Available online 29 March 2011.
Pushing the refractive index of metamaterials into the negative regime has been a hotly pursued area of research. However, some researchers have been looking into how high they can make this value. Now, a team of South Korean researchers have managed to create a material with an incredibly high index in the terahertz regime [Choi et al., Nature (2011) 470, 369].
Theoretical structures have previously been proposed for high refractive index materials, but until now they have not been physically realized. The new material produced by Prof Bumki Min and co-workers is a broadband, large area, free standing construct that possesses a peak refractive index of 38.6 in the 0.5 THz regime. Compared to the host material, which possesses a far more conventional index of 1.8, this is a considerable step-up.
The refractive index is increased by maximizing the effective permittivity, which is in turn dependant on the polarization. Each unit cell, measuring 60 μm2, consists of an “I” shaped metallic patch embedded in a dielectric material (polyimide). By placing the patches close together they can interact like capacitors; in the presence of THz radiation charge can accumulate on each arm, producing a large electric polarization and so raising the permittivity.
|Full-size image (13K) |
High-quality image (56K)
The final metamaterial is free standing and extremely flexible.
Reprinted by permission from Macmillan Publishers Ltd: Nature 470, 369, © 2011.
Higher values of the refractive index could be possible by using a dielectric with a higher refractive index, or by decreasing the separation between the metallic patches. However, there are limitations, as Min explains “this refractive index increase will accompany the lowering of [the] resonance frequency (and peak refractive index frequency) as well. This means your available bandwidth of high refractive index decreases. If we are not concerned with that frequency lowering, the limit of [the] positive index will be determined by the tunnelling length scale of electrons.”
The team is aiming to eventually produce a high index metamaterial in the infrared or visible range. Min told us that these “high refractive index metamaterials would provide another way of achieving sub-wavelength resolution in an imaging system”. However, they are currently focusing their efforts on two-dimensional isotropic metamaterials, and are investigating how the angle of the incident light affects the material.