Optical Switching of $\chi^{(2)}$ in Diamond Photonics

Diamond's unique physical properties make it a versatile materials for a wide range of nonlinear and quantum photonic technologies. However, unlocking diamond's full potential as a nonlinear photonic material with non-zero second-order susceptibility $\chi^{(2)}<br>eq0$ requires symmetry breaking. In this work, we use a nanoscale cavity to demonstrate second-harmonic generation (SHG) in diamond, and demonstrate, for the first time, that the magnitude of the diamond's effective $\chi^{(2)}$ strongly depends on the electronic configuration of defects in the diamond crystal, such as nitrogen vacancy centres. The modification of $\chi^{(2)}$ arises from photoionisation from the negative to neutral charge state, and is manifested by quenching of SHG upon green illumination. Toggling the green illumination allows for optical switching of the device's $\chi^{(2)}$. Optical control of $\chi^{(2)}$ by defect engineering opens the door for second-order nonlinear processes in diamond.