Abstract
Radiation absorbers have increasingly been attracting attention as crucial components for controllable thermal emission, energy harvesting, modulators, etc. However, it is still challenging to realize thin absorbers which can operate over a wide spectrum range. Here, we propose and experimentally demonstrate thin, broadband, polarization-insensitive and omnidirectional absorbers working in the near-infrared range. We choose titanium (Ti) instead of the commonly used gold (Au) to construct nano-disk arrays on the top of a silicon dioxide (SiO2) coated Au substrate, with the quality (Q) factor of the localized surface plasmon (LSP) resonance being decreased due to the intrinsic high loss of Ti. The combination of this low-Q LSP resonance and the propagating surface plasmon (PSP) excitation resonance, which occur at different wavelengths, is the fundamental origin of the broadband absorption. The measured (at normal light incidence) absorption is over 90% in the wavelength range from 900 nm to 1825 nm, with high absorption persisting up to the incident angle of ~40°. The demonstrated thin-film absorber configuration is relatively easy to fabricate and can be realized with other properly selected materials.
Highlights
Radiation absorbers have increasingly been attracting attention as crucial components for controllable thermal emission, energy harvesting, modulators, etc
Thin-film broadband radiation absorbers are extremely difficult to realize due to natural constraints: very high absorption occurs only near resonances whose bandwidth is necessarily limited, while strong mismatch in dielectric properties with environment causes high reflection at an interface
Highly-efficient broadband absorption is ascribed to the excitation of low-Q localized surface plasmon (LSP) resonance supported by titanium (Ti) nano-disks, and the generation of propagating surface plasmon (PSP) resonance
Summary
Radiation absorbers have increasingly been attracting attention as crucial components for controllable thermal emission, energy harvesting, modulators, etc. There have been other methods addressing this issue, such as adiabatic nanofocusing of gap surface plasmon modes excited by the scattering off subwavelength-sized wedges[23,24] and the excitation of slow-wave modes in tapered alternating metal-dielectric multiple thin films[25,26,27,28] Despite these significant achievements, it remains challenging to realize efficient light absorbers with a large bandwidth, high efficiency and fabrication simplicity, since the fabrication of the aforementioned broadband absorbers involved focused ion beam milling (FIB) or complicated multiple steps of electron beam lithography (EBL), which are time-consuming and costly. Numerical simulations show that the absorption performance is insensitive to the polarization of incident light, and high absorption persists when the incident angle is less than 40° Such a planar device is much easier to fabricate compared with other broadband absorbers with complicated structures. This design concept is general and can be applied to other frequencies with properly selected materials, such as highly-doped semiconductors in THz frequency
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