Sketch of an optimized optical antenna: A cavity is located inside; the electrical fields during operation are coded by the colour scale. Current patterns are represented by green arrows. (Thorsten Feichtner)
Atoms and molecules can be made to emit light particles (photons). Without external intervention, this process is inefficient and undirected. If it were possible to influence the process of photon creation fundamentally in terms of efficiency and emission direction, new technical possibilities would open up, such as tiny multifunctional light pixels that could be used to build 3D displays, reliable single-photon sources for quantum computers or optical microscopes to map individual molecules.
Nanometer-sized “optical antennas” are a well-known approach. They are able to send photos in a specific direction with high efficiency. The idea goes back to Nobel Laureate Richard P. Feynman, who envisioned nanoscale antennas during a speech at the California Institute of Technology in 1959 and was way ahead of his time.
Feynman triggered a rapid development of nanotechnology. This enables building antenna for visible light today. The dimensions and structural details of these antennas can be controlled precisely at a size of around 250 nanometers.
The form of these optical antennas has previously been inspired by established models from radio communication and radio technology. The antennas used are usually made of specially shaped metal wires and metal rod arrays due to the wavelengths in the centimeter range.
It is possible to construct antennas for light waves using metal nanorods to influence the creation and propagation of photons. The analogy between radio waves and light waves is limited.
While macroscopic radio antennas have a high-frequency generator connected to the antenna through a cable. The link at the nanometer scale of light wavelength has to be contactless. But atoms and molecules that act like photon sources don’t feature connecting cables to hook them up to an optical antenna.
This difference, combined with a few other problems due to the high frequency of light that has made it impossible to produce and subsequently control photons with optical antennas in a satisfactory manner.
Physicists from Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany, have solved this problem and established a set of rules for optimized optical antennas.
These new rules could help build antennas for light so that the photons’ birth and their propagation can be controlled precisely, according to Thorsten Feichtner, researcher at JMU’s Institute of Physics in Professor Bert Hecht’s team.
"The idea behind this is based on the principle of similarity," the Würzburg physicist explains. "What's new in our research is that the currents of the free electrons in the antenna have to fulfill 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 antennas for light built with the help of the new rules extract far more photons from an emitter than previous antenna types from radio technology.
A paper on this research was published in Physical Review Letters.
