Point-spread function optimization in isoSTED nanoscopy

isoSTED microscopy with water-immersion lenses and background reduction

Fluorescence microscopy is an excellent tool to gain knowledge on cellular structures and biochemical processes. Stimulated emission depletion (STED) microscopy provides a resolution in the range of a few 10 nm at relatively fast data acquisition. As cellular structures can be oriented in any direction, it is of great benefit if the microscope exhibits an isotropic resolution. Here, we present an isoSTED microscope that utilizes water-immersion objective lenses and enables imaging of cellular structures with an isotropic resolution of better than 60 nm in living samples at room temperature and without CO2 supply or another pH control. This corresponds to a reduction of the focal volume by far more than two orders of magnitude as compared to confocal microscopy. The imaging speed is in the range of 0.8 s/μm3. Because fluorescence signal can only be detected from a diffraction-limited volume, a background signal is inevitably observed at resolutions well beyond the diffraction limit. Therefore, we additionally present a method that allows us to identify this unspecific background signal and to remove it from the image.

Pupil function design for multifocal confocal, STED, and isoSTED microscopy

Point scanning super-resolution microscopy techniques such as stimulated emission depletion (STED) microscopy are powerful tools to observe biological samples at sub-diffraction limited resolution in three dimensions. However, scanning the sample with only a single beam limits the imaging speed in these microscopes. Here, we propose a concept to increase this speed by introducing highly flexible multifocal illumination and detection. We introduce phase patterns in the objectives' pupil planes to create arrays of foci in the sample plane with negligible loss of laser power. High uniformity of these foci's intensities is achieved by iteratively applying a weighted Gerchberg-Saxton phase retrieval algorithm. We characterize the performance of this iterative approach numerically and present simulation results that demonstrate the high quality of the focus arrays for future implementations in laser-scanning STED and isoSTED microscopes. The same approach can also be applied in diffraction-limited confocal laser scanning microscopy.

Three-dimensional adaptive optical nanoscopy for thick specimen imaging at sub-50-nm resolution

Understanding cellular organization demands the best possible spatial resolution in all three dimensions. In fluorescence microscopy, this is achieved by 4Pi nanoscopy methods that combine the concepts of using two opposing objectives for optimal diffraction-limited 3D resolution with switching fluorescent molecules between bright and dark states to break the diffraction limit. However, optical aberrations have limited these nanoscopes to thin samples and prevented their application in thick specimens. Here we have developed an improved iso-stimulated emission depletion nanoscope, which uses an advanced adaptive optics strategy to achieve sub-50-nm isotropic resolution of structures such as neuronal synapses and ring canals previously inaccessible in tissue. The adaptive optics scheme presented in this work is generally applicable to any microscope with a similar beam path geometry involving two opposing objectives to optimize resolution when imaging deep in aberrating specimens.

Inverse design of optical needles with central zero-intensity points by artificial neural networks

Optical needles with central zero-intensity points have attracted much attention in the field of 3D super-resolution microscopy, optical lithography, optical storage and Raman spectroscopy. Nevertheless, most of the studies create few types of optical needles with central zero-intensity points based on the theory and intuition with time-consuming parameter sweeping and complex pre-select of parameters. Here, we report on the inverse design of optical needles with central zero-intensity points by dipole-based artificial neural networks (DANNs), permitting the creation of needles which are close to specific length and amplitude. The resolution of these optical needles with central zero-intensity points is close to axial diffraction limit (∼1λ). Additionally, the DANNs can realize the inverse design of several types on-axis distributions, such as optical needles and multifocal distributions.

z-STED Imaging and Spectroscopy to Investigate Nanoscale Membrane Structure and Dynamics

Super-resolution stimulated emission depletion (STED) microcopy provides optical resolution beyond the diffraction limit. The resolution can be increased laterally (xy) or axially (z). Two-dimensional STED has been extensively used to elucidate the nanoscale membrane structure and dynamics via imaging or combined with spectroscopy techniques such as fluorescence correlation spectroscopy (FCS) and spectral imaging. On the contrary, z-STED has not been used in this context. Here, we show that a combination of z-STED with FCS or spectral imaging enables us to see previously unobservable aspects of cellular membranes. We show that thanks to an axial resolution of ∼100 nm, z-STED can be used to distinguish axially close-by membranes, early endocytic vesicles, or tubular membrane structures. Combination of z-STED with FCS and spectral imaging showed diffusion dynamics and lipid organization in these structures, respectively.

Aberrations in stimulated emission depletion (STED) microscopy

Like all methods of super-resolution microscopy, stimulated emission depletion (STED) microscopy can suffer from the effects of aberrations. The most important aspect of a STED microscope is that the depletion focus maintains a minimum, ideally zero, intensity point that is surrounded by a region of higher intensity. It follows that aberrations that cause a non-zero value of this minimum intensity are the most detrimental, as they inhibit fluorescence emission even at the centre of the depletion focus. We present analysis that elucidates the nature of these effects in terms of the different polarisation components at the focus for two-dimensional and three-dimensional STED resolution enhancement. It is found that only certain low-order aberration modes can affect the minimum intensity at the Gaussian focus. This has important consequences for the design of adaptive optics aberration correction systems.

Improved lateral resolution with an annular vortex depletion beam in STED microscopy

We report on the experimental demonstration of improved lateral resolution in stimulated emission depletion (STED) microscopy using an annular depletion beam configuration. A tight and finely tuned doughnut focal spot can be created by annular vortex illumination. Its application in STED microscopy for enhanced lateral resolution is systematically investigated by imaging 40 nm fluorescent beads. An improved resolution with more than 20% reduced effective point spread function of the imaging system determined by the full width at half-maximum compared to that of the conventional STED is achieved. The proposed scheme also demonstrates its resolving capability for biological samples. The principle holds great potential in the research fields of biological superresolution imaging as well as STED-based nanolithography and high-density optical data storage.

Aberrations in 4Pi Microscopy

The combination of two opposing objective lenses in 4Pi fluorescence microscopy significantly improves the axial resolution and increases the collection efficiency. Combining 4Pi microscopy with other super-resolution techniques has resulted in the highest three-dimensional (3D) resolution in fluorescence microscopy to date. It has previously been shown that the performance of 4Pi microscopy is significantly affected by aberrations. However, a comprehensive description of 4Pi microscope aberrations has been missing. In this paper, we introduce an approach to describe aberrations in a 4Pi cavity through a new functional representation. We discuss the focusing properties of 4Pi systems affected by aberrations and discuss the implications for adaptive optics schemes for 4Pi microscopes based on this new insight.