Focusing Inside Left-Handed Material Studied

Daniel S. Burgess

A team of physicists at Massachusetts Institute of Technology in Cambridge has reported the results of its experiments on internal focusing in negative refractive index materials. Also called left-handed materials to describe the way they refract an incident ray through an angle to the left rather than to the right of the normal line, the materials have been the subject of greater investigation since John B. Pendry of Imperial College London published a series of equations four years ago suggesting that they could be used to create flat-surfaced "superlenses" with perfect resolution.

Last year, the MIT scientists reported that a flat slab of one class of these materials, comprising plated copper split rings and wires in a circuit board substrate, exhibited a focusing effect on microwaves. To better understand the phenomenon, they recently sought to verify theoretical predictions regarding left-handed lenses; that is, that the electric field inside the lens displays negative curvature and that the focal distance is linearly dependent upon the frequency of the transmitted radiation.

In the new work, they compiled a profile of the electrical field inside a 40 × 7-cm slab exposed to 10.3-GHz microwaves, taking point measurements inside each of the cells of the slab with an inserted dipole antenna. The measurements are inconsistent with positive refractive index materials, displaying negative curvature, no broad collimation and no broadened power-law decay.

The researchers also performed transmission measurements to examine the relationship between focal distance and frequency. They varied the frequency of the microwave source between 10.3 and 10.42 GHz, at each frequency performing one-dimensional scans to find the region of highest power beyond the output side of the slab. They found that focal distance linearly increased with frequency.
Further experimental work and material improvements will be required, they note, before theoretical predictions of subwavelength focusing can be tested in this group of left-handed materials.