Real Time Magneto-optical Imaging using Liquid Crystal Modulator

Abstract

Polarization modulation technique has been applied for magneto-optical (MO) imaging to measure MO values quantitatively. In this paper, we report on a real-time MO imaging using the polarization modulation method with a liquid crystal modulator. Measurements of MO images and hysteresis Kerr loop of patterned Bi,Ga substituted yttrium iron garnet film were demonstrated by using the technique. A frame rate of 1 s was achieved in this experiment. K Key wor ds : magneto-optical imaging, garnet thin film, real time measurement, liquid crystal modulator 1 1. I ntro duction Recent years, quantitative and real time measurements are one of the most important issues in the magneto-optical (MO) imaging to study dynamics of magnetic properties in magnetic materials. We have recently developed an MO microscope using a polarization modulation technique, and demonstrated that quantitative MO measurements are possible with this technique [1][2][3] .In this technique, quantitative images of MO rotation and ellipticity can be obtained by using three images taken with three different polarized lights; i.e., the linearly polarized light (LP), the right-circularly polarized light (RCP) and the left-circularly polarized light (LCP). However, there is a drawback that it takes a several seconds at least for measurements because of a procedure to modulate optical polarization states by mechanical rotation of a quarter wave plate. In order to realize a real time MO imaging at the video rate (30 frames/sec) using this technique, an optical modulator that works faster than 30 ms is required. In this study, we used a liquid crystal modulator (LCM), which can switch the optical polarization state only by changing an AC voltage applied to the device. In this paper, we report on introduction of LCM to the MO microscope and MO imaging for a patterned Bi,Ga-substituted yttrium iron garnet (YBFGO) thin film 4) . 2 2. M MO microscope Figure 1 shows an optical setup for the MO microscope used in this experiment. In order to utilize the optical modulation method with a polarizer, an LCM and an analyzer were installed with an optical axis of /4, 0 and 0, respectively. An objective lens with a long working distance (Mitsutoyo, G Plan APO 50) was employed. Digital images of 640 480 pixels are taken by a high speed CCD camera (Hamamatsu, C9300-201) with a 12-bit A-D converter, which can directly transfer 160 digital images/sec to RAM in the computer. A halogen lump was used as a light source, and wavelength was selected by using interference filters. We prepared an LCM employing a commercially available liquid crystal (ZLI-4792) and ITO-coated glass plates. The retardation was varied by an application of AC voltage of 0 -10 V with a frequency of approximately 100 Hz to the LCM in this experiment. The voltages corresponding to the LCP, the LP, the RCP were determined by measuring an extinction ratio of a maximum intensity Imin and a minimum intensity Imax, of the output light. Figure 2 shows extinction ratios for a wavelength of 550 nm. Maxima and a mini...