THE DETERMINATION OF THE OPTICAL CONSTANTS OF SOME SILICATE GLASSES AT MILLIMETRE AND SUBMILLIMETRE WAVELENGTHS

Abstract

This work has two interlocking themes. It is primarily concerned with the development of precise, broad band, Fourier transform spectrometric techniques for the determination of the optical constants of solids at millimetre and submillimetre wavelengths. One such technique cannot easily accomodate the wide range of optical constants found in solids, leading to specimens which range from the virtually transparent to the virtually opaque, and it was therefore necessary to develop transmission and reflection techniques. The intercomparison of these techniques, their particular experimental difficulties and susceptibilities to random and systematic error, was performed by using each method to determin'the optical constants of soda lime silica glass over as wide a spectral range as possible. Previous to this work there had been no systematic study of this important material at these wavelengths and,thus, this investigation gave the second theme of this work, the quantitative determination of the optical constants of soda lime silica glass at millimetre and submillimetre wavelengths. The results of this study are presented in four chapters. First, in chapter 5, power transmission Fourier transform spectrometry has been used to investigate the spectral variation of the optical constants of the glass between 3 and 50cm, using an analysis of channel spectra to give the refractive index. This chapter also contains the results of measurements made with a grating instrument between 1000 and k000cm. Secondly, in chapter 6, the results of the first direct determinations of the optical constants by dispersive transmission Fourier transform spectrometry are presented and shown to indicate the presence of a hitherto unknown loss process below 20er that is tentatively associated with a similar process previously observed in fused silica. Thirdly, in chapter 7, dispersive reflection Fourier transform spectrometry has been used to determin the optical constants up to 360em, well into the midinfrared region of opacity of this glass. The results of chapters 6 and 7 provide the first quantitative description of the optical constants of this glass betweem 3 and 360em Finally, in chapter 8, the dispersive reflection measurements are extended to simple binary and ternary silicate glasses, and these results used to identify the contributions to the mid-infrared absorption of the metal ions associated with the various metal oxide additives of the glass. From this it was possible to account for all of the absorption in soda lime silica glass as a superposition of contributions from the near-infrared bands of the SiO1 network of the glass and the four main metal ions.