Infrared Metamaterial Absorber Shows Promise in Radiative Cooling

Researchers from the State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) have proposed a broadband, polarization- and angle-insensitive metamaterial absorber (MA) that covers the longwave infrared (LWIR) band. The design supports radiative cooling applications, as well as infrared imaging and thermal detection.

MAs that are based on subwavelength periodic patterns can achieve perfect absorption at any target wavelength, depending on the design of their geometric parameters. Because they are ultrathin and due to their near-field enhancement, MAs are widely used in photodetection, solar energy harvesting, and gas detection, as well as in thermal emitters and infrared imaging.

However, given the resonant nature of plasmon excitation, common MAs are only able to achieve perfect absorption in a narrow wavelength range.

In the current work, the researchers succeeded in inducing antiparallel currents in the upper and lower metal layers of the MA; the incident light, coupled with the metal pattern of the MA, excited localized surface plasmon resonance. The induction ensured that energy was confined in the MA.

Schematic of the metamaterial absorber described by a team of researchers from the State Key Laboratory of Applied Optics, CIOMP, CAS. The absorber pertains to the LWIR portion of the spectrum — giving it implications for radiative cooling. Courtesy of OU.

Common broadband MAs are based on the combination of multi-localized surface plasmon resonance modes and the intrinsic absorption of lossy dielectric layers. In addition, rationally designed 2D metal grating structures can also excite propagating surface plasmon resonances (PSPR) at the metal-dielectric interface.

The researchers introduced PSPR into the patterned metal-dielectric-metal sandwich structure of MA. The top pattern of the MA consisted of two alternating ring sizes, with the four adjacent rings forming a “super unit,” which made the MA two distinct periods.

Therefore, the MA excited multiple modes of surface plasmon resonance, and the lossy metal enabled the four absorption peaks with wide full width at half maximum. The researchers obtained a perfect broadband absorption covering the LWIR band with this design.

Compared with previous broadband absorbers, the researchers said, the excitation of hybrid modes can achieve broader wavelength band absorption with a more straightforward structure. In addition, the energy of the incident light is all concentrated in the metal layer of the MA , which enabled the researchers to flexibly select the material of the dielectric layer according to the application scenario.

The research was published in Optics Express (www.doi.org/10.1364/OE.446655).