External cavity system generating broadly tunable terahertz radiation in mid-infrared quantum cascade lasers

An external cavity system, comprising:

• a difference-frequency generation quantum cascade laser source designed with integrated laser gain and optical nonlinearity for mid-infrared lasing and terahertz generation, respectively;
• a beam splitter configured to split mid-infrared laser emission into two beams of light directed to a first and a second diffraction grating, wherein the first diffraction grating is configured to tune and reflect mid-infrared emission at a first wavelength, wherein the second diffraction grating is configured to tune and reflect mid-infrared emission at a second wavelength;
• and
• a lens configured to collimate mid-infrared radiation from the laser source onto the beam splitter as well as focus the mid-infrared radiation reflected from the first and second diffraction gratings into an active region of the laser source whereby a tunable THz DFG takes place in the active region at a frequency determined by the first and second diffraction gratings.