&Bullet; physics 14, 84
Theorists have shown that patterned surfaces can convert the emission of a hot object into a polarized, focused beam of light.
Machined surfaces can be used to manipulate laser light, focus it on a specific point, or give it a specific polarization. Now researchers have developed a recipe for creating surfaces that can be used to manipulate thermal radiation, such as the glow of a candle or a hot wire  . Previous work has shown some degree of control over thermal radiation, such as collimating it into a beam. However, the new work provides designs for surfaces that can convert the radiation from a hot object into a beam of light with any frequency, polarization, or wavefront shape. The result would be a laser-like emission from a simple supply of heat.
The ability to manipulate thermal radiation is not new. With pinhole diaphragms, lenses and filters, for example, the light from a light bulb can be converted into a focused beam of polarized, monochromatic light. However, this approach is not only bulky, but also inefficient because it loses much of the original light. Andrea Alù and his colleagues from the City University of New York have now developed a more efficient and compact method for manipulating thermal radiation. You envision using a surface that can be placed on an object to convert its heat into light that is emitted in a specific direction with a specific polarization and a near-monochromatic spectrum.
The team’s strategy is based on the burgeoning field of meta-surfaces, which are arrangements of tiny optical elements that act in a coordinated manner on incident light. The team’s thermal meta-surface design consists of an array of silicon pillars, the size of which is smaller than the desired output wavelength. Each column has an elliptical cross-section so that the thermal radiation emitted along its longitudinal axis differs from that emitted along its short axis. By choosing the orientation of each column, the researchers were able to shape the heat emission from point to point along the surface.
But there is a catch. This type of local control – the foundation of Metasurface technology – should only work if the emission of a column is coherent with the emission of its neighbors (the light waves are synchronized). This coherence can be expected when the pillars are excited by a laser, but it is not expected when the excitation is thermal.
Alù and his colleagues solved this problem by incorporating a well-known collective resonance effect that occurs on patterned surfaces such as grids  . This effect temporarily traps light at the surface, creating “crosstalk” between separate regions, causing their emitted light to be synchronized. By arranging the pillars in a large area, double-layer pattern, Alù’s team found that they could create this non-local behavior while maintaining local control based on the orientation of individual emitters. “Our suggestion is a way of giving a meta-surface really complete control over thermal emissions,” says Alù.
The researchers created a kind of “alphabet” of columnar patterns that engineers could program onto a surface to achieve any desired heat emission performance. The team simulated a handful of possible arrangements, for example one that converts the thermal emission into a near-monochromatic beam and another that focuses it on one point. They also developed a meta-surface that can rotate the wavefront – called orbital angular momentum – of an object’s thermal emission. “This is way beyond what anyone could do before,” says Al.
The ability to control heat emission could lead to new light sources that do not require electricity. “The dream is to have a kind of radiation that comes close to a laser, but comes from a heat source that you can simply heat up,” says Alù. He and his colleagues write in their work that their designs correspond to the current manufacturing possibilities, since tiny pillars are often made with lithographic techniques. It will be a challenge to align the pillars exactly according to the recipe. Another complication will be stacking columns in double layers, as some of the team’s designs require. Still, there are simpler patterns that the researchers want to create as a demonstration of their technique. And they see an early application for their meta-surfaces in the control of LED light, which is not thermal radiation but has the same incoherence.
University of Wisconsin-Madison photonics expert Zongfu Yu says Alù and his colleagues have demonstrated the essential role of nonlocal effects in controlling heat emission. “This work illustrates the design principle in an intuitive way with recipes that others can easily follow to design new spotlights.” Yu believes these meta-surfaces could be useful for lighting, cooling and energy applications.
Michael Schirber is the corresponding editor for physics based in Lyon, France.
- AC monitoring et al., “Thermal meta-surfaces: Complete emission control through a combination of local and non-local light-matter interactions”, Phys. Rev. X11, 021050 (2021).
- J.-J. Met et al., “Coherent light emission by thermal sources”, nature416, 61 (2002).