in Diamond Photonics*

The wide bandgap, low multi-photon absorption, large thermal conductivity and moderate refractive index make diamond an attractive material for photonic applications [1]. However, diamond is a centrosymmetric material – whose vanishing χ⁽²⁾ prohibits second-order nonlinear interactions. Despite this limitation, we have demonstrated second-harmonic generation (SHG) in a doubly-resonant microdisk cavity [2]. This normally forbidden process can arise from several sources of crystal symmetry breaking whose relative contributions are typically difficult to disentangle. By introducing an auxiliary green laser “control” field that we observe to have the effect of modulating χ(2) experienced by a telecom input. We credit the observation of SHG to the presence of charged crystal defects, such as substitutional nitrogen and nitrogen-vacancy (NV) centres. These charged defects create local electric fields EDC, which couple to the intrinsic χ⁽³⁾ to form an effective χ⁽²⁾_eff=3E_DCχ⁽³⁾. The combination of the telecom input and the green laser quenches the SHG signal, which we attribute to the photoionization from the negative to the neutral NV centre [3]. This photoionization alters EDC, and consequently diminishing χ⁽²⁾_eff. Removing the green laser recovers the original SHG signal, allowing for deterministic optical switching of the device’s χ⁽²⁾. Unlocking diamonds full potential as a χ⁽²⁾ material via defect engineering opens the door for the realisation of frequency conversion, modulators, and optical switching via χ (2) engineering, thereby opening the door for second-order nonlinear processes – a valuable addition to the diamond photonics toolbox.

[1] P. K. Shandilya et al., J. Lightwave Technol. 40, 7538-7571 (2022)

[2] S. Flågan et al., arXiv:2412.06792 (2024)

[3] P. K. Shandilya et al., arXiv:2411.10638 (2024)