CSIRO researchers reported that metallic nanoparticles can be used as components of computers powered by light rather than electric currents.
The nanoparticles can control and manipulate the flow of light in photonic circuits in computers that should be much more powerful than their electronic counterparts.
Nanoscale optical circuits: controlling light using localized surface plasmon resonances
T.J. Davis, K. C. Vernon, D. E. Gómez
CSIRO Materials Science and Engineering, Normanby Road, Clayton VIC 3168, Australia
We discuss the idea of metallic nanoparticles as elements in optical circuits for controlling and manipulating light at the nanoscale. The nanoparticles act like passive electrical components, controlling the flow of energy and its resonances.
Localized surface plasmon (LSP) resonances are surfacecharge oscillations that are associated with metallic nanoparticles. The oscillation frequencies occur in the infrared and visible regions of the optical spectrum and can be excited by light. Because the LSP oscillations are the result of the collective motion of the conduction electrons in the metal they provide a means by which light can be captured, manipulated and re-emitted. II. METALLIC NANOPARTICLE S AS OPTICAL COMPONENTS
Plasmonic nanoparticles may play an important role as components in optical circuits. A gold nanorod in the shape of a rectangular prism of width 30 nm, length 100 nm and thickness 20 nm has a geometric cross sectional area of 0.003 m2. However, at resonance, its optical cross sectional area is about 0.076 m2 which is some 25 times larger. This means the nanoparticle efficiently captures optics optical energy so that we can think of it as an optical antenna – capturing optical energy and efficiently reradiating it.
Because the LSP is an electrical phenomenon, it is possible to modify the behaviour of the resonances by placing two or more nanoparticles in close proximity . Their mutual electric fi elds overlap and influence the resonances in a variety o f ways, sometimes leading to unexpected results. One example is the formation of “ dark modes ”, which are closely related to an effect known as the plasmonic equivalent of electromagnetically-induced transparency that was demonstrated recently . The resonances of this type form part of the class of Fano resonances .
Nanoparticle structures can be used to mimic the behaviour of electrical circuits. One example is the plasmonic equivalent of the Wheatstone bridge circuit used in electrical engineering . The plasmonic circuit is expected to show similar properties as the Wheatstone bridge, being sensitive to optical phase shifts and producing a polarized light signal related to the degree of imbalance in the circuit. Configurations of nanoparticles are analogous to passive electrical components that react in a linear fashion to the applied light fields. Modern electronics relies heavily on nonlinear devices, such as diodes for controlling the frequency content of signals or transistors for providing gain and electrical modulation of signals. Can we create devices that do the same for light? Optical control of light is relatively difficult because light does not interact with itself and optical nonlinearity is usually achieved only with intense light fields. Surface plasmons, on the other hand, are an electrical phenomenon that can interact strongly with electrons in nearby materials, such as excitons in semiconductor nanocrystals . By converting the light energy into electric energy, we open up the possibility of creating devices that can manipulate this energy and re-radiate it as a modified light signal (Fig. 1).
 T. J. Davis, K. C. Vernon, and D. E. Gomez, Phys. Rev. B79, 155423 (2009)
 N. Liu, L. Langguth, T. Weiss, J. Kästel, M. Fleischhauer, T. Pfau, and H. Giessen, Nature Materials 8, 758-762 (2009)
 T. J. Davis, D. E. Gomez, and K. C. Vernon, Nano Lett. 10, 2618-2625 (2010)
 T. J. Davis, K. C. Vernon, and D. E. Gomez, J. Appl. Phys. 106, 043502 (2009).
 D. E. Gómez, K. C. Vernon, P. Mulvaney, T. J. Davis, Nano Lett. 10, 274-278 (2010).
Tim Davis, firstname.lastname@example.org