An ultrathin, flat lens that focuses light without imparting the distortion associated with conventional lenses has been created by applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS).
The flat lens is essentially two dimensional, at a mere 60 nanometres thick, yet approaches the ultimate physical limit in terms of focusing power. The device, which is surprisingly simple to manufacture, operates at telecom wavelengths (the range commonly used in fiber-optic communications). The findings have been published online in the journal Nano Letters.
Principal investigator and Professor of Applied Physics at SEAS, Federico Capasso said: “Our flat lens opens up a new type of technology. We’re presenting a new way of making lenses. Instead of creating phase delays as light propagates through the thickness of the material, you can create an instantaneous phase shift right at the start of the lens. It’s extremely exciting.”
Capasso and colleagues create the flat lens by plating an ultrathin wafer of silicon with a nanometre thin layer of gold. They then strip away part of the gold layer, leaving behind an array of V-shaped structures which are evenly spaced in rows across the surface. When Capasso’s team shines a laser onto the flat lens, these structures act as nanoantennas, capturing the incoming light, holding it briefly and then releasing it again. This delay effect changes the direction of the light, in a similar way to a thick glass lens, but with a fundamental distinction.
Optical aberrations such as the “fish-eye” effect, which result from conventional wide-angle lenses, are eliminated by the flat lens. This results in a completely accurate image, which does not require corrective technology.
The nanoantenna array or ‘metasurface’ can be tuned for specific wavelengths of light simply by changing the size, angle and spacing of the antennas.
Lead author Franceso Aieta said: “In the future, we can potentially replace all bulk components in the majority of optical systems with just flat surfaces.”