Mode Matching for Optical Antennas

Sketch of an optimized optical antenna: A cavity is located inside; the electrical fields during operation are coded by the colour scale. (Source: T. Feichtner)

Without external inter­vention photon emission is inefficient and undirected. If it was possible to influence the process of photon creation funda­mentally in terms of efficiency and emission direction, new technical possi­bilities would be opened up such as tiny, multi­functional light pixels that could be used to build three-dimen­sional displays or reliable single-photon sources for quantum computers or optical micro­scopes to map indi­vidual molecules. Nanometre-sized optical antennas are capable of sending photons in a specific direction with high efficiency. The dimensions and structural details of such antennas can be controlled precisely at a size of around 250 nano­meter.

The form of these optical antennas has previously been inspired by established models from radio communi­cation and radio tech­nology. The antennas used there are usually made of specially shaped metal wires and metal rod arrays due to the wave­lengths in the centimeter range. It is in fact possible to construct antennas for light waves using metal nanorods to influence the creation and propa­gation of photons, but the analogy between radio waves and light waves is limited. While macro­scopic radio antennas have a high-frequency generator connected to the antenna via cable, the link at the nanometer scale of a light wave length has to be contactless. But atoms and molecules that act as photon sources do not feature connecting cables to hook them up to an optical antenna.

It is this major difference, combined with a number of other problems that are due to the high frequency of light, that has made it impossible so far to produce and subse­quently control photons with optical antennas in a satis­factory manner. Physicists from Julius-Maxi­milians-Uni­versity Würzburg in Bavaria, Germany, have now solved this problem and established a set of rules for opti­mized optical antennas. The new rules could help build antennas for light so that both the photons’ birth and their subsequent propa­gation can be controlled precisely, at least theore­tically, according to Thorsten Feichtner, a researcher at JMU’s Institute of Physics.

“The idea behind this is based on the principle of simi­larity,” the physicist explains. “What’s new in our research is that the currents of the free electrons in the antenna have to fulfil two simi­larity conditions at the same time. Firstly, the current pattern in the antenna must be similar to the field lines in the direct vici­nity of a light-emitting atom or molecule. Secondly, the current pattern must also match the homo­geneous electrical field of a plane wave as best as possible so that each photon can reach a distant receiver.” The novel antennas for light built with the help of these new rules extract far more photons from an emitter than previous antenna types derived from radio tech­nology. (Source: JMU)

Reference: T. Feichtner et al.: Mode Matching for Optical Antennas, Phys. Rev. Lett. 119, 217401 (2017); DOI: 10.1103/PhysRevLett.119.217401

Link: Nano-Optics (B. Hecht), Dept. of Experimental Physics 5, Univ. of Würzburg, Würzburg, Germany

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