Thermal diffusion dynamics as a contrast mechanism in mid-infrared photothermal microscopy
Embargo Date
2025-09-05
OA Version
Citation
Abstract
The mid-infrared contains the molecular fingerprint region where characteristic vibrational resonances of many natural compounds are present, including proteins, lipids, nucleic acids and a variety of polymers. However, the optical resolution of conventional infrared microscopy is limited to values at best > 3 μm due to the long wavelengths used. Mid-infrared photothermal microscopy has proven itself in recent years to be a powerful tool in infrared imaging with the ability to resolve chemical signatures at the submicron level. The combination of state-of the art visible and or near infrared photodetectors along with the large absorption cross sections present in the mid-infrared, makes photothermal microscopy a powerful technique for chemical imaging of high specificity and sensitivity.
So far most of the research has been focused on optimizing the photothermal microscope abilities in terms of resolution, contrast and speed. However often contrast based purely on absorption differences can be limiting in more complicated environments where features of interest are buried under also absorbing background. Thus, there is an ongoing need for a new imaging contrast mechanism complementary to absorption
In this dissertation we explore thermal diffusion dynamics as an additional contrast mechanism complementary to absorption. The technique of VIPPS (Vibrational Infrared Photothermal and Phase Signals Imaging) is presented that utilizes phase sensitive lock-in detection to obtain contrast from features with similar absorption but varying thermal diffusion properties. This enables high contrast imaging of absorbing microparticles in an also absorbing embedding medium with sub diffraction limited resolution. The technique also paves the way for imaging protein signatures in cell cultures embedded in physiologically relevant environments (i.e., fibroblast cells grown in a collage extracellular matrix). It was found that the plasma and membrane interfaces act like thermal barriers while different regions of the cell experience different diffusion properties. The sensitivity of the Amide I band was also utilized for imaging secondary protein conformations.
Complementary to VIPPS, time resolved photothermal imaging is demonstrated with time gated boxcar detection. The ability to obtain hyper-temporal image stacks of the heating and cooling processes and as well as highly localized thermal transient profiles, enables a two-dimensional mapping of characteristic time decay constants and a visualization of thermal confinement effects. The technique was used to study interfacial heat transfer dynamics in water embedded nanoparticles and animal extracted axon-bundles.