1217 ISSN 1063-7826, Semiconductors, 2019, Vol. 53, No. 9, pp. 1217–1221. © Pleiades Publishing, Ltd., 2019. Russian Text © The Author(s), 2019, published in Fizika i Tekhnika Poluprovodnikov, 2019, Vol. 53, No. 9, pp. 1244–1249. Second-Harmonic Generation of Subterahertz Gyrotron Radiation by Frequency Doubling in InP:Fe and Its Application for Magnetospectroscopy of Semiconductor Structures V. V. Rumyantsev a, *, K. V. Maremyanin a , A. P. Fokin b , A. A. Dubinov a , V. V. Utochkin a , M. Yu. Glyavin b , N. N. Mikhailov c , S. A. Dvoretskii c , S. V. Morozov a , and V. I. Gavrilenko a a Institute for Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod, 603950 Russia b Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, 603950 Russia c Institute for Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *e-mail: rumyantsev@ipm.sci-nnov.ru Received April 24, 2019; revised April 29, 2019; accepted April 29, 2019 Abstract—The possibility of obtaining intense terahertz radiation due to second-order lattice nonlinearity in indium-phosphide crystals doped with iron is discussed. As a source of radiation, subterahertz gyrotrons are considered. It is shown that the efficiency of frequency doubling can reach 3%, which opens up the possibility for developing a new generation of terahertz radiation sources. The possibility of applying second-harmonic radiation for the magnetospectroscopy of semiconductor structures with quasi-Dirac dispersion is demon- strated. Keywords: gyrotrons, frequency doubling, terahertz radiation, indium phosphide, second-order nonlinearity DOI: 10.1134/S1063782619090185 1. INTRODUCTION In recent decades, an unprecedented increase in activity in the so-called terahertz (THz) range of the electromagnetic spectrum has been observed around the world. The giant spread of studies into the possibil- ity of using THz radiation in a variety of very import- ant applications suggests that today this range proves to be at the forefront of investigations and develop- ments (see, for example, [1–3]). In the THz range, there occur the rotational spectra of many organic molecules including vibrations of biologically import- ant collective modes of DNA and proteins, as well as phonon resonances of crystal lattices, which make possible the development of new methods for the spectroscopy of biological and semiconductor struc- tures. The use of THz radiation for diagnosis and ther- apy in biology and medicine are of particular appeal. Unlike X-rays, this radiation is nonionizing and causes no tissue damage, which makes possible a diagnosis that is harmless to humans including that of cancer and burns. The difficulty in creating efficient terahertz sources is related to the fact that, in the terahertz range, well- developed methods for generating optical and micro- wave radiation are poorly applicable. A separate prob- lem lies in creating narrow-band sources of intense THz radiation. Powerful sources of terahertz radiation based on charge-carrier transport are synchrotrons and free-electron lasers; however, a high cost and large sizes prevent their wide use even for purely scientific applications. The classical devices of vacuum elec- tronics, such as traveling-wave tubes and backward- wave tubes, barely achieve the frequency limit of 1 THz, and their radiation power in this range does not exceed 1 mW. The difficulties in increasing the operating frequency of such vacuum sources are asso- ciated with those in manufacturing a small-scale deceleration system [4]. In gyrodevices, the radiation frequency is limited by the magnetic-field strength (to generate radiation at a frequency of 1 THz at the fun- damental harmonic, a field of ~38 T is required) and by the necessity of selection when working at gyrofre- quency harmonics, which usually requires complex technical solutions. On the other hand, since the equivalent temperature of 1 THz radiation is only 47.6 K, the thermal relaxation of levels at room tem- perature leads to the rapid destruction of inversion in lasers. Thus, quantum-cascade lasers (QCLs), which occupy the leading positions among compact semi- conductor sources in the mid-infrared (IR) range, operate at terahertz frequencies only under conditions of cryogenic cooling. Nowadays, the highest operating temperatures of THz QCLs are 117 K in the continu- ous-wave mode [5] and 199.5 K with pulsed pumping XXIII INTERNATIONAL SYMPOSIUM “NANOPHYSICS AND NANOELECTRONICS”, NIZHNY NOVGOROD, MARCH 11–14, 2019