Quasi-isotropic 3-D resolution in two-photon scanning microscopy

One of the main challenges in three-dimensional microscopy is to overcome the lack of isotropy of the spatial resolution, which results from the axially-elongated shape of the point spread function. Such anisotropy gives rise to images in which significant axially-oriented structures of the sample are not resolved. In this paper we achieve an important improvement in z resolution in two-photon excitation microscopy through spatial modulation of the incident beam. Specifically, we demonstrate that the design and implementation of a simple shaded ring performs quasi-isotropic threedimensional imaging and that the corresponding loss in luminosity can be easily compensated by most available femtosecond lasers. The outcome looks particularly relevant to nano-fabrication and optical manipulation.