Large-Scale, Bandwidth-Adjustable, Visible Absorbers by Evaporation and Annealing Process
Optical absorbers have received a significant amount of attention due to their wide range of applications in biomedical sensing, solar cell, photon detection, and surface-enhanced Raman spectroscopy. However, most of the optical absorbers are fabricated with high-cost sophisticated nanofabrication techniques, which limit their practical applications. Here, we introduce a cost-effective method to fabricate an optical absorber by using a simple evaporation technique. The absorbers are composed of evaporated nanoparticles above a silver (Ag) mirror separated by a silicon oxide layer. Experimental results show over 77% absorption in the wavelength range from 470 to 1000 nm for the absorber with isolated Ag nanoparticles on the top. The performance of the absorber is adjustable with the morphology and composition of the top-layer nanoparticles. When the top layer was hybrid silver-copper (Ag-Cu) nanoparticles (NPs), the absorption exceeding 90% of the range of 495–562 nm (bandwidth of 67 nm) was obtained. In addition, the bandwidth for over 90% absorption of the Ag-Cu NP absorber was broadened to about 500 nm (506–1000 nm) when it annealed at certain temperatures. Our work provides a simple way to make a highly efficient absorber of a large area for the visible light, and to transit absorption from a narrow band to broadband only by temperature treatment.
KeywordsMetasurfaces Visible absorbers Bandwidth-adjustable Copper-silver alloy
Atomic force microscopy
Local surface plasma resonances
Scanning electron microscopy
Sub-wavelength absorbers have attracted considerable attentions due to their light and thin features which enable their wide applications ranging from biochemical sensing [1, 2], and enhanced spectroscopies to solar cells [3, 4, 5]. Classical metal-insulator-metal (MIM) absorbers consist of top-layer metallic resonators and a bottom metal mirror separated by a spacer layer. The absorption of light can be maximized when a large number of plasmonic nanostructures are exposed to incident light with suitable frequency [6, 7]. As the absorption is associated with excitation of local surface plasma resonances (LSPRs) of the patterned structures, it is possible to adjust the absorption by changing the structural design [8, 9, 10]. In addition, changing the material of the spacer layer results in the change of absorption. Some phase-change materials like Ge2Sb2Te5 [11, 12, 13] and VO2 [14, 15] and electrically tunable graphene [16, 17, 18, 19] are typically used to adjust absorption. These ways break the limitations of the material’s inherent response spectrum [20, 21]. Due to the extremely fine features of the resonators, nanofabrication methods are commonly used to fabricate plasmonic absorbers. DUV lithography [22, 23, 24], nanoimprint lithography [25, 26], and electron beam lithography are mostly used nanofabrication techniques. Due to the flexibility of nanofabrication technique, various kinds of metallic structures such as gratings and nanoparticles have been fabricated and investigated for their absorption [27, 28, 29, 30]. However, these nanofabrication techniques are expensive and complicated and not suitable for fabrication over large areas, hindering commercialization of optical absorbers. In addition, once the absorbers are fabricated, their absorption bandwidth is not easy to adjust. Recently, direct evaporation or sputtering of non-uniform nanoparticles have been introduced as low-cost methods for fabrication of plasmonic absorbers [31, 32]. These methods are promising to act as a low-cost fabrication method for optical absorbers and need to be further investigated. Especially, the fabrication of bandwidth-adjustable absorbers with the evaporation methods has not been reported.
In this work, we investigate the methods of evaporation to fabricate optical absorbers numerically and experimentally. Broadband and narrow-band absorbers were controlled by the composition of the evaporated metals. The nanoparticles were evaporated above the Ag mirror with a SiO2 spacer layer in between. Broadband absorption was obtained with Ag-only nanoparticles, and narrow-band absorption was obtained with hybrid Ag-Cu nanoparticles. The absorption can be converted from narrow- to broadband with the Ag-Cu nanoparticle (NP) absorber by changing the annealing temperature.
Fabrication of Metasurfaces
The surface patterns were examined by scanning electron microscopy (Hitachi SU8010) and atomic force microscopy (Dimension EDGE).
The fabricated absorbers were measured with the portable spectrometer (Ocean Optics) for their reflectance. The light source is a 100-W halogen lamp. The light shines normally to the sample surface with a hybrid fiber and a holder. The measured reflection spectra were normalized to the reflection of a blank aluminum mirror.
Numerical simulations were performed with finite-element method (FEM)-based commercial software package, CST Microwave Studio. Dispersion parameters of the Ag and Cu were obtained from literature . The thickness of ground plane and dielectric layer are 150 nm and 90 nm, respectively. Unit cell boundary condition is applied in x- and y-directions. In the z-direction, we chose an open boundary condition. The polarization of the incident light is along the x-direction. As the thickness of the metallic ground plane is greater than its skin depth, the transmittance can be neglected. Then the absorption can be simplified as A(ω) = 1 − R(ω), where R is reflectance. To model the random distribution features of metallic nanoparticles, we changed the size and height of the particles in the simulation. The overall absorption spectrum was an enveloped profile of each individual nanoparticle simulated.
Results and Discussions
In conclusion, we have demonstrated fabrication of plasmonic absorbers simply with an evaporation method. Broadband and adjustable band absorbers were fabricated by controlling the composition of the evaporated nanoparticles. Broadband absorption was achieved with pure Ag nanoparticles on the top, and bandwidth-adjustable absorption was achieved with hybrid Ag-Cu nanoparticles on the top. The Ag-Cu NP absorber demonstrated single-frequency absorption before annealing and the absorption became broadband when annealed at a certain temperature. The absorption is > 90% in a wavelength range of 506–1000 nm, which covers both the visible and near-infrared ranges. Our work has provided a simple and low-cost fabrication technique to make large-area visible absorbers. In addition, the high absorption is accompanied with a huge local field enhancement, which makes our absorbers suitable for surface-enhanced Raman scattering (SERS) and other surface spectroscopies.
Thanks to Dr. Hou from Sichuan university for his support in testing.
This work was supported by the Opening Foundation of State Key Laboratory of Optical Technologies on Nano-Fabrication and Micro-Engineering, Institute of Optics and Electronics, Chinese Academy of Sciences; 973 Program of China (2013CBA01700); Sichuan Provincial Department of Education (16ZA0047); Sichuan Science and Technology Program (2018JY0439, 2018JY0616).
XYL, WDC, and YRS completed the preparation, characterization, and analysis of the samples. XYL performed the simulation and wrote the manuscript draft. LL and WSY advanced the theoretical considerations and refined the details of the draft. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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