Tapered velocity coupling method for optical waveguide Raman scattering spectroscopy. Application to iron(III) phosphate thin films

The application of the tapered velocity coupling method was proposed to measure optical waveguide Raman (OWG-R) spectra of a thin film, which shows large attenuation due to absorption and/or scattering. The method was applied to record the spectra of an iron(III) phosphate thin film (ca. 0.2 μm) deposited on a K+-doped glass waveguide. The thin film was fabricated to have a tapered structure at both ends. One of the TE and TM modes of the 514.5-nm Ar+ laser line in the glass waveguide was adiabatically transferred to the thin film through the tapered structure, so that the optical field intensity becomes concentrated almost completely within the thin films. Theoretical calculations indicated that the adiabatically coupled TE-O mode has its intensity maximum near the interface between the film and the K+-doped glass waveguide, while the TM-O mode has its maximum near the surface (or the interface between the films and air). The OWG-R spectra measured with the TE-O and TM-O modes give rise to a P-O stretching band near 1060 and 1030 cm-1, respectively, suggesting differences in structure and/or composition between the interface part and the surface. Raman spectra of iron phosphate treated at various temperatures (100-200 degrees C) indicated that a conversion from an amorphous to a crystalline state induced by the treatment causes the shift of a P-O stretching band from 1049 to 1030 cm-1. Furthermore, the OWG-R results indicate a difference in crystallinity between the surface and interface regions of the iron phosphate thin film; i.e., the surface region exists in a crystalline state, giving the OWG-R band at 1030 cm-1, and the interface assumes an amorphous state, giving the band at 1060 cm-1. This conclusion was supported by X-ray diffraction measurements of iron phosphate thin films treated under various temperatures. 13 refs., 7 figs