The creation of a negative refractive index — responsible for a host of interesting phenomena, including negative refraction, the inverse Doppler effect and reverse Čerenkov radiation — usually requires a material with simultaneous negative permittivity and negative magnetic permeability. The problem is that this is hard to achieve in practice, especially at optical wavelengths.
It now seems that manmade chiral metamaterials may provide an answer. Two separate research collaborations in the United States and Europe have now independently provided experimental proof that such chiral materials can be fabricated to provide a negative index in the terahertz and microwave regions.
Shuang Zhang and colleagues at the University of California, Berkeley, and Oklahoma State University report a negative index at terahertz frequencies with the assistance of a man-made chiral metamaterial (Phys. Rev. Lett. 102, 023901; 2008), while Eric Plum at the University of Southampton and an international team of collaborators report a negative index due to chirality at gigahertz frequencies (Phys. Rev. B 79, 035407; 2008).
Both results make use of chirality — a structural design that does not show mirror symmetry. Some naturally occurring materials, such as milk or sugar solutions, have extremely weak chirality. Now chirality in man-made metamaterials that is orders of magnitude stronger than in those natural materials has been demonstrated.
Chirality plays a part in all fields of physics. In optics, it is known for its effect on the spin of photons: that is, its influence on left-handed and right-handed circularly polarized light. In conventional materials, these two circular polarizations propagate with the same phase velocity.
According to Zhang, once chirality is introduced, this behaviour is changed, with one polarization propagating with a faster phase velocity and the other with a slower phase velocity. “If the chirality is really strong, one circular polarization will experience very slow phase velocity and even finally becomes negative. As a result, the refractive index can be negative even though the permittivity and permeability are not simultaneously negative, as required by the conventional negative index materials consisting of small metallic coils and rods”, he told Nature Photonics.
The Zhang group used this effect to observe a negative index between 1.06 and 1.27 THz in a sample measuring 1.5 cm × 1.5 cm composed of an array of micrometre-scale gold resonators (image above) with bridges roughly 20 μm long elevated by pillars 4.5 μm tall.
The results are likely to inspire further work into the potential for chiral metamaterials, such as isotropic and active designs. The US team is now working towards a new type of chiral metamaterial that can flip the handedness of the chiral structures upon illumination by a control light beam.
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Pile, D. Chirality-assisted negative index. Nature Photon 3, 133 (2009). https://doi.org/10.1038/nphoton.2009.10