A two-dimensional meta material composed of silver elements will have a negative refractive index for light in the most energetic region of the visible spectrum, 400 to 500 nanometers (violet, blue and light blue), according to computer simulations by the team of Sergey Belan, Vladimir Parfenyev, and Sergey Vergeles from the Landau Institute for Theoretical Physics in the Russian Academy of Sciences, Moscow; Belan is also affiliated with the Moscow Institute of Physics and Technology, in Dolgoprudny. Writing in the journal Optical Material Express, the researchers hint at its potential for compact optical. [DOI: 10.1364/OME.5.002843]
The computations using the COMSOL Multiphysics package on a two-dimensional meta lattice composed of pairs of closely spaced silver cylinders with a radius of the order of 100 nanometers suggest a refractive efficiency of 70 percent and a reflective efficiency of 80 percent for violet to blue light. Earlier meta materials are usually three-dimensional and have much more complex geometries. Moreover, they have been demonstrated to operate at microwave wavelengths, which limits their potential applications.
The silver lattice meta material works through diffraction, splitting incident light into rays depending on angle of incidence, wavelength and the period of the lattice. The structure of the unit cell determines how the energy of the incident light is distributed between the rays. In a material with a negative refractive index, all but one of the diffracted rays are suppressed and the remainder emerges in the desired direction. That direction is counterintuitive given our experience with the appearance of a stick dipped into a swimming pool where water has a natural, positive refractive index.
At the mechanistic level, light's interaction with the pairs of metal cylinders is due to a plasmon resonance effect. The researchers have demonstrated that the extraordinary optical response of the proposed material arises due to excitation of the plasmonic modes in the gaps between cylinders, Belan told Materials Today. Tweaking the lattice allows a negative refractive index to be manifest for wide range of angles of incidence. This suggests potential applications in controlling signals in ultra-compact devices for optical telecommunications and future computing.
The next step will be to manufacture with very high tolerance the requisite smooth metal cylinders with their less than 10 nanometer separation for laboratory testing of the meta material's true potential. "We are now working on optimization of the proposed design in order to improve efficiency," Belan told us. "Specifically, we are testing in simulations the metal rods with a non-circular cross-section. In addition, we are looking for collaboration with experimental groups capable of realizing negative refraction in the laboratory."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".