Plasmonic nanocircuits enable optics on a chip

Functional Plasmonic Nanocircuits with Low Insertion and Propagation Losses

We experimentally demonstrate plasmonic nanocircuits operating as subdiffraction directional couplers optically excited with high efficiency from free-space using optical Yagi-Uda style antennas at λ0 = 1550 nm. The optical Yagi-Uda style antennas are designed to feed channel plasmon waveguides with high efficiency (45% in coupling, 60% total emission), narrow angular directivity (<40°), and low insertion loss. SPP channel waveguides exhibit propagation lengths as large as 34 μm with adiabatically tuned confinement and are integrated with ultracompact (5 × 10 μm2), highly dispersive directional couplers, which enable 30 dB discrimination over Δλ = 200 nm with only 0.3 dB device loss.

The publication “Functional Plasmonic Nanocircuits with Low Insertion and Propagation Loss” was published in the issue 13 (9), 4539-4545 2013 of the journal Nano Letters (ACS) (download PDF or via doi:10.1021/nl402580c,arxiv:10.1021/nl402580c). It is the result of an international collaboration between a research group at the Max Planck Institute for the Science of Light and theUniversity of Erlangen-Nuremberg(Germany) and a research group at theCalifornia Institute of Technology (USA).

In this paper we demonstrate that it is possible to operate extremely compact optical circuits on the nanoscale, a size scale that makes it compatible and potentially competitive with state-of-the-art electronic microchips, while substantially reducing the limiting factor of heating loss and while strongly increasing the efficiency to funnel infrared laser light into these circuits with a novel design of optical nanoantennas.

Circuits on microchips today process data with electrons, running on small wires as close as few tens of nanometers (1/1000th of a hair diameter) next to each other. Electrons mutually influence their flow  when doing so. The speed by which it is possible to clock these currents of electrons is therefore limited and energy loss leads to excessive heating of the newest micro processors which also makes the large fans in your computer necessary. IBMIntel and many international research institutes are therefore intensely researching for an alternative for several years now. They are working to implement optical communication instead of copper wires not only into the communication between computers, but also within computer components and even within the chips[1,2].

Why optics? Photonic circuits do not suffer the shortcoming of limited packing due to coupling. However, photons obey the laws of electromagnetic radiation where Abbe’s law limits focussing and confinement to a size scale of only micrometers (1000 times a nanometer) and far above the current integration size scale of electronic integrated circuits. Plasmonics enables to scale circuitry for light down to the size scales of electronic nano-circuits which solves this problem[3]. Unfortunately, plasmonics comes with a new downside: Loss of power and conversion to heat. Light first experiences loss when being funneled into the circuit, again when it is guided through nano-scale waveguides.

We have targeted this issue by developing unprecedentedly efficient optical nanoantennas that follow the design principle of Yagi-Uda for radio-antennas and that are resonant for the electromagnetic field of infrared light at the telecommunication wavelength (1550 nm)[4]. These antennas feed a focused laser beam into waveguides. Such plasmonic waveguides usually consume most of the inserted power over a short length of few µm, strongly limiting the applicability of plasmonics, in case of high confinement that is necessary for tight integration. We demonstrate a circuitry scheme where highly confining plasmonic functional units are interconnected with low loss plasmonic waveguides that allow for large-scale connections on the size scale of tens to a hundred micrometers. We demonstrate a useful application of such circuits by implementing so-called directional couplers that allow for wavelength discrimination over a very short length of only a few micrometers where they have been seveal millimeters large in the past.

With this work we lay the foundations for future applications where optical signals will be modulated electronically in such nanoscale small circuits[5] or where these currents of photonics might even interact, light switching light[6] and where there are direct plasmon-sources (recently reported on a conference) and detectors[7] integrated. Combined with the results, recently published by other researchers it may also become possible to build quantum computers on a plasmonic-photonic chip[8] into this plasmonic platform.

This article is available in English and in German.

References

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  8.  R.W. Heeres, L.P. Kouwenhoven, and V. Zwiller, "Quantum interference in plasmonic circuits", Nature Nanotechnology, 2013. http://dx.doi.org/10.1038/nnano.2013.150