26 August 2015 Optimization of mid-IR photothermal imaging for tissue analysis
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Photothermal imaging in the mid-infrared enables highly sensitive, label-free microscopy by relying on bond-specific characterization of functional groups within the samples. In a pump-probe configuration, the mid-infrared (mid-IR) pump laser is tuned to characteristic vibrational modes and through localized absorption thermal changes in the refractive index are induced. The shorter wavelength probe scatter can be detected with lock-in technology, utilizing highly sensitive detectors at telecommunication wavelengths. This mitigates the need of complex detector technology as required for traditional infrared spectroscopy/Fourier Transform Infrared Spectroscopy.

The presented photothermal system integrates a high brightness quantum cascade laser that can be tuned continuously over a spectral range of interest with a fiber probe laser.

Fiber laser technology features a compact footprint and offers robust performance metrics and reduced sensitivity to environmental perturbations compared to free-space laser configurations. In systematic spectroscopy studies where the probe laser parameters were modified, we demonstrate that the signal-to-noise ratio can be significantly enhanced by utilizing a mode-locked laser compared to a continuous-wave laser. With a raster-scanning approach, photothermal spectroscopy can be extended to hyperspectral label-free mid-infrared imaging to combine spectral content with localized sample details. By tuning the pump laser to the amide-I absorption band around 1650 cm-1 in biological tissue samples, the spectral characteristics can provide insight into the secondary structure of proteins (e.g. amyloid plaques; alpha-helix, beta-sheet). We present the versatility of our mid-IR photothermal system by analyzing histopathological tissue sections of cancerous tissue in a non-contact, non-destructive approach with good sensitivity.
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Atcha Totachawattana, Atcha Totachawattana, Shyamsunder Erramilli, Shyamsunder Erramilli, Michelle Y. Sander, Michelle Y. Sander, } "Optimization of mid-IR photothermal imaging for tissue analysis", Proc. SPIE 9584, Ultrafast Nonlinear Imaging and Spectroscopy III, 95840C (26 August 2015); doi: 10.1117/12.2187652; https://doi.org/10.1117/12.2187652

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