Transient Spectral Fluctuations of Single Molecules Revealed using an Optical Antenna

Abstract

Fluorescence spectra and lifetimes of single molecules reflect their specific local environments, which is not available from the ensemble-averaged measurements. Usually, a low signal-to-noise ratio of single-molecule spectroscopy impedes access to transient emission spectra of single molecules at room temperature, especially for the molecules with a low quantum yield. Here, we realize detection of transient single-molecule spectra with an effective integration time of ∼2.14 milliseconds at room temperature of methylene blue (MB) or crystal violet (CV) molecules. In the experiments, the transient emission spectra detection of single molecules is achieved through the enhanced signal using a plasmonic antenna, the suppressed background noise due to a low quantum yield, and the short dwelling time of molecular adsorption on the antenna surface. The transient spectra show apparent fluctuation in the emission intensity and shifts of the peak maximum. Interestingly, the emission spectrum presents a narrow band which can be fitted well with a single Lorentzian lineshape. The specific molecule configuration should dominate the transient spectral lineshape mainly, which is supported by the numerical ab initio simulations. These spectral fluctuations should attribute to the transitions between metastable minima in the molecular potential-energy surface. These findings provide us more understanding of the transient behaviors of single molecules at the solid-aqueous interfaces.