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1/14/14

 

Optical antennas heat up

In recent years scientists have been taking advantage of the collective sloshing of electrons in metals (called plasmons) to build nanoscale optical antennas and manipulate light at length scales that are small compared to its wavelength in free space. These plasmons do come with a cost, however. Just as the sloshing motion of water in a bathtub eventually damps out, leaving the water slightly warmer, plasmon excitations can dissipate, warming up the metal structure. This kind of heating can be useful, as in photothermal medical treatments based on nanoparticles, or it can be an inconvenience; experimentally, it is generally quite difficult to measure the temperature changes that take place due to plasmons.

Led by postdoc Joseph Herzog (now at the University of Arkansas), the Natelson group has succeeded in determining the plasmonic contribution to heating when near-infrared light is shined on an optical antenna structure. The electrical resistance of the antenna itself is used as a thermometer, and the plasmon contribution to heating can be isolated because the plasmon oscillations in that structure are preferentially excited by light of a particular polarization. These measurements and associated computational modeling show how critically important the metal electrodes are for carrying away the optically-generated heat, and provide insights relevant for the control and engineering of such optical heating effects.

J. B. Herzog, M. W. Knight, and D. Natelson, “Thermoplasmonics: Quantifying plasmonic heating in single nanowires”, Nano Lett. (in press, 2014). http://dx.doi.org/10.1021/nl403510u

 

 Optical Antenna 2

 Left: An electron micrograph of a nanowire optical antennal. Center: Calculated temperature increase due to plasmonic heating when the nanowire is illuminated with near-infrared light polarized transverse to the wire. Right: The polarization dependence of the heating effect, showing that the heating is greatest when the light is polarized transverse to the wire (up/down in the plot and the images), and that the plasmon resonance nearly vanishes when the wire is made considerably narrower.