In Situ Hilbert-Transform Spectral Analysis of Pulsed Terahertz Radiation of Quantum Cascade Lasers by High-Tc Josephson Junctions

We have studied the applicability of high- Tc Josephson junctions (JJs) and Hilbert-transform spectral analysis (HTSA) for characterization of pulsed terahertz (THz) radiation. We have reconsidered the extension of HTSA into the THz range, including an impact of such emerging factors as nonequilibrium voltage fluctuations and dc Joule heating in high- Tc JJs, and found an unexpected weak temperature dependence of Josephson linewidth δf and a spectral dependence δf(f) with a minimum. Due to this analysis, we have chosen YBa2Cu3O7-x bicrystal JJs for experimental study. The JJs have analyzed pulsed THz radiation from quantum cascade lasers (QCLs) located in the same cryogenic environment at 50 K. The spectra recovered by the JJ with Rn = 43 Ω consist of a line with a central frequency around 2.2 THz, which have a symmetrical form and the linewidth of 90 GHz close to the intrinsic Josephson linewidth of this JJ. The spectra measured by the JJ with  Rn  = 7 Ω and the intrinsic Josephson linewidth of 16 GHz consist of several lines at the frequencies fi around 2.2 THz, which are due to the excitation of different resonant modes in the active volume of the QCL. The difference between the fitted central frequencies f 2 - f 1 of the strong lines and the ratio of their intensities are in agreement with the spectrum of the same QCL measured by Fourier spectroscopy. The developed technique paves the way for detailed and rapid characterization of pulsed THz sources in the frequency domain.