Optical activity, i.e. the ability to rotate the polarization state of light, can also occur in structures that are not chiral at all, a property of fundamental importance for the diagnostic applications. This effect can be artificially obtained by means of metasurfaces, i.e materials surface with suitable nanostructures under appropriate light injection. In addition it is possible to obtain an optical response from metasurfaces that exhibits a nonlinear response such as second- order circular dichroism thanks to the recent successes in nanotechnology, opening the way to new functionalities on a spatial scale of few hundred on nms. In what follow we describe some properties of chiral metasurfaces.
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