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What Is a Zone Plate?

By Ray Hawk
Updated May 16, 2024
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A zone plate is a flat, circular medium of material used for focusing light or other electromagnetic waves, such as x-rays, using principles of diffraction. They are often referred to as Fresnel zone plates and are related to the fresnel lens, both of which are named after a 19th century French engineer, Augustin-Jean Fresnel, who studied the nature of optics. Diffraction grating effects with a zone plate or fresnel lens have applications in photography, microscopy, and gamma-ray holography, as well as for potential space-based antenna systems.

Zone plates use the principle of diffraction to bend a wave of light or other energy, such as sound or quantum level matter waves of free neutrons and helium atoms, by bending their angle of incidence as they impact on transparent and opaque mediums. This creates a level of constructive interference with the light waves where they come to focus beyond the zone plate, which can increase resolution for certain aspects of the light or energy wave. To process all of the electromagnetic radiation impacting on a surface in this manner, a zone plate is made up of concentric circles that alternate between reflective or opaque qualities and transparent or light qualities, which gives it the appearance of a bull's eye.

A special type of zone plate where the dark and light rings fade into each other will create a single focal point, which has been used with gamma rays in the field of medical imaging holography. The idea is being researched for the imaging of regions around tracer isotopes introduced into the body in nuclear medicine. As the radioactive source illuminates a zone plate, the plate casts a shadow that can be recorded on photographic film at a smaller size than the actual source. This image precisely reflects the interference pattern created by the zone plate in three dimensions, and the photographed image can later be illuminated with ordinary light to reconstruct the image and examine the structure around the isotopes in detail.

X-ray microscopy is one of the primary research arenas for the use of diffraction grating devices such as zone plates. This is because traditional lens materials like glass will reflect x-rays or only weakly diffract them instead of focusing them, due to their small wavelength size, and zone plates must be constructed on a nanometer scale to achieve the desired focusing effect. Typically an x-ray zone plate has a a circular diameter of about 4 millimeters and zone thicknesses of between 50 to 300 nanometers. Such zone plate lenses can focus x-ray beams down to a resolution as fine as 10 nanometers, or 10 billionths of a meter. By comparison, a typical molecule of water, or H2O, is roughly 1 nanometer in diameter. This makes it possible to study biological materials, crystals, and other structures at the atomic level with a fine degree of optical resolution.

Using zone plates made of 1-millimeter-thick tungsten to capture high energy x-rays with energy levels up to 250,000 electron volts (250 keV) in size, in space-based antenna systems has been researched from 1968 to 2003. This goes beyond the ability of conventional lens materials, which cannot capture photons above 10 keV. Two-zone plates were used in tandem in one experiment, with a diameter of 2.4 centimeters containing 144 concentric zones, placed 30 centimeters apart in the telescope. They demonstrated a resolution of around 30 arcseconds, with no arago spot in the shadow casting process for the x-rays. An arago spot, or Poisson spot, is a typical energy point that appears at the shadow center of a Fresnel diffraction pattern where constructive interference occurs between energy wavelengths. Zone plate reflector antennae for spacecraft are seen as a technological leap forward from traditional parabolic antenna, being of much lower cost and weight, with high-gain performance characteristics and efficiencies for capturing up to 95% of incident radiation.

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