Evolutionary Optimization of Optical Antennas

What is it about?

Optical antennas do not work exactly like the well-known radio-frequency antennas. Metal behaves different for electromagnetic waves at visible frequencies and plasmons (coupled oscillations of light and electrons) emerge. Here, to find the optimal geometry of an optical antenna, an evolutionary algorithm was devised, which represents a planar structure by an array of cubes. The resulting antenna geometry can be reduced to a simple shape. This is different from radio-frequency designs, which it surpasses. It is a combination of two fundamental electric dipole resonators and one fundamental magnetic split-ring resonator, called the split-ring antenna.

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Why is it important?

Evolution can solve problems, where a fundamental understanding is not yet achieved. In this work it was not only used to find a good solution for the problem of nano-focusing by means of optical antennas, but also to learn something about the fundamental physical principles. The final geometry is better as both its constituents (two-rod dipole and split-ring) and can be used for e.g. surface enhanced Raman scattering. The method itself is very flexible. Unpublished results show e.g. a 90° rotation of the polarization in the focus of the antenna while keeping a very strong field enhancement.