Detecting stimulated-Raman responses of few molecules using Optical Forces

TL;DR

This paper introduces a novel near-field microscopy technique that detects stimulated Raman responses of few molecules using optical forces, eliminating the need for far-field detection and enabling imaging of very small molecular quantities.

Contribution

It demonstrates stimulated Raman nanoscopy of few molecules via optical forces, using AFM cantilever vibrations and non-collinear beams, advancing molecular imaging methods.

Findings

Detected fewer than 20 molecules at maximum field enhancement.

Improved signal-to-noise ratio with second eigenmode AFM vibration.

Implemented stimulated Raman nanoscopy with non-collinear pump and probe beams.

Abstract

We demonstrate the stimulated Raman near-field microscopy of few molecules, measured only using near-field optical forces thereby eliminating the need for far-field optical detection. The molecules were excited in the near-field without resonant electronic enhancement. We imaged gold nanoparticles of 30 nm diameter functionalized with a self-assembled monolayer (SAM) of 4-nitrobenzenethiol (4-NBT) molecules. The maximum number of molecules detected by the gold-coated nano-probe at the position of maximum field enhancement could be fewer than 20 molecules. The molecules were imaged by vibrating an Atomic Force Microscope (AFM) cantilever on its second flexural eigenmode enabling the tip to be controlled much closer to the sample thereby improving the detected signal-to-noise ratio when compared to vibrating the cantilever on its first flexural eigenmode. We also demonstrate the implementation of stimulated Raman nanoscopy measured using photon-induced force with non-collinear pump and stimulating beams which could have applications in polarization dependent Raman nanoscopy and spectroscopy and pump-probe nano-spectroscopy particularly involving infrared beam/s.