Non-volatile photorefractive tuning and green light generation in a diamond cavity

Single-crystal diamond nanocavities have tremendous potential for use in quantum and nonlinear optical technologies. The ability to precisely control their resonant frequencies is essential for many applications, and $\textit{in situ}$ tuning is particularly desirable. In this work, we demonstrate deterministic and non-volatile resonance tuning of a diamond nanocavity. We observed a photorefractive effect in concert with the generation of third-harmonic light within the device. This effect blue-shifted the cavity resonance frequency by $20.9\,\text{GHz}$, exceeding the cavity linewidth. The shift corresponded to a fractional change in refractive index of $-10^{-4}$, and its relaxation occurred over several tens of hours. Although photorefraction is a second-order nonlinear effect and has previously not been observed in diamond owing to its vanishing $\chi^{(2)}$, the observed behaviour is consistent with the generation of non-zero $\chi^{(2)}$ by electric fields from charged crystal defects. This observation could enable the realisation of diamond frequency converters and electro-optical modulators that rely on second-order nonlinearity.