Opening a new way to manipulate electric fields

New research makes it possible to drastically manipulate the electric displacement fields, resulting in effective dielectric constants over 3 million.

Article  |  Spring 2017

The dielectric constant is a measure of how polarized materials are under electric fields and is the most fundamental electric property of materials. For example, the classification of materials into conductors, semi-conductors, and insulators can be based on their dielectric constants. Moreover, together with the magnetic permeability, they determine the refractive index and the impedance of materials, which are the two main parameters that prescribe how electromagnetic waves behave inside materials in general. Hence, developing materials with tailored dielectric constants is one of the most sought-after research goals in optical materials science and related engineering areas.

Dielectric constants of common materials are limited below ~100, with some specially formulated materials such as CaCu₃Ti₄O₁₂ having unusually high values near 10,000 at low frequencies only. Using electromagnetic resonances, one can further increase the dielectric constant near the resonance frequencies. Since larger dielectric constants mean slower propagation, this resonance effect was used to slow down light in previous works. However, the dielectric constants were also very dispersive—their values sensitively depend on the frequency of electromagnetic waves, which was unavoidable since the principle utilized resonances directly. Hence, different frequency components propagate at different speeds, and temporal pulses deform severely while propagating inside these materials.

An alternative method using the electric field localization principle has also been developed. Jonghwa Shin (currently at Materials Science and Engineering, KAIST) used quasi-static boundary conditions to localize E fields into tiny gaps between metallic objects while the D field is almost uniform everywhere. The macroscopically averaged (homogenized) E field is greatly reduced compared to the mesoscopic value in the gap while the D field remains unchanged. Using this non-resonant principle, Prof. Bumki Min’s research group (Mechanical Engineering Department, KAIST) was able to realize a terahertz high index metamaterial with dielectric constants larger than 400 over a broad range of terahertz frequencies (Nature, 2011). Further enhancing the dielectric constants requires the manipulation of D fields, but this has remained a very elusive goal due to the solenoidal nature of D vector fields.

Recently, Prof. Shin, in collaboration with Prof. Yong-Hee Lee (Physics Department, KAIST) proposed a geometric way to manipulate the local D field. The principle is to fill the space with narrow dielectric regions sandwiched between metallic plates according to a geometric design similar to space-filling curves in mathematics. Its effect is such that local D fields are highly non-uniform and they accumulate constructively to produce a macroscopic D field that can be thousands of times larger than the local values. As this principle relies only on geometry, it was experimentally confirmed that the resulting dielectric constants, over 3 million in value, remained nearly constant over a broad frequency range. The corresponding refractive index was over 1800 at microwave frequencies. At a telecommunication wavelength of 1.55 microns, a focusing resolution smaller than 66 nm was numerically confirmed.

High refractive index materials have potential applications in diverse areas since the refractive index affects many aspects of wave propagation. For example, a 10 times larger index signifies a 10 times smaller wavelength, a 10 times better imaging and lithography resolution, 100 times higher absorption limit for thin films, 100 times faster light emission, and so on. The fact that the developed index is almost dispersionless makes it possible to apply the technology to applications requiring large bandwidth of operation. Scientifically, it is interesting that the homogenized D field can be drastically different from local values both in magnitude and direction. Together with previously known E field manipulation schemes, it is expected to be used for developing other original concepts in effective medium theory.

The principle and main results of this research can be found in an article entitled “Broadband giant-refractive-index material based on mesoscopic space-filling curves,” published on August 30^th, 2016 in Nature Communications (http://dx.doi.org/10.1038/ncomms12661).