Exploring the Capabilities of Optical Photothermal Infrared Imaging Systems

Abstract

Optical Photothermal Infrared (O-PTIR) spectroscopy is a pump-probe technique that exploits the local refractive index change in the sample upon absorption of infrared (IR) radiation. This refractive index perturbation is probed by a second laser, whose wavelength is in the visible spectral range, thereby enhancing resolution beyond the limits achieved in conventional infrared spectroscopy [2]. Due to its high chemical specificity, non-destructive nature and sub-micron resolution, O-PTIR imaging has gained increasing popularity in both nanoparticle research and life sciences [1, 3, 5]. In a transmission configuration, the technique is capable of optical sectioning and can even be used for measurement in aqueous environments, e.g. within the channels of a microfluidic chip. However, accommodating a microfluidic chip in a confocal microscope system necessitates certain design changes which come at the expense of spatial resolution and the size of the instrument. On top of that, although similar to optical imaging in the radial plane, the photothermal signal has a two-lobed shape in axial direction [3, 4], making photothermal z scans inherently prone to artifacts. An O-PTIR prototype dedicated to imaging within microfluidic channels was developed at TUW. We will explain specific design choices for O-PTIR imaging in aqueous environments and support them by theoretical evidence. Building on results from nanolens theory [4], we will then present theoretical and experimental data to investigate the imaging capabilities of O-PTIR transmission systems.

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