Thursday, 1 October 2009

Creating The Emperors New Clothes

Copyright of University of Surrey
The concept of creating an invisibility cloak has split the science community in opposite directions, many still considering it a work of fiction. Provocative speculation, media attention and only a few years of experimental data have built up an opposition to this latest development in science.

The basis of the invisibility cloak stems from a material having a negative refractive index, a ratio that controls the angle of refraction. This was first conceptualised forty years ago by V. G. Veselago[i], who predicted that a material would have a negative refractive index when both the permeability and permittivity are negative. In effect, Doppler shift would be reversed and higher frequencies would have longer instead of shorter wavelengths. Most importantly, light passing through would appear to move in the opposite direction as energy flow, bending back towards the source. Due to no known naturally occurring material possessing these qualities, it was only after Prof. John Pendry proposed the use of metamaterials[ii], an artificial composite material whose properties are ‘fabricated from repeated elements, specifically engineered to produce a desired electromagnetic behavior’ [iii] that any real developments evolved. 2001 was the year these theories were experimentally verified, when light was successfully refracted by an angle corresponding to that of a negative refractive index[iv].

Since then, there has been much interest surrounding these so called left handed metamaterials. Research is continually probing the properties and discovering more unusual phenomena, such as a laser pulse appearing on the other side of the metamaterial before it even passes through[v]. This was implemented by passing femtosecond pulses through the material in an interferometer. Analysis of the time delay found it to be negative; the laser pulse arrived before it even started to propagate. Although an attractive notion, the reality of their results could lie in the use of laser pulses which are composed of many waves of differing velocities. The apparent area of no activity behind the metamaterial may be due to the summing and not the absence of these waves.

Apart from research into new fields, metamaterials are also advancing current ones such as the creation of higher resolution. As the metamaterial bends light backwards it focuses both the rays and the finer details of the evanescent electromagnetic near-fields that do not propagate[vi]. In effect, this would be the dawning of a perfect lens.

The most prolific prospect for metamaterials has to be the proposed fabric of invisibility. The fact that light can be bent away from the material is the foundation for this project. However, it still remains uncertain how feasible this really is as experiments have not yet extended to visible frequencies[vii].

Despite initial hesitance to accept the infant findings, it seems like metamaterials will soon be a large part of future technology. With both media and public eager for a cloak of invisibility, scientists may soon weave the gap between science fiction and reality, by creating the real Emperor’s clothes.
[i] V.G. Veselago, The electrodynamics of substances with simultaneously negative values of e and ยต, Sov. Phys. Usp. 10, 509 (1968)
[ii] J B. Pendry, A.J. Holden, D.J. Robbins, and W.J. Stewart, Magnetism from Conductors and Enhanced Nonlinear Phenomena, IEEE Trans. Microwave Theory Tech. (1999)
[iii] D.R. Smith, in a UCSD press release April 5th (2001)
[iv] R. Shelby, D.R. Smith, S. Schultz, Science 77 (2001)
[v] G. Dolling, C. Enkrich, M. Wegener, C.M. Soukoulis, S.Linden, Simulatneous Negative Phase and Group Velocity of Light in a Metamaterial, Science Vol 312(2005)
[vi] D.R. Smith, J.B. Pendry, M.C.K. Wiltshire, Metamaterials and Negative Refractive index; Science magazine, Vol 305
[vii] R. A. Shelby, D. R. Smith, S. Schultz ,Experimental Verification of a Negative Index of Refraction