Optical frequency comb by giant nonlinear capillary waves

Nonlinear optical processes are essential for modern photonics and they are possible mostly when light is produced by a high-power laser. However, nonlinear effects of non-optical origin, such as those observed in acoustical and mechanical systems, are many orders of magnitude stronger than optical nonlinearities, and therefore they can be induced with energy that is much lower than that of a laser pulse. Here, we experimentally confirm our theoretical prediction of the possibility to convert giant acoustic and capillary wave nonlinearities into optical signals, thereby effectively reproducing the result of a conventional nonlinear-optical interaction. We excite highly nonlinear capillary Faraday waves on the surface of a thin layer of ethanol and we reflect a beam of low-power, incoherent light from these waves to produce an optical frequency comb. Our results can be used in many areas of photonics, including new classes of biomedical sensors that do not rely on high laser powers.