Photofabrication is the manufacture of devices through the action of light on a photosensitive material. In its most usual form an image is projected onto a layer of photoresist which, when developed, acts as a mask for the selective processing of the underlying substrate. In coherent photofabrication the pattern to which the photo-resist is exposed is generated in situ as a pattern of interference fringes, produced by the interaction of coherent beams of light. The device is then an interferogram recorded as a modulation in the surface of the photoresist layer and may either be used as it stands or may require further processing. Items which have successfully been made in this way include spectroscopic diffraction gratings of very high quality, grid polarisers for the near infrared, a novel type of antireflection surface, grating couplers for thin film integrated optics and a selectively absorbing surface which may be useful for the harnessing of solar energy.
It has long been known that zone plates can be used to form an image but they have been little used in practice because a lens generally performs better. Since a zone plate distributes light into several diffracted orders the efficiency is low and since the focal length is inversely proportional to wave-length the chromatic aberration is enormous. However, despite these drawbacks the zone plate does possess some attractive features: it is a thin structure so it produces no distortion in the image; it is less bulky and lighter than a lens (which would be particularly important for many infrared applications), and can be replicated so it is amenable to inexpensive mass production. We suggest that there are 3 main reasons why zone plates might become more widely used: a change of requirements; improvements in manufacturing technology; and new applications which take advantage of the unique features of the zone plates.
Blazed zone plates have been manufactured by recording in photoresist the circular fringes transmitted by a Fabry Perot Interferometer. The fringe pattern is scanned through one order by an axial translation of one of the mirrors and a series of exposures is recorded in such a way that a triangular groove profile is generated. Their numerical aperture is limited by the properties of the camera used to photograph the fringes and this limits the range of application of the technique.
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