Rapid imaging of chaotic modes in optical microcavities

Identifying optical modes in chaotic cavities is crucial for exploring and understanding the physical mechanisms inside them. Compared with free spectral range estimation, the direct imaging technique has the capability of providing more precise mode information, but it is extremely time-consuming and susceptible to environmental perturbations. Here we report a high-speed imaging technique for visualizing field distributions in chaotic microcavities. When a silicon microdisk is excited by a femtosecond laser, free carriers are locally generated, thereby reducing the refractive index. Under a constant laser power, the spatial distribution of mode inside the silicon microdisk is proportional to its wavelength shift and can be precisely identified by comparing it with numerical simulation. With the assistance of a galvanometer, imaging a mode profile only takes a few hundred milliseconds to a few seconds, orders of magnitude faster than previous reports. The impacts of slight fabrication deviations on spectra have also been identified.