Mode Mapping Photonic Crystal Nanocavities with Q>5×105 Using Free-Carrier Absorption

We demonstrate a nonlinear photomodulation spectroscopy method to image the mode profile of a high-$Q$ photonic crystal resonator (PhCR). This far-field imaging method is suitable for ultrahigh-$Q$ cavities which we demonstrate on a $Q=619\phantom{\rule{0.1em}{0ex}}000$ PhCR. We scan the PhCR surface with a 405-nm pump beam that modulates the refractive index by local thermal tuning, while probing the response of the resonance. We enhance resolution by probing at high power, using the thermo-optical nonlinearity of the PhCR. Spatial resolution of the thermo-optical effect is typically constrained by the broad thermal profile of the optical pump. Here we go beyond the thermal limit and show that we can approach the diffraction limit of the pump light. This is due to free carrier absorption that heats up the PhCR only when there is overlap between the optical pump spot and the optical mode profile. This is supported with a thermo-optical model that reproduces the high-resolution mode mapping. Results reveal that the observed enhanced resolution is reached for surprisingly low carrier density.