A Switch for Light-Wave Electronics

Atoms in silicon dioxide are hit by the light wave, causing the electrons around each atom to oscillate. At the end of the cycle the absorbed energy is returned to the light wave (Source: C. Hackenberg)

Atoms in silicon dioxide are hit by the light wave, causing the electrons around each atom to oscillate. At the end of the cycle the absorbed energy is returned to the light wave (Source: C. Hackenberg)

Light waves could in principle be used to drive future transistors. Since the electro­magnetic waves of light oscillate at petahertz frequencies, opto­electronic computers could attain switching rates 100,000 times higher than current digital electronic systems. However, to achieve this goal, we will need a better under­standing of the sub-atomic electron motion induced by the ultra­fast electric field of light. Now a team led by Ferenc Krausz, who holds a Chair in Experimental Physics at LMU and is a Director of the Max Planck Institute for Quantum Optics in Garching, in collaboration with theorists from Tsukuba University in Japan, has used a novel combination of experimental and theo­retical techniques, which for the first time provides direct access to the dynamics of this process.

Electron movements form the basis of electronics, as they facilitate the storage, processing and transfer of information. State-of-the-art electronic circuits have reached their maximum clock rates at some billion switching cycles per second, as any further increase is limited by the heat generated in the process of switching power on and off. The electric field of light changes its direction a trillion times per second and is able to mobilize electrons in solids at this rate. This means that light waves can form the basis for future electronic switching, provided the induced electron motion and its influence on heat accu­mulation is precisely understood. Krausz and his team had already shown that it is possible to mani­pulate the electronic properties of matter at optical frequencies.

As in these earlier experiments, the researchers have now employed extremely intense laser pulses, each lasting for a few femto­seconds to perturb electrons in glass. The light pulse consists of a single oscil­lation of the field, so the electrons are moved left and right only once. The full temporal charac­terization of the light field after trans­mission through the thin glass plate for the first time yields direct insight into the electron dynamics induced by the light pulse in the solid on an attosecond scale.

This measurement technique reveals that electrons react to the incoming light within a few tens of atto­seconds. The duration of the delay in the response in turn determines the amount of energy trans­ferred between light and matter. Since it is possible to measure the energy exchanged within one light cycle for the first time, the parameters of the light-matter interaction can be precisely determined and optimized for ultrafast signal processing. The greater the degree of rever­sibility in the exchange and the smaller the amount of energy left behind in the medium after passage of the light pulse, the more suitable the inter­action becomes for future light field-driven electronics.

To obtain a detailed under­standing of the observed phenomena, and identify the most appropriate set of experi­mental parameters for that purpose, the experiments were backed up by a novel simulation method based on first principles developed at the Center for Compu­tational Sciences at University of Tsukuba. The theorists there used the K computer, currently the fourth fastest super­computer in the world, to compute electron motions within solids with unpre­cedented accuracy.

The researchers succeeded in optimizing the energy con­sumption by carefully tuning the amplitude of the light field. At certain field strengths energy is trans­ferred from the field to the solid during the first half of the pulse cycle and is almost completely re-emitted during in the second half of the oscil­lation period. These findings confirm that a potential switching medium for future light-driven electronics would not overheat. The ‘cool relation­ship’ between glass and light might thus provide an oppor­tunity to drama­tically accelerate electronic signal- and data processing to its ultimate limits. (Source: LMU)

Reference: A. Sommer et al.: Attosecond nonlinear polarization and light–matter energy transfer in solids, Nature, online 23 May 2016; DOI: 10.1038/nature17650

Link: Attosecond Physics (F. Krausz), Max Planck Institute of Quantum Optics, Garching, Germany

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