1 What we expect from weakly dissipating materials at the range of plasmon resonance frequencies B. S. Luk’yanchuk 1 , T. C. Chong 1 , L. P. Shi 1 , M. I. Tribelsky 2 , Z. B. Wang 3 , L. Li 3 , C.-W. Qiu 4 , C. J. R. Sheppard 5 , J. H. Wu 6 1 Data Storage Institute, Agency for Science, Technology and Research, Singapore 2 Moscow State Institute of Radioengineering, Electronics and Automation, Moscow Russia 3 School of Mechanical, Aerospace and Civil Engineering, University of Manchester, UK 4 Department of Electrical and Computer Engineering, National University of Singapore 5 Division of Bioengineering, National University of Singapore 6 School of Mechanical Engineering, Xi’an Jiaotong University, China Abstract - Development of modern materials, including nanoclusters, cluster assembled materials and metamaterials is among the actual challenges for the development of future nanotechnologies. Here we discuss the peculiarities of far-field and near-field light scattering by plasmonic nanoparticles, and possible applications of weakly dissipating materials. Over the last few years many peculiarities of light scattering have been found for nanoparticles in the regime of plasmon resonances. Optical excitation of localized plasmons is accompanied by inverse process - transformation of localized resonant plasmons into scattered light. When radiative damping prevails over the dissipative damping, the effects of anomalous light scattering result in sharp giant optical resonances and complicated near-field structure of the Poynting vector field, see e.g. [1-4]. Here we present peculiarities of far-field and near-field light scattering by plasmonic nanoparticles with weak dissipation and anisotropy. Lord Rayleigh [5] performed the first analysis of light scattering by small scatters and found formulas for a spherical particle and thin cylinder (Rayleigh scattering). Later it was shown that these formulas are certain limits of the exact solution of Maxwell equations, obtained by Mie [6] for spherical particles and of a similar solution for a thin wire [7]. These formulas however contain resonance denominators. For weakly dissipating materials it results in unreasonably large scattering cross-sections at the plasmon resonance frequencies, which is explained by violation of the applicability conditions for the Rayleigh approximation [1-4,8]. The thorough analysis of the exact Mie solution in this case reveals a number of unexpected features of the light scattering, such as giant optical resonances with an inverted hierarchy (the quadrupole resonance occurs more intensively than the dipole, etc.), a complicated near-field structure with vortices, and unusual frequency and size dependencies of the scattered light [1-4,8]. Probably, the most intriguing feature is related to the great sensitivity of the Poynting vector field to small variations in light frequency, see e.g. Fig. 1, where transforms of the field in the vicinity of dipole resonance are shown. With sufficient deviation from the dipole plasmon resonance frequency (or with sufficiently high dissipation) one returns back to the field distribution typical of the Rayleigh scattering [9]. This provides a surprisingly powerful tool for manipulating with energy flows at nanoscales with help of the weakly dissipating materials. An even more fascinating effect was found in the vicinity of quadrupole resonance, where extra high sensitivity of the angular distribution of scattering light can be seen [10, 11]. A very small variation in the incident light frequency changes the scattering diagram from forward scattering to backward scattering, as shown in Fig. 2. Recently, it has been revealed [12] that this effect occurs identical to the well-known Fano resonances in quantum physics [13]. In this case the localized plasmons (polaritons), exited by the incident light in the scattering particle, are equivalent to the quasi-discrete levels in the Fano approach, while the radiative decay of these excitations plays exactly the same role as tunneling from the quasi-discrete levels in the quantum problem. As a result the resonance may have a typical N-shaped line with a local maximum, corresponding to the constructive interference of different eigenmodes and a local minimum, corresponding to the destructive one. In particular, the destructive interference may result in considerable, or even complete suppression of the scattering along any given direction. In the vicinity of the dipole resonance we always get a Lorenzian scattering contour [1], while in the vicinity of the quadrupole resonance an asymmetric Fano resonance profile [12] may be observed. Thus, the famous Fano resonance was, in fact, have been hidden in the exact Mie solution [14]. We also discuss light scattering by a spherical particle with radial anisotropy by extending the Mie theory to the diffraction by an anisotropic sphere, including both electric and magnetic anisotropy ratio [16, 17]. It is shown that radial anisotropy may lead to great modifications in scattering efficiencies and field enhancement, elucidating the importance of anisotropies in control of scattering. Even a small variation in the anisotropy plays an important role in enhancing or suppression of scattering efficiencies, see in Fig. 3 a, b. Authorized licensed use limited to: National University of Singapore. Downloaded on May 16, 2009 at 01:31 from IEEE Xplore. Restrictions apply.