Dominik Wildanger

STED microscopy with a supercontinuum laser source.

We report on a straightforward yet powerful implementation of stimulated emission depletion (STED) fluorescence microscopy providing subdiffraction resolution in the far-field. Utilizing the same super-continuum pulsed laser source both for excitation and STED, this implementation of STED microscopy avoids elaborate preparations of laser pulses and conveniently provides multicolor imaging. Operating at pulse repetition rates around 1 MHz, it also affords reduced photobleaching rates by allowing the fluorophore to relax from excitable metastable dark states involved in photodegradation. The imaging of dense nanoparticles and of the microtubular network of mammalian cells evidences a spatial resolution of 30-50 nm in the focal plane, i.e. by a factor of 8-9 beyond the diffraction barrier.

A compact STED microscope providing 3D nanoscale resolution

The advent of supercontinuum laser sources has enabled the implementation of compact and tunable stimulated emission depletion fluorescence microscopes for imaging far below the diffraction barrier. Here we report on an enhanced version of this approach displaying an all‐physics based resolution down to (19 ± 3) nm in the focal plane. Alternatively, this single objective lens system can be configured for 3D imaging with resolution down to 45 × 45 × 108 nm in a cell. The obtained results can be further improved by mathematical restoration algorithms. The far‐field optical nanoscale resolution is attained in a variety of biological samples featuring strong variations in the local density of features.

Simultaneous multi-lifetime multi-color STED imaging for colocalization analyses.

We describe a STED microscope optimized for colocalization experiments with up to three colors. Two fluorescence labels are separated by their fluorescence lifetime whereas a third channel is discriminated by the wavelength of fluorescence emission. Since it does not require a second STED beam, separating by lifetime is insensitive to drift and thus optimally suited for colocalization analyses. Furthermore, we propose a setup having a second STED beam for long duration multicolor recording.

Creation and nature of optical centres in diamond for single-photon emission—overview and critical remarks

A huge variety of optical colour centres can be found in diamond, emitting in its whole wide transparency range. Although several of these centres have been demonstrated as single-photon emitters, none of them meets all of the requirements of an ideal single-photon source. In this view, we discuss the properties of prominent optical centres, such as the nitrogen vacancy, the silicon vacancy or the so-called NE8 centre, as well as recently found centres ascribed to defects containing Ni, Si, Cr and Xe. Besides suitable intrinsic properties, it is necessary for practical applications that optical centres can be created artificially on demand. Of all known methods, only ion implantation allows for the most controlled creation of such defect centres. In this paper, we discuss how nanoscalability, that is, the nanometre placement and the deterministic creation of optical centres, can, could or cannot be achieved by the available ion implantation techniques. A fine analysis of individual optical centres is now possible, thanks to the recently developed subdiffraction optical microscopy methods.

Dual‐Color STED Microscopy at 30‐nm Focal‐Plane Resolution

Owing to its sensitivity and noninvasiveness, far-field fluo-rescence microscopy would be almost ideal for biologicalimaging if the resolution of its established variants were notlimitedbydiffractiontoDr l=ð2n sinaÞ,withldenotingthewavelength of light, n the index of refraction, and a theaperture angle of the objective lens.

Far-field fluorescence nanoscopy of diamond color centers by ground state depletion

We report on two modalities of lens-based fluorescence microscopy with diffraction-unlimited resolution relying on the depletion of the fluorophore ground state. The first version utilizes a beam with a deep intensity minimum, such as a doughnut, for intense excitation followed by mathematical deconvolution, whereas in the second version, a regularly focused beam is added for generating the image directly. In agreement with theory, the subdiffraction resolution scales with the square root of the intensity depleting the ground state. Applied to the imaging of color centers in diamond our measurements evidence a resolving power down to ≈7.6 nm, corresponding to 1/70 of the wavelength of light employed. Our study underscores the key role of exploiting (molecular) states for overcoming the diffraction barrier in far-field optical microscopy.

Solid immersion facilitates fluorescence microscopy with nanometer resolution and sub-ångström emitter localization.

Exploring the maximum spatial resolution achievable in far-field optical imaging, we show that applying solid immersion lenses (SIL) in stimulated emission depletion (STED) microscopy addresses single spins with a resolution down to 2.4 ± 0.3 nm and with a localization precision of 0.09 nm.

A STED microscope aligned by design

STED microscopes are commonly built using separate optical paths for the excitation and the STED beam. As a result, the beams must be co-aligned and can be subject to mechanical drift. Here, we present a single-path STED microscope whose beams are aligned by design and hence is insensitive to mechanical drift. The design of a phase plate is described which selectively modulates the STED beam but leaves the excitation beam unaffected. The performance of the single-beam setup is on par with previous dual-beam designs.

Nanoscale engineering and optical addressing of single spins in diamond.

The control of positioning of single atoms in solid becomes relevant for emerging technologies like spintronics and quantum information processing. [ 1–4 ] Quantum computing imposes strict requirements on the positioning of quantum bits (qubits). [ 5 ] The interaction between qubits needs to be stronger than the coupling to the environment in order to allow coherent quantum gates. Spins associated with defects in diamond are particularly promising candidates for solid state information carriers [ 6–8 ] owing to their long coherence time (seconds for nuclear spins and milliseconds for electron spin [ 9 ] ). Furthermore, when associated with color centres having spin-selective optical transitions (like the nitrogenvacancy (NV) centre), individual spins can be readout and polarized using optical techniques. Although spectacular experiments including two and three spins entanglement [ 10 ] or ultrafast control of spin qubits [ 11 ] were demonstrated recently, fully scalable architecture of diamond quantum registers requires the ability to create arrays of electron spins with a few nanometers accuracy. The NV centre in diamond consists of a vacancy and a nitrogen in the adjacent lattice position. It exists in two charge states: neutral and negatively charged. The energy structure

Dark state photophysics of nitrogen-vacancy centres in diamond

Nitrogen-vacancy (NV) colour centres in diamond are attractive fluorescence emitters owing to their unprecedented photostability and superior applicability to spin manipulation and sub-diffraction far-field optical microscopy. However, some applications are limited by the co-occurrence of dark state population and optical excitation. In this paper, we use fluorescence microscopy and correlation spectroscopy on single negatively charged NV centres in type IIa bulk diamond to unravel the population kinetics of a >100s long-lived dark state. The bright-dark state interconversion rates show a quadratic dependence on the applied laser intensity, which implies that higher excited states are involved. Depopulation of the dark state becomes less effective at wavelengths above 532nm, resulting in a complete fluorescence switch-off at wavelengths >600nm. This switch is reversible by the addition of shorter wavelengths. This behaviour can be explained by a model consisting of three dark and three bright states of different excitation levels, with the most efficient interconversion via the respective higher excited states. This model accounts for