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Nano-lens goes cup-shaped

05 Aug 2009

Self-assembled device overcomes diffraction limit of light and resolves features as small as 200 nm.

A new type of self-assembled nano-lens that overcomes the diffraction limit of light has been developed by researchers in Korea. The lens, which is made of cup-shaped organic molecules, allows features as small as 200 nm to be resolved. The device might find use in nano- and bio-imaging, optical nanolithography and other nano-optical applications.

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

Kwang Kim of the Pohang University of Science and Technology and colleagues have now overcome this limit by making nanoscale lenses that are self-assembled from cup-shaped organic molecules. The devices deflect light in curved beams, resulting in very short focal lengths that allow information related to the evanescent waves emitted by the object to be accessed. This allows near-field magnification that can resolve features as small as 200 nm.

The very short focal points of the nano-lens come thanks to a combination of two different optical effects, explains Kim. The first is the extremely high curvature of the lens, which strongly bends light to form a focus close to the lens. The second is the diffraction of light as it is transmitted through the nanoscale circular aperture of the lens, and subsequent interference with light bending around the exterior of the lens.

Revealing the wave nature of light

"These diffraction and interference phenomena are related to the wave nature of light, which is revealed when a lens is reduced to the nanoscale," he told nanotechweb.org. "This wave nature is neglected for regular-sized lenses in which light is considered to simply travel in straight lines."

Kim and colleagues made their lenses from calixhydroquinone (CHQ). The researchers began by dissolving 10 mg of CHQ powder in 2 ml of acetone and the same of water. Slow evaporation of the solution led to the self-assembly of various CHQ structures, including nano-lenses as small as 50 nm in diameter.

The devices might be used in nano- and bio-imaging, optical nanolithography, nano-optical devices and Raman lensing, among other applications, say the researchers.

The team now plans to integrate the lenses into nanoscale systems. For example, replacing the lenses in conventional optical systems with nano-lenses could scale down entire experiments to the nanoscale for lab-on-a-chip technology.

The new nano-lenses are entirely different from so-called super-lenses that also overcome the diffraction limit. Super-lenses are made of complicated metamaterials whereas a nano-lens is a simple, real lens, stresses Kim.

The work was reported in Nature.

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