Super-Resolved FRET Imaging by Confocal Fluorescence-Lifetime Single-Molecule Localization Microscopy.

Fluorescence Resonance Energy Transfer (FRET)-based approaches are unique tools for sensing the immediate surroundings and interactions of (bio)molecules. FRET imaging and Fluorescence Lifetime Imaging Microscopy (FLIM) enable the visualization of the spatial distribution of molecular interactions and functional states. However, conventional FLIM and FRET imaging provide average information over an ensemble of molecules within a diffraction-limited volume, which limits the spatial information, accuracy, and dynamic range of the observed signals. Here, an approach to obtain super-resolved FRET imaging based on single-molecule localization microscopy using an early prototype of a commercial time-resolved confocal microscope is demonstrated. DNA Points Accumulation for Imaging in Nanoscale Topography with fluorogenic probes provides a suitable combination of background reduction and binding kinetics compatible with the scanning speed of usual confocal microscopes. A single laser is used to excite the donor, a broad detection band is employed to retrieve both donor and acceptor emission, and FRET events are detected from lifetime information.

Interacting polariton fluids in a monolayer of tungsten disulfide.

Atomically thin transition metal dichalcogenides (TMDs) possess a number of properties that make them attractive for realizing room-temperature polariton devices1. An ideal platform for manipulating polariton fluids within monolayer TMDs is that of Bloch surface waves, which confine the electric field to a small volume near the surface of a dielectric mirror2-4. Here we demonstrate that monolayer tungsten disulfide can sustain Bloch surface wave polaritons (BSWPs) with a Rabi splitting of 43 meV and propagation lengths reaching 33 µm. In addition, we show strong polariton-polariton nonlinearities within BSWPs, which manifest themselves as a reversible blueshift of the lower polariton resonance. Such nonlinearities are at the heart of polariton devices5-11 and have not yet been demonstrated in TMD polaritons. As a proof of concept, we use the nonlinearity to implement a nonlinear polariton source. Our results demonstrate that BSWPs using TMDs can support long-range propagation combined with strong nonlinearities, enabling potential applications in integrated optical processing and polaritonic circuits.