Abstract
Chiral plasmonic nanoparticles can exhibit strong chiroptical signals compared to the corresponding molecular response. Observations are, however, generally restricted to measurements on stationary single particles with a fixed orientation, which complicates the spectral analysis. Here, we report the spectroscopic observation of a freely diffusing single chiral nanoparticle in solution. By acquiring time-resolved circular differential scattering signals we show that the spectral interpretation is significantly simplified. We experimentally demonstrate the equivalence between time-averaged chiral spectra observed for an individual nanostructure and the corresponding ensemble spectra, and thereby demonstrate the ergodic principle for chiroptical spectroscopy. We also show how it is possible for an achiral particle to yield an instantaneous chiroptical response, whereas the time-averaged signals are an unequivocal measure of chirality. Time-resolved chiroptical spectroscopy on a freely moving chiral nanoparticle advances the field of single-particle spectroscopy, and is a means to obtain the true signature of the nanoparticle’s chirality.
Highlights
Chiral plasmonic nanoparticles can exhibit strong chiroptical signals compared to the corresponding molecular response
We have reported the measurement of a circular differential scattering spectrum acquired from a single chiral nanoparticle that is freely suspended in a liquid
Brownian showed that the time-averaged and thereby rotationally averaged circular differential scattering intensity (CDSI) spectrum unequivocally arises from chirality intrinsic to the single particle, whereas contributions due to linear birefringence (LB) and linear dichroism (LD) cancel out
Summary
Chiral plasmonic nanoparticles can exhibit strong chiroptical signals compared to the corresponding molecular response. We experimentally demonstrate the equivalence between time-averaged chiral spectra observed for an individual nanostructure and the corresponding ensemble spectra, and thereby demonstrate the ergodic principle for chiroptical spectroscopy. A difference in absorption (strictly, extinction) is known as circular dichroism (CD) Both are unequivocal signatures of the presence of chirality when observed from an isotropic solution or suspension. The examined particle remains stationary (immobilized to a surface) and oriented with respect to the input light In this case contributions from linear dichroism (LD) and linear birefringence (LB) can cause nonzero circular differential intensities[16], which are exacerbated by coupling effects with the substrate[17] or the optics[12,13]. Deducing the same information from a single plasmonic nanoparticle as from traditional CD spectroscopy enables novel sensing techniques with strongly enhanced sensitivities
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