MULTIPLE REFLECTORS AS NARROW-BAND AND BROAD-BAND VACUUM ULTRAVIOLET FILTERS

We report on the development of a new coating design for applications in the vacuum ultraviolet that yields significantly higher reflectivity over selectable bandwidths. We demonstrate that the concept can be used to fabricate high-performance narrow-band and broadband reflection filters, whose spectral properties can be greatly enhanced by utilizing several of these filters in tandem. For example, we have fabricated a narrow-band filter at the location of the O I 135.6-nm line with a 3.2-nm bandwidth, a peak transmittance of 39.3%, and out-of-band wavelength blocking of better than 10(-4) %. The principle of our design approach is to use a combination of high (H) and low (L) refractive-index dielectric pairs so that H + L = lambda(r)/2, where H/L < 1. H and L designate the optical thicknesses of high- and low-index film materials. This kind of choice for the high-low ratio reduces the effects of absorption for the H films for which the extinction coefficient in the vacuum ultraviolet is much higher than for the low-index film material MgF2. The reduced absorption of multilayers with H/L < 1 results in a significant increase in reflectivity compared with the classical quarter-wave stack for which H/L = 1.