Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators

A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules.