Single-frequency terahertz source based on difference frequency generation in an enhancement cavity

We demonstrate a continuous wave, single-frequency terahertz (THz) source with a linewidth less than 100 kHz. The emitted THz power at 1.9 THz exceeds 100 μW. The source is based on difference frequency generation within an optical enhancement cavity. Two single-frequency semiconductor disc lasers with infrared emission wavelengths spaced by 6.8 nm are used as pump lasers for the nonlinear conversion. To increase the efficiency of the THz generation, the infrared intensity is enhanced by a factor of 100 by phase locking of the two lasers to the external cavity.