MS Thesis Defense Di Huang PHO315

Title: HIGH RESOLUTION PHOTOTHERMAL IMAGING OF PMMA FILMS AND PHOTOTHERMAL SPECTROSCOPY OF PHEMA HYDROGEL Abstract: Mid-infrared region is a promising region for detection of organics. Many vibrational modes, including bending and stretching, locate in this region. Photothermal spectroscopy and imaging in the mid-infrared region is an emerging new method for non-contact detection of molecular groups and doing hyperspectral imaging for organics materials. Our approach to photothermal spectroscopy and imaging utilizes an erbium doped fiber laser (EDFL) with near-infrared laser beam to detect the photothermal induced changes in the refractive index. These 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 including pHEMA are very interesting organics materials. They are very interesting polymer for contact lens, drug delivery and soft tissue replacement. PHEMA hydrogels can imbibe different amount of water by swelling and it has a thermal property that the sample will undergo a glass transition when it reaches a specific temperature. In this thesis, a photothermal signal has a turning point at specific QCL drive current for 1728cm-1 and 1702cm-1, which linearly decreases with the sample temperature. Additionally in this thesis, the photothermal imaging based on a raster scanning mechanism on a PMMA USAF target sample is demonstrated and ideal parameters for high resolution photothermal imaging are determined. A resolving power much smaller than the mid-infrared diffraction limit is found.