Recent developments in label-free SR techniques can be divided into different types. The first type involves techniques that use complex microstructures in the near field of the sample. This type can also be divided into two main groups. The first group concept is to use the microstructures in the near field to create special illumination of the sample. The special light contains information on the subwavelength features that could be detected further away. A few examples of this group are hyperstructured illumination, plasmon illumination, plasmon polariton illumination, and time-reversal mirror techniques. The concept of the second group is that the microstructures located in the near field will manipulate light from ambient illumination or the sample itself. With this method, the subwavelength features will still be projected to the image in the far field. An important collection of methods in this group creates superlenses, which are made from thin slabs with negative permittivity and/or permeability. Superlenses enhance the amplitude of the evanescent fields, so they can be detected far from the sample. One advantage is that a whole image of the object is projected to the image plane and the acquisition can be done without scanning. The variety of different ways to create superlenses, such as metamaterials, flat metal films, photonic crystals, and others, enables imaging from the visible to the terahertz frequency regime.
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