Reviewing Photon Absorption Remote Sensing (PARS): An Emerging Approach for Label-free Absorption Microscopy Across Biological Scales

Label-free optical absorption microscopy techniques have evolved as effective tools for non-invasive chemical specific structural, and functional imaging. Yet most label-free microscopy modalities target only a fraction of the contrast afforded by optical absorption interactions. This work reviews an emerging optical absorption microscopy technique, Photon Absorption Remote Sensing (PARS), which simultaneously captures the dominant light matter interactions occurring as pulsed light is absorbed by specimens. In PARS, scattering, attenuation, and radiative and non-radiative relaxation processes are collected during each optical absorption event. This provides a complete representation of the absorption interaction, enabling unique measurements presented as the total absorption (TA) and quantum efficiency ratio (QER). By capturing the complete interaction, PARS bridges many specific challenges associated with label-free imaging, recovering a wider range of biomolecules than independent radiative or non-radiative modalities. To show the versatility of PARS, a range of biological specimens are imaged, from single cells to in vivo living subjects. These examples of label-free histopathological imaging, and vascular imaging illustrate fields where PARS may have profound impacts. Overall, PARS may provide comprehensive, and otherwise inaccessible, label-free visualizations in biological specimens, representing a new source of data to develop new AI and machine learning methods for diagnostics and visualization.