Application of High-Tc Josephson Detectors for Spectral Characterization of THz Quantum Cascade Lasers

We have demonstrated the potential of high-
T_c
Josephson junctions (JJ) for broadband spectral characterization of electromagnetic radiation from terahertz (THz) quantum cascade lasers (QCLs). We have used frequency-selective Josephson detection with Hilbert-transform spectral analysis (HTSA) and classical detection in YBa
₂
Cu
₃
O
_7-x
bicrystal junctions to measure and interpret the spectra of QCL radiation from a few tens of GHz to 2.5 THz. The dominant part in the measured spectra concentrates near the designed frequency of 2.2 THz and includes several lines with the frequencies of the spatial modes in the QCL. We have found that, in addition to expected 2.2 THz radiation, QCL spectra demonstrate a low-frequency part with the frequencies
f
< 400 GHz and an intermediate region with increasing intensity for the frequencies from 500 GHz to 2 THz. We have found that intensities and spectral contents of three parts non-monotonically depend on the injection current of QCLs. We interpret the recovered spectra as an interplay of designed QCL operation with nonlinear effects of 2nd and 3rd orders in the active volume of QCLs. Compared to conventional techniques, the advantages of high-
T_c
JJs with HTSA are in frequency-domain operation and extended ranges, including THz, sub-THz, and GHz frequencies of QCL nonlinear dynamics.