Negative index materials without magnetic resonances
with Yi-Fan Chen and
We showed theoretically how the refractive index in a left-handed medium is computed using causality arguments and predicted that there can be a negative index of refraction without the need for a magnetic resonance. This is unlike all existing left-handed media that require meta-materials to obtain an appreciable magnetic permeability at optical frequencies.
In our system, as is true for almost all media at optical frequencies, the magnetic permeability is taken to be 1, but the dielectric function is chosen to have two resonances. These correspond to two different atomic, molecular, or luminescent species. One of the two is assumed to be inverted (pumped into an excited state). If expressed as simplified Lorentz oscillators, then the dielectric function now has an additional term due to the second species. It follows that the dielectric function now has two zero-pole pairs, as opposed to one zero-pole pair of a single Lorentzian oscillator (for a single resonance). Each of the pairs contributes a phase to the dielectric function and hence to the refractive index function. Choosing the location of the zeros and poles correctly will cause the real part of the refractive index function to become negative, without any contribution from the permeability!
The method described in the references is general, and can be used to determine the refractive index (real and imaginary) of any medium, not 'just' negative-index systems.
[1] Y.-F. Chen, P. Fischer, and F.W. Wise, “Negative refraction at optical frequencies in nonmagnetic
two-component molecular media”, Phys. Rev. Letters, 95 (2005),
067402.
[2] Y.-F. Chen, P. Fischer, and F.W. Wise, “Sign of the refractive index in a gain medium with negative permittivity and
permeability”, J. Opt. Soc. Am. B, 23 (2006), 45.
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