We present a stimulated emission depletion (STED) microscope that provides 3‐D super resolution by simultaneous depletion using beams with both a helical phase profile for enhanced lateral resolution and an annular phase profile to enhance axial resolution. The 3‐D depletion point spread function is realised using a single spatial light modulator that can also be programmed to compensate for aberrations in the microscope and the sample. We apply it to demonstrate the first 3‐D super‐resolved imaging of an immunological synapse between a Natural Killer cell and its target cell. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We imaged core-shell nanoparticles, consisting of a dye-doped silica core covered with a layer of gold, with a stimulated emission depletion, fluorescence lifetime imaging (STED-FLIM) microscope. Because of the field enhancement provided by the localized surface plasmon resonance of the gold shell, we demonstrate a reduction of the STED depletion power required to obtain resolution improvement by a factor of 4. This validates the concept of nanoparticle-assisted STED (NP-STED), where hybrid dye-plasmonic nanoparticles are used as labels for STED in order to decrease the depletion powers required for subwavelength imaging.

Plasmonic nanoparticles influence the absorption and emission processes of nearby emitters due to local enhancements of the illuminating radiation and the photonic density of states. Here, we use the plasmon resonance of metal nanoparticles in order to enhance the stimulated depletion of excited molecules for super-resolved nanoscopy. We demonstrate stimulated emission depletion (STED) nanoscopy with gold nanorods with a long axis of only 26 nm and a width of 8 nm that provide an enhancement of up to 50% of the resolution compared to fluorescent-only probes without plasmonic components irradiated with the same depletion power. The nanoparticle-assisted STED probes reported here represent a ~2x10 3 reduction in probe volume compared to previously used nanoparticles. Finally, we demonstrate their application toward plasmon-assisted STED cellular imaging at low-depletion powers and we also discuss their current limitations. Key-words: STED nanoscopy, nanorods, super-resolution, plasmonic nanoparticles, bio-imagingThe diffraction limit has ceased to be a practical limit to resolution in far-field microscopy, following the demonstration of STED, 1,2,3 RESOLFT 4 and localisation microscopies 5,6,7 and the subsequent development of a plethora of super-resolved nanoscopy techniques. 8 In particular, stimulated emission depletion (STED) nanoscopy, which builds on the advantages of laser scanning confocal microscopy, is a powerful technique for super-resolved imaging in complex biological samples including live organisms. 9,10 STED nanoscopy uses stimulated emission to turn off the spontaneous fluorescence emission of dye molecules, typically overlapping a focused excitation beam with a "doughnut" shaped beam that deexcites emitters to the ground state everywhere except for the area within the centre of the doughnut, thus providing theoretically diffraction-unlimited resolution in the transverse plane by reducing the fullwidth half-maximum (FWHM) of the point spread function. By increasing the power of the depletion beam the emission region can be can be drastically reduced -theoretically allowing for sub-nanometre resolution -with resolutions of less than 10 nm being demonstrated. 11 The scaling of resolution with the square root of the depletion beam power means that relatively high-power lasers are typically used for STED nanoscopy. In practice, however, the use of high-power irradiation can result in problems such as photobleaching of the fluorophores and phototoxicity, and so the achievable resolution is compromised by the need to limit the intensity of the depletion laser radiation. Furthermore, high power lasers can add cost and complexity to STED microscopes and so the requirement for high power depletion beams presents challenges for parallelizing STED measurements 12,13 in terms of the lasers required, thus, limiting the potential for faster super-resolved imaging.To some extent the issue of photobleaching can be addressed with the development of more robust fluorescent labels such as quantum dots 14 as ...

We demonstrate a simplified set-up for STED microscopy with a straightforward alignment procedure that uses a single spatial light modulator (SLM) with collinear incident excitation and depletion beams to provide phase modulation of the beam profiles and correction of optical aberrations. We show that this approach can be used to extend the field of view for STED microscopy by correcting chromatic aberration that otherwise leads to walk-off between the focused excitation and depletion beams. We further show how this arrangement can be adapted to increase the imaging speed through multibeam excitation and depletion. Fine adjustments to the alignment can be accomplished using the SLM only, conferring the potential for automation.

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