The world's smallest lasers are just a few thousandths of a millimetre across, and they offer vast banks of light sources for optical telecommunications that fit onto a single silicon chip. Among the key components of these devices are mirrors made from sandwiches of different materials in layers only hundreds of atoms thick. Making them is a delicate business that needs high vacuums and high temperatures. But researchers at the Massachusetts Institute of Technology (MIT) describe in the journal Macromolecules [13 July] a variety of plastic multilayer mirrors that assemble themselves.
Lasers consist of a cavity in which the light is emitted, bordered at each end by mirrors that let the light bounce back and forth in the cavity, amplifying it, before it finally bursts out through one of the mirrors (which is rendered only partially reflective for this purpose). In big lasers (those that can sit on a desktop), the mirrors tend to be metallic surfaces like those on the bathroom wall. But a stack of alternating layers of two materials with different refractive indexes will also act as a mirror if the layers are about the same thickness as the wavelengths of light - just a few hundred millionths of a millimetre.
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