Recent work by Associate Professor David Masiello and colleagues was highlighted in a November 7 article in Nature Photonics. The research was also highlighted in Chemical & Engineering News and in a News & Views feature article in Nature Photonics.
Measurement of the two distinct components—scattering and absorption—of a single nanoscale object’s optical extinction provides fundamentally important and complementary information on how that object processes light: either scattering it back to the far-field or converting it into internal excitation. Today, various techniques exist to measure the scattering from individual nanoscale objects, all relying on the detection of scattered photons in regions of zero background. Measuring their absorption, however, is much more complicated due to the fundamental inability to detect extremely small reductions in transmission over statistical fluctuations in the number of photons. This means that the spectroscopic signature of the vast majority of molecules—specifically, those that are transformed into dark states through photoreactions—is difficult to access.
To overcome this challenge, researchers in the Masiello group and the Goldsmith group at the University of Wisconsin–Madison devised a new experimental route to measure the absorption spectra of individual, nonemissive nanoscale objects by photothermal contrast in an optical microresonator cavity.
Photothermal spectroscopies function by inferring an object’s absorption from the localized temperature increase and resulting refractive index inhomogeneity produced by the excited object’s nonradiative decay. In their work, the team coupled individual plasmonic nanorods to an ultrahigh-quality optical microresonator cavity and succeeded in determining the nanorod’s absorption spectrum by monitoring the temperature-dependent attometer shifts in the resonance frequency of microresonator’s whispering gallery modes. These exceedingly small but detectable resonance shifts correspond to temperature increases of ~100 nK (measured at room temperature!), making their absorption spectrometer simultaneously one of the world’s best thermometers. Suprisingly, the nanorod’s absorption spectrum revealed a dense array of sharp Fano interferences arising from its interaction with the whispering gallery modes of the microresonator, allowing the team to deeply explore the hybridization of plasmonic and photonic cavity modes.
This collaborative effort brought together the creativity and talents of several graduate students and postdocs in multiple departments between the two institutions. The results were achieved following years of hard work involving both theorists and experimentalists. Future directions will explore the feasibility of this system to serve as a platform for studying quantum physics at room temperature.