Nonlinear Optical Processes in Centrosymmetric Systems by Cavity-Induced Symmetry Breaking

Nonlinear optical processes associated with even-order nonlinear susceptibilities are critical for both classical and quantum technologies. Inversion symmetry, however, prevents nonlinear optical responses mediated by even-order susceptibilities in several material systems that are pertinent for applications in nanophotonics. Here, we demonstrate induced nonlinear optical processes, namely, second- and fourth-harmonic generation that are naturally forbidden in an inversion symmetric system, by coupling to a photon mode of an optical cavity. For the coupled system, we control the inversion symmetry breaking by changing the light-matter coupling strength, which at the same time allows tuning of the nonlinear conversion efficiency. We find that the harmonic generation yield can be significantly increased by increasing the light-matter coupling strength in an experimentally feasible way. In addition, we find that the harmonic conversion efficiency is increased in the cavity-induced setting as opposed to using intense pump fields. Our work constitutes a step forward in the direction of realizing physically forbidden nonlinear optical processes in centrosymmetric materials widely adopted for applications in integrated photonics.