Optical microresonators as single-particle absorption spectrometers

• Published in: Nature photonics Vol. 10; no. 12; pp. 788 - 795
• Main Authors: Heylman, Kevin D., Thakkar, Niket, Horak, Erik H., Quillin, Steven C., Cherqui, Charles, Knapper, Kassandra A., Masiello, David J., Goldsmith, Randall H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-12-2016; Nature Publishing Group
• Subjects: 119/118; 140/125; 639/624/1075/1083; 639/624/1107/527/1989; 639/624/399/1097; 639/638/11/2257; 639/925/357/354; Absorption spectroscopy; Applied and Technical Physics; Arrays; Biological materials; Electrons; Gold; Lasers; Nanorods; Nanostructure; Photonics; Physics; Quantum Physics; Resonators; Spectrometers; Thermometers; United States > US
• ISSN: 1749-4885; 1749-4893
• DOI: 10.1038/nphoton.2016.217

Optical measurements of nanoscale objects offer major insights into fundamental biological, material and photonic properties. In absorption spectroscopy, sensitivity limits applications at the nanoscale. Here, we present a new single-particle double-modulation photothermal absorption spectroscopy method that employs on-chip optical whispering-gallery-mode (WGM) microresonators as ultrasensitive thermometers. Optical excitation of a nanoscale object on the microresonator produces increased local temperatures that are proportional to the absorption cross-section of the object. We resolve photothermal shifts in the resonance frequency of the microresonator that are smaller than 100 Hz, orders of magnitude smaller than previous WGM sensing schemes. The application of our new technique to single gold nanorods reveals a dense array of sharp Fano resonances arising from the coupling between the localized surface plasmon of the gold nanorod and the WGMs of the resonator, allowing for the exploration of plasmonic–photonic hybridization. In terms of the wider applicability, our approach adds label-free spectroscopic identification to microresonator-based detection schemes.
Single-particle double-modulation absorption spectrometers based on whispering-gallery-mode microresonators achieve sub-100-Hz sensitivity to photothermal resonance shifts and allow for the study of arrays of Fano resonances in the context of plasmonic–photonic hybridization.