All smoke and metamaterials

An illusion device, placed near but not enclosing an object of arbitrary shape, manipulates and transforms light scattered off the object so as to give it the appearance of a completely different object.

Seeing is believing — a naive assumption in the case of an illusion device proposed by Lai and colleagues at the Hong Kong University of Science and Technology, and described1 in Physical Review Letters. The new device has the power to 'act at a distance' and therefore covertly alter an object's appearance such that it has no apparent physical connection to the light scattered by the object — although this becomes increasingly difficult to achieve the farther the illusion device is from the object. Lai and colleagues1 outline a mathematical formalism proving that it is theoretically possible to grab the rays of light emitted by a given object and to reconstruct them so that they seem to come from a completely different object.

But how is this possible? Our brains interpret light reaching our eyes as if it consists of a stream of particles travelling in a straight line from the source. For most practical purposes, this is an excellent approximation. But light has in fact a wave-like character, and the straight-line approximation to light propagation sometimes breaks down. Over the past decade, scientists working in the field of optics have been investigating circumstances in which this breakdown takes an extreme form.

The story begins in 1968, when a Soviet scientist, Victor Veselago, showed2 that a flat slab of negatively refracting material, one that bends light in the 'wrong' direction, could act as a lens so that objects placed on one side of the slab are seen on the far side as images of exactly the same apparent size as the object. His long-neglected work has attracted much interest over the past decade, partly because the new technology of metamaterials — artificially engineered materials that owe their unique properties to their internal physical structure rather than their chemical composition — enables negative refraction to be experimentally achievable. Other remarkable properties of Veselago's lens have also been revealed. For example, it has been shown that its resolution is not constrained by the normal limits of wavelength, as in a conventional lens (an ordinary lens cannot focus light on an area smaller than the square of the light's wavelength), but only by the perfection with which it can be manufactured3.

Another relevant development is a very powerful design tool for optical systems4. Imagine that the optical system in question, such as a lens, is embedded in a rubber medium. We then stretch and pull the rubber, taking with it all the rays of light passing through the lens, until the rays are travelling in the desired directions. Of course, the electric and magnetic fields associated with the light no longer obey Maxwell's equations for the propagation of electromagnetic waves. But this can be remedied by altering two properties of the distorted medium: the electric permittivity and the magnetic permeability. In fact, if we carefully record the changed shape of the rubber medium by measuring the distortion of an embedded system of coordinates, the relationship between the old and new coordinates gives the prescription we need. This tool, known as transformation optics, was originally applied to generalizing Veselago's lens into a magnifying glass, but attained notoriety when it was used to design a 'cloak of invisibility'5,6.

Figure 1 shows this transformed lens in operation. An object, in this case a cup, is enclosed by a negatively refracting lens of radius r2, which projects an image of the cup into the volume occupied by an outer sphere of radius r1. Note that everything outside the shaded annulus, the lens, is free space. It is as if the negatively refracting material 'annihilates' the adjoining space out as far as radius r1, and the inner sphere of radius r3 containing the cup expands to fill this space (Fig. 1a). This system is the starting point for Lai and colleagues' illusion device1.

Figure 1: An illusion device in action.

a, A spherical lens system incorporates negatively refracting material — one that bends light in the opposite direction to conventional materials — within the shaded annulus of radius r2 (blue). The outer annulus of radius r1 contains only free space, and the innermost sphere of radius r3 contains a cup. An external observer would see the contents of the inner sphere magnified to fill a sphere of radius r1. b, In Lai and colleagues' illusion device1, the lens is designed to eliminate any light scattering from a spoon located in free space in the outer annulus: an 'antispoon' in the shaded annulus would effectively cancel any light scattering from the spoon itself. An external observer would see no trace of the spoon, only a magnified image of the cup — the illusion object. A ray of light (red) is shown scattering from the spoon.

The authors1 observe that negatively refracting material, appropriately structured, can annihilate any adjoining positively refracting material7. Transformation optics provides the recipe and metamaterials provide the means. If, as in the example suggested by Lai et al., a spoon is placed in the free space beside the illusion device, and an 'antispoon' incorporated in the annulus of radius r2 is designed to cancel any light scattering from the spoon itself, the inner sphere containing the cup expands to fill this space and effectively replaces the spoon with a cup (Fig. 1b).

But how can the illusion device take control of rays that never strike it? In reality, rays are formed from a collection of waves, and the best approximation to a ray that waves can make is a 'Gaussian beam', which actually has a finite lateral extent. These fuzzy tails on the rays will have a slight overlap with the device which is designed to be peculiarly sensitive to them. The device responds by launching a second ray (not shown in Fig. 1) calculated to cancel the effect of the first.

Potential applications of Lai and colleagues' illusion device abound, ranging from altering the radar signature of an aircraft to creating large shadows using only small objects. However, it should be noted that this illusion concept is extremely demanding of material properties, and some of the more exotic applications are likely to remain in the realm of theory.


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Pendry, J. All smoke and metamaterials. Nature 460, 579–580 (2009). https://doi.org/10.1038/460579a

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