Electrical engineers at the University of Texas (Austin) have demonstrated what they claim is the world's smallest silicon modulator. The device features a photonic-crystal waveguide with an electrode configuration that they hope will make it easily manufacturable. Such a compact modulator might be the key to building practical all-silicon lasers, they said. Photonic crystals are periodic structures in silicon. They are usually defined by a regular set of holes that interact with photons in the same way that the much smaller periodic structure of silicon atoms interacts with electrons. The structures can produce superior optical cavities when optical bandgaps are introduced into silicon. The architecture realizes a Mach-Zehnder interferometer (MZI), which modulates by comparing the phase at the end of two arms of a signal path. One is unobstructed and the second is electrically controlled by the photonic-crystal waveguide, thereby enabling an electrical control signal to modulate the optical signal. To make the design the world's smallest, and thus manufacturable, the two electrical connections (one on each side of the 80-micron-long photonic crystal in one arm of the MZI) had to go through an extra step that created an external connection to the central electrode. The ultracompact silicon electro-optical modulator works by slowing light down in one of the two arms of a Mach-Zehnder interferometer, thereby shifting its phase to control the frequency of modulation of the optical signal passing through the photonic crystal. The electrical signal injected just .15 milliampere into one arm of the dual photonic crystals to achieve a 92 percent modulation depth in optical communication wavelengths of 1.3 to 1.55 microns.