Fast Switching with Tailored Light Waves

Illustration of how electrons can be imagined to move between two arms of a metallic nanoantenna, driven by a single-cycle light wave. (Source: U. Konstanz)

Contemporary electronic components, which are traditionally based on silicon semi­conductor tech­nology, can be switched on or off within picoseconds. Standard mobile phones and computers work at maximum frequencies of several gigahertz while indi­vidual transistors can approach one terahertz. Further increasing the speed at which electronic switching devices can be opened or closed using the standard technology has since proven a challenge. A recent series of experi­ments conducted at the University of Konstanz demons­trates that electrons can be induced to move at sub-femto­second speeds, i.e. faster than 10-15 seconds, by mani­pulating them with tailored light waves.

“This may well be the distant future of electronics”, says Alfred Leitens­torfer, Professor of Ultra­fast Phenomena and Photonics at the University of Konstanz. “Our experi­ments with single-cycle light pulses have taken us well into the attosecond range of electron transport”. Leitens­torfer and his team believe that the future of elec­tronics lies in integrated plasmonic and opto­electronic devices that operate in the single-electron regime at optical – rather than microwave – frequencies. “However, this is very basic research we are talking about here and may take decades to implement”, he cautions.

The challenge for the inter­national team of theoretical and experi­mental physicists from the University of Konstanz, the University of Luxem­bourg, CNRS-Université Paris Sud and the Center for Materials Physics and Donostia Inter­national Physics Center in San Sebastián who colla­borated on this project was to develop an experimental set-up for mani­pulating ultrashort light pulses at femto­second scales below a single oscillation cycle on the one hand, and to create nano­structures suited for high-precision measurements and mani­pulation of electronic charges on the other.

The experi­mental set-up involved nano­scale gold antennae as well as an ultrafast laser capable of emitting one hundred million single-cycle light pulses per second in order to generate a measurable current. The bowtie design of the optical antenna allowed for a sub-wavelength and sub-cycle spatio-temporal concen­tration of the electric field of the laser pulse into the gap of a width of six nm. As a result of the highly nonlinear character of electron tunnelling out of the metal and acce­leration over the gap in the optical field, the researchers were able to switch electronic currents at speeds of approxi­mately 600 atto­seconds.

“This process only occurs at time scales of less than half an oscil­lation period of the electric field of the light pulse”, explains Leitens­torfer – an observation that the project partners in Paris and San Sebastián were able to confirm and map out in detail by means of a time-dependent treatment of the electronic quantum structure coupled to the light field. The study opens up entirely new oppor­tunities for under­standing how light interacts with condensed matter, enabling observation of quantum phenomena at unpre­cedented temporal and spatial scales. Building on the new approach to electron dynamics driven at the nanoscale by optical fields that this study affords, the researchers will move on to inves­tigate electron transport at atomic time and length scales in even more sophis­ticated solid-state devices with pico­meter dimensions. (Source: U. Konstanz)

Reference: M. Ludwig et al.: Sub-femtosecond electron transport in a nanoscale gap, Nat. Phys., online 23 December 2019; DOI: 10.1038/s41567-019-0745-8

Link: Ultrafast Phenomena and Photonics, University of Konstanz, Konstanz, Germany

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