Optical Interference Filters By Sol-Gel Processing
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OPTICAL INTERFERENCE FILTERS BY SOL-GEL PROCESSING J.L. KEDDIE AND E.P. GIANNELIS Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. ABSTRACT Optical interference filters have been synthesized by sol-gel. The selected filter is a multilayer of alternating TiO2 and SiO2 films. In transmission, the filter edge depends on the angle of incident light, which can be tailored through control of thickness and refractive index of the individual films. Theoretical modeling of the filter with film thicknesses obtained by Rutherford backscattering spectrometry is in good agreement with the experimental optical response. INTRODUCTION Optical filters that reflect or absorb light at selected wavelengths while transmitting in other regions are used to eliminate the halo effect on conventional cathode ray tubes (CRT) thereby increasing light output [1]. The presence of the halo inadvertently degrades the performance of the CRT display by decreasing the high spatial frequency contrast. This problem becomes exceedingly detrimental in high performance, high resolution displays, such as those used in defense systems. Although antireflection (AR) coatings have been used for reducing the problem, they are inadequate when the incident light is at a high angle, such as with diffuse light sources like phosphors [1,2]. Using an interference filter, light emitted by the phosphor normal to the faceplate is transmitted, while light at an angle from the normal is reflected. Interference filters usually consist of a multilayered structure with layers alternating between high and low refractive index materials. One such filter, consisting of a Ti0 2 /SiO2 multilayer, can increase the on-axis brightness of a CRT screen by a factor as high as 1.5 [3]. In the synthesis of a multilayer via sol-gel processing, several problems must be surmounted. One is the control of thickness and refractive index (optical thickness) of individual films that determine the optical characteristics of the filter. Two is the minimization of flaws and cracks. Three, and often overlooked, is the possibility of interdiffusion of the films, resulting in poorly defined interfaces and subsequently poor optical performance. We report here the sol-gel synthesis of optical interference filters consisting of up to 16 alternating layers of Si0 2 and TiO2 . Rutherford backscattering spectrometry (RBS) is used to probe film interdiffusion and to determine the thickness of individual layers in the filter. EXPERIMENTAL PROCEDURE Si02 sols were prepared according to the literature except that the synthesis was conducted at room temperature [4]. TiO2 sols with molar ratios Ti:PrOH:H 2 0:HC1 of 1:11.4:1:1.8 were prepared by adding a solution of deionized H20 in propyl alcohol to an acidified solution of titanium isopropoxide also in propyl alcohol [5]. Sols, diluted with the appropriate amount of alcohol, passed through 0.20 pm filters to eliminate any particulates before deposition. In some TiO2 sols, half of the propanol was substituted by toluene. One
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