Ultrashort light pulses for 'lightwave' computers

German and US researchers have demonstrated extremely short and configurable pulses of light, in a method that moves electrons faster and more efficiently than electrical currents — with reliable effects on their quantum states.

The researchers showed that they could control the peaks within the laser pulses and also twist the light. Their study has been published in the journal Nature Photonics.

Electrons moving through a semiconductor in a computer occasionally run into other electrons, releasing energy in the form of heat. But the concept of lightwave electronics proposes that electrons could be guided by ultrafast laser pulses. While high speed in a car makes it more likely that a driver will crash into something, high speed for an electron can make the travel time so short that it is statistically unlikely to hit anything.

“In the past few years, we and other groups have found that the oscillating electric field of ultrashort laser pulses can actually move electrons back and forth in solids,” said Professor Rupert Huber from the University of Regensburg, who led the recent experiment. “Everybody was immediately excited because one may be able to exploit this principle to build future computers that work at unprecedented clock rates — 10 to 100,000 times faster than state-of-the-art electronics.”

But first, Professor Rupert Huber and his team needed to be able to control electrons in a semiconductor. This required them to mobilise groups of electrons inside a semiconductor crystal using terahertz radiation — the part of the electromagnetic spectrum between microwaves and infrared light.

The researchers shone laser pulses into a crystal of the semiconductor gallium selenide. These pulses were very short at less than 100 femtoseconds, or 100 quadrillionths of a second. Each pulse popped electrons in the semiconductor into a higher energy level — which meant that they were free to move around — and carried them onward. The different orientations of the semiconductor crystal with respect to the pulses meant that electrons moved in different directions through the crystal — for instance, they could run along atomic bonds or in between them.

“The different energy landscapes can be viewed as a flat and straight street for electrons in one crystal direction, but for others, it may look more like an inclined plane to the side,” said Fabian Langer, a doctoral student at Regensburg. “This means that the electrons may no longer move in the direction of the laser field but perform their own motion dictated by the microscopic environment.”

When the electrons emitted light as they came down from the higher energy level, their different journeys were reflected in the pulses. They emitted much shorter pulses than the electromagnetic radiation going in. These bursts of light were just a few femtoseconds long.

Inside a crystal, they are quick enough to take snapshots of other electrons as they move among the atoms, and they could also be used to read and write information to electrons. For that, the researchers would need to be able to control these pulses — and the crystal provided a range of tools.

“There are fast oscillations like fingers within a pulse,” said Professor Mackillo Kira from the University of Michigan, whose group worked with researchers at the Philipp University of Marburg to interpret Huber’s experiment. “We can move the position of the fingers really easily by turning the crystal.”

The crystal could also twist the outgoing light waves or not, depending on its orientation to the incoming laser pulses.

Because femtosecond pulses are fast enough to intercept an electron between being put into an excited state and coming down from that state, they can potentially be used for quantum computations using electrons in excited states as qubits.

“For example, here we managed to launch one electron simultaneously via two excitation pathways, which is not classically possible,” said Professor Kira. “That is the quantum world. In the quantum world, weird things happen.”

An electron is small enough that it behaves like a wave as well as a particle — and when it is in an excited state, its wavelength changes. Because the electron was in two excited states at once, those two waves interfered with one another and left a fingerprint in the femtosecond pulse that the electron emitted.

“This genuine quantum effect could be seen in the femtosecond pulses as new, controllable, oscillation frequencies and directions,” Professor Kira said. “This is of course fundamental physics. With the same ideas you might optimise chemical reactions. You might get new ways of storing information or transmitting information securely through quantum cryptography.”

Professor Huber is particularly interested in stroboscopic slow-motion cameras to reveal some of the fastest processes in nature, such as electrons moving around within atoms.

“Our crystalline solids make for fantastic light sources in this field — with unprecedented possibilities for pulse shaping,” he said.

Image caption: A semiconductor crystal has shown an unprecedented capacity to shape ultrashort laser pulses. Image credit: Fabian Langer, Regensburg University.