Novel applications of nanogratings for high resolution microscopy
This dissertation leverages the recent advances in fabricating nanogratings with highly tunable birefringent properties to address some important challenges in high resolution microscopy. Out-of-focus features of a sample cause blurring in wide-field microscopes. Polarized illumination coded Structured Illumination Microscopy (picoSIM) addresses this problem by performing optical sectioning, that too in a single exposure. Using nanogratings improves the light efficiency of picoSIM by 70%, while simultaneously making the setup compact. It also extends picoSIM to reflective microscopy applications by allowing the use of incoherent light. The first practical implementation of picoSIM for material imaging with a field-of-view of 100 µm x 128 µm at a fast acquisition rate of 154 fps is demonstrated. Reliably visualizing three-dimensional biological structures requires an extended depth-of-field (EDF). However, the axial elongation of focus is accompanied by loss of lateral resolution. To address this challenge, a beam shaping device called Hybrid filter based on nanogratings is proposed. This device combines radial polarization with a binary phase filter for pupil modulation to achieve EDF at high numerical aperture. Two designs of this device obtained by different methodologies are fabricated and compared. Axial elongation of 1.69 and 2.66 and lateral resolution improvement of 17.8% and 22.7% compared to a Gaussian focus is demonstrated. Aligning a STimulated Emission Depletion (STED) microscope can be challenging due to the phase mask needed to generate the depletion beam. A nanogratings based Quadrant Retarder Plate has been proposed in this work to reduce the manufacturing complexity. This wavelength-selective device can potentially simplify a STED microscope by combining the paths of the excitation and depletion beams. Preliminary results obtained with a prototype of this device are presented and suggestions to improve its performance are discussed.
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