Abstract-Spectral purity and tunability of terahertz quantum cascade laser sources based on intracavity difference-frequency generation

Luigi Consolino.  Seungyong Jung,  Annamaria Campa, Michele De Regis, Shovon Pal, Jae Hyun Kim, Kazuue Fujita, Akio Ito, Masahiro Hitaka, Saverio Bartalini, Paolo De Natale, Mikhail A. Belkin, Miriam Serena Vitiello,

http://advances.sciencemag.org/content/3/9/e1603317


Terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically pumped semiconductor sources capable of operating at room temperature across the 1- to 6-THz range. Despite tremendous progress in power output, which now exceeds 1 mW in pulsed and 10 μW in continuous-wave regimes at room temperature, knowledge of the major figure of merits of these devices for high-precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. By exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute center frequency of individual emission lines of these sources with an uncertainty of 4 × 10−10. The unveiled emission LW (400 kHz at 1-ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications that require narrow-LW THz sources.

Abstract-Spectroscopic Study of Terahertz Generation in Mid-Infrared Quantum Cascade Lasers

• Yifan Jiang
• , Karun Vijayraghavan
• , Seungyong Jung
• , Aiting Jiang
• , Jae Hyun Kim
• , Frederic Demmerle
• , Gerhard Boehm
• , Markus C. Amann
•  & Mikhail A. Belkin

http://www.nature.com/articles/srep21169

Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation are currently the only room-temperature mass-producible diode-laser-like emitters of coherent 1–6 THz radiation. Device performance has improved dramatically over the past few years to reach milliwatt-level power output and broad tuning from 1.2 to 5.9 THz, all at room-temperature. Terahertz output in these sources originates from intersubband optical nonlinearity in the laser active region. Here we report the first comprehensive spectroscopic study of the optical nonlinearity and investigate its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of 2 or greater depending on the spectral position of the mid-infrared pumps for a fixed THz difference-frequency. We have also measured for the first time the linewidth for transitions between the lower quantum cascade laser states, which is critical for determining terahertz nonlinearity and predicting optical loss in quantum cascade laser waveguides.