Plasmonic nanocircuits enable optics on a microchip

(Nanowerk Spotlight) Researchers from Friedrich-Alexander University Erlangen-Nuremberg (FAU), the Max Planck Institute for the Science of Light (MPL), and California Institute of Technology (Caltech), 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.
This research is published in the current issue of Nano Letters (" Functional Plasmonic Nanocircuits with Low Insertion and Propagation Losses"; open access).
 plasmonic nanocircuits operating as subdiffraction directional couplers
The researchers 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 x 10 µm2), highly dispersive directional couplers, which enable 30 dB discrimination over Δλ = 200 nm with only 0.3 dB device loss. (©American Chemical Society)
Circuits on micro chips today process data with electrons, running on small wires as close as few tens of nanometers 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 microprocessors, which also makes the large fans in your computer necessary. IBM, Intel and many international research groups are therefore intensely researching for an alternative for several years now. They are working to implement optical communication instead of copper wires not only for the communication between computers, but also within computer components and even within the chips1,2.
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 focusing 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 problem3. Unfortunately, plasmonics comes with a new downside: Loss of power and conversion to heat. Light first experiences loss when it is funneled into the circuit, again when it is guided through nano-scale waveguides.
The researchers 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 micrometers, strongly limiting the applicability of plasmonics, in case of high confinement that is necessary for tight integration.
The researchers 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. They 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 the researchers lay the foundations for future applications where optical signals may be modulated electronically in such nanoscale small circuits5 or where these currents of photons might even interact, light switching light6. Even direct plasmon-sources and detectors7 are currently in development. Combined with the results, recently published by other researchers it may also become possible to build quantum computers on a plasmonic-photonic chip8 based on this plasmonic circuitry platform.
1. N. Engheta, "Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials", Science, vol. 317, pp. 1698-1702, 2007.
2. D. Miller, "Device Requirements for Optical Interconnects to Silicon Chips", Proceedings of the IEEE, vol. 97, pp. 1166-1185, 2009.
3. M.L. Brongersma, and V.M. Shalaev, "The Case for Plasmonics", Science, vol. 328, pp. 440-441, 2010.
4. L. Novotny, and N. van Hulst, "Antennas for light", Nature Photonics, vol. 5, pp. 83-90, 2011.
5. J.A. Dionne, K. Diest, L.A. Sweatlock, and H.A. Atwater, "PlasMOStor: A Metal-Oxide-Si Field Effect Plasmonic Modulator", Nano Letters, vol. 9, pp. 897-902, 2009.
6. D. Powell, "Light flips transistor switch", Nature, vol. 498, pp. 149-149, 2013.
7. D. Ly-Gagnon, K.C. Balram, J.S. White, P. Wahl, M.L. Brongersma, and D.A. Miller, "Routing and photodetection in subwavelength plasmonic slot waveguides", Nanophotonics, vol. 1, 2012.
8. R.W. Heeres, L.P. Kouwenhoven, and V. Zwiller, "Quantum interference in plasmonic circuits", Nature Nanotechnology, 2013.
By Arian Kriesch, [email protected], Tel. +49 9131 8520343, Particle Center Cluster of Excellence EAM, University of Erlangen

Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on