Tunable Nanoscale Localization of Energy on Plasmon Particle Arrays

We demonstrate experimentally that plasmon resonant nanoparticle chains exhibit a nanoscale localized response to unfocused light that can be controlled by tuning the incident wavelength. Confocal microscopy shows that field concentrates on just a few nanoparticles at either the front or back side of the plasmon chains. Our experiments clearly demonstrate that plasmon chains are nanoscale receivers and concentrators of light analogous to multielement radio wave antennas. This analogy inspires new design rules for many other photonic functionalities, like guiding and redirecting light at the nanoscale. With the advent of novel techniques to structure materials on the nanoscale, the study of noble metal nanostructures for nanoscale photonics has gained tremendous momentum. 1,2 The key property of metal nanostructures is that light is coupled to resonant motion of the free electron plasma in the metal, which allows metal nanostructures to enhance and confine electromagnetic fields. Such plasmonic materials are therefore promising for photonic applications in nonlinear optics,3 for the control of optical signals on subwavelength length scales, 1,2,4-6 but also as biocompatible labels in microscopy7 and as agents for locally enhanced photothermal medical therapies. 8 To achieve the goal of locally enhancing electromagnetic fields in a controlled fashion, researchers have focused on so-called plasmon antennas. 9-11 These antennas operate by the local enhancement of electric field near sharp edges of metals and the strong field that can be generated in narrow gaps between subwavelength metal islands. Chains of coupled resonant metal particles have been proposed by Quinten et al. 4 and Brongersma et al. 5 to allow transport of confined plasmon excitations from particle to particle at nanometer length scales. An intuitive picture is that waveguiding occurs below the diffraction limit and without radiation loss via instantaneous “quasi-electrostatic” near-field dipole-dipole interactions. 5,12,13 Recently, however, several theoretical reports have pointed out that the dispersion relation for such guiding can be phase-matched to the dispersion relation of free photons around the arrays, even for arrays with subwavelength spacings as small as 75 nm.14-17 Around this condition, plasmon nanoparticle chains couple particularly strongly to incident light. Moreover, because interaction between particles occurs via (retarded) far-field contributions, the local response of plasmon chains is expected to be extremely sensitive to the frequency of incident light due to interference effects. 14,18This effect can be used to capture light from an incident plane wave and create strongly localized tunable energy distributions as first proposed by Herna ́ndez et al. 14 In this letter, we use confocal microscopy to image the local distribution of electromagnetic intensity near plasmon chains with subwavelength spacing in response to illumination along the array axis. The response is not spread out evenly over the entire array but is located either at the front few particles or at the backmost particles of the array. The exact distribution can be controlled via the incident wavelength.14 Such a strong local response to far-field radiation on a single site of an array is equivalent to the response of radio wave antennas in which an array of resonant scatterers is used to optimize constructive interference for maximum signal on a single electrically connected element. 19,20Plasmon particle chains are equivalent to such multielement radio wave antennas apart from the source of the resonance of individual buildings blocks. Whereas in the case of RF antennas, the resonance derives its existence entirely from the scatterer size, in the optical case, resonant scattering requires a material resonance. Our work thus demonstrates a new paradigm for nanoantenna design: instead of relying on edges or gaps to create field singularities, one can engineer the interferences in radiative coupling between many resonant antenna elements to efficiently funnel light into a nanoscale