Tiny nano-electromagnets turn a cloak of invisibility into a possibility

A team of researchers at the FOM institute AMOLF has succeeded for the first time in powering an energy transfer between nano-electromagnets with the magnetic field of light.

This breakthrough is of major importance in the quest for magnetic ‘meta-materials’ with which light rays can be deflected in every possible direction. This could make it possible to produce perfect lenses and, in the fullness of time, even ‘invisibility cloaks’.

The artificial ‘meta-materials’ studied by the researchers consist of very small U-shaped metal ‘nano-rings’. The electromagnetic field of light drives charges back and forth, thereby inducing an alternating current in each U shape. The tiny opening at the top of the ring makes sure that the current zooms around at the frequencies of light. In this way, each ring becomes a small but strong electromagnet, with its north and south poles alternating 500 billion times per second.

Physics Review Letters: Electric and Magnetic Dipole Coupling in Near-Infrared Split-Ring Metamaterial Arrays

We present experimental observations of strong electric and magnetic interactions between split ring resonators (SRRs) in metamaterials. We fabricated near-infrared planar metamaterials with different inter-SRR spacings along different directions. Our transmission measurements show blueshifts and redshifts of the magnetic resonance, depending on SRR orientation relative to the lattice. The shifts agree well with simultaneous magnetic and electric near-field dipole coupling. We also find large broadening of the resonance, accompanied by a decrease in effective cross section per SRR with increasing density due to superradiant scattering. Our data shed new light on Lorentz-Lorenz approaches to metamaterials.

4 page pdf on arxiv

In conclusion, we have measured large resonance shifts as a function of density in SRR arrays resonant at  = 1.4 μm. These shifts are due to strong near-field electrostatic and magnetostatic dipole coupling. Furthermore, we observe electrodynamic superradiant damping that causes resonance broadening and an effective reduction of the extinction cross section per SRR. Since the data show that the response of SRR arrays is not simply given by the product of the density
and polarizability of single constituents, we conclude that a Lorentz-Lorenz analysis to explain effective media parameters of metamaterials ‘atomistically’ is not valid. The fact that the Lorentz-Lorenz picture is invalid has important repercussions: It calls for a shift away from the paradigm that the highest polarizability per constituent is required to obtain the strongest electric or magnetic response from arrays of electric or magnetic scatterers. Our experiments show that increasing the density of highly polarizable constituents to raise the effective medium response is ineffective, since superradiant damping limits the achievable response. To strengthen  or μ, we propose that one ideally finds constituents that have both a smaller footprint and a smaller polarizability per constituent. We stress that even if constituent coupling modifies  and μ,
we do not call into question reported effectivemediumparameters or the conceptual validity thereof per se. The effective medium regime only breaks down when constituent coupling is so strong that collective modes of differently shaped macroscopic objects carved from the same SRR array have very different resonance frequencies or widths. In this regime interesting interesting physics comes into view, particularly regarding active devices. Specific examples are array antennas for spontaneous emission and ‘lasing spasers’, where the lowest-loss array mode will lase most easily.

Very strong interaction
The researchers made an important discovery by measuring how much light passes through a thick grid of these electromagnets. It appears that when the tiny currents of the rings are actuated by light the nano-magnets also influence each other and can power each other.

The researchers have also shown for the first time that the interaction with the magnetic field of light is very strong in these materials; just as strong as the interaction with the electrical field in the best ‘classical’ optical materials. This improved understanding of the nano-magnets and their interaction with light gives the researchers all the ingredients they need to disperse light along arbitrary paths.

Fast magnetic fields
We are all familiar with rod-shaped magnets: they are described as ‘dipolar’, with a north pole and a south pole, and the tendency to attract each other’s opposite poles and repel similar poles. We also know that, just like a compass, magnets align themselves along a magnetic field. This is how you can manipulate magnets with magnetic fields, and – vice versa – you can exercise control over magnetic fields using magnets. This commonplace intuition works particularly well for slowly changing magnetic fields, but not for those in a state of rapid flux.

Handicap for optics
Light is an electromagnetic wave consisting of a very rapidly fluctuating electrical field and an associated magnetic field. In principle, you can direct electromagnetic waves at will by manipulating both the electrical field and the magnetic field. But at the very high frequencies of light (500 THz – 500 billion vibrations per second), atoms scarcely respond to magnetic fields. This is why normal materials only control the electrical field of light and not the magnetic field, and is also why normal optical devices (lenses, mirrors and glass fibres) are handicapped in the way they work. But this type of control is actually possible with these artificial ‘meta-materials’.