Submicron simultaneous IR and Raman microscopy (IR+Raman): breakthrough developments in Optical Photothermal IR (O-PTIR) combined with Raman provide new capabilities

Abstract

We introduce a new approach to infrared microspectroscopy – Optical-Photothermal Infrared (O-PTIR) Spectroscopy. Based on the principles of the photothermal effect (in response to IR radiation), we describe and illustrate how the novel use of an optical probe (532nm laser) enables significant enhancements in IR micro-spectroscopy, with ~20x improvement in spatial resolution compared to traditional IR microscopes (FTIR or direct QCL). Furthermore, this configuration enables the generally preferred and practical reflection (far-field) mode analysis without exhibiting any scatter artefacts and poor SNR of traditional systems, with O-PTIR reflection spectra being equivalent to FTIR transmission spectra. Due to the unique systems architecture, namely, the use of a visible probe beam to extract IR spectroscopic information, we show how this same visible laser can be used simultaneously as a Raman excitation source, thus delivering IR and Raman microspectroscopy from the same spot at the same time with the same spatial resolution. As a performance proof, IR spatial resolution of 400nm across a polymer interface has been obtained, together with 900nm single polymeric particle detection from a microplastics model system from a dried saline solution. Reflection mode fiber analysis of 10 micron and 800nm fibers shows reflection spectra are unaffected by fiber diameter to produce high quality FTIR-transmission mode comparable data. Finally, we demonstrate live cell (epithelial cheek cells) IR spectroscopic imaging analysis in water with submicron intracellular resolution.