Mid-Infrared Optical Photothermal Interferometric Microscopy of Substrate-Supported Samples

An interferometry model is proposed and evaluated experimentally for interpreting image contrast arising in optical photothermal mid-infrared (O-PTIR) microscopy. For samples extending over several micrometers in dimension prepared on planar supported substrates (which includes most analyzed experimentally), existing scattering-based theoretical formulations become computationally intractable for structures beyond simple spherical geometries. An alternative thin-film model is proposed for interpreting both the time-averaged (DC) reflectivity and the photothermally induced change in reflectivity (AC) from localized, transient heat deposition. This optical photothermal interferometry analysis (OPTIA) approach for interpreting O-PTIR microscopy measurements enabled recovery of the absolute thickness of individual <i>Synechocystis</i> colonies following transfer to IR-transparent substrates. In addition, large signal-to-noise enhancements in O-PTIR measurements were both predicted and observed for measurements performed on silicon substrates relative to calcium fluoride, attributed to a combination of a higher overall reflectivity as well as enhancements in the contributions arising from optical interference between the sample and substrate responses. Leveraging these interference effects has the potential to significantly increase the accessible information content in O-PTIR microscopy measurements and lays the foundation for additional measurement approaches specifically designed to exploit the native interferometry arising for substrate-supported samples.