ZP_Salon

Diffractive optics elements: Though both a lens and a zone plate are optical elements which converge a light, their bases of the convergence are different in that the lens makes use of the refraction phenomenon and the zone plate utilizes the diffraction and interference phenomena. These optical elements have been used originally for converging visible lights but, with the progress of science and engineering, there appear requirements to converge or transport various electromagnetic waves other than the visible light such as the radio wave, the infrared light, the ultraviolet light, the X-ray, the gamma ray and so on. In this situation a very difficult problem is encountered, that is, there is no appropriate material of a lens except for electromagnetic waves with a wavelength near that of the visible light. Thus the chance of the zone plate becomes on. As the light wave passes through the transparent zones which can be a vacuum space without any material, the zone plate can be used as an optical element for any kind of the electromagnetic waves and even for any particle beam. The zone plates I described up to now are made of a film because a transparent film is harmless in the case of the visible light and the existence of the film is advantageous as supporting material of the zone plate. As is imagined easily a zone plate made of concentric annular zones ruins if it is produced by cutting zones from a metal plate. Therefore, in such a case some supporting structures are necessary.

Photon Sieve: As a natural extension of a zone plate with supporting structures we can replace a transparent zone by a train of pinholes on the annular zone. The train of pinholes becomes the original zone if the pinholes are placed densely. In 2001 this kind of optical elements was proposed by L. Kipp, et al. and named as photon sieve. The photon sieve has more advantages other than the above described one. As the resolution of the image produced by a zone plate is determined by the width of the outermost zone (the narrowest zone) and the higher resolution is obtained for the narrower width, a zone plate with a larger number of zones is desirable to attain higher resolution. When a zone plate for the short wavelength electromagnetic waves or particle beams is produced the zone plates are usually processed by lithography, and the limiting precision of lithography is considered to be 20 - 40 nm (0.00002 - 0.00004 mm) at present. By the way in the case of a photon sieve it is theoretically proved that the diameter of a pinhole can be made larger than the width of the corresponding transparent zone of a zone plate, which means that the number of “zones” of a photon sieve can be made larger than a zone plate and the resolution of a photon sieve can be made higher. Moreover, in the case of a photon sieve, the number of pinholes on a zone can be decreased smoothly toward the outer boundary (apodization), the intensity of side lobes of the converged image can be reduced and the converging performance can be improved.

A zone plate

A zone plate with supports

A photon sieve

Both the zone plate and the photon sieve are based on the diffraction and the interference phenomena which are rather difficult to understand intuitively. Though the zone plate has been studied as an optical element for a visible light from the beginning the photon sieve was invented for non-homeliness affairs, that is, for converging electromagnetic waves other than visible lights. Therefore, the zone plate is not popular among photographers and the photon sieve is not popular furthermore . However, as both the zone plate photography and the photon sieve photography are based on the same principle it is possible to take a photograph by using the photon sieve. Though, in the case of the zone plate photography variable parameter of the element is only the number of zones for the given light frequency, the given focal length , and the given type (Fresnel type or Gabor type), in the case of the photon sieve there are a lot of parameters to be determined additionally, that is, the size of pinhole, the radial and the circumferential distributions of the pinholes, which may cause surprises in a photograph. Please, start to see photographs taken by using “standard” photon sieves which are shown in the gallery.

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