Theoretical Analysis of Optically Selective Imaging in Photoinduced Force Microscopy

TL;DR

This paper provides a theoretical analysis of photoinduced force microscopy (PiFM), demonstrating enhanced sensitivity and resolution in imaging composite molecular systems through localized electric field effects and wavelength-dependent responses.

Contribution

It introduces a comprehensive theoretical model for PiFM using the discrete dipole approximation, revealing new insights into its sensitivity and resolution capabilities for molecular imaging.

Findings

Enhanced sensitivity for resonant molecules due to field enhancement.

Ability to observe forbidden optical transitions with high resolution.

PiFM images vary significantly with wavelength and localized field structures.

Abstract

We present a theoretical study of the measurements of photoinduced force microscopy (PiFM) for composite molecular systems. Using the discrete dipole approximation, we calculate the self-consistent response electric field of the entire sample including the PiFM tip, substrate, and composite molecules. We demonstrate a higher sensitivity for the PiFM measurement on resonant molecules than by the previously obtained tip-sample distance dependency $z^{-4}$ owing of the multifold enhancement of the field between the localized electric field induced at the tip-substrate nanogap and the molecular polarization. The enhanced localized electric field induced at the tip-substrate nanogap in PiFM allows high-resolution observation of the forbidden optical electronic transition in dimer molecules. We investigated the wavelength dependence of PiFM for dimer molecules and obtained images at incident light wavelengths corresponding to allowed and forbidden transitions. We reveal that these PiFM images drastically change with the frequency-dependent spatial structures of the localized electric field vectors and resolve different types of nanoparticles beyond the resolution for the optically allowed transitions. This study demonstrates that PiFM provides multifaceted information based on microscopic interactions between nanomaterials and light.