Light where she wants it: Engineer makes a wave of photonics advances

In a wave of recent advances, electrical engineering Assistant Professor Jelena Vuckovic’s research group has unveiled novel solid state structures that can localize and manipulate light on the “nanoscale” of hundreds of billionths of a meter. These innovations might be just what industry needs to deliver faster, more powerful, and more secure computing and communications when conventional methods can no longer handle the job.

“There is an industry goal to increase processor clock speeds to 20 gigahertz (cycles per second) around 2010, but at such speeds, there is a problem with transmitting electrical signals over wires,” says Vuckovic, who joined the Stanford faculty in 2003. “One of the options is to introduce optical interconnects, but that requires optical devices which operate at rates faster than 20 gigahertz.” Vuckovic’s recent work has produced nanoscale devices to manipulate light in optical integrated circuits and to produce very fast, power-efficient lasers.

The work could also contribute to making quantum communications and computing a reality. Quantum computers, while still years or decades away from practical use, could someday use individual particles of light called photons to greatly ease certain computing tasks, such as factoring large numbers. On the other hand, quantum communications is already finding practical uses by enabling much more secure data transmission by relying on the quantum mechanical properties of individual photons. Vuckovic’s lab has produced new devices that can control the emission of single photons.

To enable these exotic applications, Vuckovic takes advantage of conventional microelectronic manufacturing techniques. By precisely etching chips of familiar materials such as silicon, gallium arsenide, and indium-phosphide, she creates photonic crystals that can localize, or contain, light so that it can be manipulated in a variety of useful ways. The crystals are built to have confined areas — picture them as tiny rooms and corridors — where only particular wavelengths of light can travel. The “rooms,” or cavities, on Vuckovic’s chips are only about two hundred nanometers long in each direction.

Controlling individual photons

In Physical Review Letters in July 2005, Vuckovic, electrical engineering and applied physics Professor Yoshihisa Yamamoto, and collaborators from Stanford and the Institute of Industrial Science at the University of Tokyo, reported building photonic crystals that can control the emission rate of individual photons and thus emit single photons with a wide range of pulse durations. In quantum communications, short single photon pulses imply fast communication rates (Vuckovic and collaborators reported single photon pulse durations of 210 trillionths of a second, which would allow for transmission of more than a billion photons, and therefore secure bits of information, in only one second). A slower emission rate (pulses as long as 8 nanoseconds), could be used for short term data caching or storage in a circuit. In a circuit operating at 40 billion cycles a second, for example, 320 cycles would go by while the data persisted in an 8 ns pulse.

The team’s discovery resulted from placing particles called quantum dots — basically artificial atoms designed to emit particular wavelengths of light — inside cavities on a photonic crystal. Dots with a wavelength well tuned to resonate in the cavity emitted short pulses. Dots that were not well tuned emitted longer pulses.

At any rate, single photons can be used to make communications more secure because, by a basic principle of quantum physics, any eavesdropper who observed the photon in transit would force it to take on particular characteristics. Once that happened the communicating parties would know someone had been listening in and could stop communicating until they could be sure the line was clear. Vuckovic and Yamamoto are among the community of researchers around the world working to make industrially useful single-photon emitting devices.

A fast, efficient laser

With the ink drying on the single-photon paper, Vuckovic’s research group was preparing a paper on another innovation, a photonic crystal laser that can be modulated — turned on an off — as much as five times faster than the best conventional solid state lasers. The laser works by channeling light through a series of photonic crystal cavities. As the beam bounces through one cavity after another, it stimulates light emission from the semiconductor material in those cavities.

Using the emission rate control technology they described in July, the team has demonstrated a laser which they expect could be modulated at speeds as fast as 100 billion times a second. Such a laser could be used to clock a computer at much faster speeds than are possible today. Such a fast modulation rate would also allow for much faster data transmission around a computer chip or a network.

This lasing method is also relatively powerful. Pumped with a few thousandths of a watt, the laser presently produces a beam with a peak output power of more than 10 millionths of a watt, which is enough to be useful in many applications. Previous photonic crystal lasers that used only one cavity consumed hundreds of millionths of a watt of power, but only produced beams with output powers in the range of a few billionths of a watt, which is too weak.

Vuckovic’s group demonstrated the laser in the lab in May 2005. The researchers are now finishing an article describing the results, which they hope will be published by the end of the year. They are also seeking a patent on it through the Stanford Office of Technology Licensing.

Finding ways to keep the computing and communications industries moving forward helps drive Vuckovic’s passion for her research. “Nanophotonic devices that I develop could help overcome the bottlenecks that the semiconductor industry will face a couple of years from now,” she says. “But they could also open new areas for that industry, such as quantum communication devices.”

Jelena Vuckovic, assistant professor of electrical engineering at Stanford University