The objective of this research project is to develop a novel multiphoton microscopy technique and to investigate its application to selected problems in cell biology which require sensitive three-dimensional imaging, in-vivo and real time. This novel technique is based on the detection of the resonant coherent non-linear optical response (four-wave mixing) of colloidal quantum dots (CQDs) to explore their application as bio-labels beyond their fluorescence properties. This method would retain many of the advantages of multiphoton fluorescence microscopy, such as intrinsic sectioning capability, and would offer additional advantages such as coherent detection free from fluorescence backgrounds. We expect the spatial resolution to be increased by a factor of two due to the optical non-linearity, resulting in a 130nm of lateral resolution at 550nm exciting wavelength and objectives with 1.3 NA. To compare advantages and disadvantages of this novel coherent optical microscopy technique with respect to confocal/multiphoton fluorescence microscopy, we will investigate a model system, namely HeLa cells, which provides well established routes for biolabelling using both dye-labelled antibodies, protein fusions with fluorescent tags and bioconjugated colloidal quantum dots. The application of this microscope has the potential to bring a significant step forward in the fundamental understanding of major areas in cell biology, which can not be fully addressed with the conventional fluorescence microscopy tools. Moreover, the dependence of fundamental electronic properties, such as homogeneous linewidth of excitonic transition and exciton-phonon interaction, on the size and composition of quantum dots will be investigated via the measurements of the transient four-wave mixing signal as a function of temperature. This study will help assessing the applicability of CQDs, beyond bioimaging, in areas such as quantum information processing, spintronics, and optoelectronics.
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