Photothermal imaging of PMMA film and photothermal spectroscopy of pHEMA hydrogel
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The mid-infrared is a promising region for detection different materials. Many vibrational modes, including bending and stretching, are located in this regime. Photothermal spectroscopy and imaging in the mid-infrared region is an emerging new method for non-contact detection of molecular groups. Our approach to photothermal spectroscopy and imaging utilizes a near-infrared erbium doped fiber laser (EDFL) to detect the photothermal induced changes in the refractive index. These changes are excited by a mid-infrared quantum cascade laser (QCL) pump beam. The probe beam is detected by a commercially available near-infrared photodetector. This method has advantages of high sensitivity, label-free detection, high spatial resolution and high signal-to-noise ratio (SNR). Hydrogels such as pHEMA are polymers that are of interest for contact lens, drug delivery and soft tissue replacement. The pHEMA hydrogel can retain water content, causing the material to swell. Additionally, pHEMA has a critical temperature at which the hydrogel undergoes a glass transition. Photothermal spectroscopy of pHEMA is demonstrated in this thesis where the presence of this glass transition temperature can be revealed. Additionally, photothermal imaging of a PMMA USAF target sample is shown and ideal parameters for high-resolution photothermal imaging are determined. In this thesis, we report a spatial resolution much smaller than the diffraction limited spot size of the mid-infrared beam.
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