Split-ring resonator

SRRs can produce the effect of being electrically smaller when responding to an oscillating electromagnetic field.

[4] [5] [6] [7] These media create the necessary strong magnetic coupling to an applied electromagnetic field not otherwise available in conventional materials.

For example, an effect such as negative permeability is produced with a periodic array of split ring resonators.

The small gaps between the rings produces large capacitance values, which lowers the resonating frequency.

[8][9][10] The split ring resonator was a microstructure design featured in the paper by Pendry et al in 1999 called, "Magnetism from Conductors and Enhanced Nonlinear Phenomena".

This capacitor and inductor microstructure design introduces a resonance that amplifies the magnetic effect.

However, the periodic construction of many SRR cells is such that the electromagnetic wave interacts as if these were homogeneous materials.

The SRR is designed to mimic the magnetic response of atoms[clarification needed], only on a much larger scale.

Also, as part of periodic composite structure, the SRR is designed to have a stronger magnetic coupling than is found in nature.

The larger scale allows for more control over the magnetic response, while each unit is smaller than the radiated electromagnetic wave.

The variations of split ring resonators have achieved different results, including smaller and higher frequency structures.

However, in 2005 it was noted that, physically, a nested circular split-ring resonator must have an inner radius of 30 to 40 nanometers for success in the mid-range of the visible spectrum.

[18][16][17] Then multiple cells of this double band configuration are fabricated onto circuit board material by an etching technique and lined with copper wire strip arrays.

[17] For this demonstration, square shaped SRRs, with the lined wire configurations, were fabricated into a periodic, arrayed, cell structure.

[17] Just before this prism experiment, Pendry et al. was able to demonstrate that a three-dimensional array of intersecting thin wires could be used to create negative values of ε.

In a later demonstration, a periodic array of copper split-ring resonators could produce an effective negative μ.

In 2000 Smith et al. were the first to successfully combine the two arrays and produce a so-called left-handed material, which has negative values of ε and μ for a band of frequencies in the GHz range.

[21] SRRs had been used to focus a signal from a point source, increasing the transmission distance for near field waves.

[22] The response, when combined with an array of identical wires, is averaged over the whole composite structure which results in effective values, including the refractive index.

In research based in metamaterials, and specifically negative refractive index, there are different types of split-ring resonators.

[24][26] On May 1, 2000, research was published about an experiment which involved conducting wires placed symmetrically within each cell of a periodic split-ring resonator array.

This effectively achieved negative permeability and permittivity for electromagnetic waves in the microwave regime.

The unit is designed to generate a large capacitance, lower the resonant frequency, and concentrate the electric field.

[28][29][30] Research has continued into terahertz radiations with SRRs[31] Other related work fashioned metamaterial configurations with fractals[25] and non-SRR structures.

These can be constructed with materials such as periodic metallic crosses, or an ever-widening concentric ring structures known as Swiss rolls.