A Fast, Coherent Look At Single-Molecule Vibrations

Ultrafast nonlinear spectroscopy in which scientists excite a molecule with optical pulses and then use a probe pulse to see what happens allows researchers to observe the motions of chemical bonds. But such measurements are usually made of large ensembles of molecules and are limited to time­scales over which the molecular motions of the ensemble remain coherent. Vartkess A. Apkarian, Eric O. Potma, and coworkers at the University of California, Irvine, show that they can now make measurements of single molecules by attaching the molecules to dumbbell-shaped silica-coated gold “nanoantennae” and using surface-enhanced coherent anti-Stokes Raman scattering, or CARS (Nat. Photonics 2014, DOI: 10.1038/nphoton.2014.143). The time-resolved single-molecule signals are not subject to dephasing, which is a defining feature of typical ensemble measurements. Instead, single-molecule CARS signals reveal characteristic phase fluctuations. This phase noise is a result of sampling statistics and can be used to determine whether a signal comes from one, two, or many molecules. Richard P. Van Duyne of Northwestern University, a pioneer in single-molecule surface-enhanced Raman spectroscopy, or SERS, says “this paper demonstrates how far we’ve come in this field, as well as the challenges we face as we work toward the goal of watching molecules react in real time.”