Atomic-scale structure changes in gallium lanthanum sulphide bulk glass and ablation-deposited thin films have been studied by the x-ray absorption fine-structure (EXAFS) technique. EXAFS spectra have been recorded at the sulphur and gallium K edges, and the lanthanum edge, and this has allowed us to construct a detailed picture of the local structure in bulk glass and thin films. The EXAFS results indicate that there is chemical disorder in the structural network of the GLS thin films, although chemical ordering is predominant in bulk GLS glass. The existence of `wrong bonds', i.e. Ga - Ga and S - S bonds, in the structure has been discussed and correlated with optical absorption experiments undertaken on the same samples to provide a consistent picture of the local structure.

Gallium -lanthanum sulphide glasses (GLS) show wide range transparency and low non radiative relaxation rates for dopant ions such as Ho", EI' etc. They also show permanent photomodification of the refractive index under visible illumination. We report laser ablation deposition of these glasses and preliminary results on film stoichiometry and deposition rate as a function of excimer laser fluence. The sulphur to metal and G&a ratios are found to have marked fluence dependencies. The films show considerably more Urbach tail absorption than bulk material. A novel method has been developed for mapping the permanent photomodifled index. INTRODUCTJONGallium-lanthanum sulphide (Ga&La$,) ('GLS') glasses [l] have a range of novel optoelectronic applications. The material is transparent from -0.65um to -lOurn, covering the important near infrared telecommunications and mid infrared 'fingerprint' spectral regions. The lanthanum content ensures very high solubility for other rare earth dopant ions such as Er3',Ho",Pr3',Nd" [2] etc which can be used as active laser or optical amplifier species. The low phonon energy of GLS glass gives rise to low non radiative relaxation rates, which opens the possibility of operating lasers in this host material in the mid infrared. Laser action to wavelengths approaching 5um may be feasible in high quality films. An unusual property of the material is that the refractive index can be substantially and permanently modified by exposure to illumination above the band gap, typically at 0.5145pm wavelength or shorter [3,4]. FILM DEPOSITION Target fabricationThe use of these large targets ensures consistency of target material over mAblation targets are manufactured as gallium lanthanum sulphide glass disks of typically 40mm diameter by 1Omm thick. any ablation experiments and freedom from target thermal damage and cracking. Gallium sulphide (70% molar) and lanthanum sulphide (30% molar) are weighed out under dry nitrogen into a vitreous carbon crucible pre placed in a silica ampoule. Particular care is taken with the source of the sulphide (Merck) to minimise contamination with oxysulphides. The silica ampoule is evacuated and heated to 1200C for two hours and then water quenched. The glass disc is clear orange/yellow, with some superficial bubbles where gas adhered to the crucible/glass interface.The discs release easily from the crucible and there is no evidence of attack. The targets are visually homogeneous and show visible and IR spectra typical of GLS glass. Energy dispersive X ray analysis shows the targets to be homogeneous and stoichiometric to within experimental error. The disc is ground down to a flat surface, cleaned, and mounted in a cup designed to provide a surface to radiate the absorbed power load and minimise conduction to the in-vacuum stepper motor drive.

With any waveguide device accurate knowledge of the optical characteristics is necessary for device design and specification; hence it is important from a practical and theoretical point of view to establish an efficient and precise method for measuring the refractive index profile of optical waveguides. For the majority of waveguide applications accurate knowledge of the absolute refractive index is rarely required but in contrast the difference between two adjacent regions is essential. The technique presented here addresses this second requirement. Measurements can be performed at any wavelength on the basis of intensity measurements of reflected signals from two adjacent points on the surface of a material and, since it is a non-contact measurement, it is also possible to record reflectance changes in situ. Samples can be heated, illuminated or processed in other ways, with a constant record of the refractive index changes which occur or are being made, mapped across an entire surface to an accuracy in of . The spatial mapping of is limited only by the focused spot diameters and the need for a separation between them, the surface quality and the noise associated with the combined detector and PSD electrical noise. We used and ZnSe at various temperatures to calibrate and measure small refractive index changes.