Refractive index gradient in the surface of a zirconium fluoride glass by exchange with chlorine
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X. S. Guo and William Landford Department of Physics, State University of New York at Albany, Albany, New York (Received 4 May 1989; accepted 30 August 1989) The refractive index of the surface of a zirconium fluoride glass was increased by exchanging fluorine in the glass with gaseous chlorine. The chlorine profile in the glass surface, as measured by Rutherford backscattering, was that expected for diffusion. The refractive index change was measured with ellipsometry, and compared to models of the concentration profile of chlorine. I. INTRODUCTION
New fluoride glass compositions transmit infrared radiation to a wavelength of 8 jxm or more, and are therefore highly attractive as fiber optic waveguides and as other optical components. In many optical applications, especially fiber optics, it is desirable to introduce a gradient or change in the refractive index at the surface of the component. This different refractive index is often introduced by melting together two different glass compositions, for example in the preform for an optical fiber. It is sometimes difficult to control the resulting index profile in this method. A refractive index gradient can also be introduced into a silicate glass by exchanging sodium ions in the glass with other ions such as silver to change the refractive index; such techniques are used in making gradient index lenses and other special optical components. No easy way to introduce a refractive index gradient or change into a fluoride glass surface has been reported. Ion exchange is difficult; the glasses are fluoride ion conductors, and other halide melts dissolve the glass. We have found that gaseous chlorine exchanges with fluorine in a zirconium-barium-lanthanum fluoride glass surface at temperatures below the glass transition temperature of about 315 °C. The resultant gradient of chloride concentration in the glass surface was measured with Rutherford backscattering (RBS), and the refractive index change was measured with ellipsometry. This change in the refractive index should provide additional impetus for optical applications of the heavy metal fluoride glasses, and the exchange of the larger chlorine ion for fluoride ion may strengthen the glass surface by introduction of a compressive stress, just as happens in silicate glasses when a large ion is introduced in place of a smaller one.
limation to eliminate impurities such as ZrC>2. Glasses were melted in a vitreous carbon crucible under an atmosphere of pure N2 gas in a resistance furnace. A maximum temperature of 800 °C was held for 20 min and then the melt was poured into brass molds preheated at 300 °C. Glasses were annealed at 300 °C for 5 min and slowly cooled to room temperature. Samples of size 10 x 10 x 3 mm3 were polished successively with 15 jum diamond paste and 0.3 fim A12O2 paste. After cleaning with acetone, glass samples were heated at 300 °C for times from 10 min to 162 h in an atmosphere of 3% Cl2 in N2. Profiles of elements in the glass were measured by Rutherford backscattering with the Dynamitron acce
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