A broadly tunable terahertz source constructed as an external cavity system using a
difference-frequency generation quantum cascade laser source. The external cavity system
includes an external diffraction grating configured to tune and reflect mid-infrared emission
at a first wavelength. The laser includes a mid-infrared feedback grating defined in the laser
waveguide of the laser to fix mid-infrared lasing at a second wavelength. Alternatively, two
external diffraction gratings may be configured to tune and reflect mid-infrared emission at a …