Progress of point-wise scanning superresolution methods

Abstract

Optical microscope has been giving impetus to the development of modern technology. As the advancement of these techniques, high resolution microscopy becomes crucial in biological and material researches. However, the diffraction limit restricts the resolution of conventional microscopy. In 1968, confocal microscopy, the first pointwise scanning superresolution method, appeared. It improves the imaging resolution, enhances the contrast, and thus breaks through the diffraction limit. Since then many superresolution methods have come into being, among which the pointwise scanning superresolution method earns reputation for its high imaging resolution and contrast. The stimulated emission depletion microscopy becomes the most prominent method with an achievable resolution of about 2.4 nm and then widely used. Besides, the newly developed fluorescence emission difference microscopy (FED) and the saturated absorption competition microscopy (SAC) have their advantages of non-constraint on fluorescent dyes, low saturated beam power, simplified optical setups, while they achieve a resolution of lower than /6. Further explorations of FED will be keen on vivo biological observations by using it, while that of SAC can concentrate on enhancing the resolution on a nanoscale and reducing the signal-to-noise ratio. In addition, the Airyscan technique in which a detector array is used for image acquisition, can serve as a complementary tool to further enhance the imaging quality of pointwise scanning superresolution method. The detector-array enables both the narrowed size of pinhole and the increasing of the acquired signal intensity by 1.84 folds. The other methods, e.g. superoscillation lens and high-index resolution enhancement by scattering, have the potentialities to obtain superresolved image in material science or deep tissues. After being developed in the past three decades, the superresolution methods now encounter a new bottleneck. Further improvement of the current methods is aimed at imaging depth, and being used more practically and diversely. In this review, we detailedly describe the above pointwise scanning superresolution methods, and explain their principles and techniques. In addition, the deficiencies and potentialities of these methods are presented in this review. Finally, we compare the existing methods and envision the next generation of the pointwise scanning superresolution methods.