Transistor laser rewrites the rules

14 May 2010

Kirchhoff's current law has been rewritten thanks to a three-port transistor laser developed at the University of Illinois, US.

While the laws of physics weren't made to be broken, sometimes they need to be revisited and revised. That's the position that researchers Milton Feng and Nick Holonyak from the University of Illinois at Urbana-Champaign, US, find themselves in following their research into transistor lasers (Journal of Applied Physics 107 094509).

"Based on an earlier charge control analysis, we have constructed a microwave circuit model of a three-port quantum-well transistor laser by extending Kirchhoff's law to include electron-photon interaction, to yield an electrical-optical form of Kirchhoff's law," explain Feng and Holonyak, from the Department of Electrical and Computer Engineering.

Transistor lasers could shape the future of applications such as high-speed signal processing, integrated circuits, optical communication and supercomputing. However, harnessing its capabilities hinges on a clear understanding of the physics of the device, and data the transistor laser generated did not fit neatly within established circuit laws governing electrical currents.

"We were puzzled," said Feng. "How did that work? Is it violating Kirchhoff's law? How can the law accommodate a further output signal, a photon or optical signal?"

Kirchhoff's current law, described by Gustav Kirchhoff in 1845, states charge input at a node is equal to the charge output. On a basic bipolar transistor, with ports for electrical input and output, the law is straightforward but the transistor laser adds a third port for optical output.

This posed a conundrum for researchers working with the laser: how were they to apply the laws of conservation of charge and conservation of energy with two forms of energy output?

"The optical signal is connected and related to the electrical signals, but until now it's been dismissed in a transistor," said Holonyak. "Kirchhoff's law takes care of balancing the charge, but it doesn't take care of balancing the energies. The question is: how do you put it all together, and represent it in circuit language?"

The unique properties of the transistor laser required Holonyak, Feng and graduate student Han Wui to re-examine and modify the law to account for photons as well as electrons, effectively expanding it from a current law to a current-energy law.

"The previous law had to do with the particles – electrons coming out at a given point. But it was never about energy conservation as it was normally known and used," commented Feng. "This is the first time we see how energy is involved in the conservation process."

Simulations based on the modified law fit data collected from the transistor laser, allowing researchers to predict the bandwidth, speed and other properties for integrated circuits. "This fits so well, it's amazing," said Feng. "The microwave transistor laser model is very accurate for predicting frequency-dependent electrical and optical properties. The experimental data are very convincing."

With accurate simulations, the team can continue exploring applications in integrated circuits and supercomputing. In their current paper, the researchers used their model to simulate a directly modulated transistor laser up to 40 Gb/s to represent how the device could be employed in an optical communication link.