By Lida Tunesi
A new paper from the lab of Professor Andrea Alù, founding director of the Photonics Initiative at the Advanced Science Research Center at The Graduate Center (CUNY ASRC), shows that it is possible to create engineered surfaces, or metasurfaces, with superior control over light emission in the form of thermal radiation and photoluminescence. Metasurfaces are a well-established technology that can manipulate light in extreme ways, such as creating very thin lenses or holograms. However, they are currently limited to working with coherent external light sources, such as lasers.
The advance could allow scientists to make custom light sources much smaller, cheaper, more efficient, and more portable.
“Custom light sources are used in all areas of science,” Alù said, “and in many areas of technology. Compact, lightweight custom sources are particularly important in applications needing portability, such as in space-based astronomy, field research for geology or biology, biomedical, and military applications.”
Metasurfaces are surfaces so thin that researchers refer to them as two-dimensional. From the surface rise “nanopillars,” tiny structures as small as 100 nanometers, or about four millionths of an inch, tall. Because these surface features are smaller than the wavelengths of the incoming light, they are able to control and shape that light in extremely precise ways. In that sense, metasurfaces are newer, more compact, and more efficient alternatives to older technologies for controlling light, such as glass lenses.
Scientists, however, are currently limited to using metasurfaces with coherent light from external sources, such as that produced by lasers. These beams are easier to control, but if scientists could have the same level of control over light emanating from the metasurface itself, this would open new possibilities.
“Thermal metasurfaces eliminate the need for bulky external light sources to illuminate metasurfaces,” Overvig said, “and instead allow a custom light source that is ultrathin and lightweight, controlled simply by heating up the metasurface.”
Thermal radiation is light produced by the random movement of charged particles like protons and electrons. Inside any object with a temperature above absolute zero, these particles move around. As they do so, their kinetic energy is converted into electromagnetic energy, and the object emits light, though not always in wavelengths visible to the human eye.
It is this random nature of thermal radiation that has made it challenging to combine with metasurfaces. Typically, thermal light contains a wide range of frequencies and emits in many different directions, and so is difficult to control.
In the new paper, though, Alù, Overvig, and Mann show that such control is possible. The metasurface nanostructures must be carefully engineered to control thermal light both at a scale smaller than the light’s wavelength and at a scale larger than the wavelength. The former allows for control over polarization state, wavefront shape, and directionality, and the latter makes it possible to wrangle light in different places and times into something more closely resembling coherent laser light. Alù’s research group is now fabricating metasurfaces based on these ideas for experimental demonstrations in the lab.
Because photoluminescent sources, like LEDs, emit light based on similar principles, these findings also allow for enhancements in consumer technology.
“We are used to incandescent bulbs, which are thermal sources, emitting bright, broad, white light all around,” Alù said. “With our findings we can envision lightbulbs, as well as LEDs, that emit customized holograms, focus light in specific directions, and tailor the polarization and directionality of emitted light at will.”
Read more about this discovery: “Customizing Thermal Emission,” Physics, June 4, 2021
Published by The Graduate Center’s Office of Communications and Marketing.