Efficient fluorescence inhibition patterns for RESOLFT microscopy

Effects of primary aberrations on the fluorescence depletion patterns of STED microscopy

Effects of primary aberrations including spherical aberration, coma and astigmatism on the three fluorescence depletion patterns mainly used in stimulated emission of depletion (STED) microscopy are investigated by using vectorial integral. The three depletion patterns are created by inserting a vortex phase plate, a central half-wavelength plate or a semi-circular half-wavelength mask within Gaussian beam respectively. Attention is given to the modification of the shape, peak intensity, the central intensity of the dark hole and the hole size of these depletion patterns in the presence of primary aberrations.

Birefringent device converts a standard scanning microscope into a STED microscope that also maps molecular orientation

Stimulated emission depletion (STED) microscopy usually employs a scanning excitation beam that is superimposed by a donut-shaped STED beam for keeping the fluorophores at the periphery of the excitation spot dark. Here, we introduce a simple birefringent device that produces a donut-shaped focal spot with suitable polarization for STED, while leaving the excitation spot virtually intact. The device instantly converts a scanning (confocal) microscope with a co-aligned STED beam into a full-blown STED microscope. The donut can be adapted to reveal, through the resulting fluorescence image, the orientation of fluorophores in the sample, thus directly providing subdiffraction resolution images of molecular orientation.

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.

Investigation of the influence of the aberration induced by a plane interface on STED microscopy

The structure of the inhibition patterns is important to the stimulated emission depletion (STED) microscopy. Usually, Laguerre- Gaussian (LG) beam and the central zero-intensity patterns created by inserting phase masks in Gaussian beams are used as the erase beam in STED microscopy. Aberration is generated when focusing beams through an interface between the media of the mismatched refractive indices. By use of the vectorial integral, the effects of such aberration on the shape of depletion patterns and the size of fluorescence emission spot in the STED microscopy are studied. Results are presented as a comparison between the aberration-free case and the aberrated cases.

Laser scanning confocal microscope with programmable amplitude, phase, and polarization of the illumination beam

We describe the design and construction of a laser scanning confocal microscope with programmable beam forming optics. The amplitude, phase, and polarization of the laser beam used in the microscope can be controlled in real time with the help of a liquid crystal spatial light modulator, acting as a computer generated hologram, in conjunction with a polarizing beam splitter and two right angled prisms assembly. Two scan mirrors, comprising an on-axis fast moving scan mirror for line scanning and an off-axis slow moving scan mirror for frame scanning, configured in a way to minimize the movement of the scanned beam over the pupil plane of the microscope objective, form the XY scan unit. The confocal system, that incorporates the programmable beam forming unit and the scan unit, has been implemented to image in both reflected and fluorescence light from the specimen. Efficiency of the system to programmably generate custom defined vector beams has been demonstrated by generating a bottle structured focal volume, which in fact is the overlap of two cross polarized beams, that can simultaneously improve both the lateral and axial resolutions if used as the de-excitation beam in a stimulated emission depletion confocal microscope.

Dark-spot formation by vector beams

Dark-spot formation is demonstrated by calculating electric-field distribution near the focus produced by focusing higher-order transverse-mode vector beams with radial and azimuthal polarizations in addition to Laguerre-Gaussian beams with linear and circular polarizations. The size of the dark spot in the radial direction varies from one beam to another, but the size in the axial direction is almost the same. The radially polarized TM02-mode beam is predicted to form a dark spot solely by an axial electric field.

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.

Three-dimensional nanoscopy of colloidal crystals

We demonstrate the direct three-dimensional imaging of densely packed colloidal nanostructures using stimulated emission depletion microscopy. A combination of two de-excitation patterns yields a resolution of 43 nm in the lateral and 125 nm in the axial direction and an effective focal volume that is by 126-fold smaller than that of a corresponding confocal microscope. The mapping of a model system of spheres organized by confined convective assembly unambiguously identified face-centered cubic, hexagonal close-packed, random hexagonal close-packed, and body-centered cubic structures.

Resolution scaling in STED microscopy

We undertake a comprehensive study of the inverse square root dependence of spatial resolution on the saturation factor in stimulated emission depletion (STED) microscopy and generalize it to account for various focal depletion patterns. We used an experimental platform featuring a high quality depletion pattern which results in operation close to the optimal optical performance. Its superior image brightness and uniform effective resolution <25 nm are evidenced by imaging both isolated and self-organized convectively assembled fluorescent beads. For relevant saturation values, the generalized square-root law is shown to predict the practical resolution with high accuracy.

Trends in fluorescence imaging and related techniques to unravel biological information

Optical microscopy is among the most powerful tools that the physical sciences have ever provided biology. It is indispensable for basic lab work, as well as for cutting edge research, as the visual monitoring of life processes still belongs to the most compelling evidences for a multitude of biomedical applications. Along with the rapid development of new probes and methods for the analysis of laser induced fluorescence, optical microscopy over past years experienced a vast increase of both new techniques and novel combinations of established methods to study biological processes with unprecedented spatial and temporal precision. On the one hand, major technical advances have significantly improved spatial resolution. On the other hand, life scientists are moving toward three- and even four-dimensional cell biology and biophysics involving time as a crucial coordinate to quantitatively understand living specimen. Monitoring the whole cell or tissue in real time, rather than producing snap-shot-like two-dimensional projections, will enable more physiological and, thus, more clinically relevant experiments, whereas an increase in temporal resolution facilitates monitoring fast nonperiodic processes as well as the quantitative analysis of characteristic dynamics.