Homodyne Photothermal Force Microscopy for Direct Optical Near-Field Mapping

Abstract

Photoinduced force microscopy enables nanoscale studies of light–matter interactions, yet photothermal effects in the probe are commonly regarded as artifacts that limit performance. Here, we show that this photothermal response can instead be exploited as a sensitive measurement principle for optical near-field mapping. Under modulated near-field illumination, partial absorption at the AFM tip apex induces a photothermal bending of the metal-coated cantilever. At slow modulation frequencies, efficient heat diffusion modulates the temperature of the entire cantilever, leading to a strongly enhanced homodyne signal. We introduce this approach as homodyne photothermal force microscopy and demonstrate its capability by imaging plasmonic silver nanocubes under evanescent illumination on a commercial AFM. Interference between the optical near-field and radiation from the induced dipole at the scatterer produces a characteristic parabolic standing-wave pattern. An analytical model based on an equivalent point dipole reproduces the observed features and enables the extraction of the complex polarizability of individual nanocubes. By reinterpreting probe photothermal motion from an artifact into a signal carrier, we establish homodyne photothermal force microscopy as a simple and powerful platform for high-resolution characterization of optical near-fields and plasmonic nanostructures.