Materials Science & Engineering B 249 (2019) 114419
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Broadband polarization-insensitive absorption by metasurface with metallic pieces for energy harvesting application Naseer Muhammada,b, Xiaopin Tangb,c,d, Tao Fue, Qiang Liub,c,d, Zhengbiao Ouyangb,c,d,
Lab of Artiﬁcially Micro- & Nano-structured Materials and Devices for Photonics, Institute of Microscale Optoelectronics, Shenzhen 518060, China THz Technical Research Center of Shenzhen University, Shenzhen Key Laboratory of Micro-Nano Photonic Information Technology, Shenzhen 518060, China c Key Laboratory of Optoelectronics Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen 518060, China d College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China e Guangxi Key Laboratory of Precision Navigation Technology and Application, Guilin University of Electronic Technology, Guilin 541004, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Metasurface Electro-optics Energy-harvesting Relative-sensitivity Photonics Plasmonics
We present a broadband absorber in tri-layer plasmonic metamaterials with an electro-optic material as a substrate to ﬁne tune the resonances. Broad resonant absorption peaks are achieved in a structure with metallic pieces. In the wavelength range from 300 nm to 2200 nm, the absorption spectral width is calculated to be 1078 nm, 566 nm, 282 nm, 119 nm, and 78 nm with absorption levels of 75%, 90%, 94.6%, 98.6%, and 99.4%, respectively. The total absorption eﬃciency, of the structure is evaluated to be 71.43%, which approaches to the absorption eﬃciency of a complex design with over 40-pair layers. The wide incident angle, polarization-insensitive, broadband absorption can be utilized for energy harvesting and solar thermals. Moreover, the use of electro-optic substrate has the advantage and potential to reduce the use of scaling structures and fabrication errors, and tune the resonant wavelengths to the desired range for imaging and infrared detection applications.
1. Introduction The extraordinary dynamic characteristics of metamaterials not available in nature, such as electromagnetic cloaking , perfect lensing , negative refraction , Fano resonances [4–6], and perfect absorbers [7–10], have attracted extensive attention in the past decade. One of the important application–perfect absorber–was demonstrated ﬁrst by Landy in 2008 . Such perfect absorbers have potential applications in diﬀerent electromagnetic regimes [9,11–14]. As the use of fossil fuel substantially causes environmental pollution to the Earth , utilizing solar energy through solar cells or solar powered electricity to replace fossil fuel is an important solution. As a result, broadband absorbers are of great interest for power harvesting and solar cells [16,17]. These broadband absorbers are resulted by exciting multiple resonant modes. So far, many research groups had proposed broadband absorption structures, but some in relatively narrow bands [7,8,16–26], some in a relatively narrow angle range or angle dependent [14,27–32], some are for selective wavelengths [31,32], and some are polarization dependent [18,28–30]. A very broadband polarizationinsensitive absorber was also reported, but as the design consists of over 40-pair gold/silicon alternating layers , it falls short due to the fabrication complexities. Hedayati et al. reported a very interesting
perfect absorber, although the structure lacks wavelength tunability and the absorption is limited to visible frequencies . Zhou et al. reported a super broadband absorber, however, such designs can cause diﬃculties and increase the cost in future production . Li et al. demonstrated an eﬃcient layer-by-layer absorber consisting of 11 stacks of (PU/TiN/PSS) which absorbs the waveband of 500 nm to 2500 nm , where PU and PSS are polymer components. Li et al. reported a simple design both theoretically and experimentally which absorbs a short waveband from 400 nm to 800 nm . Another eﬃcient widest absorber was exploited that can absorb from visible just up to 1200 nm  and it is limited by the angle of incident, also the design lacks waveband tunability and geometric ﬂexibility. Lack of tunability in the broadband absorbers limit their applications to speciﬁc waveband. Therefore, an eﬀective and simple design of absorber consisting of few layers with tunability in the wide solar spectrum range from 300 nm (shorter) to 2000 nm (longer) and above is a challenge. In this letter, we present and investigate highly tunable broadband absorption in three dimensional tri-layer, polarization-independent design based on plasmonic metamaterials, as shown in Fig. 1. The structure possesses highly broadband absorption in both visible and near infrared range. The broad spectral width of the resonant peaks increase the absorption eﬀectiveness of the design. An electro-optic
Corresponding author at: College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. E-mail address: [email protected]
https://doi.org/10.1016/j.mseb.2019.114419 Received 2 August 2018; Received in revised form 7 August 2019; Accepted 6 September 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 249 (2019) 114419
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rings is go = 15 nm; the horizontal and vertical splits in the outer ring are xo = yo = 30 nm; the horizontal and vertical splits in the inner ring are xi = yi = 15 nm; the length of each rod set in parallel to one side of the outer ring is l2 = 180 nm, it’s width is w2 = 30 nm, and gap g1 between the outer square ring and the rods is 20 nm; the resonator thickness in −z to z-direction is 30 nm; the thickness of the dielectric substrate is st = 70 nm because for absorption at shorter wavelengths, the role of metallic part is more important (plasmon decay and Ohmic losses) ; the thickness of base metallic reﬂector, which is used to reduce the transmission T (T = 0), is mt = 830 nm; θ is the incident ﬁeld angle; and Vo is the input voltage. In simulations, Drude model has been used for the dielectric constant of gold with a plasma frequency of 1.37 × 1016(rad/s) . Lithium tantalite (LiTao3) is used as a suitable electro-optic substrate material, which ﬁne tunes the dielectric environment to the structure. The varied refractive index n due to the Pockels eﬀect by applying external electric ﬁeld Ei can be calculated by the formula  n = no + 0.5no3γEi, where Ei = Vo/st. The electro-optic coeﬃcient γ = 8 pm/V is considered. The ordinary refractive index of Lithium tantalite is no = 2.175 under the x-polarized (0° incident angle unlike the selective angle in [31,32]) electromagnetic beam propagating in z-direction. The simulation space is ﬁlled with air.
Fig. 1. Schematic of the three dimensional absorber.
material is used as a substrate to ﬁne tune the resonances and achieve outstanding absorption eﬃciency. The geometric parameters are varied to modify the design versatility. The spectral widths of 1078 nm, 566 nm, 282 nm, 119 nm, and 78 nm are observed with absorption levels of 75%, 90%, 94.6%, 98.6%, and 99.4%, respectively. The total absorption eﬃciency, deﬁned as the ratio of the total absorbed energy by a device to the total solar energy radiated to the device on the Earth surface, is calculated to be as high as 71.43%. Our values approach to the absorption eﬃciency of the complex multilayer absorber with over 40-pair layers working in the wavelength range of 300 nm to 2000 nm and above . The proposed plasmonic absorber can be used for solar cells, and its broad wavelength range is promising for solar thermals . Furthermore the electro-optic based tunability distinct from that in  will decrease the fabrication errors, and avoid scaling of designs, and moreover the resonant wavelengths can be adjusted to a desired range for outstanding results in infrared detection and imaging.
2.2. Proposed fabrication process Thanks to the fabrication techniques, it is possible to fabricate small nanoparticles (down to few nanometers) with high dimensional precision [40,41]. The metallic pieces can be fabricated by advanced techniques, e.g., electron-beam lithography (EBL). In the fabrication process, ﬁrst, the pattern of the metallic pieces can be shaped in a PMMA layer by the EBL on the surface of lithium tantalite substrate backed by a thick metallic reﬂector. Then the 20 nm thick metal ﬁlm can be deposited on the pattern, and at the ﬁnal-step life-oﬀ process, the upper layer of the metal can be removed and the desired resonator metal pattern will leave above on the substrate .
2. Methods and parameters
2.3. Total solar light absorption eﬃciency
As the solar energy spectrum is a non-uniform distribution in a wide range of wavelengths, to calculate the actual total absorption eﬃciency of our designed structure we apply the eﬃciency integration formula: ηT = ∫ I(λ)A(λ)dλ/∫ I(λ)dλ , where ηT is total eﬃciency, the weight coeﬃcient I(λ) is the solar intensity spectrum distribution on the earth surface, and A(λ) is the absorption coeﬃcient of the device deﬁned by the intensity proportional to the square of the electric ﬁeld. Physically, the total absorption eﬃciency is the ratio of the total absorbed energy by a device to the total solar energy radiated to the device on the Earth surface.
We use ﬁnite-element method by COMSOL Multiphysics software with RF module to simulate the structure proposed. The square rings split into metallic pieces were utilized as the top layer of the unit cell. Each side length of square unit cell is 500 nm. The Floquet periodic boundary conditions were applied to the simulation area which repeats the unit-cell inﬁnitely both in x- and y-directions. The geometric parameters are: the side length of the outer square ring is l = 310 nm, and its width is w = 42 nm; the side length of the inner square ring is l1 = 196 nm, and its width is w1 = 23 nm; the gap between the two
Fig. 2. Absorption spectra of (a) inner horizontal-vertical split ring, (b) outer ring (without splits), (c) split double rings, and (d) T attached in y-direction of l2 = 180 nm. 2
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3. Results and discussion
ring increases, the interference between the resonant modes of the inner ring increases. The increase in interference of resonant modes further increases the Fano dips between the ﬁrst three resonant modes around 660 nm, 793 nm, and 925 nm. The spectral width for the absorption level of 90% decreases because of increase in the Fano dips as shown in Fig. 3b. Furthermore, we increase the length of the T-head from l2 = 180 nm–220 nm. As we increase the l2 value, the absorption level of the resonant mode increases. The spectral width at 75%, 97% and 99.4% is calculated to be respectively about 1078 nm, 181 nm (652 nm–682 nm, 857 nm–962 nm, 1686 nm–1732 nm), and 78 nm (871 nm–949 nm) at l2 = 200 nm. The spectral width of l2 = 220 nm at 75% of absorption level is 1078 nm which is higher than the spectral width at 75% of a recently reported absorber . The total absorption eﬃciency ηT for Fig. 3b is calculated, the lower value is 69.74% and the higher one is 71.43% calculated at l2 = 210 nm and 190 nm respectively. The total eﬃciency in our work approaches to the absorption eﬃciency of complex multilayer absorbers, e.g., the 40-pair-layer structure working in 300 nm to 2000 nm  and the layer-by-layer absorber with 11 stacks of (PU/ TiN/PSS) in the 500 nm to 2500 nm waveband . To observe the tuning eﬀect, diﬀerent input voltage Vo from 0 V to 400 V is applied unlike that in  with an interval of 100 V to observe the spectral response of the design, as shown in Fig. 4a. The refractive index of the electro-optic substrate varies as the input voltage changes. The resonant wavelengths show sensitivity to variation in the refractive index of substrate and shift from 648 nm, 921 nm, 1142 nm, 1636 nm, and 2028 nm to 697 nm, 1006 nm, 1234 nm, 1933 nm, and 2228 nm, respectively. The resonant wavelengths are tuned with changing the spectral response of the absorption spectra. This voltage based tuning sensitivity can be a useful parameter for broadband optical ﬁltering. The relative sensitivity Sr can be calculated by Sr = (Δλ/λ)/ΔVo, where Δλ is the change in resonant wavelength, λ is average wavelength in the band of observation, and ΔVo is the change in the input voltage. The highest relative sensitivity of 4.2 × 10−4 V−1 is calculated at the resonant mode around 1636 nm which shifts to 1933 nm. This can be useful in devices for diﬀerent wavelength ranges without scaling. To see the inﬂuence of incident angle on the absorption, the absorption spectra dissimilar to that at selective wavelengths in [31,32] under diﬀerent incident angles are shown in Fig. 4b, indicating that the absorption is insensitive to the incident angles. The broad resonances at wide-incident angle can provide a strong candidature for energy localization applications. The mechanism of increasing the absorption bandwidth is that the design introduces a number of resonant structures, of which each has a
For better understanding the mechanism of the structure, we ﬁrst present the results of simple structures that are part of the whole structure. Fig. 2a shows the absorption spectra of only the inner split ring. The absorption peaks arise due to the interaction of the resonant modes supported by each piece of the inner split ring and other two layers. The splits in rings can be useful for reducing the amplitude of reﬂection . Fig. 2b shows the absorption spectra of only the outer split ring. The absorption peaks are resulted by the interference of resonant modes on the outer surface and in the inner cavity of the outer ring. The absorption becomes higher because in shorter wavelengths the metallic layers have high Ohmic losses and plasmon decay , also the nanogaps localize some amount of light and decrease the amount of reﬂection back to the space [26,43] and thus more energy is absorbed. Next we insert the inner split ring to the outer split ring, resulting in broadband absorption spectra as shown in Fig. 2c. It is also worth to note that the splits in rings can reduce the amplitude of reﬂection . Further, 4 nanorods arranged in two T-head in y- direction, the inner split ring, and the outer split ring are combined, resulting an even wider absorption spectrum, as shown in Fig. 2d. At last, we consider the full structure in Fig. 1 in which the inner ring, the outer ring, and the 4 T-heads are integrated to form the whole structure. The absorption spectra are shown in Fig. 3a, indicating more absorption peaks and wider absorption band. The spectral width of the ﬁrst broad resonant mode increases, new modes appear around 1138 nm, 1558 nm, and 1970 nm as shown in Fig. 3a. The peak absorption levels of the resonant modes for l2 = 180 nm are 98.7%, 96.3%, 99.8%, 98.1%, 98.9%, and 90.2% around 660 nm, 777 nm, 918 nm, 1138 nm, 1558 nm, and 1970 nm, respectively. The spectral widths of all the resonant modes are collectively 422 nm for the absorption level of 90%. When we increase the length of T-head l2, the spectral width of absorption further increases. The calculated spectral widths of all resonant modes at the absorption level of 90% are collectively 444 nm, 527 nm, and 566 nm for l2 = 190 nm, 200 nm, and 210 nm, respectively. The spectral width at 90% of absorption level is higher than that previously reported in tri-layer plasmonic structures [21,22–24]. The spectral width for the absorption level of 97% and 99.6% is calculated to be 107 nm (863 nm–970 nm) and 50 nm (880 nm–930 nm) for l2 = 220 nm and 210 nm, respectively. These spectral widths are broader than previously reported values reported in [7,16–18,20,21]. In Fig. 3a the total absorption eﬃciency ηT lies between lower 62.55% for l2 = 180 nm, to upper 63.44% for l2 = 190 nm. From Fig. 3b we can see that, as the internal circumference of inner
Fig. 4. Absorption spectra at diﬀerent input voltages Vo in the case t1 = 18 nm and l2 = 190 nm (a), and at diﬀerent incident angle θ of applied electromagnetic beam (b).
Fig. 3. Absorption spectra of diﬀerent T-head length l2 at (a) t1 = 23 nm and (b) t1 = 18 nm. 3
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diﬀerent resonance wavelength; as a result, the combined absorption has a widely enlarged bandwidth, promoting the total absorption level greatly. The input voltage based tunability is eﬀective to reduce the fabrication complexities and can alter resonant wavelengths to the desired wave-band without scaling the design.
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4. Conclusion In this paper, a broadband absorber in tri-layer plasmonic metamaterials is presented and demonstrated numerically. The structure shows broadband absorption in both visible and near infrared range. The large spectral width of resonances increases the absorption eﬀectiveness of the design. The geometric conﬁgurations are changed to characterize the design versatility. The broad absorption peak levels are observed above 99.9% of absorption. The highest absorption for spectral widths of 1078 nm, 566 nm, 282 nm, 119 nm, and 78 nm are observed to be 75%, 90%, 94.6%, 98.6%, and 99.4% respectively, in wavelength range of 300 nm to 2200 nm, and the total absorption efﬁciency of the structure is obtained to be 71.43% by taking the integration of the wavelengths absorbed by the structure on the solar energy spectrum. The electro-optical material substrate can help tuning the position of high absorption peaks so that fabrication complexity can be reduced. The relatively simple design, wide incident angle, polarization-insensitive, ultra-broadband absorption can be used in solar thermals, energy harvesting. Furthermore the resonant wavelength can be tuned to desired range by applying diﬀerent input voltage unlike thermal tuning  to electro-optic substrate for infrared detection and imaging applications. Declaration of Competing Interest The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper. Acknowledgements This work is supported by the NSFC (Grant Nos.: 61275043, 60877034, 61605128, and 61307048), GDNSF (Grant No.: 2017A030310455), and SZSF (Grant No.: JCYJ20170302151033006). Data availability All materials, data and speciﬁcations are clearly mentioned in “Methods and Parameters”. References  D. Schurig, J. Mock, B. Justice, S.A. Cummer, J.B. Pendry, A. Starr, et al., Metamaterial electromagnetic cloak at microwave frequencies, Science 314 (2006) 977–980.  N. Fang, H. Lee, C. Sun, X. Zhang, Sub–diﬀraction-limited optical imaging with a silver superlens, Science 308 (2005) 534–537.  V.M. Shalaev, Optical negative-index metamaterials, Nat. Photon. 1 (2007) 41–48.  N. Muhammad, A.D. Khan, Z.-L. Deng, K. Khan, A. Yadav, Q. Liu, et al., Plasmonic spectral splitting in ring/rod metasurface, Nanomaterials 7 (2017) 397.  N. Muhammad, A.D. Khan, Tunable Fano resonances and electromagnetically induced transparency in all-dielectric holey block, Plasmonics 10 (2015) 1687–1693.  N. Muhammad, A.D. Khan, Electromagnetically induced transparency and sharp asymmetric fano line shapes in all-dielectric nanodimer, Plasmonics 12 (2017) 1399–1407.  P. Rufangura, C. Sabah, Dual-band perfect metamaterial absorber for solar cell applications, Vacuum 120 (2015) 68–74.  H. Tao, C. Bingham, D. Pilon, K. Fan, A. Strikwerda, D. Shrekenhamer, et al., A dual band terahertz metamaterial absorber, J. Phys. D: Appl. Phys. 43 (2010) 225102.  Y. Liu, Y.Q. Zhang, X.R. Jin, S. Zhang, Y.P. Lee, Dual-band infrared perfect absorber for plasmonic sensor based on the electromagnetically induced reﬂection-like eﬀect, Opt. Commun. 371 (2016) 173–177.  C.F. Guo, T. Sun, F. Cao, Q. Liu, Z. Ren, Metallic nanostructures for light trapping in energy-harvesting devices, Light Sci. Appl. 3 (2014) e161.  N.I. Landy, S. Sajuyigbe, J. Mock, D. Smith, W. Padilla, Perfect metamaterial