1. Introduction

Since the first demonstration by Landy et al [1], metamaterial absorbers (MAs) have attracted increasing scientific and industrial interest due to various light harvesting applications [2–26]. Generally speaking, MAs can be divided into narrowband absorber and broadband absorber in terms of the absorption bandwidth. The narrowband MAs are widely used in plasmonic sensors [8–10] and detectors [11–14], while the broadband MAs are broadly utilized in solar thermal energy converters [2–7], cloaking devices [15], thermal emitters [16–19], and artificial colors [21,22,25]. Indeed, considerable work has been done to enlarge the absorption band and increase the absorptivity for the broadband MAs. For example, Wang et al. demonstrated an absorber using silicon nano-cone arrays based on Mie resonances achieving absorption of 95% over the broadband wavelength from 300 nm to 2000 nm [20]. Guo and associates reported a nano-structured absorber based on the special dispersion relationship with flat average absorption greater than 80% within the visible range of 400-700 nm [21]. Later, they proposed a semiconductor layer absorber with coupled multi-cavity resonance effect realizing wide-angle, polarization-independent light absorption in the visible regime [22]. Tun et al. developed a broadband MA composed of phase change materials and separated thin gold (Au) squares for uniform absorption in the whole visible region [23]. Ko Aydin proposed a MA consisting of a metal-insulator-metal stack configuration utilizing multi-cavity resonance effects yielding an average absorption of 71% over the visible range from 400 nm to 700 nm [24]. Qian and associate fabricated a MA consisting of elaborately designed stepped nickel (Ni) nanopillars realizing high absorptivity of 96% over the entire visible wavelength band of 400-760 nm [25]. However, most of the aforementioned broadband absorbers are usually based on rigid substrate and one or more different structured materials. Moreover, complicated device configurations and fabrication procedures still constitute significant problematic to achieve practical applications due to the time-consuming and costly fabrication technology, such as electron beam lithography or focused ion beam milling. Herein, new MA structures that can be achieving broadband high absorption in the whole visible spectrum regime should be developed to avoid the complicated architecture topography and the high-cost fabrication procedures.

Here, we proposed and demonstrated an integrated MA composed of low-cost homogenous only one material of periodic non-noble Ni cylinder arrays, which is obviously different from most of the reported MAs composed of composite materials with specific structures. The simulated optimal polarization-independent absorbance of the proposed MA is as high as 90% from the wavelength of 400 nm to 650 nm, and the achieving nearly perfect absorption of 99.8% at the specific wavelength of 580 nm at normal incidence. The proposed optimal MA was fabricated by the low-cost double-beam interference lithography followed by soft nano-imprinting and electroforming technology. It had an average absorption of 92% over the entire visible wavelength band from 400 nm to 700 nm and the absorbance still remained beyond 70% when the incident angles varying from 0° to 60°. The presented fabrication approach is simple and cost-effective, which make it potentially applicable for solar-thermal energy harvesting, structure color, thermoelectric, and imaging.

2. Design and simulations

The schematic diagram of the proposed MA is presented in Fig. 1. Two-dimension periodic Ni nano-cylinder array is uniformly arranged on a substrate with same material of metal Ni. The simulations are performed by employing the rigorous coupled wave analysis (RCWA) method [27]. The absorption can be calculated by A = 1- T - R, where A, T and R depict absorption, transmission, and reflection, respectively. The metal Ni is employed because of its own properties, such as strong decay and high melting point (1455°C) for specific applications (e.g., solar thermal absorbers). Furthermore, the adopted Ni is cost-effective compared with that of the previously reported MAs with the noble metals, implying that the proposed structure is directly compatible with inexpensive manufacture technology, offering additional appealing features for various potential applications. Notably, the suggested nano-cylinder array is symmetric along the X and Y directions, indicating that the desirable polarization-independence could be deservedly obtained. The structure parameters close related to the absorption quality of the MA, namely, the period (p), the width (w), the height (h) and the fill factor (f = w/p) of the cylinder array, are optimized to be 410 nm, 205 nm, 380 nm and 0.5, respectively. The absorption as function of the wavelength over the visible range from 400 to 700 nm at normal incidence is presented in Fig. 2. It can be clearly seen that three distinctive resonance peaks locate at 410 nm (point B₁), 450 nm (point B₂) and 580 nm (point B₃), respectively, and substantial broadband absorption at normal incidence exceeding 90% (from 400 to 650 nm) is obtained. Moreover, the maximal absorption approaches to 99.8% at the specific wavelength of 580 nm, clearly demonstrating excellent broadband high absorption properties (average absorption is 94%) in the visible region.

 

Fig. 1 The schematic architecture of the designed MA.

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Fig. 2 The simulated absorption (black), reflection (red) and transmission (blue) properties of the proposed MA at normal incidence.

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To probe the effects of various geometric parameters and materials on the performance of light absorption, the designed architecture was systematically analyzed. The absorption spectrum curves for several commonly used metals (Ti, Ag and Al) are calculated with the same optimized structured parameters and are presented in Fig. 3. It can be seen that the suggested MAs with Ti (average absorption > 95%) and Ni (average absorption > 94%) exhibit desirable comparable absorption capabilities compared with that of the same architecture with the materials of Ag (average absorption > 69%) and Al (average absorption > 64%). Although Ti has better absorption performance and is cheaper than Ni, we still choose metal Ni. This is because nickel is a commonly used material in electroforming technology, providing a great potentiality to be widely used in the high-throughput manufacture technology in the near future. The obvious discrepancy for various structured metal material could be attributed to the metal’s intrinsic dispersion property. Besides, it is noted that the strong averaged absorption cannot be realized by the flat monolayer Ni film (average absorption > 40%), clearly verifying the validity of our proposed MA structure.

 

Fig. 3 Absorption spectra with different metals.

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To explain the underlying mechanism of strong and broadband absorption of the proposed MA, the magnetic and electric field distributions under normal incidence at the specific resonant peaks B₁, B₂, and B₃ depicted in Fig. 2 and non-resonant wavelengths (@ 650 nm) are simulated and described in Fig. 4. As apparently displayed in Fig. 4(a), the electric field is mainly confined in the grooves and extends far from the surface. The former indicates that the slot mode resonance plays a role, while the latter indicates that it is the effect of Rayleigh-Wood anomalies [30]. Rayleigh-Wood anomalies is a passing-off of a spectral diffraction, and the wave vectors of Rayleigh-Wood anomalies mode are governed by the well-known grating formula $K_{\text{RWA}}=K_{\text{inc},\text{x}}\pm \text{mG}$, where $\text{ G}=\raisebox{1ex}{$2\text{π}$}\!\left/ \!\raisebox{-1ex}{$\text{p}$}\right.$, $K_{\text{inc},\text{x}}=\raisebox{1ex}{$\text{ω}$}\!\left/ \!\raisebox{-1ex}{${\text{c}}_0$}\right.\sin \text{θ}$, and m is an integer that defines diffraction order. According to the above equations, Rayleigh-Wood anomalies resonance is sensitive to incident angle and the period (p) of the cylinder array in nature. Therefore, the absorption peak associated with RWA appears at around 410 nm, as shown in Fig. 2. In Fig. 4(c) and Fig. 4(e), the magnetic field intensity is concentrated within the grooves and the top surface of the Ni array, which allows the slots modes that exist within the grooves to couple to one another leading to a supported surface wave [31,32]. At non-resonant wavelengths of 650 nm, as plotted in Fig. 4(g), the magnetic field supported along the surface of the Ni array is almost out of sight and the magnetic field is totally confined within the groove. From these three graphs, we can draw the conclusion that the slot modes and the coupling effect dominate the absorption response. As depicted in Fig. 4(d), Fig. 4(f) and Fig. 4(h), the strong electric fields are localized at the corners of the Ni arrays, indicating the supported surface plasmon polaritons [32].

 

Fig. 4 The magnetic and electric near-field distributions for the proposed MA at the specific wavelengths at normal incidence.

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To further identify the nature of these near-field optical modes, the Poynting vector distribution at specific resonant wavelengths of 450 nm and 580 nm are calculated and are presented in Fig. 5. Note that the majority of the incident light energy flow flood into the nano-groove and penetrated into the metallic Ni nano-cylinder sidewalls, resulting in the absorb light by collective excitation of electrons in the form of the excited surface plasmons.

 

Fig. 5 The simulated Poynting vector distributions excited by the specific resonant wavelength of (a) 450 nm, and (b) 580 nm. The arrows depict the direction of the light energy flow and the white lines display the interface of the cylinder arrays.

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Moreover, to investigate the angular dependence properties of the proposed MA, the simulations of the absorption spectra under oblique incident angle for the TM- and TE- polarized light are performed and displayed in Fig. 6. The average absorption of the two polarizations at visible frequency are still retain beyond 70%, resulting in the angle-robust absorption with the incident angles varying from 0° to 60°.

 

Fig. 6 Simulated angular absorptions of the MA in Fig. 1 for TM- and TE- polarized light. The incident angle is varied from 0° to 60° in 15° steps.

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The influence of the height h and duty cycle f (f = w/p) of Ni cylinder arrays on the absorption performances are dedicatedly simulated and are detailed in Fig. 7. It is obvious that the absorption peak around the wavelength of 410 nm remain unchanged regardless of the variation of the structure parameters of h and f, clearly illustrating that Rayleigh-Wood anomalies play an important role in the strong absorption at 410 nm. In addition, as plotted in Fig. 7(a) and Fig. 7(b), the absorption peak changes slightly around 450 nm as the increase of duty cycle. This could be ascribed to the weakening of Rayleigh-Wood anomalies and the enhanced slot modes. In contrast, the absorption peak at 600 nm displays red-shifted feature as the values of h and f increase monotonically, implying that the demonstrated slot modes become dominate role and steer the incident photons absorption characteristics as the increasing of the wavelength. These results are in good agreement with the near-field magnetic and electric field distributions discussed in Fig. 4.

 

Fig. 7 Calculated absorption characteristics as function of the wavelength for the parameter of (a) height h, and (b) duty cycle f.

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3. Experimental section

Figure 8 graphically illustrates the fabrication process of the proposed MA. In a typical process, the photoresist (AZ MIR-701) layer of ~400 nm was firstly spin-coated onto a pre-cleaned glass substrate and then periodic cylinder arrays were generated by using double-beam interference lithography techniques and subsequently developed in NaOH solutions (6‰) for 6 s and dried by electric blow drier. As a consequence, the periodic cylinder arrays are successfully produced on the glass. To fabricate the cost-effective MA, soft nano-imprinting lithography and electroforming technology are adopted to fabricate the MA. Subsequently, the UV resin (D10, PhiChem) was drop-cast onto the fabricated glass template, and PET substrate was placed onto the UV resin and imprinted under a constant pressure of 1.5 bar for 15 s with a UV illumination of 500 mJ/cm2 at a wavelength of 395 nm. Subsequently, by carefully peeling off the PET, the periodic cylinder arrays (UV resin) constructed on the PET substrate was obtained [28,29]. Finally, industrial-grade electroforming technology was employed to obtain the integrated Ni cylinder arrays. The SEM images of proposed MA with different scales are shown in Figs. 9(a)-9(c). It is remarkable that that the height, the period and the diameter of Ni cylinder arrays are almost 372 nm, 405 nm and 185 nm, respectively. It should be noted that some defects in Ni metamaterial absorber can be observed [Fig. 9(a)], which are mainly stemmed from the imperfect demolding process due to the innate viscosity of the adopted UV-resin. This minor obstacle would be solved for employment of some new materials with the rapid progress of the nanofabrication technologies.

 

Fig. 8 Schematic of the fabrication processes of the proposed MA with nano-cylinder arrays.

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Fig. 9 SEM images of the proposed MA with different scales (a-c) and the side-view of the sample (d).

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To probe into the viability of the proposed MA, the angle-resolved absorption spectra for TM- and TE- polarized light are measured employing the spectrophotometer (LAMBDA 750) and are shown in Fig. 10(a) and Fig. 10(b), respectively. The average measured absorption of the two polarizations from 400 to 700 nm at normal incidence is about 92%, slightly less than the theoretical value (94%). Notably, the absorption nearly keeps as a constant until the incident angle reaches up to 60°, directly illustrating the broadband polarization-independent and wide-angular absorption. This experimental absorption results correspond very well with the prediction of the simulations, clearly verifying the validity and the accuracy of our method. The obtained MA clearly shows “black” appearance at different viewing angles (0°, 30°, 60°) and even was rolled on the surface of the ballpoint pen, illustrating the desired strong light absorption [Figs. 10(c)-10(d)]. Besides, it is noteworthy that there is negligible discrepancy between the calculated and measured absorption spectra, which mainly originates from the fabrication error of the sample and the background testing signal.

 

Fig. 10 The measured incident angle resolved spectrum response of the MA for (a) TM and (b) TE polarization, respectively. (c) The optical images of the fabricated MA taken with indoor ambient light at different incidence of 0°, 30° and 60°. (d) The MA rolled on the surface of the ballpoint pen.

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4. Conclusions

In summary, an integral broadband, polarization-independent absorber at visible frequency has been demonstrated. The measured average absorption exceeds 92% over the wavelength ranges from 400 to 700 nm. Moreover, the absorption still remains beyond 70% when the incidence changes from normal to 60° oblique. The proposed MA can be easily obtained without involving ion or electrochemical etching of metal procedures, and can be easily integrated into other optoelectronic devices. The new optical and technical features make the presented ultrathin homogenous meta-surface nickel absorber an alternative candidate for applications in solar system and radiation thermal devices in the foreseeable future.