Optical frequency combs from high-order sideband generation

We report on the generation of frequency combs from the recently-discovered phenomenon of high-order sideband generation (HSG). A near-band gap continuous-wave (cw) laser with frequency f was transmitted through an epitaxial layer containing GaAs/AlGaAs quantum wells that were driven by quasi-cw in-plane electric fields F between 4 and 50 kV/cm oscillating at frequencies f between 240 and 640 GHz. Frequency combs with teeth at f = f + nf (n even) were produced, with maximum reported n > 120, corresponding to a maximum comb span > 80 THz. Comb spectra with the identical product f × F were found to have similar spans and shapes in most cases, as expected from the picture of HSG as a scattering-limited electron-hole recollision phenomenon. The HSG combs were used to measure the frequency and linewidth of our THz source as a demonstration of potential applications.