On-chip molecular electronic plasmon sources

19 Oct 2016. NUS scientists have developed on-chip molecular electronic plasmon sources without using optical elements for ultra-high speed electronics.

A research team led by Prof Christian NIJHUIS from the Department of Chemistry, NUS comprising researchers from the Institute of Materials Research and Engineering and the Institute of High Performance Computing, both research institutes under the Agency for Science, Technology and Research (A*STAR), has developed a new type of on-chip plasmon source. This device is developed using tunnel junctions which have a layer of molecules sandwiched between two electrodes. The current flow across these junctions is controlled by the molecular structure and occurs on time-scales of 10-15 seconds. This current excites plasmons which are seen as light flashes trapped in the form of collective oscillations of electrons at the electrode-dielectric interface. The team has demonstrated full molecular electronic control over the plasmon launching, frequency, and bias-selectivity. At a frequency of 200-300 THz, these plasmon sources can be used potentially in ultra-high speed electronics.

The clock rate of microprocessors, currently at around 3GHz, is limited by the speed of information flow through the electronic circuits. This has experienced minimal increase in recent years. Photonic elements which can carry information at the frequency of light have huge bandwidths but these elements are physically large in size and cannot be easily integrated with conventional complementary metal-oxide-semiconductor (CMOS) processing technology.

Plasmonics combine the benefits of photonics and nano-electronics and could potentially provide a technological breakthrough. Most approaches for on-chip plasmon sources shrink conventional light sources, such as LEDs, to excite plasmons indirectly. These are usually large and complicated devices. The approach by the research team converts electrical signals directly into plasmonic signals at molecular length scales. Their plasmon sources which operate at frequencies of hundreds of THz could pave the way for future nanoscale optoelectronic information processing and computing applications. Their results also open up ways to excite and control plasmons at the molecular level.

Following this proof-of-concept study, the researchers plan to further downscale their device to single molecule junctions and connect the plasmon sources to other circuit components (e.g., plasmonic waveguides, on-chip plasmon detectors, etc.) to build plasmonic-electronic circuitry. They are also exploring new molecular architectures to improve molecular electronic control over the plasmons.

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Figure shows an artist’s impression of a molecular electronic plasmon source. The molecular tunnel junction developed composed of a molecular layer sandwiched between two metal electrodes. When bias was applied to the junction, electrons tunnel through the tunnel barrier and excite surface plasmons which propagate along the gold bottom electrode. Polarisation orientation of the plasmon emission was controlled by the tilt angle of the molecule. [Image credit: TAO Wang.]



Du W; Wang T; Chu HS; Wu L; Liu RR; Sun S; Phua WK; Wang LJ; Tomczak N*; Nijhuis CA*, "On-chip molecular electronic plasmon sources based on self-assembled monolayer tunnel junctions", NATURE PHOTONICS Volume: 10 Issue: 4 Pages: 274-280 DOI: 10.1038/NPHOTON.2016.43 Published: 2016.