Quantum tunneling between two plasmonic resonators links nonlinear quantum optics at 245 terahertz

Singapore researchers have successfully designed and fabricated electrical circuits that can operate at hundreds of terahertz frequencies, which is tens of thousands times faster than today’s state-of-the-art microprocessors.

This novel invention uses a new physical process called ‘quantum plasmonic tunnelling’. By changing the molecules in the molecular electronic device, the frequency of the circuits can be altered in hundreds of terahertz regime. The new circuits can potentially be used to construct ultra-fast computers or single molecule detectors in the future, and open up new possibilities in nano-electronic devices.

A focused electron beam (in yellow) was used to characterise the structures and to probe the optical properties of two plasmonic resonators bridged by a layer of molecules with a length of 0.5 nm. (Image credit: Tan Shu Fen, National University of Singapore)

Science – Quantum Plasmon Resonances Controlled by Molecular Tunnel Junctions

Abstract

Quantum tunneling between two plasmonic resonators links nonlinear quantum optics with terahertz nanoelectronics. We describe the direct observation of and control over quantum plasmon resonances at length scales in the range 0.4 to 1.3 nanometers across molecular tunnel junctions made of two plasmonic resonators bridged by self-assembled monolayers (SAMs). The tunnel barrier width and height are controlled by the properties of the molecules. Using electron energy-loss spectroscopy, we directly observe a plasmon mode, the tunneling charge transfer plasmon, whose frequency (ranging from 140 to 245 terahertz) is dependent on the molecules bridging the gaps.

Editors Summary – Controlling Quantum Plasmonics

Electron tunneling across cavities could potentially induce a quantum mechanical plasmon mode that would be important in nano-electronics, catalysis, nonlinear optics, or single-molecule sensing, but has been expected to occur only at length scales beyond the reach of current state-of-the-art technology. Using a system of plasmonic dimers comprising silver nanocubes bridged by a molecular self-assembled monolayer, Tan et al observed quantum plasmonic tunneling between the resonators and were able to tune the frequency of this tunneling plasmon resonance via selection of the molecular tunnel junctions. Moreover, the effects were observed at length scales that are technologically accessible

Research Results

In this landmark study, the research team demonstrated that quantum-plasmonics is possible at length scales that are useful for real applications. Researchers successfully fabricated an element of a molecular electronic circuit using two plasmonic resonators, which are structures that can capture light in the form of plasmons, bridged by a layer of molecules that is exactly one molecule thick. The layer of molecules switches on the quantum plasmonic tunneling effects, enabling the circuits to operate at terahertz frequencies.

Dr Bosman used an advanced electron microscopy technique to visualise and measure the opto-electronic properties of these structures with nanometer resolution. The measurements revealed the existence of the quantum plasmon mode and that its speed could be controlled by varying the molecular properties of the devices.

By performing quantum-corrected simulations, Dr Bai confirmed that the quantum plasmonic properties could be controlled in the molecular electronic devices at frequencies 10,000 times faster than current processors.

Explaining the significance of the findings, Asst Prof Nijhuis said, “We are very excited by the new findings. Our team is the first to observe the quantum plasmonic tunneling effects directly. This is also the first time that a research team has demonstrated theoretically and experimentally that very fast-switching at optical frequencies are indeed possible in molecular electronic devices.”

The results open up possible new design routes for plasmonic-electronics that combines nano-electronics with the fast operating speed of optics.

31 pages of supplemental material

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