Multiphoton imaging with vibrational contrast and high spatial resolution

• Main Author: Cohn, Keith Ryan
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2007
• Subjects: Optics
• ISBN: 9780542983146; 0542983141

Imaging with mid-infrared (mid-IR) radiation is useful for spectroscopic purposes because light of this energy corresponds to vibrational resonances in molecular systems. However, diffraction limits the spatial resolution of conventional imaging techniques to approximately the wavelength of the source light, which often proves insufficient for samples with subwavelength features. We have explored three novel avenues of circumventing the diffraction barrier by achieving subwavelength resolution with vibrational sensitivity. The first, the transient aperture, employs near-field infrared microscopy with a transient optically induced probe. Photoinduced reflectivity in semiconductors is used to generate a relatively large transient mirror with a small aperture in its center. Properties of this probe have been studied and the first images obtained using the technique are presented. Resolution better than one-fifth the wavelength with 6.25 μm light is demonstrated. Among the advantages of this technique are the elimination of distance feedback loops required for physical near-field probes, ease in simultaneous visible imaging, and a high scanning rate limited primarily by the pulse repetition rate of the laser system. The second method substitutes the transient aperture probe with a transient solid immersion lens. The transient lens is formed by photoinducing a Fresnel zone plate structure onto the surface of a semiconductor wafer with a high index of refraction. Lenses with comparable numerical apertures have been tested using gallium phosphide and silicon wafers, and their focusing properties studied. We demonstrate that transient solid immersion lenses provide the same high numerical apertures as conventional solid immersion lenses. The last technique involves coherent anti-Stokes Raman scattering (CARS) microscopy. CARS is a nonlinear four wave mixing process that uses visible or near-IR light to probe vibrational resonances. The anti-Stokes photons occur in the visible range, imparting to the technique resolution commensurate with visible imaging. We report on and produce images demonstrating the successful implementation of a novel, wide-field beam geometry that acquires the entire field of view simultaneously.