Ricerc@Sapienza

Optical frequency combs are one of the most remarkable inventions in recent decades.
Originally conceived as the spectral counterpart of the train of short pulses emitted by mode-locked
lasers, frequency combs have also been subsequently generated in continuously pumped
microresonators, through third-order parametric processes. Quite recently, direct generation of
optical frequency combs has been demonstrated in continuous-wave laser-pumped optical resonators
with a second-order nonlinear medium inside. Here, we present a concise introduction to such
quadratic combs and the physical mechanism that underlies their formation. We mainly review
our recent experimental and theoretical work on formation and dynamics of quadratic frequency
combs. We experimentally demonstrated comb generation in two configurations: a cavity for second
harmonic generation, where combs are generated both around the pump frequency and its second
harmonic and a degenerate optical parametric oscillator, where combs are generated around the pump
frequency and its subharmonic. The experiments have been supported by a thorough theoretical
analysis, aimed at modelling the dynamics of quadratic combs, both in frequency and time domains,
providing useful insights into the physics of this new class of optical frequency comb synthesizers.
Quadratic combs establish a new class of efficient frequency comb synthesizers, with unique features,
which could enable straightforward access to new spectral regions and stimulate novel applications.