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A team of researchers including engineers from the University of Michigan have showed that they can control peaks within laser pulses and also twist the light. They demonstrated, extremely short, configurable "femtosecond" pulses of light that can lead to future computers that run up to 100,000 times faster than today's electronics. The method moves electrons faster and more efficiently than electrical currents and with reliable effects on their quantum states. It is being considered as a step toward "lightwave electronics" and, in the more distant future, quantum computing.
Electrons moving through a semiconductor in a computer, for instance, occasionally run into other electrons, releasing energy in the form of heat. But a concept called 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, researches have found that the oscillating electric field of ultrashort laser pulses can actually move electrons back and forth in solids. But first, researchers need to be able to control electrons in a semiconductor. This work takes a step toward this capability by mobilizing groups of electrons inside a semiconductor crystal using terahertz radiation - the part of the electromagnetic spectrum between microwaves and infrared light.
The researchers shined 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.
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, researchers would need to be able to control these pulses and the crystal provides a range of tools. Since there are fast oscillations like fingers within a pulse, researchers could move the position of the fingers 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.
The 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. 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 interfere with one another and leave 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.
A paper on the work, titled "Symmetry-controlled temporal structure of high-harmonic carrier fields from a bulk crystal," is published in Nature Photonics. Click here to read the published paper.
