Nanobead Superlens Provides Superb Resolution

Resolves data on DVD

Crossing a so far impassable threshold, scientists fashioned a “superlens” from an all-dielectric (non-conducting) metamaterial, obtaining a resolution five times better than that provided by the best optical microscopes.

Authors of the study, published in the 12 August issue of Science, say the semispherical lens made of millions of “nanobeads” (of scales in the billionths of a meter) could resolve the stored data in the form of dots and lines 45 nanometers across in the 100nanometer-wide grooves of a Blu-ray DVD, a resolution well outside the capabilities of existing optical microscopes.

Metamaterials are those possessing electromagnetic properties not found in nature, which can be produced by state-of-the-art technologies at considerable cost. Because they have negative refraction indexes, it is theoretically possible to use them to view even individual atoms in visible (optic) light, to construct “invisibility cloaks” which are hoped to hide objects from sight by bending the light around them, and to build optical computers. But inevitable losses associated with metals used in their manufacture limit the performance of prototypes. Losses in metamaterials can be minimized at microwave and terahertz frequencies, but in higher wavelengths of optical (visible) light ranging from 390 to 740 nanometers, losses build up and obstruct the practical use of these materials. Hence, viewing objects smaller in size than half the wavelength of visible light (about 200 nanometers) could not be possible until now.

Although all-dielectric metamaterials developed in recent years allowed some progress, but lenses or cloaks made of these remained as 2-D planar surfaces or stacks of them with limited practical use. (See: Pushing Light Beyond the Frontiers / pp.30-35) Another obstacle rising in front of practical applications is that these products can function in highly-constrained, easily deformable conditions.

But the nanomaterials used by the group led by Dr. Zenbo Wang of Bangor University, UK, and Professor Limin Wu of the Fudan University in China were beads just 15 nanometers across, made of cheap and readily available titanium oxide (TiO₂).

Using millions of these nanobeads and squeezing them in a centrifuge and coating them with an oily solution, the team fashioned these nanobeads into a 3-D semispherical, metamaterial-based “solid immersion lens” (mSIL) with tunable base area and height. When the material is placed between the lens of an optical microscope and sample to be studied with the tip of a syringe, “Each sphere bends the light (from the optical telescope)  to a high magnitude and splits the light beam, creating millions of individual beams of light. It is these tiny light beams which enable us to view previously unseen detail”, Dr Wang explains. 

Because their use of relatively cheap raw materials and production costs, nanobead metamaterial could be used in microscopy without the need for dyes, stains or lasers which can alter the studied material and find a wide field of application in medicine as well as applications in optical communication networks, harnessing of solar energy and invisibility cloak studies, the authors point out.