zoneplating

A zone plate is a type of diffractive optical element that focuses light. It consists of a series of concentric rings, similar to an Fresnel zone plate. These rings are alternately opaque and transparent, or have varying refractive indices, and are designed to cause constructive interference of light at a focal point. The zones are spaced such that the path difference from the center of the zone plate to the edge of each ring differs by half a wavelength of the incident light. This arrangement causes light passing through the alternating zones to interfere constructively, creating a focal point.

Unlike lenses, which refract light to a focus, zone plates diffract light. This diffraction process means that zone plates can have multiple focal points, both positive (real) and negative (virtual), as well as a significant amount of light that is not focused. The focal length of a zone plate is determined by the radius of its outermost zone and the wavelength of the incident light. They are often used in applications where conventional refractive optics are not suitable, such as in X-ray microscopy due to the high refractive indices needed for X-rays. Zone plates can also be designed to operate at a specific wavelength. Their fabrication often involves lithographic techniques.