Beam Modulation for Aberration Control and Signal Enhancement in Tip-Enhanced Raman Spectroscopy.

Abstract

Tip-enhanced Raman spectroscopy (TERS) provides the sensitivity required to obtain the vibrational fingerprint of few molecules. While single molecule detection has been demonstrated in UHV experiments, the sensitivity of the technique in ambient, liquid and electrochemical conditions is still limited. In this work, we present a new strategy to increase the signal-to-noise in TERS by spatial light modulation. We iteratively optimize the phase of the excitation beam employing two different feedback mechanisms. In one optimization protocol, we monitor the spectral changes upon aberration correction and tight far-field focusing. In a second protocol, we use a phase-optimization strategy where TER spectra are directly used for feedback. Far-field tight focusing results in average signal enhancements of a factor of 3.5 in air and has no impact on TER signals obtained from solid/liquid interfaces. Using the TER spectrum as direct feedback, we obtain average signal enhancements between a factor of 2.6 in liquid and 4.3 in air. In individual cases, some bands increase by more than one order of magnitude in intensity upon spatial light modulation. Importantly, phase modulation in addition allowed the retrieval of bands that were initially not discernible from the noise. The proposed beam-modulation strategy can be easily implemented in existing TERS instruments and can help to push the detection limit of the technique in applications where the signal-to-noise level is low.