OPTICS: Negative-index materials form strange images

Theorists have predicted many interesting effects for materials with negative index of refraction n, including perfect lensing, a reversed Doppler shift, or reversed Cherenkov radiation.

May 1st, 2006
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Theorists have predicted many interesting effects for materials with negative index of refraction n, including perfect lensing, a reversed Doppler shift, or reversed Cherenkov radiation. Using ray-tracing calculations, researchers at the Universität Karlsruhe, the DFG-Center for Functional Nanostructures, and the Forschungszentrum Karlsruhe (all in Karlsruhe, Germany) and the Universität Braunschweig (Braunschweig, Germany) have shown with geometrical optics that objects embedded in negative-index materials are full of surprises. These objects can exhibit reversal of apparent velocity, distortions in shape, and even loss of connectivity.

Although artificial negative-refractive-index materials have been fabricated, they have not yet entered the visible spectrum and suffer from small sample size, anisotropies, strong frequency dependence, and excessive loss. However, using a ray-tracing program, one can learn much about how objects would appear in an isotropic negative-refractive-index material. A ray-tracing program calculates the image of a scene by simulating how the light rays travel. POV-Ray 3.6 software (www.povray.org) is suitable for this task because its use of Fresnel equations and the laws of refraction and reflection are compatible with a refractive index less than zero.


The calculated ray-tracing image of a metal rod in an empty glass (left) is compared to a metal rod in a glass of water with index of refraction n = 1.2 (center), showing ordinary refraction effects. Replacing the water by a fictitious negative-refractive-index material (right) with n = -1.2 causes unusual refraction effects.
Click here to enlarge image

Using this software, researchers confirmed that a convex-shaped negative-index lens acts like a concave-shaped positive-index lens. Another example that shows unusual properties is a metal rod in a glass of negative-index “water” (see figure). Compared to a rod in a regular glass of water, the rod in the negative-index water appears broken, bends in the opposite direction, and the bottom of the glass cannot be seen. All these effects are caused by the unusual refraction properties at the air/negative-index “water” interface.

In another experiment, a ball dropped into a pool of water sinks to the bottom with constant velocity. In regular water, the ball would appear to be getting smaller to an observer looking into the pool. However, ray-tracing calculations show that the ball dropped into a pool of negative-index water would appear to move toward the observer, would grow infinitely large in size, would flip sides, and eventually move away from the observer. Even more complex, if the ball is replaced by a long object, that object will appear to have different magnification factors along its length due to its different depths in the water. As the object falls, the magnification changes can distort the object significantly, causing the object to appear disconnected in places. In fact, if the metal rod in the negative-index water is viewed carefully near the air-water interface, it would exhibit a trumpet shape rather than a cylindrical shape-again caused by the depth dependence of the magnification factor.

“Even though we do not yet have these isotropic negative-refractive-index materials for the visible in our lab, these simulated images can help both scientists and laymen get an intuitive and visual understanding of this new class of materials,” said researcher Gunnar Dolling. “However, given the fast development of this field, I am optimistic that negative-index materials for the visible will be fabricated within the near future.”

Gail Overton

REFERENCE

1. G. Dolling et al., Optics Express14(5) 1842 (March 6, 2006)

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