  Optical frequency combs are key to optical precision measurements. While most
frequency combs operate in the near-infrared regime, many applications require
combs at mid-infrared, visible or even ultra-violet wavelengths. Frequency
combs can be transferred to other wavelengths via nonlinear optical processes,
however, this becomes exceedingly challenging for high-repetition rate
frequency combs. Here, it is demonstrated that a synchronously driven high-Q
microresonator with a second-order optical nonlinearity can efficiently convert
high-repetition rate near-infrared frequency combs to visible, ultra-violet and
mid-infrared wavelengths providing new opportunities for microresonator and
electro-optic combs in applications including molecular sensing, astronomy, and
quantum optics.
