Coherent destruction technology to unleash semiconductor optical switching speeds

Coherent destruction technology to unleash semiconductor optical switching speeds

PAUL MORTENSEN

Scientists from the Hitachi Cambridge Laboratory and the Microelectronics Research Centre of the Cavendish Laboratory of Cambridge University in the U.K. claim that they have demonstrated--for the first time--the coherent destruction of charge carriers or electrons and holes in a semiconductor using ultrafast light pulses.

The work, according to the research team, establishes coherent destruction as an operating principle for optoelectronic devices and promises that the speed at which optical switches in semiconductors may be able to operate could virtually be unlimited.

"The operating speed of current optoelectronic devices is limited by the time taken for electrons to disappear--usually approximately 1 nanosecond in semiconductors," says research team member Albert Heberle of the Hitachi Cambridge Laboratory. This time can be decreased to several picoseconds in extremely small semiconductor elements or in semiconductor materials that contain a high density of defects, but there are practical limitations to such approaches.

"However, by employing synchronized light pulses, we are able to generate and destroy electrons--an effect known as `coherent destruction`--on the time scale of our laser pulses, currently at approximately 100 femtoseconds," adds Heberle.

The team of researchers began their joint efforts in 1989, focusing on quantum effect devices. Approximately 25 scientists at the two locations are involved in collaborative research projects, which include new structures of quantum devices, fabrication of nanostructures and a range of instrumentation, including femtosecond laser pulse measurements, scanning probe and electron microscopes as well as modeling and simulations using high-performance computers.

Speed limitation

According to the researchers, there is a fundamental limitation to the speed of current semiconductor optical switches and photodetectors. When photons of light interact with a semiconductor, they create mobile charges or electrons and holes. These charges are responsible for the electrical current in a photodetector and for modifying optical properties in an optical switch. The electrons and holes remain within the device for several nanoseconds.

Because light pulses up to a million times shorter can be generated by femtosecond lasers, such "slow" semiconductor devices do not use the full potential bandwidth of optical communications.

One approach to faster-operating semiconductors has been to minimize the size of the semiconductor element, so that electrons and holes can quickly escape to the contacts, thereby leaving the device ready to accept a new pulse. But the speed is limited to several picoseconds.

A second approach concerns the production of a semiconductor material that contains a high density of defects. When electrons and holes are generated by a light pulse falling on this material, they are rapidly trapped by the defects, resetting the device for the next pulse. Again, the speed is limited to 1 picosecond.

In addition, the ultimate speed of such devices is limited by factors such as fabrication techniques, the maximum electric field that can be sustained in the material and the maximum density of defects that can be produced without diminishing the semiconducting properties of the device.

Destruction demonstration

To demonstrate coherent destruction, the team uses phase-locked pairs of 100-femtosecond infrared pulses. The first pulse creates electrons and holes, and the second is timed to destroy them again. The success of this approach depends on accurately synchronizing the two pulses. The two synchronized pulses are created by splitting the light beam from an ultrafast pulsed laser and are then sent to the semiconductor along different paths. The arrival times are controlled by varying the length of the path traveled by each pulse. The two paths are stabilized to an accuracy of a few nanometers by using an arrangement of piezoelectric devices.

In the first experiment, 70% of electrons and holes were destroyed within 200 femtoseconds. Further increases in the speed and in the fraction of carriers destroyed are expected in modified experiments now underway. Presently, coherent destruction is observed at liquid helium temperatures (-269°C) in high-quality gallium arsenide quantum-well samples. High quality and low temperature reduce the scattering of electrons and holes, which cause loss of coherence. Future work is aimed at reducing these operating constraints. For example, the team plans to reduce scattering further by using nanostructure devices such as quantum dots.

The work on coherent destruction is still in the initial demonstration stage. Significant technical barriers have yet to be overcome before a practical room-temperature device can be realized, notes team leader Jeremy Allam. Present results, though, suggest that a new generation of coherent optically controlled semiconductor devices is possible. q

Paul Mortensen writes from Japan and Australia.

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