Stepwise quantum phonon pumping in plasmon-enhanced Raman scattering

TL;DR

This paper theoretically predicts and experimentally demonstrates a stepwise increase in plasmon-enhanced Raman scattering intensity with laser power, revealing nonlinear phonon pumping behavior at the quantum level.

Contribution

It introduces a quantum model of phonon pumping in PERS and provides experimental evidence of stepwise intensity dependence on laser power.

Findings

Observation of stepwise PERS intensity increase with laser power

Quantum prediction of phonon number state dependence

Experimental validation using gold nanosphere-plane antennas

Abstract

Plasmon-enhanced Raman scattering (PERS) becomes nonlinear when phonon pumping and phonon-stimulated scattering come into play. It is fundamental to the understanding of PERS and its photobleaching behavior. By quantization of the molecular vibration coherent state into phonon number states, we theoretically predict a stepwise dependence of PERS intensity on laser power. Experimental evidence is presented by measuring a monolayer of malachite green isothiocyanate molecules sandwiched in individual gold nanosphere-plane antennas, under radially polarized laser excitation of sub-microWatt powers.