoveries

This review highlights the development and diverse applications of surface-emitting ring quantum cascade lasers. These lasers, featuring a ring-shaped cavity and a second-order distributed feedback grating, enable controlled surface emission, stable single-mode operation, and collimated, circularly symmetric beams. These advancements have expanded their use in scientific and industrial applications, with significant progress in extending their operation from the mid-infrared to the terahertz regime. Innovations such as microring designs, phase-shifted gratings and buried heterostructures have enhanced their scalability, beam quality, and suitability for continuous-wave operation at room temperature, unlocking opportunities in spectroscopy, imaging, and communication. On-chip optical solutions and integrated architectures combining emission and detection within a single chip further pave the way for compact and efficient systems for remote sensing and gas monitoring. Progress in miniaturization, thermal stability, and wavelength consistency strengthens the role of ring-based lasers in industrial, environmental, and medical diagnostics. These advancements, alongside scalable fabrication techniques and enhanced modulation schemes, position ring quantum cascade lasers as key enablers for the next generation of robust, energy-efficient sensors tailored to diverse scientific and industrial needs.

On this special occasion as we celebrate the 30th anniversary of quantum cascade lasers (QCLs), the author would like to reflect his own journey from the previous research on far-infrared superconducting devices to the development of THz QCLs. The initial phase of this journey spanned over 12 years (1990–2002) till we finally reached the lasing threshold, and it involved three generations of graduate students (Jurgen Smet, Bin Xu, and Ben Williams). The journey was not totally smooth, but full of joy from learning new things that are quite different from the author’s previous background to the eventual witness of laser operation of the very first THz QCL developed in the author’s group. This article is mostly a memoir of the author’s own process in the development of THz QCLs with many unpublished episodes, and it is by no means intended as a review article of the overall field. As such, work from other groups working in the field will not be cited thoroughly.

We demonstrate mid-infrared multiplexers based on evanescent couplers in In 0.53 Ga 0.47 As/InP ridge waveguides. Multiplexing of λ = 5.2 µm and λ = 8 µm input wavelengths in TM 00 modes to a single TM 00 output was achieved with 0.7 dB insertion loss. The demonstrated multiplexing bandwidth is significantly broader than is achievable using typical arrayed waveguide gratings, while displaying comparable insertion loss. These devices will be essential toward the development of broadband multi-color mid-infrared photonic integrated circuits for multi-species gas sensing and multi-band mid-infrared free-space communications.

The development of novel mid-infrared (MIR) devices and systems is crucial for addressing applications in biomedical analysis, chemical reaction-monitoring, or high-bitrate free-space telecommunication. Combining multiple functional elements on one chip into complex miniaturized photonic integrated circuits (PICs), is the next step in these developments, yet limited by existing material and technology constraints. In this work, we introduce a new concept for realizing fully monolithic MIR-PICs based on low-loss on-chip plasmonic guiding and beam combining. The core of our study demonstrates a monolithic beam combiner by integration of active quantum cascade (QC) devices at ∼8 µm (laser and detector) with tailored passive waveguides based on weakly-coupled Ge/Au plasmonics and on-chip micro-mirror optics. The on-chip gold-coated micro-mirrors enhance the directional control and beam combining capabilities of the plasmon waveguides while minimizing energy dissipation typically associated with tight plasmon confinement. We discuss the MIR-PIC beam combiner design, micro-fabrication, and characterization and compare it to the routing concept of simple plasmonic Ge/Au y-couplers exploiting strong-confinement.

Unipolar quantum optoelectronics is emerging as a promising semiconductor platform for developing mid-infrared applications, particularly spectroscopy and free-space communications. In this work, we present a proof of principle of a unipolar quantum optoelectronic hybrid Lidar – Radar for the measurement of the position and speed of a moving target. The system operates at 9 µm wavelength and is composed of a quantum cascade laser, a Stark modulator and a metamaterial quantum well infrared photodetector. The laser amplitude is modulated with a chirped radio-frequency subcarrier, and the backscattered light is detected coherently on the metamaterial quadratic receiver. With our system we have measured a ranging distance of 150 cm with an intrinsic resolution of 15 cm. Our demonstration is a first milestone towards combined communication and ranging systems in a transparency window of the atmosphere.

Terahertz (THz) technology is experiencing rapid progress thanks to recent advancements in sources and detectors. We present a 3.9 THz quantum cascade laser operating in a compact high heat load (HHL) housing. The device features a loop mirror back reflector and a dry-etched surface-emitting antenna that shapes the far-field pattern to optimize coupling to user-defined devices. A peak power of 1.8 mW and average power of 4.5 μW at 185 K are measured experimentally, and the device output is easily detected with a room-temperature THz camera.

Coherent ultrafast detection has become an important method to study the temporal response of terahertz (THz) quantum cascade lasers (QCLs), bringing insights into the dynamics of modelocking and frequency comb operation in these complex structures. Coherent detection has been typically based on the use of nonlinear crystals and electro-optic sampling, which are less sensitive to QCLs operating at high THz frequencies. This is because their response drops rapidly with frequency owing to phase matching conditions. Here, we develop coherent detection based on THz photoconductive antennas in vertical quarter-wavelength cavities, where we can freely engineer the spectral response to enhance the THz detection at frequencies greater than 2 THz. We develop thick low temperature grown GaAs that is transferred onto polymer and metal coated substrates to create a cavity with an THz response that exceeds the response of non-cavity detectors. This vertical THz cavity also permits the planar electrode geometry to be designed independently. Indeed, we show that the THz cavity can be combined with large surface area single contact electrodes to further enhance the spectral response. Although this proof-of-principle coherent detection is not fully optimised, it is used to coherently resolve the temporal response of a double-metal THz QCL operating at 3 THz. This approach opens up perspectives to tune the response of THz photoconductive antennas and enhance their spectral response at a desired frequency.

Photonic bound states in the continuum (BICs) are optical modes that remain highly localized despite co-existing with radiating waves in the continuum, attracting considerable attention for both fundamental studies and technological innovations. Conventional single-mode BIC lasers predominantly focus on maximizing the Q-factor of a specific mode, often overlooking the critical role of Q-contrast – the difference in Q-factors between the highest-Q BIC mode and competing modes – which is crucial for achieving stable single-mode lasing. In this study, we present a compact, high Q-contrast BIC laser, enabled by strategically optimizing the alignment of the TM 1 band of the core domain with the shell domain to confine the high Q-factor mode within the core. Using a quantum cascade laser chip operating in the terahertz (THz) regime, this design achieves a Q-contrast ratio of approximately 2.3, resulting in stable single mode lasing across the dynamic region with a side-mode suppression ratio of ∼20 dB. These findings underscore the pivotal role of Q-contrast in photonic lasers, with promising implications for applications in THz lasers, sensors, harmonic signal generators and modulators.

Increasingly, unipolar quantum optoelectronic devices such as quantum cascade lasers are employed for the targeted generation of dynamic waveforms in the mid-infrared and terahertz regime. These include for example short-pulse trains, frequency combs and solitons. For the theoretical investigation and targeted development of these devices, suitable semiclassical models such as Maxwell–Bloch type equations have been developed, which employ a two- or multilevel density matrix description for the electron dynamics and a classical propagation equation for the optical resonator field. Unipolar devices typically utilize quantized conduction band states as optical levels. For quantum well and wire structures, the electron states are additionally characterized by a wavevector associated with free motion in the non-confined directions. This degree of freedom can give rise to nonparabolicity effects as well as Bloch gain, both leading to gain asymmetry and linewidth enhancement. However, fully accounting for the wavevector greatly increases the computational cost of the density matrix approach. Here, we introduce an effective discrete-level density matrix model, which includes these effects via correction factors obtained by suitable wavevector averaging. These parameters can be extracted from carrier transport simulations along with other required input data, yielding a self-consistent model. Coupling the effective density matrix description to optical propagation equations results in an effective Maxwell-density matrix approach, which is well-suited for dynamic simulations of quantum optoelectronic devices.

We describe structure localization and dissipative solitons in a semiconductor laser with a saturable absorber exhibiting gain/absorption recovery times shorter than the photon lifetime. Under assumptions compatible with QCL characteristics and graphene-based absorbers, we study the existence and stability of solitons, along with their dynamical behavior. Numerical simulations confirm the robustness of our predictions. This evidence hints at promising pathways to realize passive mode locking in ultrafast lasers, implying highly valuable application prospects.

Terahertz quantum cascade laser (QCL) frequency combs are a disruptive technology for spectroscopy, imaging, and quantum technologies in this spectral range. For advanced development and tailoring of frequency comb properties, detailed modeling of the QCL device is necessary. Recent achievements in the field of THz QCLs have unveiled that custom modifications of the laser cavity by dispersion engineering or tapered waveguide sections significantly influence the laser’s behavior. In this article, we present a numerical model based on the Maxwell-density matrix formalism that captures such cavity effects in detail, yielding a better understanding of the QCL dynamics and opening the possibility of designing cavities for custom laser applications. We show that waveguide engineering in terms of dispersion compensation and field enhancement can stabilize an unlocked multimode state into frequency comb operation and even shape its properties, such as the bandwidth or the mode spacing of harmonic frequency combs.

The development of on-chip optical frequency comb devices paves the way for novel applications in environmental tracking, fast ranging and smart communication solutions. Recently, a new type of frequency comb device, based on a modulated ring quantum cascade laser, was introduced and demonstrated. Here we present a rigorous theoretical study of this type of device, also known as the quantum walk comb laser. We show that resonant phase modulation of a fast gain laser with a dispersive circular cavity is sufficient to support a broadband comb. This method requires the gain to have a sufficiently fast recovery time to support quasi-instantaneous suppression of intensity fluctuations. When this condition is met, the modulation leads to quantum walk dynamics, and then to stabilization onto a stable and controllable frequency comb. We show this type of dynamics through simulations using realistic parameters and reveal the impact of higher-order contributions from gain and dispersion. We also study the resilience of this type of mode-locked laser to noise injection and show its superiority to that of active mode-locking. We believe that this work will allow the development of comb devices with high wall-plug efficiency, arbitrary output spectral shaping and increased stability properties.