Optical Switching of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mn>2</mml:mn> <mml:mo stretchy="false"

Diamond's unique physical properties make it a versatile material for a wide range of nonlinear and quantum photonic technologies. However, unlocking diamond's full potential as a nonlinear photonic material with nonzero second-order susceptibility, χ^{(2)}≠0, requires symmetry breaking. In this Letter, we use a nanoscale cavity to demonstrate second-harmonic generation (SHG) in diamond, and demonstrate, for the first time, that the magnitude of diamond's effective χ^{(2)} strongly depends on the electronic configuration of defects in the diamond crystal, such as nitrogen-vacancy centers. The modification of χ^{(2)} arises from photoionization 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 χ^{(2)}. Optical control of χ^{(2)} by defect engineering opens the door for second-order nonlinear processes in diamond.