Theoretical Analysis of Optically Selective Imaging in Photoinduced Force Microscopy

We present a theoretical study on the measurement of photoinduced force microscopy (PiFM) for composite molecular systems. Using discrete dipole approximation, we calculate the self-consistent response electric field of the entire system, including the PiFM tip, substrate, and composite molecules. We demonstrate a higher sensitivity for PiFM measurement on resonant molecules than the previously obtained tip-sample distance dependency, z-4, owing to multifold enhancement of the localized electric field induced at the tip-substrate nanogap and molecular polarization. The enhanced localized electric field in PiFM allows high-resolution observation of forbidden optical electronic transitions in dimer molecules. We investigate the wavelength dependence of PiFM for dimer molecules, obtaining images at incident light wavelengths corresponding to the 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 yields multifaceted information based on microscopic interactions between nanomaterials and light.