Analytical and Experimental Investigations of Electromagnetic Field Enhancement Among Nanospheres With Varying Spacing

[+] Author and Article Information
Li-Hsin Han, Wei Wang, John Howell

Department of Mechanical Engineering, University of Texas at Austin, Austin, TX 78712

Yalin Lu, R. J. Knize

Physics Department, Laser Optics Research Center, USAF Academy, Colorado Springs, CO 80840

Kitt Reinhardt

 AFOSR/NE, 875 North Randolph Street, Suite 326, Arlington, VA 22203

Shaochen Chen1

Department of Mechanical Engineering, University of Texas at Austin, Austin, TX 78712scchen@mail.utexas.edu


Corresponding author.

J. Heat Transfer 131(3), 033110 (Jan 23, 2009) (6 pages) doi:10.1115/1.3056574 History: Received July 11, 2008; Revised October 07, 2008; Published January 23, 2009

A modified Mie scattering theory was used to calculate the enhancement of electromagnetic (EM) field between gold nanospheres. The simulation result showed that the density of EM-energy in the space between neighboring nanospheres increases drastically as the interparticle space decreases. Simulated absorption-spectra also showed a peak-shifting from the visible to the infrared region when decreasing the nanosphere spacing. We used our previous experiment to verify the analytical results; the experiment was conducted by using a photodeformable microshell, which was coated with gold nanospheres. Made of photoshrinkable azobenzene polyelectrolytes, the microshells supported the gold nanospheres and gave the tunability of the interparticle spacing among the nanospheres. Upon irradiation of ultraviolet light, the microshells shrank and reduced the interparticle space. The absorption-spectra of the gradually shrinking microshells showed significant changes; a peak-broadening from the visible to the near-infrared region and a drastically enhanced water-absorption were observed in the experimental spectra. The experimental results confirmed the analytical analysis based on the modified scattering theory.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

A coordinate-system for the multiple-spheres. (Rji,θji,ϕji) denotes the central point of sphere i with respect to the center of sphere j.

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Figure 2

The simulated enhancement of energy density between two close spheres in vacuum. (a)–(c) illustrate the intensity ratio of the enhanced field to the incident light; the wavelength is 700 nm in these plots and the interparticle spaces in the plots are (a) 10 nm, (b) 5 nm, and (c) 1 nm. (d) shows the spectra of field-enhancement at the center of different gaps, from 10 nm to 0.1 nm.

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Figure 3

The simulated absorption-spectra of the bispherical system of different gaps. The medium is vacuum. The absorption spectrum of a single particle is included for comparison.

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Figure 4

(a) The spectra of water at the center of different particle gaps. Note the rising peak at 970 nm, which indicates the enhancement of water-absorption. This change was observed in our experiment. (b) A logarithmic plot of (a).

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Figure 5

The photodeformable microshell(s). (a) The SEM picture of one microshell before the removal of its silica core. The inset shows a close look at the surface nanospheres. (b) The SEM picture of a dried, collapsed microshell. (c) A nonirradiated microshell in water, under an optical microscope. (d) Microshells after the irradiation by UV light. The shell-diameters reduced about 35%.

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Figure 6

(a) The experimental spectra of the microshells under different stages of irradiation, from 0 min to 14 min. The triangle indicates the raising peak of water-absorption at 970 nm. The broadening of absorption-peak at 560 nm indicates the change in particle-interaction during the shell-shrinking, as shown by the simulation. (b) The spectrum of diluted 40 nm gold nanospheres in water. This figure is used to compare with the spectra of (a).



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