Bidirectional Phase Sensitivity in Holographic Phototransient Microscopy

Mid-infrared photothermal microscopy combines the chemical specificity of infrared absorption with the spatial resolution of visible light detection, but practical implementations face a persistent trade-off between forward-scattering (FWS) and backward-scattering (BWS) detection geometries. FWS provides quantitative, shape-independent phase contrast but requires two-sided optical access, which is difficult to achieve in aqueous or thick samples. BWS offers convenient single-sided access, but its signals are strongly distorted by depth-dependent interference for micrometer-scale objects. Here, we present a bidirectional femtosecond mid-infrared pump-probe holographic microscope capable of switching between FWS and BWS geometries within a single instrument and use it to introduce and validate a new imaging modality, internal forward scattering (IFS). IFS exploits the back reflection generated at the top surface of the mid-infrared-transparent sample substrate as an internally generated forward-scattering illumination wave, isolated from the directly backscattered field via temporal coherence gating. Using polystyrene beads on CaF₂ substrates in air, water, and a refractive-index-matched glycerol-water mixture, we show that IFS reproduces the signal magnitudes and temporal dynamics of true FWS measurements while retaining the mechanical simplicity and single-sided accessibility of BWS. These results establish IFS as a practical, quantitative alternative to conventional FWS and BWS geometries for photothermal and, more broadly, quantitative phase imaging, with direct relevance to single-sided imaging of biological or solvent-contained specimens.