Recently, researchers from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) have proposed a design concept for a tunable broadband perfect absorber based on the non-split coupling of Epsilon-near-zero (ENZ) and localized surface plasmon resonance (LSPR) modes. The sub-wavelength single-layer ITO ellipsoidal shell array is designed to excite spatially separated ENZ and LSPR modes and enable more than 98% light absorption in the range of 1435–1680 nm. A centimeter-level absorber is fabricated by low-cost self-assembly technology and features polarization-independent, wide-angle, and tunable resonant absorption. The related results were published in Applied Surface Science under the title of "Polarization-Independent, Tunable, Broadband Perfect Absorber Based on Semi-Sphere Patterned Epsilon-Near-Zero Films" on April 29, 2022.

Optical absorption is critical in many linear and nonlinear optical applications. In recent years, planar ENZ materials have provided effective solutions for various narrowband, broadband, tunable perfect absorbers. However, planar ENZ materials for perfect absorption face significant polarization dependence and oblique incidence problems, which limit the application scenarios of perfect absorbers based on planar ENZ materials. The combination of ENZ films with metasurfaces is thought to solve the above polarization and angle dependence problems. However, the reported metasurface schemes inevitably involve complex designs, multi-step fabrication, and expensive FIB or EBL techniques, which limit the device size to the order of hundreds of microns.

This work proposes a broadband perfect absorber scheme based on semi-sphere patterned ENZ films. The core idea of the perfect absorber is to utilize the unique ultrathin film and nanoparticle characteristics of the semi-ellipsoidal shell ENZ film to excite spatially separated ENZ and LSPR resonance modes to achieve broadband perfect absorption. The unique structural symmetry enables the absorber to exhibit polarization-independent and wide-angle features, while the active tunability of ENZ materials endows the absorber with tunability.

In addition, the perfect absorber is compatible with self-assembly technology, which can realize the fabrication of centimeter-scale or even wafer-scale devices at a low cost. In addition to ITO materials, the perfect absorber scheme proposed in this paper is suitable for other ENZ materials with both ENZ characteristics and plasmon characteristics, such as CdO, AZO, TiN, etc., which is conducive to realizing broadband perfect absorbers in more wavelength ranges.

Figure 1. (a) Schematic diagram of the excitation unit of the ITO semi-ellipsoid shell and the realization principle of the tunable absorption band. (b) Schematic diagram of the structure of the broadband perfect absorber. (c) Microscopic topography of the cross-section of the device. (d) Reflectance spectrum and picture of the device. (Image by SIOM)

Figure 2. (a) Reflection spectrum of unpolarized light varies with the angle of incidence. (b) Reflectance spectra of different polarization states under 20° incident conditions. (Image by SIOM)

Article website:
https://doi.org/10.1016/j.apsusc.2022.153551

Contact:
WU Xiufeng
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: xfwu@siom.ac.cn
Web: http://english.siom.cas.cn/