Narrow linewidth single-frequency terahertz source based on difference frequency generation of vertical-external-cavity source-emitting lasers in an external resonance cavity.

We demonstrate a continuous wave, single-frequency terahertz (THz) source emitting 1.9 THz. The linewidth is less than 100 kHz and the generated THz output power exceeds 100 µW. The THz source is based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency vertical-external-cavity source-emitting lasers with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and provide pump photons for the nonlinear downconversion. It is demonstrated that the THz source can be used as a local oscillator to drive a receiver used in astronomy applications.