Conventional, positive-refractive-index lenses create images by capturing the light waves emitted by an object and then bending them. However, objects also emit "evanescent" waves that contain a lot of information at very small scales about the object. These waves are much harder to measure because they decay exponentially and never reach the image plane -- a threshold in optics known as the diffraction limit.

In 2000, John Pendry of Imperial College in London suggested that a material with a negative refractive index -- that is, one that bends light in the opposite direction to an ordinary material -- could capture and "refocus" these evanescent waves. This idea of a perfect lens or "superlens" came over 30 years after Russian physicist Victor Veselago first speculated that negative index materials could exist. In such a superlens, electromagnetic waves that reach the surface of a negative refraction lens excite a collective movement of surface waves, such as electric oscillations -- also known as "surface plasmons". This process enhances and recovers the evanescent waves.

In 2003, Zhang's group showed that optical evanescent waves could indeed be enhanced as they passed through a silver superlens. Now they have taken this work one step further and have imaged objects as small as 40-nm across with their superlens, which is just 35-nm thick (see figure). In contrast, current optical microscopes can only resolve objects down to around 400-nm, which is about one tenth the diameter of a red blood cell.

"Our work provides a new imaging method that can beat the optical diffraction limit and that has tremendous potential to revolutionize a wide range of technologies," says Zhang. These include detailed biomedical imaging in real-time and in vivo, optical lithography to make higher density electronic circuits and faster fibre-optic communications.

"This paper represents a very critical step forward," David Smith of Duke University in the US told PhysicsWeb. "It provides confirmation of Pendry’s original conjecture that a negative refractive element can focus near-fields and demonstrates clearly that evanescent refocusing occurs to create an image."

"The work is a remarkable accomplishment," says Pendry. "Although superlensing has previously been demonstrated at microwave frequencies, this is the first true super resolution at optical frequencies -- where the greatest rewards in terms of applications are to be had. I am extremely pleased with this result."