Open Access Publications from the University of California

## Nanophotonics with surface enhanced coherent Raman microscopy

• Author(s): Fast, Alexander
Herein, a novel technique, wide-field surface enhanced coherent anti-Stokes Raman scattering (\textit{wf}SE-CARS) is presented. This technique allows for isolating weak vibrational signals in nanoscopic proximity to the surface by using chemical sensitivity of coherent Raman microspectroscopy (CRM) and field confinement from surface plasmons supported on a thin gold film. Uniform field enhancement over a large field of view, achieved with surface plasmon polaritons (SPP) in \textit{wf}SE-CARS, allows for biomolecular imaging demonstrated on extended structures like phospholipid droplets and live cells. Surface selectivity and chemical contrast are achieved at $\sim$ 70 fJ/$\mu$m$^2$ incident energy densities, which is over five orders of magnitude lower than used in conventional point scanning CRM.
Finally, a platform for coherently interrogating single molecules is presented. Single-molecule limit SE-CARS on non-resonant molecules is achieved by means of 3D local field confinement in the nanojunctions between two spherical gold nanoparticles. Localized plasmon resonance of the dimer nanostructure confines the probe volume down to 1 nm$^3$ and provides the local field enhancement necessary to reach single-molecule detection limit. Nonlinear excitation of Raman vibrations in SE-CARS microspectroscopy allows for higher image acquisition rates than in conventionally used single-molecule surface enhanced Raman spectroscopy (SERS). Therefore, data throughput is significantly improved while preserving spectral information despite the presence of the metal. Data simultaneously acquired from hundreds of nanoantennas allows to establish the peak enhancement factor from the observed count rates and define the maximum allowed local-field that preserves the integrity of the antenna. These results are paramount for the future design of time resolved single-molecule studies with multiple pulsed laser excitations, required for single-molecule coherence manipulation and quantum computing.