Time-resolved infrared photothermal imaging: From transient observations towards the steady-state

Abstract: Mid-infrared photothermal microscopy is a highly promising imaging technique that enables spatially resolved vibrational fingerprinting. The combination of infrared induced heating with optical readout at visible wavelengths provides excellent spatial resolution while retaining the spectral observations of conventional infrared imaging. Most current implementations rely on long-duration illumination periods, to ensure sufficient heating and hence large signals. However, undesirable processes such as heat-diffusion degrade spatial resolution and the interplay between heat-induced refractive index changes and sample expansion adds additional uncertainties. Fundamentally, these issues stem from the difficulties associated with separating non-equilibrium and photoacoustic contributions from purely thermal signals. Highly time-resolved observations hold great promise for addressing these issues and are imperative for enabling future imaging modalities in this exciting temporal window. Here, we provide this much needed insight by employing widefield phototransient holography to phase-resolve optical responses from pico- to tens of nanoseconds following vibrational overtone excitation. We observe rapid transient-induced phase shifts, followed by heat-induced coherent expansion and thermalisation dynamics. Our observations provide a direct link between the photoacoustic and photothermal realms, thus not only offering much-needed insight for rationally optimising these exciting technologies but also avenues towards future all-optical stiffness and super-resolution imaging modalities.