Real-time studies of mark formation processes in phase-change and magneto-optical media using a two-laser tester.

Abstract

We have equipped a commercial polarized-light microscope with two laser diodes operating at 643 nm and 680 nm. The two laser beams are focused simultaneously and coincidentally on the sample through the microscope objective lens. We use one of the lasers in the pulsed mode to write marks on the sample, and the other laser in a low-power cw mode to monitor the changes in reflectivity and/or the state of polarization upon reflection from the sample. The system has a liquid crystal wave-plate for adjusting the optical phase-shifts, an electro-magnet, a hot plate, a computerized XY-stage, two polarizing beam-splitters, and both single-ended and differential detection modules for each of the laser wavelengths.The dynamics of phase-transition (i.e., melting, crystallization, and amorphization upon rapid cooling from melt in the case of PC media, and ferrimagnetic-to-paramagnetic in the case of MO media) are readily observed in the 160 MHz-bandwidth signal picked up by the detectors. Similarly, changes in the magnetic state of the MO media under a focused spot (whether due to the change of temperature or caused by magnetization reversal), are monitored in real-time.We describe the dynamics of phase transformation in PC media as well as the observed behavior of magnetization of the MO media within the region of the hot spot. These observations are made over periods ranging from 30 ns up to several microseconds, with a resolution of only a few nanoseconds. We also outline the methods of extracting from the observed data the physical parameters of the media such as the specific heat, thermal conductivity, temperature-dependence of the optical constants, and the probabilities of nucleation and growth.