Evaporation of thin metal films by laser radiation

An optical fuse contains a thin metal film that reflects ordinary light, but is damaged when the light intensity becomes too high, so that the light no longer reflects. Optical fuses can be used to protect eyes or sensors against dangerous laser radiation. In order to be suitable for use in an optical fuse, a metal film should have high reflectance when exposed to ordinary light intensities, but very quickly get damaged, so that the reflectance becomes low when exposed to dangerous light levels. In this diploma work, different metals and film thicknesses have been investigated, to find suitable films for this purpose. This has been done by making simulations of for instance how quickly different metal films are vaporised. From those simulations aluminium and tin were chosen for a closer study. A number of films were made, and the reflectances were measured in a spectrometer. Then the films were exposed to laser pulses where the incoming, reflected and transmitted light energy was measured. The results show that tin is vaporised faster than aluminium and therefore is a very interesting metal for use in optical fuses. Suitable film thicknesses are of the order of a couple of tens of nanometers and depend on the metal and the demands on the reflectance of ordinary light.