plasmonic circuits

	Plasmonic circuits - the optical Wheatstone bridge

When metal nanoparticles supporting localized surface plasmons (LSPs) come close to one another, the electric fields from the LSPs interact, modifying the resonances and phases. The resonances in metal nanoparticles are electrical in nature and can be modelled using electric circuit concepts [1, 2]. The metal rod looks like an inductor with resistance (since charge can flow up and down it like an electric current) and the surface charges at either end of the rod couple through their electric fields, and therefore look like capacitance. The resonating rod then looks like an LC circuit. Coupling between the pairs of nanorods is through the electric fields, and therefore is capacitive.

The key point is that the electric currents and charges oscillate at extremely high frequencies - the frequency of light. This suggests that configurations of metal nanoparticles can be used as circuit elements in extremely high speed electronic circuits.

An example of a circuit is the Wheatstone bridge. This is used with direct currents for finding an unknown resistance. Variations on the circuit are used in sensors where an AC voltage developed between the two arms of the bridge can be zeroed and then small changes in the circuit components (such as due to temperature or pressure) produce large changes in the voltage. This is an example of a null circuit which can be very sensitive.

We have devised a plasmonic equivalent of the Wheatstone bridge [3]. This can be analysed using our theory of coupling of plasmonic particles which provides a method for designing plasmonic circuits. This circuit can be made very much smaller than the wavelength of light (below 100 nm in width) but is capable of measuring the phase differences in a light beam.

This concept opens up the field of optical circuits based on LSPs. In effect, we are developing ways of controlling light at the nanoscale using electronic engineering concepts.

Above - a subwavelength array of plasmonic circuits that, together, form a plasmonic metamaterial. Below - the Wheatstone bridge circuit and the plasmonic equivalent.

References

[1] N. Engheta, A. Salandrino, A. Alu: "Circuit Elements at Optical Frequencies: Nanoinductors, Nanocapacitors, and Nanoresistors" Physical Review Letters 95, 95504 (2005); [2] T. J. Davis: "Modeling and fabrication of tuned circuits for optical meta-materials" Proc. of SPIE Vol. 6038 (2006); [3] T. J. Davis, K. C. Vernon, D. E. Gomez: "A plasmonic "ac Wheatstone bridge" circuit for high-sensitivity phase measurement and single-molecule detection" Journal of Applied Physics 106, 043502 (2009)