Ultrafast widefield optical sectioning microscopy by multifocal temporal focusing.

The need for optical sectioning in bio-imaging has amongst others led to the development of the two-photon scanning microscopy. However, this comes with some intrinsic fundamental limitations in the temporal domain as the focused spot has to be scanned mechanically in the sample plane. Hence for a large number of biological applications where imaging speed is a limiting factor, it would be significantly advantageous to generate widefield excitations with an optical sectioning comparable to the two-photon scanning microscopy. Recently by using the technique of temporal focusing it was shown that high axial resolution widefield excitation can be generated in picosecond time scales without any mechanical moving parts. However the achievable axial resolution is still well above that of a two-photon scanning microscope. Here we demonstrate a new ultrafast widefield two-photon imaging technique termed Multifocal Temporal Focusing (MUTEF) which relies on the generation of a set of diffraction limited beams produced by an Echelle grating that scan across a second tilted diffraction grating in picosecond time scale, generating a widefield excitation area with an axial resolution comparable to a two-photon scanning microscope. Using this method we have shown widefield two-photon imaging on fixed biological samples with an axial sectioning with a FWHM of ~0.85 µm.

Near-isotropic 3D optical nanoscopy with photon-limited chromophores.

Imaging approaches based on single molecule localization break the diffraction barrier of conventional fluorescence microscopy, allowing for bioimaging with nanometer resolution. It remains a challenge, however, to precisely localize photon-limited single molecules in 3D. We have developed a new localization-based imaging technique achieving almost isotropic subdiffraction resolution in 3D. A tilted mirror is used to generate a side view in addition to the front view of activated single emitters, allowing their 3D localization to be precisely determined for superresolution imaging. Because both front and side views are in focus, this method is able to efficiently collect emitted photons. The technique is simple to implement on a commercial fluorescence microscope, and especially suitable for biological samples with photon-limited chromophores such as endogenously expressed photoactivatable fluorescent proteins. Moreover, this method is relatively resistant to optical aberration, as it requires only centroid determination for localization analysis. Here we demonstrate the application of this method to 3D imaging of bacterial protein distribution and neuron dendritic morphology with subdiffraction resolution.

Multilayer three-dimensional super resolution imaging of thick biological samples.

Recent advances in optical microscopy have enabled biological imaging beyond the diffraction limit at nanometer resolution. A general feature of most of the techniques based on photoactivated localization microscopy (PALM) or stochastic optical reconstruction microscopy (STORM) has been the use of thin biological samples in combination with total internal reflection, thus limiting the imaging depth to a fraction of an optical wavelength. However, to study whole cells or organelles that are typically up to 15 microm deep into the cell, the extension of these methods to a three-dimensional (3D) super resolution technique is required. Here, we report an advance in optical microscopy that enables imaging of protein distributions in cells with a lateral localization precision better than 50 nm at multiple imaging planes deep in biological samples. The approach is based on combining the lateral super resolution provided by PALM with two-photon temporal focusing that provides optical sectioning. We have generated super-resolution images over an axial range of approximately 10 microm in both mitochondrially labeled fixed cells, and in the membranes of living S2 Drosophila cells.