In a dark room, a spark jumps between two wires, and for an instant, there is light. Physicists have done something similar, but a million times smaller: at the boundary of two transparent crystals, they drew a conducting path and left a break just a few atoms wide. When a direct voltage was applied across this tiny gap, it began to glow.
The light was not born from heat, as in a light bulb, but from the electric field itself. Inside the gap, the field was enormous — more than ten million volts per meter, as if a thundercloud were squeezed between your fingertips. Such a field makes electrons in the crystal oscillate twice as fast as usual, releasing energy as light at twice the frequency. The glow's brightness grew quadratically with voltage: double the voltage, and the light became four times brighter. At one volt, the brightness changed nearly fourfold, and it all happened without delays or hysteresis, like a switch.
This gap can be redrawn — trace the path elsewhere, change the shape of the break, and the light will be polarized differently. Such a tunable light source, smaller than the wavelength, could be used in optical chips, where light is controlled as easily as current in a transistor.
Doubled-frequency light is the same effect that makes a green laser pointer glow green: inside, an infrared laser beam passes through a crystal that doubles its frequency.