Latest advances in optical frequency combs based on quadratic non-linearity

Second-order non-linear susceptibility ($\chi ^2$χ2) is the cause of many non-linear processes in optics including second harmonic generation, sum frequency generation and difference frequency generation. Optical frequency combs based on $\chi ^2$χ2, i.e. quadratic frequency combs, hold the promise of reducing the devices power consumption and footprint since $\chi ^2$χ2 materials present a stronger non-linearity than $\chi ^3$χ3 materials, which suggests that higher efficiencies can be achieved. In this work, we will revisit the latest quadratic combs experimental advances, concerning power threshold, span and wavelength conversion efficiency.