"We have for the first time demonstrated a high-speed, ultracompact silicon electro-optic modulator based on a photonic crystal waveguide," claimed engineering professor Ray Chen. "So far only our group and IBM have demonstrated such devices, but our is operating 1,000 time faster." Chen performed the work with researchers Wei Jiang, Xiaonan Chen, Lanlan Gu, and Li Wang at the University of Texas Microelectronics Research Center.

The photonic waveguide was constructed from a hexagonal lattice photonic crystal with a period of 400 nanometers and an air hole diameter of 220 nanometers. They were patterned using electron-beam lithography and drying etching.

University of Texas optical waveguide

The architecture achieved a Mach-Zehnder interferometer (MZI), which modulates light by comparing the phase at the end of two arms of a signal path. One is unobstructed, the second is electrically controlled by the photonic-crystal waveguide. By slowing light in the controlled arm, the phase is shifted. That enables an electrical control signal to modulate the optical signal and control the frequency modulation of the optical signal passing through the photonic crystal.

"Our device is the fastest p-i-n- [p-type/undoped intrinsic/n-type] diode based MZI modulator ever developed on silicon," Chen claimed. "Plus, it maintains the lowest driving power of any silicon modulator to date."

The ultimate goal of such silicon waveguides and modulators is to reduce power consumption in CMOS chips by replacing metal interconnection layers with optical interconnects. That could enable higher speeds while consuming less power. Today's optical modulators require lots of power to achieve high speed, but photonic crystal modulators consume less silicon real estate and have much lower driving voltage for the same modulation output, according to Chen.

"We exploited the slow light effect in photonic crystal waveguides to shorten the interaction length 10 times, thereby reducing the electric current about 10 times for the given current density, yielding much lower voltage and power," said Wei Jiang.

The team claims to have also gained insights into the principles governing photon-electron interaction in silicon photonic waveguides. "One principle we have discovered is that there is a minimum current density, set by the fundamental nature of light and certain intrinsic property of silicon, for high speed silicon modulators," said Wei Jiang. "For modulation at gigabits per second, this minimum current density is 10,000 amps per square centimeter, which falls in the so-called high-injection regime of a silicon diode. This explains why previous silicon modulators ran into high voltage and high power issues at gigabits per second" speeds.

The researchers will next seek an additional 10-fold increase in speed. They also plan to demonstrate on-chip lasing by pumping the silicon modulator with an off-chip laser.

One hurdle to overcome in silicon photonics, is the insertion loss of the photonic crystal itself, which is still high for the device. The Texas researchers will attempt to reduce the loss with a special coupling structure that adds an additional mask layer and several other processing steps.

"We have made progress separately on this special coupling, but have yet to integrated it into our modulator," Wei Jiang added.

The team predicts that it will take about a year to optimize its design to achieve its 10-Gbps milestone.