Two approaches for a simpler STED microscope using a dual-color laser or a single wavelength

Abstract : Stimulated emission depletion (STED) is a well-known super-resolution method. In a STED microscope, a doughnut-shaped beam is superimposed with the excitation beam and keeps the fluorophores in the periphery of the excitation spot in a dark state by stimulated emission, thus effectively improving the spatial resolution in a scanning configuration. This technique requires a complex setup since two laser beams, generally from different sources need to be perfectly aligned. In this work we propose two STED configurations that will simplify the setup and reduce the total cost of such a system. The basic idea in both cases is to use the same laser source for both excitation and stimulated emission depletion. In the first setup we have developed an original two-color source based on a microchip Nd-YAG laser. This microchip laser simultaneously delivers sub-ns pulses at two wavelengths, 355 nm (excitation) and 532 nm (depletion), which are generated by harmonic conversion from an Nd-YAG laser emission and offer the advantage of being intrinsically aligned and synchronized. Further work consisted in determining suitable dyes for this particular source. We have built a microscope setup based on this laser source and obtained images with an improved resolution. The confirmation of the reduction of the excitation volume is showed by Fluorescence Correlation Spectroscopy (FCS) measurements. However, the performance of this system is limited by chromatic aberrations. The combination of Selective Plane Illumination Microscopy (SPIM) with STED is considered. In the second setup the chromatic aberrations are no longer a problem since the same wavelength is used for two photon excitation and one photon depletion. By playing on the duration of the pulse (thus the instantaneous intensity), one of these two processes can be favored. Fluorescence was excited by two photon absorption with a femtosecond pulse, then depleted by one photon stimulated emission with a stretched pulse. We used the Time Correlated Single Photon Counting (TCSPC) method to study the depletion efficiency of DCM dye in solution and numerical simulations show that this method can be applied to super-resolved microscopy. In the end we present the preliminary images obtained with a home-built Two-photon Single wavelength STED microscope and the resolution improvement obtained. Further improvements are to be made to the custom microscope. In this work we have experimentally implemented, for the first time, two concepts meant to simplify the STED setups by using original sources.
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Ancuţa Teodora Şcheul. Two approaches for a simpler STED microscope using a dual-color laser or a single wavelength. Other [cond-mat.other]. Université de Grenoble, 2013. English. ⟨NNT : 2013GRENY040⟩. ⟨tel-01558306⟩

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