In recent years, the availability of intense continuous radiation sources, such as electron synchrotrons and laser-induced hot plasmas, has given rise to a renewed interest in soft x-ray and x-uv reflectivity measurements. Such studies were, for a long time, hindered because of mainly two reasons. First, it was really difficult to generate soft x-rays in the laboratories and second, there was no possibility for practical implementation and design of optical systems, such as focusing elements, mirrors, etc. associated with that particular wavelength region. Soft x-rays, with wavelength range usually from 10 to about 200 angstroms, can produce images of higher resolution than visible light due to their shorter wavelength. For years, physicists have wanted to construct an x-ray microscope that would exploit the ability of soft x-rays to detect small structures. The need for such an instrument is clear. The resolution of light microscopes is limited by the comparatively long wavelength of visible light. Although transmission electron microscopes have much higher resolution, they are weak in penetrating power and are, therefore, limited to very thin specimens. Therefore, transmission electron microscopy involves extreme care in sample preparation. Such preparations which might alter the very structure of a biological sample, would not be required in x-ray microscopy. The difficulties in constructing an x-ray microscope, however, have proved to be irresistible, because of the fact that soft x-rays cannot be brought together to form an image. In other words, soft x-rays cannot be made reflected from any known single surface at normal or near-normal incidence. The only possibility that existed until now employs grazing incidence, the only form of focusing x-ray optics. But their quality (resolution) has been limited because they must be machined in the form of a paraboloid or hyperboloid. Lenses of the kind used in ordinary optical microscopes cannot be made for use at wavelengths less than about 1000 angstroms. There are two reasons for this. First, there is only a tiny difference in the refractive indices among the different materials at soft x-ray wavelengths. Second, soft x-rays are strongly absorbed by all materials and cannot penetrate any conceivable lens, used in ordinary optical microscopes or telescopes. A major advance in x-ray optics holds a great promise both in the fields of high resolution scanning x-ray microscopy, lithography and substantial improvements in the quality of x-ray telescopes. Recent improvements in the techniques for quality control of evaporated and sputtered films have led to the interest in the controlled fabrication of multilayered structures known as 'Layered Synthetic Microstructures', to be used as mirrors for the extreme ultraviolet and soft x-ray regions. These can be produced with virtually any layer spacing greater than approximately 10 angstroms and they have a considerably high diffraction efficiency at normal or near-normal incidence. This remarkable enhancement in normal-incidence reflectivities at x-uv domain of the electromagnetic spectrum leads to another innovative application of these microstructures, i.e. the production of x-ray lasers with high gain-length products, where the enhanced normal-incidence reflectivity of the multilayers has been applied for multiple pass gain of the laser media by increasing the effective path lengths of the plasma columns. The present article covers the theoretical considerations, development and different techniques of controlled fabrication of layered synthetic microstructures along with their potential applications in the fields of x-ray spectroscopy, microscopy, x-ray laser production and lithography.
Optics for coherent X-ray applications
