Sub-Wavelength Lensing

Sub-wavelength lensing in flat panels of left-hand materials (LHM) has been seen in two new experiments. What this means is that a planar sheet---and not something that has to be machined into a traditional lens shape---can be used to focus light into a tight spot. The size of this spot, furthermore, is less than half the wavelength of the light being used.

Getting around the venerable "diffraction limit" (whereby an object smaller than the wavelength is difficult to image) would be a boon to optics (in the microwave range, for example, wireless communications would benefit at the level of cell phones and base stations) and is normally achieved only by parking the object very close to the source of the illumination.

Left-handed materials (so called because the "right-hand rule" used by physicists to picture the relation between a light pulse's electric and magnetic fields and its line of propagation is here reversed) possess a negative index of refraction. This fact, in turn, means that a light ray approaching from air into the LHM material will be deflected not toward but back and away from a line drawn perpendicular to the surface of the material. It is this bizarre deflection that leads to novel optical effects.

When the idea of the LHM phenomenon was first propounded, many felt that such materials could not exist. Even after the first experiments were reported (Update 476) skepticism lingered. Later more evidence arrived showing preliminary lensing effects with flat panels, the hallmark of LHM optical abilities (Update 628).

Now, two groups have more direct evidence for flat-panel lensing and for better-than-wavelength focusing. George Eleftheriades and his colleagues at the University of Toronto (gelefth@waves.toronto.edu; 416-946-3564; see his website), using a material devised from printed metallic strips mounted on a plane and sandwiched between two patterned sheets, show that a source of microwaves can be lensed better than the diffraction-limit would allow, but not into a "perfect focus" called for in some LHM theories.On the positive side, the energy losses in the material which some commentators had predicted would hamper prospective LHM lenses (and their potential use in medical imaging or radar sets, say), were actually quite minimal.

Meanwhile, Vladimir Kissel and his associates at the Institute for Theoretical and Applied Electromagnetics in Moscow (kis_v@mail.ru, +7(095)4842644) have also observed "superresolution" in their lensing of microwaves with a flat panel, achieving a spatial resolution as good as one-tenth the wavelength. (Toronto group, Grbic and Eleftheriades, Physical Review Letters, upcoming article; Moscow group, Lagarkov and Kissel, Physical Review Letters, 20 February 2004)