Optical Constant Determination of Magneto-optical Thin Film Using Surface Plasmon Resonance Phenomenon

Abstract

Recent research identifies amorphous rare-earth-transition-metal thin films as most promising magneto-optical (MO) erasable storage materials. While writing is achieved by thermomagnetic means, reading is performed using polar Kerr magneto-optic effects. Generally the polar Kerr effect of MO films is quite small, being a fraction of a degree in rotation of polarization. Multiple dielectric protective layers can also enhance the magneto-optic Kerr effect [1,2]. The refractive index n and extinction coefficient k are required for the design of multilayer structures but little data exists for magneto-optical materials. We developed a method to measure the optical constants using the surface plasmon resonance phenomenon, which is based on the high sensitivity of that resonance to the optical constants [3,4], The configuration is shown in Fig.l. Light polarized with electric field parallel to the plane of incidence (TM) is incident within the glass upon the inside surface of the MO thin film. At angles of incidence greater than the critical angle for total reflection, an evanescent wave penetrates the metal and excites the natural modes of oscillation of the density of electrons at the surface of the MO film. At certain angle of incidence such that the component of the optical wave vector parallel to the metal surface is equal to the wave vector of the surface plasma wave, resonant coupling occurs for a properly chosen thickness of the film. Since, because the angle of incidence is beyond the critical angle, no wave propagates into the third medium (air), surface plasma waves are detected by noting the deep minimum in the internal reflectance. Fig. 1 shows diagramatically the arrangement of its apparatus.