Physicists in the UK have created a Luneburg lens – a lens able to focus light from all directions equally well – on a silicon chip. The device could one day find applications in on-chip Fourier optics, which are used by the telecoms industry to perform tasks from noise reduction to data compression.
Most practical lenses have aberrations, which means that their ability to focus light deteriorates when the incident light is off-axis. But in the Luneburg lens, proposed over 60 years ago, focusing is equally good wherever the light comes from.
But creating Luneburg lenses has proved tricky. They require the refractive index – the property that governs how light is bent by a lens – to vary throughout the device, with a maximum √2 (roughly 1.4) greater than the minimum. With today’s technology it is impossible to dope a material with impurities to achieve this level of refractive-index contrast. Researchers have tried to make approximate versions in the past, but they haven’t been totally successful.
Now, Ulf Leonhardt and colleagues at the University of St Andrews have created a Luneburg lens for infrared light with a silicon waveguide. “It was thought to be impossible to build a Luneburg lens in the visible spectrum, or nearby, at a reasonable cost,” says Juan Miñano, an optics researcher of the Technical University of Madrid, who was not involved with the research. “Fortunately Leonhardt is not stopped by any well-accepted thought and is delighting us with this achievement.”