Physicists in Germany have made a new type of photonic crystal by fine-tuning the magnetic, rather than the electric, properties of a material. Stefan Linden of the Karlsruhe Research Institute and colleagues at Karlsruhe University made the crystals from pairs of gold wires, which acted as artificial magnetic atoms. The discovery has opened up new ways to manipulate light on the nanoscale, the scientists claim (Phys.Rev.Lett. 97 083902).
Photonic crystals are nanostructured materials in which periodic variations of some property – usually, the material’s electric permittivity – produce a “photonic band gap”. This affects how photons propagate through the material. This effect is similar to how a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In particular, photons with wavelengths or energies in the photonic band gap cannot travel through the crystal, which allows scientists to control and manipulate the flow of light by introducing carefully selected defects.
Until now, all photonic crystals operating with visible light have worked by modifying a material’s electric permittivity – a measure of the extent to which a material concentrates electrostatic lines of flux. Although the same effects are expected for periodic modulations of the magnetic permeability (μ) – which is a measure of how a material responds to a magnetic field – all known natural substances have a μ of 1 for visible light. This means that researchers have not been able to make photonic crystals that operate through variations in the magnetic permeability.
Now, however, Linden and colleagues have found a way round this problem by using “metamaterials”. These are composite structures made from tiny rods, ensembles of metal rings and the like, in which the individual components act as “artificial atoms”. Metamaterials therefore have very different properties from their component parts, including values of μ not equal to 1.
In the current work, the researchers used pairs of gold wires a mere 220 nanometres wide and 100 micrometres long, separated by a 50 nanometre thick layer of magnesium fluoride, to create a one-dimensional periodic lattice of artificial “magnetic atoms”. This was then placed on a quartz-based slab, which acts as a waveguide to channel light along certain paths, to create a 1D “magnetic” photonic crystal.
“Our findings are a proof of principle for the concept of a magnetic photonic crystal,” says Linden. “However, there still is a long way till we can utilize it as a real-world application.”
The ability to use both electrical permittivity and magnetic permeability will give physicists more design freedom. It could even lead to new effects such as three-dimensional photonic bands – a prerequisite if photonic crystals are to fulfil their potential – made of stacks of one-dimensional magnetic photonic crystals. The team is now trying to fabricate 3D metamaterials based on its 1D structures.
