Quantifying the capabilities of infrared photothermal heterodyne imaging

Infrared spectroscopy is often used to identify chemical specimens by measuring vibrational

transitions, but the diffraction limit of infrared light restricts its spatial resolution to the micrometer range. Alternative methods that permit nanometer-scale imaging are often experimentally complex and therefore impractical for certain applications.

However, a recently developed technique, called infrared photothermal heterodyne imaging (IR-PHI), provides a simpler way to image nanostructures. This all-optical, tabletop experiment uses a shorter-wavelength laser to indirectly measure infrared absorption, achieving spatial resolution of a few hundred nanometers – over an order of magnitude better than the diffraction limit of infrared light.

Pavlovetc et al. sought to better understand the capabilities of IR-PHI by determining the detection limit and signal contrast mechanism of the technique.

The authors measured polystyrene (PS) and poly(methyl methacrylate) (PMMA) beads with IR-PHI and discovered a correlation between the contrast of the signal and difference in the temperature-induced changes to the size and refractive index of the PS and PMMA beads. Understanding the signal contrast mechanism will allow future IR-PHI measurements to provide quantitative information about a sample, such as the concentration of molecular species.

“Our work puts the technique on a quantitative footing,” said author Ilia Pavlovetc. “More importantly, having an understanding of the different processes that contribute to IR-PHI signal contrast will enable future studies aimed at improving its sensitivity.”

Next, the authors plan to use IR-PHI to study even smaller structures and analyze IR-PHI measurements of more structurally complex samples.

Source: “Infrared photothermal heterodyne imaging: Contrast mechanism and detection limits,” by Ilia M. Pavlovetc, Eduard A. Podshivaylov, Rusha Chatterjee, Gregory V. Hartland, Pavel A. Frantsuzov, and Masaru Kuno, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5142277 .