Invisibility in visible light has to remain tiny?

Manipulation of light rays has entered a new era since the first proposal based on matamaterials. Such materials feature negative refraction index. The reason is because they have both negative dielectric constant and magnetic permittivity. This double-fold negativeness makes an unusual left-handed system of electric field, magnetic field and the energy current vector (which defines the light propagation direction), thus, when considering the continuity of these fields along the tangent at the interface of two meeting media, the incident light shall be deflected in a counter-intuitive manner. This behavior has been utilized to make cloaks that enables the undetection of things beneath it. Up to now, all designs apply to only longwavelength detecting rays. Is it plausible to make a cloak that works under visible light, and how ?

A recent work published in PRL seems detering the interest. It claims that, cloaks for visible light, if feasible, shall be very tiny and can not cover large objects. Nonetheless, there are oppositions to this claim, arguing that, it is rational only for resonant-type devices. So, how will this dubious topic advance further ? Let's see.

Science 25 June 2010:
Vol. 328. no. 5986, p. 1621
DOI: 10.1126/science.328.5986.1621-a

Even so, a broadband cloak cannot be much bigger than the wavelengths at which it works, Johnson and colleagues argue. In a paper in press at Physical Review Letters, they consider a simple scenario in which a pulse of light with a range of wavelengths descends on a flat object covered by a cloaking layer. If the object were not there, the light pulse would take more time to reach the surface and bounce back. So to hide the object, the cloak must delay the light pulse. And for the cloak to do that correctly over the entire wavelength range, its thickness must increase in proportion to the height of the hidden object, Johnson argues.

The thicker the cloaking layer, however, the longer the light pulse will remain in the material and the more light the cloak will absorb or scatter. If the cloak is too thick, that light loss becomes noticeable. Johnson and colleagues estimate that researchers might someday beat down the losses enough to cloak a meter-sized object at microwave wavelengths. At optical wavelengths, the losses are orders of magnitude too high to conceal such a large object, they say. A cloak for infrared or visible light cannot be more than a few micrometers across, they conclude.

Not everyone is convinced. Johnson's argument applies only to resonant systems, Pendry contends; it does not prove you cannot make a large nonresonant cloak. "It's not Moses descending from the mountain and saying you can't do it," Pendry says. "It's a rider saying that there may be some complications." Johnson says the result is general.

Cloaking is only one application for the concept of "transformation optics" that Pendry has pioneered, and others could prove more important. Still, it would be disappointing if all you could hide in your personal invisibility cloak were an eyelash.