The Three-Decade Journey of Quantum Cascade Lasers

Dear Colleagues,

We are pleased to invite you to join us in celebrating the remarkable progress made over nearly 30 years in the field of Quantum Cascade Lasers (QCLs), specifically covering the mid- and far-infrared (mid-IR; terahertz) spectra. The widespread implementation of QCLs in real-world applications, such as environmental sensing, process control, and combustion diagnostics, underscores their significant impact.

This Special Issue aims to present the milestones achieved and the latest hot topics related to QCL research. Given the primary focus of Photonics on devices, it is fitting to compile these advancements here.

In this Special Issue, original research articles, reviews, and comments are welcome. Research areas may include (but are not limited to) the following topics: the physics of the intersubband transition, quantum transport simulations in QCLs, state-of-the-art mid-IR and THz QCL experiments, frequency noise and stabilization of QCLs, surface-emitting photonics configurations, frequency combs, multifrequency generation techniques for THz QCLs, and an extensive illustration of the various applications of QCLs.

We look forward to receiving your contributions.

Prof. Dr. Manijeh Razeghi
Dr. Li Wang
Dr. Quanyong Lu
Prof. Dr. Hideki Hirayama
Guest Editors

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• intersubband transition
• quantum transport models
• quantum cascade lasers
• mid-infrared/terahertz
• nonlinearities
• frequency comb
• spectroscopy

Title: MO VPE grown InGaAs/InAlAs quantum cascade lasers emitting at ~7.7 μm
Authors: Maciej Bugajski 1; Andrzej Kolek 2; Grzegorz Hałdaś 2; Włodek Strupiński 3
Affiliation: 1 Łukasiewicz Institute of Microelectronics and Photonics, al. Lotników 32/46, Warszawa 02-668, Poland 2 Department of Electronics Fundamentals, Rzeszów University of Technology, al. Powstańców Warszawy 12, Rzeszów 35-959, Poland 3 Vigo Photonics, Poznańska 129/133, 05-850, Ożarów Mazowiecki, Poland
Abstract: The 7-8 μm spectral region attracts a considerable interest in recent years. The methane absorption lines at 7.7 μm are clear and well distinguished and are easily accessible for QCL spectroscopy. Another strong methane absorption band at around 3.3 μm requires ICLs, which technology is less mature than QCL’s. In this study we designed and fabricated InGaAs/InAlAs quantum cascade lasers emitting at ~7.7 μm. The lasers are based on In0.590Ga0.410As/ In0.370Al0.630As heterostructures with strain compensation. The active region is based on a diagonal transition and three-phonon-resonance design. The layer sequence of a single period, starting from the injection barrier, was: 3.10, 2.52, 1.23, 5.77, 0.74, 5.01, 0.95, 4.80, 1.60, 4.49, 1.27, 3.79, 1.29, 3.33, 1.60, 2.89, 1.89, 3.01nm. Barriers are marked in bold font, quantum wells are marked with normal font. The underlined layers were doped to a concentration of n = 1.5x1017cm-3. The thicknesses of individual layers are given in nanometers. The active core consisted of 40 repetitions of the basic segment. The laser structures were grown by metalorganic vapor phase epitaxy (MO VPE) on a conductive InP substrate (n = 1.5-2.0x1018cm-3). The lower waveguide consisted of 1.5-μm InP layer with concentration of n = 1.5x1017cm-3 and 2.5 μm InP layer with concentration of n = 3.0x1016cm-3. The upper waveguide consisted of the same layers, only in reverse order. The entire structure was finished with a heavily doped (n=8.0×1018 cm-3) InP layer with a thickness of 1.0 μm, acting as a plasmonic reflector, and a cup layer of lattice-matched InGaAs with a thickness of 0.2 μm and concentration of n=2.0×1019 cm-3.The epitaxial structures were characterized by high resolution X-ray diffraction (HR XRD) and transmission electron microscopy (TEM). Prior to making devices we performed calculations based on the self-consistent NEGF method to analyze the carrier transport and gain properties of the QCL structures. The NEGF method includes both coherent and incoherent phenomena and consequently allows for the derivation of realistic electro-optic characteristics of the lasers (Fig.2 and Fig.3). Lasers with a ridge waveguide with mesa width of W = 5, 6, 7, 8, 9, 10 μm were fabricatied using standard photolitography and reactive ion etching. The dielectric insulation of the side walls of the mesas was provided by a Si3N4 layer. On the epitaxial side, the electrical contact was a Ti/Pt/Au alloy, on the substrate side, an AuGe/Ni/Au contact was used. For the measurements the structures were mounted with the epitaxial side up on AlN spacers and then on a copper cooler. The electrooptic and spectral characteristics are shown in Fig. 4 and Fig. 5, respectively. A total of 20 lasers with mesa width of W = 5, 6, 7, 8, 9, 10 μm and a resonator length of L = 2 mm were tested. The results of measurements of current-voltage characteristics and optical power as a function of current as well as measurements of spectral characteristics of lasers show reasonable agreement with theoretical predictions.