Recent progress in terahertz difference‐frequency quantum cascade laser sources

Abstract Terahertz quantum cascade laser (QCL) sources based on intra‐cavity difference frequency generation are currently the only electrically pumped monolithic semiconductor light sources operating at room temperature in the 1–6‐THz spectral range. Relying on the active regions with the giant second‐order nonlinear susceptibility and the Cherenkov phase‐matching scheme, these devices demonstrated drastic improvements in performance in the past several years and can now produce narrow‐linewidth single‐mode terahertz emission that is tunable from 1 to 6 THz with power output sufficient for imaging and spectroscopic applications. This paper reviews the progress of this technology. Recent efforts in wave function engineering using a new active region design based on a dual‐upper‐state concept led to a significant enhancement of the optical nonlinearity of the active region for efficient terahertz generation. The transfer of Cherenkov devices from their native semi‐insulating InP substrates to high‐resistivity silicon substrates resulted in a dramatic improvement in the outcoupling efficiency of terahertz radiation. Cherenkov terahertz QCL sources based on the dual‐upper‐state design have also been shown to exhibit ultra‐broadband comb‐like terahertz emission spectra with more than one octave of terahertz frequency span. The broadband terahertz QCL sources operating in continuous‐wave mode produces the narrow inter‐mode beat‐note linewidth of 287 Hz, which indicates frequency comb operation of mid‐infrared pumps and thus supports potential terahertz comb operation. Finally, we report the high‐quality terahertz imaging obtained by a THz imaging system using terahertz QCL sources based on intra‐cavity difference frequency generation.