Recent progress in terahertz difference-frequency quantum cascade laser sources

• Published in: Nanophotonics (Berlin, Germany) Vol. 7; no. 11; pp. 1795 - 1817
• Main Authors: Fujita, Kazuue, Jung, Seungyong, Jiang, Yifan, Kim, Jae Hyun, Nakanishi, Atsushi, Ito, Akio, Hitaka, Masahiro, Edamura, Tadataka, Belkin, Mikhail A.
• Format: Journal Article
• Language: English
• Published: De Gruyter 01.11.2018
• Subjects: difference frequency generation; integrated optics devices; intersubband transitions; quantum cascade laser; terahertz
• ISSN: 2192-8606; 2192-8614
• DOI: 10.1515/nanoph-2018-0093

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.