Phase-sensitive photothermal optical coherence tomography: decoupling optical and thermal tissue properties

Photothermal optical coherence tomography (PT-OCT) has emerged as a promising extension of OCT for molecular contrast imaging, yet conventional amplitude-based PT-OCT is responsive, as opposed to being specific, to molecular contrast due to the multifactorial dependence of phase signals. Here, we present a phase-sensitive extension of PT-OCT that exploits PT phase to generate molecular contrast based on intrinsic thermal transport properties. We develop a theoretical model that links the PT-OCT phase response to the sample's thermal diffusivity and effusivity, and show that, in contrast to amplitude-based PT-OCT, the phase response is largely invariant to excitation intensity and optical attenuation. Systematic phantom experiments validate these predictions, demonstrating that while PT-OCT amplitude scales with absorption strength and excitation power, the phase signal remains independent of excitation conditions and imaging depth, instead encoding intrinsic thermal diffusivity. Using lipid- and water-rich phantoms, we further show that phase contrast provides additional separation between constituents with overlapping absorption spectra and reveals smooth interfacial transitions arising from effusivity mismatches. These results demonstrate that phase contrast can distinguish materials that are indistinguishable in conventional amplitude-based PT-OCT imaging. Together, these findings establish phase-sensitive PT-OCT as a practical extension of OCT for label-free imaging of tissue thermal properties, enabling high-contrast mapping governed by intrinsic thermophysical parameters rather than optical absorption alone. The approach has potential applications in cardiovascular diagnostics, oncology, and materials characterization, although translation to in vivo imaging will require improved phase stability and noise suppression.