Nov 15, 2012
Metalens doubles up
Researchers at the University of Birmingham in the UK and Paderborn University in Germany have designed the world's first dual-polarity "metalens" for visible light. The device is made of an array of gold nanorods on a glass surface and can function as both a convex or concave lens – and so magnify or demagnify objects – simply by switching the polarization state of the light passing through it. The lens, which is just tens of nanometres thick, could not only be ideal for imaging on very small length scales but might also be used to increase the information processing capacity of optical circuits.
Lenses are key components of telescopes, microscopes and cameras and are also used in industry in optical lithography to manufacture silicon chips and integrated circuits. A conventional lens, which is usually made of glass or another transparent material, has a curved surface and a fixed focal length. It is either convex (that is, it converges light and magnifies an object) or concave (it diverges light, making the object being observed appear smaller).
The new lens is flat and consists of an array of gold nanorods on top of a glass surface. This rod array allows the researchers to either magnify or demagnify an object depending on whether they pass light with a left- or right-handed circular polarization through the device. The lens is also just tens of nanometres thick, compared with several millimetres for traditional lenses made of glass alone. It has an aperture of 80 microns and a focal length of 60 microns.
The gold nanorods (each only 200 nm long) are made using electron-beam lithography and placed, with carefully controlled orientations, on to the glass surface of the lens. "The entire rod pattern (which itself is called a 'metamaterial') works like a lens because each rod controls the local wavefront of the input light beam through its orientation," explained Birmingham team leader, Shuang Zhang. Metamaterials are manmade artificial materials for controlling light in unconventional ways and are especially engineered to have bizarre optical properties – not found in nature – such as negative refraction. They have already been used to make devices like invisibility cloaks.
Improving optical circuits
The researchers say that the lens could help increase the information processing capacity of optical circuits because it can simultaneously function as two devices for different polarizations of the input light. "As such, the polarization-encoded information can be demultiplexed and processed separately," added Paderborn team leader, Thomas Zentgraf. "This might even lead to important applications in quantum optics where photons entangled in polarization states can be more effectively manipulated," he told nanotechweb.org.
The metalens measures just a few tens of microns across but its size is currently limited by the writing speed of the electron-beam lithography system employed by the researchers. They hope to improve things, however, and say that they are already looking into new nanofabrication techniques for making large-area metalenses for real-world practical applications. "The fact that the lens is flat also means that it might easily be integrated into other nanophotonic devices using conventional fabrication techniques," said Zhang.
The current work is described in Nature Communications.
About the author
Belle Dumé is contributing editor at nanotechweb.org.