Laser‐induced fluid motion on a dye/polymer layer for optical data storage

Abstract

The interaction of highly focused light with active dye/polymer layers has received considerable attention in these days because it creates micron-size pits useful as optical data storage sites. Pit formation during laser marking takes place in a very short period of time, usually between 50 and 500 nanoseconds (ns). It consists of the following steps: the material heats, then melts, and finally flows in the lateral direction. In other words, the highly focused laser pulse results in an uneven temperature profile on the surface of an active dye/polymer layer; the variation in surface tension from one point to another creates tangential stresses on the free surface and a capillary fluid motion is therefore induced. As a result, the free surface of the pit center depresses and a bulge is formed around the pit. To be a good recording medium, an active dye/polymer layer should have high light absorption and reflectivity a t the wavelength of the emitting laser beam. The greater the absorption, the faster the marking. The encoded data can be read back based on the difference in the surface reflectivity between the surroundings and the pit.The detailed mechanism of pit formation is not fully understood yet, but it is a typical transport phenomenon problem for a chemical engineer. Surprisingly, however, almost no attention has been given to this issue by chemical engineers; most of the work has been done by optical physicists and published in physical journals. For example, Novotny and Alexandru (1 979) calculated the temperature profile during the light-induced marking process and described the mechanism of structure change in dye/polymer systems. Wrobel et al. (1982) found that the surface tension forces were unimportant in rapid pit formation in thin polymer films. Broer and Vriens (1983) presented models to describe the hole-opening process, and concluded that surface tension is one dominant force during the process. Suh and coworkers investigated the writing mechanism in thin-textured inorganic films, and believed the surface tension gradient force to be one of the most effective driving forces to overcome the energy barriers for forming a pit (Suh and Craighead, 1985;. Chung (1986) derived the governing equations for the transitional laser-induced fluid motion in extremely thin dye/polymer layers. The temperature rise in an optical layer is calculated based on the complicated Poynting vector theorem and Maxwell equations. Chung found that there are two factors controlling the pit formation, namely, the surface tension force and the gravitational force, the former being much more important than the latter.In this paper we attempt to extend our previous work to simulate the transitional pit formation in a relatively thick optical recording layer, while the temperature profile of the recording layer is determined using a conventional chemical engineering method. Calculations will be compared to the previous experimental results to determine if the model is meaningful. TheoryIn addition to the assumptions used in the...