Incandescent light bulbs and other thermal radiation sources can produce coherent, polarized and directed emissions with the help of a structured thin film known as a metasurface. Created by Andrea Alù and colleagues at the City University of New York (CUNY), US, the new metasurface uses a periodic structure with tailored local perturbations to transform ordinary thermal emissions into something more like a laser beam – an achievement heralded as “just the beginning” for thermal radiation control.
Scientists have previously shown that metasurfaces can perform tasks such as wavefront shaping, beam steering, focusing and vortex beam generation that normally require bulky traditional optics. However, these metasurfaces only work with the highly coherent light typically emitted by lasers. “There is a lot of hype around compactifying optical devices using metasurfaces,” says Alù, the founding director of CUNY’s Photonics Initiative. “But people tend to forget that we still need a bulky laser that is exciting them.”
Unlike lasers, most light sources – including LEDs as well as incandescent bulbs and the Sun – produce light that is highly incoherent and unpolarized, with spectra and propagation directions that are hard to control. While it is possible to make thermal emissions coherent, doing so requires special silicon carbide materials, and the emitted light has several shortcomings. Notably, a device designed to emit light to the right will also emit it to the left – a fundamental symmetry known as reciprocity.
Some researchers have argued that reciprocity fundamentally limits how asymmetric the wavefront emitted from such structures can be. However, in 2021 members of Alù’s group showed theoretically that a metasurface could produce coherent thermal emission for any polarization, travelling in any direction, without relying on special materials. “We found that the reciprocity constraint could be overcome with a sufficiently complicated geometry,” Alù says.
Smart workarounds
The team’s design incorporated two basic elements. The first is a periodic array that interacts with the light in a highly non-local way, creating a long-range coupling that forces the random oscillations of thermal emission to become coherent across long time scales and distances. The second element is a set of tailored local perturbations to this periodic structure that make it possible to break the symmetry in emission direction.
The only problem was that this structure proved devilishly difficult to construct, as it would have required aligning two independent nanostructured arrays within a 10 nm tolerance. In the latest work, which is described in Nature Nanotechnology, Alù and colleagues found a way around this by backing one structured film with a thin layer of gold. This metallic backing effectively creates an image of the structure, which breaks the vertical symmetry as needed to realize the effect. “We were surprised this worked,” Alù says.
The final structure was made from silicon and structured as an array of rectangular pillars (for the non-local interactions) interspersed with elliptical pillars (for the asymmetric emission). Using this structure, the team demonstrated coherent directed emission for six different polarizations, at frequencies of their choice. They also used it to send circularly polarized light in arbitrary directions, and to split thermal emissions into orthogonally polarized components travelling in different directions. While this so-called photonic Rashba effect has been demonstrated before in circularly polarized light, the new thermal metasurface produces the same effect for arbitrary polarizations – something not previously thought possible.
Reconfigurable metasurface steers incoherent light in less than a picosecond
According to Alù, the new metasurface offers “interesting opportunities” for lighting, imaging, and thermal emission management and control, as well as thermal camouflaging. George Alexandropoulos, who studies metasurfaces for informatics and telecommunication at the National and Kapodistrian University of Athens, Greece but was not involved in the work, agrees. “Metasurfaces controlling thermal radiation could direct thermal emission to energy-harvesting wireless devices,” he says.
Riccardo Sapienza, a physicist at Imperial College London, UK, who also studies metamaterials and was also not involved in this research, agrees that communication could benefit and suggests that infrared sensing could, too. “This is a very exciting result which brings closer the dream of complete control of thermal radiation,” he says. “I am sure this is just the beginning.”
