Artificial materials with negative refractive index are called left handed materials becaue of the unusual electromagnetic wave propagation in them. In these material the wave travel backwards while the energy propagates along the incident direction, contrary to the naturally available materials. Photonic crystals and metamaterials are currently being investigated for such left handedness of EM wave, while a few other artificial materials are feasible.
We have initiated a new research program on Left-handed Metamaterials, in colaboration with Dr. John Derov, and supported by AFRL, Hanscom.
What are Left-Handed Metamaterials?
In a medium with effective permittivity ε and effective permeability m both negative (ε < 0, μ < 0), the index of refraction n is real (albeit negative, n < 0) and radiation can propagate. This medium is classified as left-handed (LHM), in contrast to the RHM that we are know so well. Novel behavior emerges as compared with what is now termed right handed materials (RHM) (all materials encountered so far in natural form are right handed).
In the 60’s, Veselago showed that LHM did not violate known physical laws, and demonstrated theoretically some of their unusual properties. Recently Pendry discussed LHM further and proposed a concrete realization at microwave frequencies. Smith, Schultz and coworkers fabricated LHM and convincingly demonstrated their unusual properties. Left handed materials are for the moment only meta materials, which means fabricated with the help of normal materials (RHM) like copper for example.
Some of the unusual properties of LHM
Wave Vector Reversal
In LHM the wave vector of a monochromatic plane wave is reversed in comparison with what it should have been for a RHM. That means that if the vector Ε is along x and the vector B is along y, in the RHM the electromagnetic wave will propagate along z, while in LHM the wave will propagate along -z (yes indeed the frame is right handed).
Negative Refraction Index
Index of refraction n being negative tells that the direction of energy propagation is opposite to the direction of plane wave motion, more clearly: a pulse traveling to the right in a left-handed material can be decomposed into the plane waves traveling to the left.
Compatibility with Maxwell’s equations is intact!
The Doppler and Cerenkov effects will also be reversed in a left handed medium: An approaching source will appear to radiate at a lower frequency (Doppler effect is visible in the ‘mpeg’ movie) and charged particles moving faster than the speed of light in the medium will radiate in a backward cone, not a forward cone.
Snell’s law get twisted, too with left handed media. The result is not only a different way of focusing waves by LHM but also the fact that the resolution of LHM made lenses will be restricted by no natural limits (uncertainty principle) and this because these lenses will restore evanescent waves!
So far there is only one combination of wires and rings that behaves like LH in a certain region of frequencies (microwaves) and only for one direction of propagation and one polarization of the wave.
Can there be devised a more isotropic medium and can we approach the visible range of frequencies? The challenge remains open.