Infrared photothermal heterodyne imaging: Contrast mechanism and detection limits

Infrared photothermal heterodyne imaging (IR-PHI) is an ultrasensitive technique for achieving super-resolution, infrared imaging throughout the mid-infrared “fingerprint” spectral region (2.5–10μm). An achieved spatial resolution of ∼300nm is up to 30-fold smaller than the infrared diffraction limit. In this study, we establish IR-PHI’s signal contrast mechanism and benchmark the technique’s capabilities using the characteristic infrared vibrational transitions of individual polystyrene (PS) and poly(methyl methacrylate) (PMMA) beads. The analysis of acquired results reveals that IR-PHI contrast originates from a competing, photothermally induced specimen size and refractive index changes to backscattering cross sections. For PS and PMMA, thermo-optic refractive index contributions dominate and are responsible for medium-free IR-PHI signal contrast. Our analysis simultaneously establishes IR-PHI’s current, medium-free peak absorption cross-sectional limit-of-detection to be σabs=3×10−18m2. This improved understanding of IR-PHI signal contrast provides a framework for future developments in super-resolution infrared absorption microscopy and spectroscopy.