Electromagnetic energy transfer and switching in nanoparticle chain arrays below the diffraction limit

Abstract

Electromagnetic energy transfer in plasmon wires consisting of chains of closely spaced metal nanoparticles can occur below the diffraction limit by means of coupled plasmon modes. Coherent propagation with group velocities that exceed 0.1 c is possible in straight wires and around sharp corners ͑bending radius much less than wavelength of visible light͒. Energy transmission through chain networks is possible at high efficiencies and is a strong function of the frequency and polarization direction of the plasmon mode. Although these structures exhibit transmission losses due to heating of about 3 dB/500 nm, they have optical functionality that cannot be obtained in other ways at a length scale Ӷ1 m.The fundamental physical properties of nanometer-size metal particles have been intensively researched for the last hundred years. 1,2 Until recently, most effort has focused on statistically large numbers of particles in disordered arrays. From this work it is well established that at the surface plasmon frequency light strongly interacts with metal particles and excites a collective electron motion, or plasmon. 2 These frequencies are typically in the visible and near-infrared part of the spectrum. 3 In nanometer size particles ͑much smaller than the wavelength of the exciting light, ͒ plasmon excitations produce an oscillating dipole field.Recent developments in particle synthesis and physical characterization of nanostructures have enabled the investigation of the optical properties of single nanoparticles 4 and ordered arrays of closely spaced nanoparticles. In such arrays collective effects and interparticle interactions play an important role. For example, it has been demonstrated that arrays of particles under broad-beam excitation show collective behavior. 5 Furthermore, numerical simulations have shown that electromagnetic ͑EM͒ energy can be transported below the diffraction limit along linear chains of closely spaced metal nanoparticles. 6 This finding could have important consequences for integrated optics which faces the fundamental limitation that, for the guiding, modulation, and amplification of light, structures are needed that have dimensions comparable to the wavelength of light. 7 The EM energy transport along chains of metal nanoparticles relies on the near-field electrodynamic interaction between metal particles that sets up coupled dipole or plasmon modes. 8 This type of coupling is analogous to the process of resonant energy transfer, which is observed in systems that contain closely spaced optically excited atoms, molecules, or semiconductor nanocrystals. [9][10][11][12] In this paper, the dispersion relation for coupled plasmon modes is developed for a linear chain of equally spaced metal particles using an analytical model that describes the near-field EM interaction between the particles in the dipole limit. The model provides estimates for the group velocity v g and the attenuation coefficient ␣ of the allowed coupled plasmon waves. Furthermore, the transport of EM energy through corners a...

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