Physicists in the US have made the first ultrathin flat lens. Thanks to its flatness, the device eliminates optical aberrations that occur in conventional lenses with spherical surfaces. As a result, the focusing power of the lens also approaches the ultimate physical limit set by the laws of diffraction.
"Imagine if you were to replace the lens in a mobile phone with a flat and ultrathin one – you could then squeeze your smartphone down to a thickness approaching that of a credit card," says team leader Federico Capasso of the Harvard School of Engineering and Applications. "Most optical components found in devices today are quite bulky because the light-beam shaping is done by changing the optical path of incident light rays, which requires changes in lens thickness. In our lens, all the beam shaping is done on its flat surface, which is just 60 nm thick."
The new flat ultrathin lens is different in that it is a nanostructured "metasurface" made of optically thin beam-shaping elements called optical antennas, which are separated by distances shorter than the wavelength of the light they are designed to focus. These antennas are wavelength-scale metallic elements that introduce a slight phase delay in a light ray that scatters off them. The metasurface can be tuned for specific wavelengths of light by simply changing the size, angle and spacing between the nanoantennas. The lens surface is patterned with antennas of different shapes and sizes that are oriented in different directions. This causes the phase delays to be radially distributed around the lens so that light rays are increasingly refracted further away from the centre, something that has the effect of focusing the incident light to a precise point.