Negative Refractive Index of Meta-Materials at Optical Frequencies
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0964-R01-02
Negative Refractive Index of Meta-Materials at Optical Frequencies S. Anantha Ramakrishna1, and Sangeeta Chakrabarti2 1 Department of Physics, Indian Institute of Technology Kanpur, Department of Physics, Indian Institute of Technology Kanpur, Kanpur, 208016, India 2 Department of Physics, Indian Insitute of Tehcnology Kanpur, Kanpur, 208016, India
ABSTRACT Scaling the performance of metamaterials to obtain negative refractive index at optical frequencies has been of great interest. One of the great barriers to the scaling is that real currents cannot be driven at very high frequencies and one is more dependent on displacement currents to generate negative magnetic permeability. Moreover to keep the dimensions of the metamaterials physically accessible, the structural lengthscales of the metamaterials begin approach the wavelength of the radiation in free space and homogenisation is often questionable. Here we will show that metamaterials such as Split ring resonators in these high frequency limits exhibit complex behaviour. Magnetic activity and Negative refractive index behaviour can, indeed, be obtained at optical frequencies but will need to be interpreted very carefully. The plasmonic nature of the metallic system and excitation needs to be considered in detail.
INTRODUCTION Since the experimental demonstrations [1, 2] of negative refractive index, first proposed by Veselago[3], the open question has been if negative refractive index is possible at optical frequencies. The large number of counterintuitive effects for wave propagation in NRM [4] including the possibility of super-lenses[4, 5] with sub-wavelength resolution have only whetted the appetite for such materials at optical frequencies. Currently the paradigm for negative refractive index is that the material should have a negative real part of the dielectric permittivity (εr < 0) and a negative real part of the magnetic permeability (µr < 0) [6]. Although several noble and alkali metals have εr < 0 at ultra-violet and optical frequencies, magnetic properties due atomic or molecular orbital currents or electronic spin tend to be negligible at optical frequencies. For negative magnetic permeability, special structures such as the split ring resonators (SRR) have been developed [7]. The operation of the Split ring resonator media can be most easily understood by considering a stack of metallic cylinders which are, let us say, placed on a square lattice. The diameters and the lattice spacing can be assumed to be small compared to the wavelength of light(λ). For an applied electromagnetic field with the time-varying magnetic field (H) along the cylinders, there will be circumferential induced currents on the metallic cylinders which will shield the interior via Lenz’s law. Thus the stack of cylinders will appear as an effectively
diamagnetic medium. Introduction of capacitive gaps in the cylinders will then make the medium resonant by the virtue of the L-C resonance that can then be induced in the structure. As the magnetic field drives the
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