Photonic comb-rooted synthesis of ultra-stable terahertz frequencies

Stable terahertz sources are required to advance high-precision terahertz applications such as molecular spectroscopy, terahertz radars, and wireless communications. Here, we demonstrate a photonic scheme of terahertz synthesis devised to bring the well-established feat of optical frequency comb stabilization down to the terahertz region. The source comb is stabilized to an ultra-low expansion optical cavity offering a frequency instability of 10^-15 at 1-s integration. By photomixing a pair of comb lines extracted coherently from the source comb, terahertz frequencies of 0.10-1.10 THz are generated with an extremely low level of phase noise of -70 dBc/Hz at 1-Hz offset. The frequency instability measured for 0.66 THz is 4.4 × 10^-15 at 1-s integration, which reduces to 5.1×10^-17 at 65-s integration. Such unprecedented performance is expected to drastically improve the signal-to-noise ratio of terahertz radars, the resolving power of terahertz molecular spectroscopy, and the transmission capacity of wireless communications.