Rub your eyes once, and what you see is a moonlit staircase somewhere in ancient Greece. Rub them again and it becomes a close-up of a dusty old book. What you're really looking at is the cross-cut section of a Bragg mirror. Under the lens of an electronic microscope, this highly reflective mirror reveals alternating layers of semi-conductor materials, each no thicker than 180 nanometers. This multi-tiered mirror channels light at a highly precise vertical angle and has become a crucial component in the production of surface emitting lasers, lasers whose beam projects vertically, rather than from the side. That's because when light enters the surface of a Bragg, a portion of it is reflected by the first layer, while the rest continues through to the second layer, where the process continues. After a light beam has bounced so many times, its particular wavelength makes the laser capable of analyzing the composition of gases down to parts per million. Such precision comes in handy when tracking pollutants in factory emissions or even scouring for water on Mars.
Pierre Grech, IES, Montpellier.