Towards Femtosecond On-chip Electronics

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Plasmonic metal antennas allow localizing and enhancing light on a nanometer scale – and also the generation of ultrafast electric pulses in macroscopic circuits, as NIM physicist Prof Alexander Holleitner and colleagues could now demonstrate. They utilized the effect of electron photoemission to let electric femtosecond-pulses propagate across a millimeter-scale chip.

Figure: Photoemission of electrons in plasmonic metal antennas. Picture: C Karnetzky

The classical theory of electronics describes circuits up to frequencies of about 100 GHz, while the so-called ‘optics’ grasp electromagnetic phenomena from 10 THz upwards. The frequency range in between is sometimes referred to as THz-gap, because it lacks efficient signal-generators, -detectors, and -converters. Another requirement for adequate THz-components is given by the miniaturization of currently build-in transistors in computers with a spatial footprint of only 10 nm. The groups of Professor Alexander Holleitner and Professor Reinhard Kienberger could verify, that nanoscale plasmonic antennas allow to generate electric pulses in a frequency range up to 10 THz and to let them propagate across a chip.  

Electron propagation using plasmonic nano-antennas

The physicists designed nanoscale antennas to be asymmetric, i.e. one side of the plasmonic metal structure is sharper than the other one. In asymmetric antennas, the sharper side emits more electrons than the flat one does such that a unipolar electric current flows across the gap of the antennas.

“All light effects are enhanced at the sharp side, also the effect of photoemission, such that we generate a small electric current. Particularly during the photoemission, a light pulse makes electrons emit from the metal surface into the vacuum.”, first-author Christoph Karnetzky explains. Most importantly, the current only flows when a light pulse excites the plasmonic antennas.

Therefore, the researchers utilized light pulses with a temporal duration of only a few femtoseconds. In the plasmonic antennas, the corresponding electric pulses occurred on an equally short time-scales. The nanoscale antennas then were integrated in much larger, macroscopic THz-circuits, so that each electric femtosecond pulse generated a THz-signal in the macroscopic circuits.

The latter were defined on sapphire chips with a millimeter extension. Corresponding pulses could propagate up to a frequency of 10 THz.  Karnetzky further: “The exciting optical femtosecond light-pulses with a frequency of about 200 THz triggered the nanoscale antennas to generate an ultrashort electric on-chip THz-signal with frequencies up to 10 THz.”

Non-lineaer THz-pulses

Holleitner and his colleagues made an astonishing observation. Both the electric and the THz-pulses depended non-linearly on the excitation power of the utilized lasers. This indicates that the photoemission in the plasmonic antennas happened because of the so-called multi-photon absorption within one light pulse.

With non-linear THz-pulses, it can be envisaged that corresponding THz-circuits allow investigating even faster tunnel-emission processes in the plasmonic antennas and to use them also for a signal-propagation within chip-based THz-circuits. Accordingly, the physicists put effort to utilize a light wavelength of about 1.5 µm, at which the absorption losses in common internet fiber optics is minor.