Nano-electromagnets interacting through the magnetic field of light

A team of AMOLF scientists at the Center for Nanophotonics has for the first time demonstrated energy transfer between nano-electromagnets, mediated by the magnetic field of light.

Their result is a breakthrough in the quest for magnetic `metamaterials’: artificial optical materials that can bend light rays in arbitrary ways. These materials are expected to enable ‘perfect lenses’ and even ‘invisibility cloaks’. AMOLF scientists Ivana Sersic, Martin Frimmer, Ewold Verhagen and Femius Koenderink published their results in Physical Review Letters.

We all know that magnets are dipoles: they have a northpole and a southpole with the property that like poles repel, and opposite poles attract each other. Similarly we know that bar magnets can be oriented by magnetic fields, similar to the alignment of compass needles. In this manner, we can manipulate magnetic fields using magnets, and conversely exert forces on magnets using magnetic fields. This intuition works very well indeed for slowly varying fields, but loses its validity when magnetic fields oscillate rapidly.

Light is an electromagnetic wave that carries an electric field and a magnetic field, both oscillating at very high frequencies (500 THz). In principle you can control light both by controlling the electric component of light, or by engineering the magnetic component of light. However, at the very high frequencies of light, atoms hardly react to magnetic fields. As a consequence, our usual optical materials only manipulate the electric field of light.

The artificial materials studied by the authors consist of very small U-shaped ‘nanorings’ made from gold. The magnetic field of light drives charges back and forth through the ring, causing an AC current in each nano-loop. The small opening on the top side of the ring ensures a resonance for current oscillations at optical frequency. Each ring is a small but very strong electromagnet, of which the north and south pole reverse every optical cycle.

By measuring how much light is transmitted by dense lattices of these electro-magnets, the researchers demonstrated that the electro-magnets are not only driven by the incident light.  As in the case of household bar magnets, the nano-rings also influence each other. The magnetic field generated by the oscillating current in one nanoring directly drives the current in nanorings around it. The researchers showed for the first time that the interaction with the magnetic field of light of these new materials is very strong:  just as strong as the interaction with the electric field of light that is obtained with the best ‘classical’ optical materials. These new findings will allow the researcher much more control over the design of metamaterials to control the flow of light.

Reference
Electric and magnetic dipole coupling in near-infrared split-ring metamaterial arrays,
I. Sersic, M. Frimmer, E. Verhagen en A. F. Koenderink.
Phys. Rev. Lett. 103, 213902 (2009)

Figure: (Left) Magnets are dipoles with north and south poles: like poles repel,while opposite poles attract each other. (Center) When and oscillating current is induced in metal U-shaped rings, each ring acts as a nanoscale electromagnet. The poles are on either side in front of and behind the ring, reversing 500 billion times a second. (Right) The researchers measured the interaction between the U-shaped electromagnets arranged in periodic lattices, which is mediated by the magnetic field of light. The rings in the electron microscopy image are 200x200 nm in size, with a 80 nm gap (~ 1000 times smaller than the thickness of a hair).