Lenses are usually thought of as being made of curved dielectrics, but must this always be the case? Rather surprisingly, the answer is "no", as Srinivas Sridhar and colleagues at Northeastern University in Boston, US, have demonstrated.
  

Flat lens

The key to creating the flat lens lies with the recent advent of materials - photonic crystals - that effectively have a negative index of refraction. A flat slab of such a substance can have an index of refraction that depends on the angle at which radiation hits it. The slab can then act as a lens, with the amazing property that there is no preferred axis and no restriction in aperture size. As the Northeastern researchers point out, the tricky part in creating the flat lens is in designing a photonic crystal with negative refraction over a wide range of angles - and low absorption.
   The two pictures on the right show that they succeeded with a structure composed of cylindrical aluminium rods. In the upper picture an image of a point source of microwaves, at a frequency of 9.3 GHz, is created on the far side of the slab. In the lower picture the source has been moved up by 4 cm and the image has moved correspondingly. This illustrates that the flat lens does not have a single optical axis and limited aperture.
   While this lens works only with microwaves - and in fact only for a narrow range of frequencies, from 9.0-9.4 GHz - the principle could herald a revolution in optics.

Further reading
P V Parimi et al. 2003 Nature 426 404.