Phase-sensitive photothermal imaging of ultrahigh-Q polyoxide toroidal microresonators

Toroidal optical microresonators are powerful platforms for quantum optics and label-free sensing and imaging. Although these microresonators are generally fabricated from thermal oxide on silicon wafers, many benefits arise from greater flexibility for fabrication of resonators on substrates other than single-crystal silicon. The ability to perform fabrication with easily deposited polysilicon while matching the superlative optical properties of thermal-oxide would address this need. In this work, toroidal microresonators were fabricated using thermal oxide grown from polysilicon (polyoxide). These resonators possess comparable quality factors to their counterparts fabricated with oxide thermally grown from single-crystal silicon (crystalline oxide). Photothermal imaging and spectroscopy showed evidence of residual silicon and adsorbed water. Phase sensitive imaging and finite-element simulation show how subtly different thermal pathways are taken in the two material systems. Polysilicon is shown to be a viable precursor for making ultrahigh-Q toroidal microresonators.