Phys. Rev. Lett. 111, 093901 (2013)
Nonlocal effects in metal optics systems are often negligible. However, with the feature sizes of structures being reduced to very small length scales in the pursuit of quantum effects and stronger electromagnetic-field enhancement, the finite wavelength of electrons can no longer be ignored. Although various models exist for predicting the behaviour on this scale, theoretical treatments of nonlocality and spatial dispersion can be cumbersome even for simple structures. Yu Luo and colleagues from Imperial College London, UK, have now demonstrated that a virtual local structure can be used to represent a nonlocal plasmonic system. They report that this technique accurately accounts for the smearing of induced charges resulting from nonlocality over a wide frequency range. The nonlocal response was reproduced to a high precision by replacing a nonlocal metal with a local metal that has an appropriate dielectric layer on its surface. Comparison of this technique with the well-known hydrodynamic treatment gave good agreement. Both these models exhibit strong effects and reveal the relevance of nonlocality for gap plasmons, coupled spheres and coupled conical tips having gaps of 1 nm, 0.1 nm and 0.5 nm, respectively. The simplicity of this approach may give a deeper understanding of plasmonic phenomena at the nanometre scale and beyond.
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