Study of Josephson Generation Harmonics in a Long Josephson Junction

We present the results of experimental study of the harmonics of Josephson radiation from a terahertz (THz) source based on a long Josephson junction (LJJ). The source consists of an integrated circuit including the LJJ, a matched transmitting antenna, and a harmonic mixer used for phase locking of the output signal. The LJJ and the harmonic mixer are both made of three-layer Nb-AlOx-Nb superconductor–insulator–superconductor (SIS) structures and operate at a temperature of 4.2 K. The antenna is located in the base superconducting Nb electrode of the microcircuit. To study the spectral composition and characteristics of the output radiation, two different methods were employed, using a THz Fourier transform spectrometer based on a broadband semiconductor detector and using a high-resolution THz spectrometer based on a heterodyne SIS receiver. The spectral composition of the radiation demonstrates the presence of both the fundamental harmonic of the Josephson effect and the higher harmonics. The spectral characteristics of the fundamental (at frequencies of 300–335 GHz) and the second (at frequencies of 600–670 GHz) harmonics, respectively, with a spectral resolution of about 0.1 MHz were studied using a SIS detector.