  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
= 619000 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.
