Black silicon: A new way to trap light

Eric Mazur, Harvard College Professor and Gordon McKay Professor of Applied Physics, and his students were studying what kinds of new chemistry can occur when lasers shine on metals, like platinum. One day, they decided to put a chip of gray silicon into a vacuum chamber, add some halogen gas, and scan it with ultrashort, ultra-intense laser pulses. Each pulse lasted a mere 100 millionths of a billionth of a second. However, the energy in a single pulse approximates focusing all the sunlight hitting Earth at one time onto a space the size of a fingernail. After more than 500 pulses, the silicon turned black. It wasn’t burned; rather, its surface had been etched by the heat and gas into a dazzling forest of billions of minute needlelike spikes. If a light is shone on such a surface, it repeatedly bounces back and forth between the spikes in a way that most of it never comes back out again. There’s the possibility that black silicon can be used for extremely fine computer and other electronic displays and for delivery of various drugs through the skin.