Fresnel Zone Plate for Optical Image Formation Using Extreme Ultraviolet and Soft X Radiation

Abstract

A new type of Fresnel zone plate has been constructed which can focus ultraviolet radiation of any wavelength down to the soft x-ray region. It consists of a set of thin circular gold bands made self supporting by radial struts, leaving the transparent zones empty. Experimental tests at 6700, 4358, and 2537 A showed that the theoretical minimum angular resolution obeys the Rayleigh criterion, sinθmin=1.22λ/D. The diameter of the zone plate is D=0.26 cm and contains 19 opaque zones, the narrowest of which measured about 20 μ across. The zone plate was better than the optimum pinhole in resolution by a factor of about 6 and in speed by a factor of 40. The zone plate produced pictures that compared favorably with those made with a lens of similar focal length and aperture. The lens was about 20 times faster than the zone plate at 4358 A, but at 1000 A the zone plate would have been far faster than the lens. Focusing tests are contemplated at 1000 A and at 100 A where lenses and mirrors, the conventional image-forming devices, may fail. The angular resolution at 2537 A was close to the theoretical value of 1.2×10−4 rad and held over a field of at least 1.75×10−2 rad, which is 2.0 times the angle subtended by the sun’s disk at the earth. A zone plate telescope, operating in the soft x-ray or extreme ultraviolet region, far above the earth’s atmosphere in an orbiting satellite, now seems possible.