Optical image of a recrystallized sapphire surface.
Using a CO2 heating laser, substrates for thin-film growth can be heated to very high temperatures. The laser can easily melt sapphire (Al2O3), which happens at a temperature of 2040 °C. After sustaining a small ‘melt pool’ on the sample surface for about 15 minutes, the laser was switched off abruptly. This rapidly freezes the molten surface, yielding a variety of objects in many shapes and sizes. In this particular image, spherical drops surround a mesa-type structure, reminiscent of a single-cell biological organism on the verge of dividing, given the two cores. The shapes and sizes of such structures are dictated by the balance of surface energies in a solid-state crystal.