The new technique is based on the transmission properties of surface plasmon polaritons (SPPs) in metallic nanostructures. Xiangang Luo of the State Key Lab of Optical Technologies for Microfabrication at the Chinese Academy of Sciences and colleagues use a silver slab containing several nanoslits with different widths. These slits excite SPPs when the light reflected from the source impinges on the slab surface.

"Since the propagation constant of SPPs in the slit region depends on the slit width, we can use the varying slit widths to manipulate the propagation behaviour of the SPPs – including their phase velocities," Luo told nanotechweb.org. This means that when the SPPs reach the slit exit, most of them are converted into light again with the desired relative phases. The researchers say that the light reflected from the metallic slab lens refocuses at the designated position and forms a subwavelength image spot.

The technique could find use in a range of applications, including integrated optics, optical data storage, biomedical sensing and subwavelength lithography, they state.

The method is very different from other subwavelength imaging techniques, such as perfect, or superlenses, in which a slab of negative refractive index medium is employed. Although the resolution of such perfect lenses goes far beyond the diffraction limit, these materials are difficult to make. Moreover, both object and image need to be confined in the near-field region, which is around tens of nanometres away from the lens.

"In our method, the imaging process is associated with the manipulation of optical phases by optimally designing the slit waveguides on a metallic slab, which transmit SPPs in different modes," explained Luo. "In contrast to the perfect lens, it can realize imaging for arbitrary object and image distances and we believe that this is a step forward for practical applications."

The researchers are now extending their method to design other nano-optical elements, such as beam deflectors and probes. "In the future, we plan to utilize these nano-optical devices to achieve integrated systems," said Luo.

The work was reported in Appl. Phys. Lett.