Forster resonance energy transfer (FRET) imaging methods provide unique insight into the spatial distribution of energy transfer and (bio)molecular interaction events, though they deliver average information for an ensemble of events included in a diffraction-limited volume. Coupling super-resolution fluorescence microscopy and FRET has been a challenging and elusive task. Here, we present STED-FRET, a method of general applicability to obtain super-resolved energy transfer images. In addition to higher spatial resolution, STED-FRET provides a more accurate quantification of interaction and has the capacity of suppressing contributions of noninteracting partners, which are otherwise masked by averaging in conventional imaging. The method capabilities were first demonstrated on DNA-origami model systems, verified on uniformly double-labeled microtubules, and then utilized to image biomolecular interactions in the membrane-associated periodic skeleton (MPS) of neurons.

In this study, we evidence hole spin mode locking in a largely inhomogeneous p-doped InAs quantum dot (QD) ensemble, g h x /g h x ≈ 34%, which allows us to reveal a long spin coherence time of T h 2 ≈ 0.8 μs. In addition, with a two-pump experiment, we demonstrate, in a low magnetic range 60-120 mT, the possibility to synchronize and tune a single subset of QDs through the distribution. Experiments are supported by an analysis within the density matrix approach.The resident spin of a carrier confined in a quantum dot (QD) is a promising candidate for quantum information systems. Until recently, most theoretical and experimental studies have been centered on the electron spin, and have demonstrated that the ultimate limitation, at low temperature and zero magnetic field, is the hyperfine interaction of an electron with a nuclear-spin ensemble. Due to the limited number of QD nuclei interacting with the electron spin, on the order of 10 5 , random fluctuations of the nuclearspin bath lead to a dephasing time on the order of ns for InAs QDs. Recently, the initialization of hole spin has been demonstrated, 1-7 and it has emerged as a more attractive candidate, with a one-order-of-magnitude weaker hyperfine coupling. 4,[8][9][10][11][12] Several studies have reported a long hole spin relaxation time T h 1 , from μs to ms, under different conditions of temperature and magnetic field. 1,13-16 In early single-hole experiments of coherent population trapping, the hole spin coherence time was estimated to be larger than 100 ns. 17 In more recent experiments in time domain, the hole-spin coherence time has been found to be 20 ns, but these single hole-spin measurements were affected by electrical-noise 18,19 or nuclear-spin 20 fluctuations. Recent spin-echo experiments allow to circumvent the inhomogeneous decoherence and measure a much longer hole-spin coherence time T h 2 , in the microsecond range between 6 and 10 T. 18 This value is of the same order of magnitude as for electrons 21 and represents an upper bound on the free-induction decay time of hole spin coherence. 22 For electron spins, the possibility to go beyond the ensemble inhomogeneities and to measure a long coherence time by using a periodic train of circularly polarized pulses has been demonstrated. A single pulse creates an ensemble of electron spins along its propagation direction and perpendicular to an external magnetic field (Voigt geometry). Each electron spin precesses about the field and, due to the inhomogeneity of the precession frequencies, the phase coherence is quickly lost. The train of pulses allows the phase synchronization of subsets of the whole spin ensemble, for which the precession frequency is a multiple of the laser repetition rate. This effect was called "electron spin mode locking." 23,24 It has allowed the measurement of an electron spin coherence time of 600 ns at 2 T, 25 and 3 μs at 6 T, 23 in an ensemble of InGaAs QDs with a relative dispersion of the in-plane electron Landé factor g e x /g e x ≈ 1%.Holes are more sensitive than...

Single-molecule localization microscopy enables far-field imaging with lateral resolution in the range of 10 to 20 nanometres, exploiting the fact that the centre position of a single-molecule’s image can be determined with much higher accuracy than the size of that image itself. However, attaining the same level of resolution in the axial (third) dimension remains challenging. Here, we present Supercritical Illumination Microscopy Photometric z-Localization with Enhanced Resolution (SIMPLER), a photometric method to decode the axial position of single molecules in a total internal reflection fluorescence microscope. SIMPLER requires no hardware modification whatsoever to a conventional total internal reflection fluorescence microscope and complements any 2D single-molecule localization microscopy method to deliver 3D images with nearly isotropic nanometric resolution. Performance examples include SIMPLER-direct stochastic optical reconstruction microscopy images of the nuclear pore complex with sub-20 nm axial localization precision and visualization of microtubule cross-sections through SIMPLER-DNA points accumulation for imaging in nanoscale topography with sub-10 nm axial localization precision.

The initialization of a resident hole spin by the absorption of a circularly polarized light at resonance involves the formation of an excited state called a trion state. For a pure heavy hole, this optical initialization is mediated by the hyperfine electron-nuclear coupling in the trion state. We show here that for a mixed-hole spin an additional mechanism for the optical initialization appears, associated to 'crossed transitions'; it becomes dominant and keeps a high level of hole spin polarization when the magnetic field screens the electron-nuclear interaction. Finally, using a simple model, we obtain a good theoretical agreement with pulsed pump-probe experiments.