'Transmogrifying' mirrors created with nanoscale patterning Arrays of tiny rectangular blocks can turn ordinary materials into metasurfaces, allowing scientists to precisely control how light interacts with the object. Fundamental Research © Pixel-Shot, Shutterstock In the last 30 years, photonics has witnessed important technological developments, becoming central to applications in navigation, remote sensing and communications. However, manipulating light typically requires the use of optical lenses which can be bulky, heavy and fragile. The EU-funded FLATLIGHT project supports the further miniaturisation of optics and optoelectronics by using an alternative system to guide and shape light. “What we’re doing is conceiving optical devices that replicate bulky optical lenses and prisms using nanostructured interfaces,” explains project coordinator Patrice Genevet. Light control His team at Côte d’Azur University's Research Center for Heteroepitaxy and its Applications (CRHEA) constructed interfaces just 1 micron thick, containing arrays of nanoscale rectangular blocks of tightly controlled size, shape and spacing. “These components shape light by changing the electromagnetic field properties, i.e. amplitude phase, polarisation and frequency, across material thinner than 100th the diameter of a human hair,” says Genevet. “The goal of FLATLIGHT is to fabricate metasurfaces operating in the visible wavelength range using nanostructured semiconductor materials that control light emission, transmission and reflection at the interface.” For example, certain nanostructures can bend light in an arbitrary direction as it passes through the interface. By precisely varying the pattern across the surface, light passing at the edges can be bent more than in the centre, focusing the beam. “These are metasurfaces, i.e. surfaces with functionalities that extend beyond classic interfaces,” adds Genevet. “Metasurface light-addressing capabilities are unique, you are not able to find similar effects in other optical devices.” One application is in vertical-cavity surface-emitting lasers (VCSELs) – tiny electronic components used to emit high-frequency, low-power laser light. These are central to applications such as the LIDAR needed for smartphone face recognition. By patterning the surface of the laser diode, Genevet and his team demonstrated that the emitted light could be shaped as desired, reducing divergence and removing the need for an additional collimation lens. Cloaking devices The nanostructures can also improve on traditional optical lenses, which suffer from inherent shortcomings known as aberrations. For example, chromatic aberration occurs as different colours of light are refracted at different magnitudes, giving the resulting image rainbow fringes. Etching nanostructures on the surface of the lens could compensate for that. The technology can also be used to make LEDs with more advanced properties such as polarisation and directional lighting, offering the potential to create compact holographic displays. “Another interesting application is cloaking,” says Genevet. “Imagine wrapping an interface around an object and reflecting the image of something else. It would be a new type of cloaking interface, capable of turning one thing into something else, bringing Harry Potter’s transmogrifying spell to reality.” The work was supported by the European Research Council. “It’s very clear that without ERC support, nothing would have happened,” notes Genevet. “It was an amazing opportunity to establish my programme, develop all of the related nanofabrication recipes, all of the conception programmes, and establish the optical characterisation lab.” Next, Genevet plans to apply for a Consolidator Grant to continue his research. “We have come up with amazing new ideas, and now we have a clear vision of what is possible and what is not.” His lab also received ERC proof of concept funding to develop a compact high-frequency LIDAR system using metasurfaces. They are now aiming to build a more comprehensive prototype with a view to commercialising this technology. Keywords FLATLIGHT, light, refraction, laser, metasurface, nanoscale, transmogrify, lenses, photonics, optoelectronics
