Mid-infrared photothermal spectroscopy with phase analysis

Abstract

Mid-infrared spectroscopy and imaging has attracted significant interest due to the presence of characteristic vibrational resonances of numerous molecular compounds in the fingerprint region. However, the imaging resolution in the midinfrared can be significantly weakened by the relatively large spot sizes of the beam used to target the absorption resonances of the sample. Photothermal spectroscopy and imaging as a label-free and nondestructive technique has shown great potential for pushing the imaging resolution to values below the mid-infrared diffraction limit while offering enhanced chemical sensitivity and specificity. Mid-infrared photothermal spectroscopy relies on a pump-probe setup, in which a shorter wavelength probe (removed from the targeted resonances) is co-aligned with the resonant pump beam. The modulated photothermal signal is then detected by a lock-in amplifier, whose output can provide information on the signal amplitude as well as phase. However, most commonly, amplitude images have been evaluated, without considering the recorded photothermal phase. We will provide a detailed analysis of the mid-infrared photothermal amplitude and phase information from a melamine bead sample embedded in a 2.5 μm-thick 4-Octyl-4’-Cyanobiphenyl (8CB) liquid crystal environment. Studies related to thermal transport phenomena as well as increases in imaging sensitivity will be presented for sub-diffraction limited resolution, label-free imaging. The concept of wavelength optimization to minimize thermal blurring effects at the zerocrossing for the out-of-phase contribution is introduced as a novel approach to enhance spatial resolution and sensitivity in mid-infrared label-free photothermal imaging.