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Aug 6, 2018 - Nano Science and Technology, Soochow University, Suzhou 215006, ..... InP nanowire arrays,” ACS Photonics 3(10), 1826–1832 (2016). 19.
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Omnidirectional broadband metasurface absorber operating in visible to near-infrared regime SHANGLIANG WU,1,2 YU GU,1,2,3 YAN YE,1,2,* HONG YE,1,2 AND LINSEN CHEN1,2,4 1

School of Optoelectronic Science and Engineering and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China 2 Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province & Key Lab of Modern Optical Technologies of Education Ministry of China and School of Optoelectronic Science and Engineering, Soochow University, Suzhou 215006, China 3 These authors contributed equally 4 [email protected] *[email protected]

Abstract: We present an omnidirectional broadband metasurface absorber whose dielectricmetal-dielectric layers are modulated by cylinder arrays. The simultaneous excitation of surface plasmon resonance and localized surface plasmon resonance affords an average optical absorption of 0.97 (0.9, experiment), with peak absorption up to 0.99 (0.984, experiment), for the wavelength range of 400-1100 nm, and absorption >0.93 (0.87, experiment) for incident angles up to 60°. The device, which is fabricated by continuously variable spatial frequency photolithography, outperforms previously reported absorbers in cost. Moreover, it exhibits considerably lower emissivity (weak absorption) in the midinfrared range, which makes it promising for energy harvesting. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction Metasurfaces, which are specially engineered materials that allow highly versatile manipulation of incident light at nanoscale levels, have found utility in the fabrication of special lenses [1,2], plasmonic detectors [3,4], color printing applications [5–10], and perfect absorbers [11–30]. Broadband near-perfect absorbers are mostly based on multiple pairs of stacked metal/dielectric resonators, which utilize the strong cavity resonance inside the dielectric film to afford absorption. Different material combinations such as nickel (Ni)aluminum oxide (Al2O3) [12], wolfram (W)-Al2O3 [13], platinum (Pt)-Al2O3 [14], silveramorphous silicon (Ag–a-Si) [15], and titanium nitride (TiN)-silicon oxide (SiO2) have been investigated as absorber materials [16]. However, for these structures, a wider absorptive bandwidth requires a corresponding increase in the total thickness, which limits their application. In this context, research efforts have been directed towards minimizing the absorber size. Meanwhile, it has been demonstrated that via assembly of resonators of varied geometries into one pixel [17–19], different resonant wavelengths can be excitated and utilized simultaneously. However, the incident angular tolerance is limited for interactions among the resonators. To achieve wider angular tolerance, absorbers incorporating patterned metals or dielectric-semiconductor-metal layers based on Mie resonance or localized surface plasmon modes have been explored [20–22], thereby resulting in absorbers consisting of stepped patterns and integrated arrays of silicon and chromium pillars. However, the process underlying the fabrication of such devices is complicated and expensive. Meanwhile, structures consisting of different patterned metal–dielectric–metal layers that utilize multiresonance phenomena have found wide applications [23–30], however, the angular tolerance of such structures still depends on multistep fabrications or the use of noble metals.

#336326 Journal © 2018

https://doi.org/10.1364/OE.26.021490 Received 28 Jun 2018; revised 27 Jul 2018; accepted 27 Jul 2018; published 6 Aug 2018

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Given thiis backdrop, in n this study, we numericallly and experiimentally inveestigate a broadband ab bsorber based on a thin mettasurface that ccomprises pattterned dielectrric-metaldielectric layeers with a period significantlly smaller thann the working w wavelength. D Due to the excitation of both surface plasmon p resonaance (SPR) annd localized SP PR, the devicee exhibits a over a broadband range from 4000 nm to 11000 nm, thereby affording near-perfect absorption an absorptivee band of 700 nm from the visible v to the nnear-infrared rrange. In this rrange, we obtain an aveerage absorptio on of 0.97 (0.9, experiment)), with a peakk absorption upp to 0.99 (0.984, experiment), and ab bsorption >0.93 3 (0.87, experriment) for inccident angles uup to 60°. Importantly, via combining g the high th hroughput conttinuously variiable spatial ffrequency photolithograp phy process with w coating tecchnology, our ddevice can be fabricated signnificantly faster, and low w cost than previously reporrted structures. Furthermore, our absorber eexhibits a significantly lower emissivity (weak abso orption) in thee mid-infraredd range. Thereefore, this a have important i appllications in fiellds such as conncentrating sollar power metasurface absorber and thermal im maging techno ology. 2. Structura al design and d experimenttal details The schematiic of our broaadband omnidiirectional absoorber is presennted in Fig. 1,, wherein SiNx-Ni layerrs are modulaated by two-d dimensional (22D) photoresisst cylinder arrrays. The height and diaameter of cylin nders are denotted as h1 and w w, respectivelyy, and the thickknesses of the Ni and SiiNx layers are denoted as h2 and h3, respeectively, whilee the period off cylinder arrays is deno oted as P. These parameters are a optimized aas follows: P = 290 nm, w = 558 nm, h1 = 90 nm, h2 = 50 nm, and h3 = 50 nm. Thee optical perfoormance of ourr device is calcculated by means of the 3D finite diffeerence time domain (3D–FDT TD) method [331]. In the calcculations, nformation asssociated with h these mateerials is derivved from Paalik [32]. the index in Subsequently, the absorptio on is calculateed as A = 1 – R – T, wherre R and T deenote the d transmission, respectively. reflection and

Fig. 1. Sch hematic of proposed broadband omnnidirectional absorrber.

The device fabrication procedure p is deepicted in Fig. 2(a). In our stu tudy, the 2D phhotoresist grating was fabricated by y means of th he fast and ssimple continuuously variablle spatial hotolithography y (SVG Corpo oration, Nanoccrystal) processs [33,34]. Firstly, the frequency ph laser beam off 351 nm is turrned into collim mated plane waave by an expaander system, secondly, this plane waave is transmittted through a 4F 4 system. Aft fter that, only ± 1 orders of ddiffracted beams are allo owed pass thro ough the objecctive lens and ffocused on thee photoresist thhat coated on the quartz substrate to fo orm a minified d interference ppattern. A pixeel filled with thhe grating ure, where diffferent grating pperiods can bee achieved by changing is formed afteer each exposu locations of the binary op ptical element.. In addition, the stage is driven by a computer

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monitored micro-stepping controller, where spatially distributed pixels obtained with its accurately changed position. Therefore, gratings can be made “pixel-by-pixel”, which is much efficiency for pixelized grating arrays' fabrication. Importantly, the smallest period that can be fabricated is about 200 nm, and this technique has excellent reproducibility as several researches based on this technique have been proposed. As an example of the process efficiency, we point out that it only takes 10 min to fabricate gratings over an area of 200 mm2, which is about 500 times faster the e-beam lithography process. In our process, the photoresist gratings were next developed in NaOH solution (6‰) and dried by means of an electric blow drier. Finally, 50 nm thick Ni and SiNx films were deposited on the gratings via magnetron sputtering and inductively coupled plasma chemical vapor deposition (ICP-CVD), respectively. The surface morphology of the fabricated absorber was characterized by means of a scanning electron microscope (SEM), as shown in Fig. 2(b). Obviously, the metallic grating arrays are neatly distributed on the quartz substrate with a period of 290 nm.

Fig. 2. (a) Schematic of fabrication processes. (b) Scanning electron microscope (SEM) image of proposed absorber.

3. Results and discussion 3.1 Performance and underlying principles Figure 3(a) depicts the calculated absorption spectrum at normal incidence, where an average absorption intensity of 0.97 is achieved in the wavelength range from 400 to 1100 nm, with three different absorption peaks at particular wavelengths, thus indicating the simultaneous excitation of multi-resonances. Such excellent characteristics are essential for absorptive devices that need to be integrated into industrial applications. Figure 3(b) shows the measured absorption spectrum, which exhibits good agreement with the calculated spectrum. In particular, we note that peaks A and B that occurred in calculations are in accordance with

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that in experiment. It is further noteworthy that the peak C on the measured response is nearly disappeared, which is mainly caused by the deposited material’s refractive index as well as the fabricated structure’s period slightly violated from the simulation ones, as the characteristic of SPR and localized SPR are highly sensitive to the environment and structural geometry.

Fig. 3. (a) Calculated and (b) measured absorption spectra of proposed absorber at normal incidence.

Next, we investigated the absorption performances related to the incident angle of light. The angle-resolved absorption of this fabricated absorber was measured by using a spectrophotometer (LAMBDA 750) with the incident angle varied from 0° to 60°. From the calculated and measured spectra shown in Figs. 4(a) and 4(b), respectively, we note that two sets of absorptions exhibit excellent angular insensitivity; the slight deviation between experiment and calculation is mainly due to errors in fabrications. To further examine how the incident angle influences the absorption, we calculated the average absorption corresponding to Figs. 4(a) and 4(b) over the angle range of 0° to 60°; this result is presented in Fig. 4(c). The average absorption is almost unchanged for incident angles up to 45°, while it gradually decreases for larger incident angles. Experimentally, even for incident angles of 60°, the average absorption is still greater than 0.87. Figure 4(d) displays optical images of the fabricated absorbers acquired under indoor ambient light illumination at angles of 20°, 40°, and 60°; we note that the absorbers appear totally black. This result indicates that the proposed device presents good angular tolerance. Moreover, the device is coated with SiNx films, which limits refraction at its interface. Consequently, the absorber exhibits wide angular tolerance.

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Fig. 4. (a) Calculated and (b) measured incident angle–resolved spectra of proposed absorber. (c) Calculated and measured average absorptions of absorber. (d) Optical images of fabricated absorber acquired at different incident angles.

In order to understand the physical mechanism underlying the broadband absorption, the contribution of each structural component of the absorber is analyzed. The calculated and measured absorption spectra of the sample during the fabrication procedure is depicted in Fig. 5. A good agreement between calculations [Fig. 5(a)] and experimental results [Fig. 5(b)] is observed. From Fig. 5(a), it can be inferred that a flat SiNx-coated Ni film affords efficient absorption at shorter wavelengths (green curve). However, the absorptions are still far from near-perfect across the entire bandwidth of interest. When the 2D grating arrays are patterned on the substrate, the absorption is greatly broadened and enhanced (black curve), and the average absorption increases from 0.65 to 0.97 in the wavelength of 400-1100 nm. Here, the Ni film acts as an absorptive layer because of the absorption is enhanced after the lossless grating is coated with 50 nm thick Ni (blue curve). Moreover, the lossless SiNx layer acts as an antireflection layer, whose effect can be observed from a comparison of the flat SiNxcoated Ni film and single Ni film (red/green curves). Therefore, the deposition thickness of the SiNx layers also plays a key role in achieving a high level of absorption in the broadband range. For instance, for an 80 nm thick SiNx layer (instead of 50 nm), the absorption peak shifts to longer wavelengths, which reduces the amount of light absorbed at shorter wavelengths. Thus, this device extends the stacked Ni/SiNx films to achieve broadband absorption in ultrathin structures.

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Fig. 5. Comparison between (a) calculated and (b) measured absorption spectra corresponding to each step of fabrication process.

To further understand the optical behavior underlying the enhanced broadband absorption, magnetic field distributions at the absorption peaks are calculated and mapped. From Fig. 6(a), without the SiNx coating, at peak A, the field is mainly localized at the sidewall and top surface of the metallic gratings, which represents the superposition of SPR and localized SPR [22]. Meanwhile, from Figs. 6(b) and 6(c), at wavelengths of the peak B and peak C, respectively, the localized SPR is formed around the sidewall of the metallic gratings, while the coupling between adjacent gratings is highly dispersive, which means that no obvious absorption peaks can be formed. For the structures with the SiNx layer, light trapping at the wavelength of peak A is due to localized SPR and slight coupling between adjacent gratings [24], wherein the field is mainly localized at the metallic grating top corners, which suppresses reection [Fig. 6(d)]. In the case of resonance at peak B, the magnetic field is mainly distributed atop the SiNx layer, as displayed in Fig. 6(e). This resonance is also attributed to SPR, and the observed red shift in the resonant wavelength is induced by the SiNx layer [in comparison with Fig. 6(a)]. The SPR can be characterized by the following dispersion relation:

kx =



λ

sin θi +

2π w  d  m = , P c ε d + m

(1)

where kx denotes the wave vector component along the grating surface, P denotes the grating period, c denotes the speed of incident light, w denotes the angular frequency of incident light, θi denotes the incident angle, and εd and εm denote the dielectric constants of the dielectric and metal, respectively [35]. Simultaneously, a significantly weaker localized SPR appears at the bottom corner of the metallic gratings.

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Fig. 6. Intensity maps of magnetic field for λ = 400, 600, and 938 nm. (a)–(c) Without SiNx coating. (d)–(f) With SiNx coating. (g)–(i) Without grating arrays.

Finally, the resonance at peak C exhibits localized SPR around the metallic grating sidewalls, as depicted in Fig. 6(f). Coupling between the localized SPRs leads to a perpendicular oscillation in magnetic fields of the bottom SiNx layers, which is strongly concentrated and enhanced relative to that in Fig. 6(c). The magnetic field distribution in this case indicates that the intensity maximum at peak C is relatively higher than that of previous resonances, which aids in compensating for the lower absorption of the structure (grating + Ni) at longer wavelengths. Therefore, via utilizing the phenomena of SPR and localized SPR, our proposed device can achieve broadband, near-perfect absorption. Remarkably, when compared with flat SiNx-coated Ni films [see Figs. 6(g)–6(i)], the stacked films modulated by 2D cylinder arrays exhibit a larger field magnitude in the SiNx region along with radical magnetic field patterns, which enables pairs of stacked Ni/SiNx layers to achieve broadband absorption in ultrathin thickness. And the comparison of our structure to the existing perfect absorbers is given in Table 1, which indicates that our developed structure as well as its fabrication has a good performance [13,20,26–28,30]. Table 1. The comparison between this structure with other absorption structures Category Our developed structure

Efficiency > 90%

Metal-dielectric grating-metal All-metal grating Stacked-films

> 90% > 90% > 90%

Fabrication method Continuously variable spatial frequency photolithography E-beam lithography or nano-imprinting Electroforming Coating technology

Cost Low

Thickness 190 nm

High High Low

> 200 nm > 200 nm > 400 nm

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3.2 Influence of period and material on absorption

As the variations of the period in the fabricated devices could impact significantly the average absorption due to spectral shifts of the different absorption peaks, we calculated and mapped the absorptive spectrum with varied periods. As plotted in Fig. 7(a), with the increasing of periods, absorptive peaks are obviously shifted, and the absorption is decreased at long wavelength. Therefore, the period for our device is set as 290 nm to obtain a better average absorption.

Fig. 7. (a) Calculated absorption spectrum for different grating shapes. (b) Calculated absorption spectra for different constituent materials.

Moreover, we also examine the absorption of this device when various coating materials are used. While the devices discussed in the previous sections used a coating layer of Ni and SiNx, we note that materials such as chromium (Cr), W, TiN, Al2O3, and zinc oxide (ZnO) can be also used as alternative materials for different applications. The absorption spectra obtained for different material combinations are plotted in Fig. 7(b), wherein the average absorption is 0.97, 0.96, 0.94, and 0.91 for Ni + SiNx, Cr + SiNx, W + ZnO, and TiN + Al2O3 coatings, respectively. Thus, this device can be used for applications such as solar thermophotovoltaics (Ni, Cr, W), and it is also compatible with CMOS (TiN) processes as the photoresist grating also can be replaced by SiO2 gratings. 3.3 Photothermal performance of absorber

As reported before, the material used for this device is heat resisting, and then its optical performance is slightly affected by the high temperature [36,37]. Therefore, this metasurface absorber can be working as a concentrating solar power device. Figure 8(a) displays the calculated absorption spectrum overlapped with the air mass 1.5 (AM1.5) solar spectrum in the range from 300 to 2000 nm. An average absorption over 0.8 is achieved over this broad range, which indicates that almost all the solar energy is absorbed by our device. More remarkably, this metasurface absorber shows very weak absorption in the mid-infrared range. From Fig. 8(b), an average absorption of 0.08 is obtained for the wavelength from 5 to 10 µm as the period of this absorber is significantly smaller than the incident wavelength. Consequently, the absorber minimizes heat dispersion through infrared radiation [38]. Therefore, this metasurface absorber can be employed as an efficient concentrating solar power device.

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Fig. 8. (a) Calculated absorption spectrum of proposed device overlapped with AM1.5 solar spectrum (300–2000 nm). (b) Calculated absorption spectrum of device in mid-infrared range.

4. Summary

In summary, we numerically and experimentally demonstrated a novel broadband near– perfect absorber based on an ultrathin metasurface. The absorber exploits the phenomena of surface plasmon resonance and localized surface plasmon resonance to afford an average optical absorption of 0.97 (0.9, experiment) over the wavelength range from 400 nm to 1100 nm, and the absorption remains at 0.94 (0.87, experiment) even for incidence angles up to 60°. Remarkably, several types of coating materials can be used in this absorber, which significantly broadens its application. Furthermore, this device effectively absorbs light in the visible and near-infrared range, with very little thermal radiation in the mid-infrared range. Our device can significantly contribute to the realization of cost-effective, large-area, broadband absorbers, which have important applications in fields such as concentrating solar power and thermal imaging technology. Funding

National Natural Science Foundation of China (NSFC) (61575132, 61107016, 61575135, 61505131); Key University Science Research Project of Jiangsu Province (14KJA510006); NFSC Major Research Program on Nanomanufacturing (91323303); Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions; BZ2016008. Acknowledgments

We thank the College of Physics and Optoelectronics of Soochow University for experimental support. Disclosures

The authors declare that there are no conflicts of interest related to this article. References 1. 2. 3. 4.

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