Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering

The motion of chemical bonds within molecules can be observed in real time, in the form of vibrational wavepackets prepared and interrogated through ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are commonly performed on large ensembles of molecules, and as such, are limited to the extent that ensemble coherence can be maintained. Here, we describe vibrational wavepacket motion on single molecules, recorded through time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The required sensitivity to detect the motion of a single molecule, under ambient conditions, is achieved by equipping the molecule with a dipolar nano-antenna (a gold dumbbell). In contrast with measurements in ensembles, the vibrational coherence on a single molecule does not dephase. It develops phase fluctuations with characteristic statistics. We present the time evolution of discretely sampled statistical states, and highlight the unique information content in the characteristic, early-time probability distribution function of the signal.Comment: 17 pages, 5 figure

SERS from the point of view of the Molecule & the Antenna

Surface Enhanced Raman Spectra (SERS) recorded on plasmonic substrate is often accompanied with broad background, origin of which is extensively deliberated. Moreover, fluctuation in the spectra is regarded as a signature of single molecule response. The plasmonic junctions typically evolve and so does the local near field, which the molecule experiences. The molecular signature indicates presence of complicated near field, as well as the background is characterized via electronic Raman of the metal. The understanding of the entire spectra is based on the molecular response, metal response and their coupling. Polarized SERS measurements on single Au or Ag nanodimer equipped with some reporter molecule shows that the emission constituting the continuum is polarized. In a metal, bulk states can only result in scattering which is unpolarized, however, the surface states of the metal can be responsible for Raman. We assign the continuua to Raman of plasmons. The polarization decomposition of the intimately related molecular and plasmonic SERS allows mechanistic assignments of the scattering process and the nature of accessible plasmons in the prototypical dumbbell antenna

Orientation-Dependent Handedness of Chiral Plasmons on Nanosphere Dimers: How to Turn a Right Hand into a Left Hand

Optical activity, which is used as a discriminator of chiral enantiomers, is demonstrated to be orientation dependent on individual, and nominally achiral, plasmonic nanosphere dimers. Through measurements of their giant Raman optical activity, we demonstrate that L/R-handed enantiomers can be continuously turned into their R/L-handed mirror images without passing through an achiral state. The primitive uniaxial multipolar response, with demonstrable broken parity and time reversal symmetry, reproduces the observations as resonant Raman scattering on plasmons that carry angular momentum. The analysis underscores that chirality does not have a quantitative continuous measure and recognizes the manipulation of superpositions of multipolar plasmons as a paradigm for novel optical materials with artificial magnetism

Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering

The motion of chemical bonds within molecules can be observed in real time, in the form of vibrational wavepackets prepared and interrogated through ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are commonly performed on large ensembles of molecules, and as such, are limited to the extent that ensemble coherence can be maintained. Here, we describe vibrational wavepacket motion on single molecules, recorded through time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The required sensitivity to detect the motion of a single molecule, under ambient conditions, is achieved by equipping the molecule with a dipolar nano-antenna (a gold dumbbell). In contrast with measurements in ensembles, the vibrational coherence on a single molecule does not dephase. It develops phase fluctuations with characteristic statistics. We present the time evolution of discretely sampled statistical states, and highlight the unique information content in the characteristic, early-time probability distribution function of the signal