Down-converting optical pulses to femtosecond electronic signals

June 27, 2018 // By Julien Happich
A team headed by Alexander Holleitner and Reinhard Kienberger, Physics professors at the Technical University of Munich (TUM), has succeeded for the first time in generating ultrashort electric pulses on a chip using metal antennas only a few nanometers in size, then running the signals a few millimeters above the surface before reading them out.

Scanning electron microscope (SEM) image of the
comb-shaped Ti/Au contacts with the asymmetric
nanojunctions. The emitter electrodes (top) feature
small triangularly-shaped indents whereas the
collector side (bottom) has smooth linear electrodes.
Scale bar is 2µm. The close-up inset shows the
asymmetry between the emitter and collector.
Scale bar is 200nm.

Conceptual illustration of the asymmetric plasmonic
antennas in operation.

Publishing their results in Nature Communications with a paper titled "Towards femtosecond on-chip electronics based on plasmonic hot electron nano-emitters", the researchers explain how they were able to leverage 1.5µm near infrared 270THz optical pulses from a telecom-compatible Er:fiber laser to "pump" and excite nanoscale asymmetric plasmonic antennas, yielding a field emission of ballistic hot electrons across the junctions, akin to electronic pulses in the THz range.

SEM image of asymmetric nanojunctions with
emitter (‘E’) and collector (‘C’) electrodes.
Inset: numerically computed field enhancement g 
within the asymmetric nanojunction for E pump = 1.3 eV.

These pulses then propagated as electronic signals in millimetre-scale strip lines arranged on the chip (gold electrodes laid on sapphire). In effect, the asymmetric nanoscale metal junctions, each consisting of a triangular-shaped emitter electrode and a plane collector electrode separated by a 90nm vacuum gap, behave like ultra-fast photo-switches that operate a plasmonically enhanced, multiphoton absorption, observed the researchers. This multiphoton absorption triggers the non-linear photoemission of electrons, from the emitter to the collector, generating THz current pulses flowing along the metal strips.

The authors anticipate that such asymmetric plasmonic antennas could be integrated in wafer-scale THz circuits for femtosecond on-chip clock and synchronization dynamics up to 10THz.

The experiments were funded by the European Research Council (ERC) as part of the "NanoREAL" project and the DFG Cluster of Excellence "Nanosystems Initiative Munich" (NIM).

Technical University of Munich -

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