Fractal Metasurface Absorbers with Octave-Spanning Bandwidth

Metasurfaces, the 2D equivalent of metamaterials, have been of great interest in the past few years. They offer a means of commercially integrable applications, and have produced novel devices involving chiroptics [1],... [ view full abstract ]

Metasurfaces, the 2D equivalent of metamaterials, have been of great interest in the past few years. They offer a means of commercially integrable applications, and have produced novel devices involving chiroptics [1], highly-efficient holograms and mirrors [2,3], and beam manipulation [4,5]. Metasurface absorbers have been an interesting topic of research in the past few years, due to many potentially useful applications, including stealth and sensing. A particular focus has been on THz absorbers, due to the lack of naturally occurring materials having strong THz response. However, although most of these achieve high-absorption magnitude, many have limited bandwidth due to the highly resonant response of the meta-atoms. Devices which have both high-efficiency and broad bandwidth are typically multi-layered, hence involving complex design and fabrication.

Here, we present a planar, ultrathin, broadband THz absorber, based on supercells (P=40μm) of fractal crosses (Fig.1). We employ the geometry-dependent frequency response of varying fractal crosses (size approximately 20μm) to cover a broad FWHM bandwidth of 2.7THz (centred at 4.1THz), with a peak absorbance of 96% (A=1–R) (Fig.3). By utilising a unique absorption mechanism based on Salisbury screen and anti-reflection responses (Fig.2), we obtain high-magnitude dispersionless absorption over wide bandwidth. This design is distinct from typical metasurface absorbers, which utilise effective current loops to tailor the effective permeability.

Experimental and simulated results are well-matched (Fig.3), where Frequency Domain calculations in CST Microwave Studio were used for numerical modelling. Both experiment and simulation were carried out at 30° incidence, showing good robustness to angle of incidence. Such a device is expected to be of great benefit to sensing technologies, and paves the way towards the development of a synthetic metasurface blackbody absorber in the THz band.

[1] M. Kenney, S. Li, X. Zhang et al., Adv. Mater. 28, 9567 (2016).

[2] G. Zheng, H. Mühlenbernd, M. Kenney et al., Nat. Nanotechnol. 10, 308 (2015).

[3] S. Xiao, H. Mühlenbernd, G. Li et al., Adv. Opt. Mater. 4, 654 (2016).

[4] L. Liu, X. Zhang, M. Kenney et al., Adv. Mater. 26, 5031 (2014).

[5] G. Zheng, G. Liu, M. Kenney et al., Opt. Express 24, 6749 (2016).