Frequency comb generation in quadratic nonlinear media

We experimentally demonstrate and theoretically explain the onset of optical frequency combs in a simple cavity-enhanced second-harmonic-generation system, exploiting second-order nonlinear interactions. Two combs are simultaneously generated around the fundamental pump frequency, with a spectral bandwidth up to about 10 nm, and its second harmonic. We observe different regimes of generation, depending on the phase-matching condition for second-harmonic generation. Moreover, we develop an elemental model which provides a deep physical insight into the observed dynamics. Despite the different underlying physical mechanism, the proposed model is remarkably similar to the description of third-order effects in microresonators, revealing a potential variety of new effects to be explored and laying the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.