High Q-factors and small mode volumes have made toroidal optical microresonators exquisite sensors to small shifts in the effective refractive index of the WGM modes. Eliminating contaminants and improving quality factors is key for many different sensing techniques, and is particularly important for photothermal imaging as contaminants add photothermal background obscuring objects of interest. Several different cleaning procedures including wet- and dry-chemical procedures are tested for their effect on Q-factors and photothermal background. RCA cleaning was shown to be successful in contrast to previously described acid cleaning procedures, most likely due to the different surface reactivity of the acid reagents used. UV-ozone cleaning was shown to be vastly superior to O2 plasma cleaning procedures, significantly reducing the photothermal background of the resonator.
The extreme temperature sensitivity of whispering-gallery-mode (WGM) microresonators holds great promise as a detection strategy for single-particle photothermal microscopy and spectroscopy. The detection limit is currently partially constrained by frequency noise from the laser used to probe the cavity resonance wavelength. We present a measurement technique capable of simultaneously detecting backscattered and transmitted light from a wavelengthlocked optical microresonator, with laser intensity noise and frequency noise partitioned into the two independent detection channels. Photothermal mapping of single absorbing nano-objects demonstrates that both methods are capable of high signal/noise, exceeding 30,000:1 in the backscattering channel for a photothermally-induced microresonator resonance shift of 93 fm.
A new method is described for measuring the absorption of light by single non-emissive nanoparticles. Individual carbon nanofibers are imaged using a photonic transducer to quantify the heat dissipated after the electronic energy is thermalized. Leveraging the high sensitivity of ultrahigh-quality-factor optical microresonators as photothermal transducers provides high sensitivity. Polarization-resolved measurements indicate that the orientation of the absorption dipole of a nanofiber matches the long axis of the fiber. The per-atom absorption cross-section is determined to be (2.9 x 10-18 cm2 /carbon atom), in close agreement with the value for bulk graphite.