398 ISSN 1063-7850, Technical Physics Letters, 2019, Vol. 45, No. 4, pp. 398–400. © Pleiades Publishing, Ltd., 2019. Russian Text © The Author(s), 2019, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2019, Vol. 45, No. 8, pp. 31–33. Room Temperature Lasing of Single-Mode Arched-Cavity Quantum-Cascade Lasers A. V. Babichev a *, A. G. Gladyshev b , A. S. Kurochkin a , V. V. Dudelev c , E. S. Kolodeznyi a , G. S. Sokolovskii a,c , V. E. Bugrov a , L. Ya. Karachinsky a,b,c , I. I. Novikov a,b,c , D. V. Denisov d , A. S. Ionov e , S. O. Slipchenko c , A. V. Lyutetskii c , N. A. Pikhtin c , and A. Yu. Egorov a a St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University), St. Petersburg, 197101 Russia b Connector Optics LLC, St. Petersburg, 194292 Russia c Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia d St. Petersburg Electrotechnical University “LETI,” St. Petersburg, 197022 Russia e OKB-Planeta PLC, Veliky Novgorod, 173000 Russia *e-mail: a.babichev@mail.ioffe.ru Received January 28, 2019; revised January 28, 2019; accepted January 31, 2019 Abstract—Single-mode lasing at room temperature in quantum-cascade lasers (QCLs) with arched cavity design has been demonstrated. The output optical power in single-mode lasing regime at ~7.7-μm lasing wavelength was above 6 mW with a side-mode suppression ratio of up to 25 dB. The QCL heterostructure for the arched cavities was grown by molecular-beam epitaxy (MBE) based on a heterojunction of In 0.53 Ga 0.47 As/Al 0.48 In 0.52 As solid alloys, lattice-matched with InP substrate, and InP layers performing the function of waveguide claddings. DOI: 10.1134/S1063785019040205 Systems of remote gas analysis and various medical applications require effective tunable single-frequency sources of laser radiation in the middle infrared (mid- IR) range with full width at half maximum (FWHM) comparable to the typical absorption linewidth (about 1 cm –1 ) of gases at room temperature. Single-mode quantum-cascade lasers (QCLs) originally reported in [2] ensured lasing in a frequency band with FWHM ~ 1 cm –1 , The main approaches to creating single-mode QCLs are based on the fabrication of a distributed feedback in the external-cavity geometry [2] or in the grating-matched geometry incorporated into a laser waveguide [3]. Use of the external-cavity design leads to greater dimensions, implies higher sensitivity to mechanical action, and requires high-precision posi- tioning of system components. The fabrication of a grating on the QSL requires using high-precision (electron-beam or holographic) lithography. Alternative approaches to the fabrication of single- mode QCLs structures are based on using photonic crystals [4], ring resonators [3], coupled cavities [5], and asymmetric Mach–Zender interferometers [6]. Results obtained using laser cavities of complicated shapes, including arched (hairpin shaped) QCLs with cavities consisting of linear and semicircular segments, were reported in [7–9]. The presence of a semicircular segment provides additional mode selection and, thus, ensures single-mode lasing regime [8]. Relative sim- plicity of implementing arched-cavity QCLs is a key factor that allows increasing the useful yield of devices and reducing their production cost. However, until the beginning of our investigations, single-mode lasing in QCLs of this type was not achieved at room tempera- ture and maximum temperature of lasing reported thus far did not exceed 240 K [8]. In the present work, we demonstrate room-temperature operation of sin- gle-mode QCLs with an arched cavity design. The QCL heterostructures were manufactured by Connector Optics LLC (St. Petersburg) using a com- mercial Riber 49 MBE system equipped with a valved cracker solid-state source of arsenic and ABI 1000 type sources of gallium and indium fluxes [10, 11]. The laser heterostructures were grown on (001)-ori- ented InP substrates doped with sulfur to n = 1 × 10 17 cm –3 , which were covered with 500-nm-thick In 0.53 Ga 0.47 As buffer layer doped to n = 5 × 10 16 cm –3 . The active region comprised 50 cascades based on In 0.53 Ga 0.47 As/Al 0.48 In 0.52 As heterojunction in a struc- ture with two-phonon resonance scattering of carriers [12, 13]. The upper waveguide cladding was 4-μm- thick InP layer doped to n = 1 × 10 17 cm –3 .