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Broadband perfect metamaterial absorber based on the gallium arsenide grating complex structure
Journal Pre-proofs Broadband perfect metamaterialabsorber based on the gallium arsenide grating complex structure Yuyin Li, Qiqi Chen, Biao Wu, Leilei Shi, Peng Tang, Guozhen Du, Guiqiang Liu PII: DOI: Reference:
S2211-3797(19)32637-3 https://doi.org/10.1016/j.rinp.2019.102760 RINP 102760
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Results in Physics
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1 September 2019 14 October 2019 14 October 2019
Please cite this article as: Li, Y., Chen, Q., Wu, B., Shi, L., Tang, P., Du, G., Liu, G., Broadband perfect metamaterialabsorber based on the gallium arsenide grating complex structure, Results in Physics (2019), doi: https://doi.org/10.1016/j.rinp.2019.102760
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Broadband perfect metamaterial absorber based on the gallium arsenide grating complex structure Yuyin Li, Qiqi Chen, Biao Wu, Leilei Shi, Peng Tang, Guozhen Du, Guiqiang Liu* Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang 330022, China E-mail: [email protected]. ABSTRACT We propose a broadband metamaterial absorber consisting of a periodic gallium arsenide (GaAs) grating array standing on a tungsten (W) film separated by an ultra-thin GaAs spacer layer. A perfect absorption broadband is achieved with an average absorption of 97% in the range from 506 nm to 1814 nm. The bandwidth with the absorption over 90% is larger than 1300 nm and the maximum absorption is up to 100%. The broadband perfect absorption results from the excited surface plasmons combined with the cavity modes. High short-circuit current density over 56 mA/cm2 and near-unity solar energy trapping are also obtained using this absorber due to the multiple resonances coupling. Moreover, this absorber requires no noble metal and is compatible with the popular nano-fabrication technologies. These offer new perspectives for achieving thin film solar cells, solar energy harvesting, magnetic recording, ultra-thin and durable optoelectronic elements.
Keywords: Metamaterials; Absorbers; Perfect absorption; Surface plasmons
1
Introduction Metamaterial devices have received great attention because of their marvelous electromagnetic performances e.g. extraordinary light absorption/transmission [1-4], Fano resonances [5,6], strong electromagnetic localization properties [7,8] and cloaking [9,10] and thus can be widely applied in ultra-sensitive sensing [11-18], perfect absorbers [19-33], surface-enhanced Raman scattering [34-36] and filters [37,38]. During the last decade, the trends to achieve high light absorption in metamaterials and plasmonic nanostructures have increased tremendously. All kind of metamaterial absorbers have been reported such as holes [26], nanopatch resonators [24], nanocones [25], multilayer structures [20,22], and grapheme-based structures [28,39,40]. In generally, there are two types of absorbers: the wide- and narrow-band absorbers [19-33,41-44]. Nevertheless, the broadband absorbers are greatly desired in many cases, e.g., thermal emitters [30], photovoltaics (PVs) [41], solar cells [42,43], and so on. Due to the reduction of non-renewable energy, the huge interest is now focused on the development of solar energy harvesting. Plenty of complex structures have been designed to build the broadband perfect absorbers. For example, based on a SiNx-α-Si-Ag three-layer film structure, a bandwidth of 300 nm with the average light absorption of 92% was obtained in the visible range due to the excitation of asymmetric multi-cavity resonances [22]. By placing a periodic cube array on the Al film, a 90% absorption bandwidth of 712.4 nm was achieved [23].Using a trapezoid plamonic nano-resonators, an average absorption of 85% was demonstrated in the range of 400-700 nm [45]. By incorporating varied geometries in one unit, strong absorption in the range of 450-900 nm was realized due to the simultaneous excitation of different resonances [46]. Although the broadband light absorption phenomena were demonstrated in these platforms, these methods may suffer from the problems such as the relatively finite absorption bandwidths (< 1000 nm), sophisticated geometries or the large requirement of noble metals. In this work, we present a broadband perfect absorber with simple geometrical configuration by combining the semiconductor gallium arsenide (GaAs) material with the refractory metal tungsten (W). The absorber consists of a thick W film substrate, an ultrathin GaAs intermediate layer and a one-dimensional GaAs grating. A broad absorption band with the bandwidth of 1308 nm in the range of 506 -1814 nm is achieved the average absorption over 97% and the maximum absorption high up to 100%. High short-circuit current density more than 56 mA/cm2 is obtained under the AM1.5 solar illumination and near-unity solar energy is trapped by this absorber due to its resonant response occurred in the ultra-broadband frequency range by the multiple resonant coupling. Therefore, the absorber proposed here can be widely applied in thin film solar cells, solar energy harvesting, magnetic recording, ultra-thin and durable optoelectronic elements.
Materials and method
2
Fig. 1. (a) Schematic diagram of the metamaterial absorber. (b) Absorption spectra of the metamaterial absorber (black), W-GaAs two-layer film (red) and the structure with a GaAs grating on the tungsten film (blue). Here, P = 500 nm, d = 240 nm, h1 =30 nm, and h2 = 120 nm. Fig. 1(a) shows the proposed broadband absorber consisting of a one-dimensional GaAs grating and a flat W substrate separated by an ultra-thin GaAs spacer layer. The width and the period of GaAs grating are denoted by d and P, respectively. The thicknesses of grating and spacer layer are marked with h2 and h1, respectively. The thickness of W film is 100 nm, enough to prevent the light transmission. This absorber is comprised by only GaAs and W materials, not noble metals. Most solar energy absorbers reported previously were based on the noble metal-dielectric
composite structures [24,27,32]. In this proposed absorber, the used semiconductor GaAs is with a good electrode section. Simultaneously, GaAs material with high dielectric constant has similar properties of noble materials [19]. Therefore, it is expected that the GaAs grating is responsible for the absorption in the shorter wavelength range. Moreover, the absorber is with geometrical configuration very simpler than those reported before [23-27,45,46]. The fabrication process of this
absorber is compatible with the popular nano-fabrication technologies. Firstly, homogeneous W and GaAs flat films with certain thickness can be deposited on a clean SiO2 substrate in turn via chemical vapor deposition [47] or physical sputtering deposition [48]. Then, a layer of photoresist would be coated on the GaAs film and etched by the electron beam lithography [24] to form one-dimensionally periodic grating. After that, GaAs material should be sputtered in the slits and on the photoresist antennas. Finally, the photoresist grating coated with GaAs materials would be removed using the lift-off method to obtain the final structure [32]. Numerical calculations are performed by the three-dimensional finite-difference time-domain ways (FDTD) [49]. Since the structure can be regarded as infinite long in the y direction, a 2D simulation model is applied to save compute space. The periodic boundary conditions are applied in the x direction and perfectly matched layers are used in the z direction. The dielectric constants of W and GaAs are taken from Palik [50]. A wide frequency plane wave with the linear polarization along the x axis is irradiated from the top of metamaterials. The transmission (T) in this absorber is equal to zero due to the opaque metal film used at the bottom. The absorption (A) of this absorber can be calculated by A = 1- R, where R denotes the reflection.
Results and discussion Fig. 1(b) shows the absorption spectrum of the optimized absorber at normal incidence 3
(marked with the absorber, black solid line). Here, the optimized period (P), width (d) and thickness (h2) of grating are 500 nm, 240 nm, and 120 nm, respectively. The thicknesses h1 of the GaAs film is 30 nm. In this work, the parameters remain invariable unless otherwise specified. For comparison, the absorption spectra of the two-layer film structure consisting of W and GaAs (marked with W-GaAs two-layer film, red dash dot line) and the structure consisting of a GaAs grating standing on the W film (marked with W-film+GaAs grating, blue short dot line) are also shown here. These three structures are with the same structural parameters. Considering the 90% absorption, only a narrow absorption band within the range of 681-932 nm is found in the two-layer film structure. When a GaAs grating is placed on the W film, a wide shoulder peak is achieved in the range of 590 -1726 nm with an absorption dip at 1074 nm (A = 79%). Interestingly, by integrating the GaAs grating and the two-layer film stacks of W and GaAs, an ultra-broad band with the absorption over 90% is observed in the wavelength region from 506 nm to 1814 nm, indicating a 90% bandwidth of 1308 nm. This absorption bandwidth is much wider than those of the metamaterial absorbers reported previously [22, 23]. The average absorption in this range is up to 97% with the maximum absorption reaching 100% in the ranges of 623-724 nm and 1666-1717 nm. The minimum absorption is 92% at λ =1250 nm. Therefore, the GaAs grating is responsible for the strong absorption.
Fig. 2. Electric field |E|2, magnetic field |H|2 distributions and current density J at the wavelengths of 641 nm (a-c), 1250 nm (b-f) and 1700 nm (g-i) in the xoz plane, respectively. To reveal the physical mechanism of the optical property observed above, the electric field intensity (|E|2), magnetic field intensity (|H|2) and current density (J) distributions in the xoz plane at λ = 641 nm, 1250 nm and 1700 nm are calculated and presented in Fig. 2. At λ = 641 nm, the electric field energy concentrating on both top corners of the grating strips demonstrates the obvious electric dipole behaviors [Fig. 2(a)]. Very weak magnetic field energy is found in the inner sides of the grating and at the interface of W film and GaAs film [Fig. 2(b)], indicating the dominant role of electric field distribution on this resonant band. The current mainly distributes in 4
the thin GaAs film and grating [Fig. 2(c)], confirming the effectiveness of GaAs material for this absorption enhancement [23]. At λ =1250 nm, the electric field energy locates at the four corners of the grating strips [Fig. 2(d)], indicating the excitation of localized surface plasmons (LSPs) [51,52]. Strong electric field energy is also observed at the interface of GaAs spacer layer and W film [Fig. 2(d)] and magnetic field energy mainly distributes in the GaAs film, especially at the interface of W film and GaAs film as shown in Fig. 2(e), which demonstrate the excitation of propagating surface plasmons (PSPs) [53,54] and the hybridized coupling effect of LSPs and PSPs [12,20,23]. In Fig. 2(f), the current mainly locates at the interface of W film and GaAs film. At λ = 1700 nm, the electric field energy mainly focuses on two sides of grating strips as well as in the GaAs spacer layer [Fig. 2(g)]. The magnetic field is mainly confined in the area below the grating strips [Fig. 2(h)] and the current is located in the bottom of grating strips, the area in the spacer layer below the grating strips and the top of the W film [Fig. 2(i)]. These indicate the excitation of LSPs and PSPs [12,20,21,23,27], the strong coupling of LSPs of grating strips and PSPs on the bottom W film and the current flowing along the lower metal layer [23].
Fig. 3. Absorption spectra of the structures by changing the materials of the bottom metal film (a) and the top layer grating (b). We further explore the effects of materials and structural parameters on its absorption performance. Fig. 3(a) shows the absorption spectra of the structure by replacing the bottom metals with several commonly used metals but keeping the structural parameters invariable. When replacing the bottom W film with refractory metals Cr or Ti, the broadband absorption performance is hardly changed. The lossy metal of Ti, Cr and W with an appropriate thickness satisfies the impedance match conditions, and thus can contribute to the acquirement of broadband absorption [20]. When the W film is replaced by Al, the absorption spectrum exhibits three wide resonant bands at around 550 nm, 786 nm and 1420 nm. While for the case replaced by Cu or Au, the absorption spectrum exhibits two narrow resonances at 801 nm and 865 nm and two wide resonances in the range of 400-704 nm and 1200-1700 nm. Strong absorption only occurs in the range of 500-1000 nm in the structure with the Al, Cu or Au film substrate. The narrow bandwidth is due to the small metal loss [23]. Fig. 3(b) displays the absorption spectra of the structures with the top GaAs, TiO2, Al2O3 or SiO2 grating. Obviously, for the grating consisting of the materials with higher dielectric constants, the absorption intensity and bandwidth are larger than those of the structures with the lower dielectric constants. Significant decreases in absorption and bandwidth 5
are thus observed in the structures with the decreased dielectric constants due to the greatly reduced excitation and limitation of the fields [19].
Fig. 4. Effects of the structural parameters on the absorption performance. (a) Period P of grating. (b) Width d of grating strips. (c) Thickness h1 of GaAs film. (d) Thickness h2 of grating. The influences of period (P), width (d) and thickness (h2) of grating and thickness (h1) of GaAs film on the absorption properties are shown in Fig. 4. As depicted in Fig. 4(a), with the decreased P, the structure behaves more like a continuous film stack [20], which leads to a wider absorption bandwidth. Contrarily, the increase of P, accompanied by intensive PSPs [14], leads to a narrower absorption band with reduced absorption. Thus, the tradeoff of P to achieve an ideal absorption is around 500 nm. The increased grating width d leads to the increased absorption bandwidth and the firstly increased and then decreased absorption intensity [Fig. 4(b)], which mainly originate from the excited PSPs and its coupling with other resonances [20,23]. The relatively larger filling factor (i.e., d/P) also leads to a wider absorption bandwidth [42]. The optimized d is equal to 240 nm. In Fig. 4(c), a gradually red-shift is observed as h1 increases. The minimum absorption firstly increases and then decreases. These are mainly due to the increased effective cavity length [20,21,23,55,56]. The ideal spectral property can be achieved when h1 is around 30-40 nm. Fig. 4(d) presents the influence of the grating thickness h2 on the absorption. A thinner grating promises a less lossy cavity and a thicker one indicates a weaker coupling [23]. As a result, the optimal performance can be obtained with h2 =120-140 nm. The parameters of grating array (p, d, h2) generally affect the plasmon resonant effects and the thickness of the intermediate layer (h1) influences the cavity modes. In Table 1, we display the light absorption features of other perfect absorbers [20-27,42]. Obviously, the bandwidth with 90% absorption obtained in our structure is larger than the absorbers based on the three-layer film structure (< 300 nm) [22], multilayer film structures with meta-surfaces (575 nm, 1110 nm, 1100 nm or 400 nm) [21,24,25,42], and complex hole structure (360 nm) [26]. Moreover, these structures [21,24-26], except for the three-layer film structure 6
[22], are all more complicated than ours. By placing a three-layer one-dimensional grating on the three-layer film stacks, an ultra-broad band (2969 nm) with 90% absorption was carried out [20] in the visible and infrared regions. Noting that the six-layers structural characteristic increases not only the cost but also the size of the sample, which is not conducive to its integrated development. Using the non-close-packed composite colloids on the Au film, an absorption bandwidth of 2851 nm was achieved by Liu et al [27] in the infrared range, not including the visible region. These hold the proposed scheme with potential application in the solar absorbers. Table 1. Comparison of structure and absorption characteristics Structures
Bandwidth
Region
References
(absorption >90%) Composite six-layer stacks
2969 nm
570-3539 nm
Ref. [20]
Composite nano-resonators on two-layer films
1110 nm
354-1426 nm
Ref. [21]
Three-layer film structure
< 300 nm
420-680 nm
Ref. [22]
Composite cube array on the Al film
712.4 nm
354-1066 nm
Ref. [23]
Three-layer film structure with a complex array
575 nm
300-875 nm
Ref. [24]
Three-layer film structure with a nanocone array
1100 nm
400-1500 nm
Ref. [25]
Complex circular hole structure
360 nm
400-760 nm
Ref. [26]
Non-close-packed composite colloids on the Au film
2851 nm
1500-4200 nm
Ref. [27]
Nanowire array on the GaAs film
400 nm
300-700 nm
Ref. [42]
Fig.5 (a) Calculated incidence angle resolved spectrum response of the absorber. (b) Short-circuit current density with the GaAs grating thickness h2. (c) Absorbed and missed energy of this absorber in the full spectral range of solar radiance AM 1.5. Absorption under different oblique angles of incident light is very important for the application of absorbers. As show in Fig.5(a). When the incidence angle increases from 0° to 55°, the perfect absorption bandwidth is always greater than 1200 nm, suggesting a promising application in solar cells and PVs. In Fig. 5(b), the effect of the thickness h2 of GaAs grating on the short-circuit current density is investigated. Assuming that each absorbed photon generates one electron-hole pair and all photo-generated carriers are collected, the short-circuit current 3000 𝑛𝑚𝑞𝜆 ℎ𝑐Φ𝐴𝑀1.5(𝜆)𝐴(𝜆)
density (Jsc) under AM1.5 solar illumination can be calculated by 𝐽𝑠𝑐 = ∫400 𝑛𝑚
[22]. Here, assuming the internal quantum efficiency is 100%, h is the Planck constant , q is the charge of the electron, λ is the wavelength of incident light, c is the speed of light, Φ𝐴𝑀1.5(𝜆) is the solar radiance AM 1.5 and A(λ) is the absorption of this absorber. When the thickness h2 of 7
grating is tuned from 80 nm to 160 nm with a step of 20 nm, the change in short-circuit current density is not regular. It is because that the short-circuit current density Jsc mainly depends on the number of resonant modes generated in the 400-3000 nm wavelength range [22,43]. The best Jsc (56.4736 mA/cm2) is obtained as h2 = 140 nm. For h2 = 120 nm, Jsc is equal to 56.3547 mA/cm2, which is much larger than the short-circuit current of 30.3 mA/cm2 reported by Meng et al [43]. The solar absorption response of the absorber is usually studied to evaluate the solar energy collection. The ideal absorber possesses a near-unity absorption band in a wide wavelength range. In this work, we carry out the solar absorption investigation by putting the absorber under the illumination of the AM 1.5 source. Fig. 5(c) shows the absorbed and missed solar energy of this absorber. The efficiency of light capture is very high although some of energy is missed. Near-unity solar energy is trapped by this absorber due to its resonant response occurred in the ultra-broadband frequency range by the multiple resonances coupling. Therefore, the absorber can hold potential applications in magnetic recording, ultra-thin and durable optoelectronic elements.
Conclusion We have proposed an ultra-broadband metamaterial absorber configured by a periodic GaAs grating array, an ultra-thin GaAs interlayer and a thick W film. This absorber exhibits the perfect broadband absorption with 90% absorption bandwidth over 1300 nm from visible to near-infrared region. The average absorption in this range is over 97%. Superior surface plasmon resonances combined with the cavity modes lead to this broadband perfect absorption. The absorber also has a high short-circuit current density over 56 mA/cm2 so that it can be widely applied in electricity. Near-unity solar energy is trapped by this absorber due to the multiple resonances coupling. Moreover, this absorber requires no noble metal and is compatible with the popular nano-fabrication technologies. These offer new perspectives for achieving thin film solar cells, solar energy harvesting, magnetic recording, ultra-thin and durable optoelectronic elements. Funding We gratefully acknowledge support from National Natural Science Foundation of China (51761015, 11564017, 11804134 and 11464019) and Provincial Natural Science Foundation of Jiangxi (20181BAB201015, 20182BCB22002 and 2018ACB21005). Declaration of Competing Interest The authors declare no conflict of interest. References [1] [2] [3] [4] [5] [6]
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Highlights 1. The broadband metamaterial absorber consists of a periodic gallium arsenide (GaAs) grating array standing on a tungsten film separated by an ultra-thin GaAs spacer layer. 2. The perfect absorption broadband is achieved with an average absorption of 97% in the range from 506 nm to 1814 nm and the bandwidth with the absorption over 90% is larger than 1300 nm. 3. High short-circuit current density over 56 mA/cm2 and near-unity solar energy trapping are obtained in this perfect absorber. [57] 10
Conflict of Interest The authors declare no conflict of interest.
[58]
Author Statement The authors claim that none of the materials in the paper has been published or is under consideration for publication elsewhere. [59]
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S2211-3797(19)32637-3 https://doi.org/10.1016/j.rinp.2019.102760 RINP 102760
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Results in Physics
Received Date: Revised Date: Accepted Date:
1 September 2019 14 October 2019 14 October 2019
Please cite this article as: Li, Y., Chen, Q., Wu, B., Shi, L., Tang, P., Du, G., Liu, G., Broadband perfect metamaterialabsorber based on the gallium arsenide grating complex structure, Results in Physics (2019), doi: https://doi.org/10.1016/j.rinp.2019.102760
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Broadband perfect metamaterial absorber based on the gallium arsenide grating complex structure Yuyin Li, Qiqi Chen, Biao Wu, Leilei Shi, Peng Tang, Guozhen Du, Guiqiang Liu* Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang 330022, China E-mail: [email protected]. ABSTRACT We propose a broadband metamaterial absorber consisting of a periodic gallium arsenide (GaAs) grating array standing on a tungsten (W) film separated by an ultra-thin GaAs spacer layer. A perfect absorption broadband is achieved with an average absorption of 97% in the range from 506 nm to 1814 nm. The bandwidth with the absorption over 90% is larger than 1300 nm and the maximum absorption is up to 100%. The broadband perfect absorption results from the excited surface plasmons combined with the cavity modes. High short-circuit current density over 56 mA/cm2 and near-unity solar energy trapping are also obtained using this absorber due to the multiple resonances coupling. Moreover, this absorber requires no noble metal and is compatible with the popular nano-fabrication technologies. These offer new perspectives for achieving thin film solar cells, solar energy harvesting, magnetic recording, ultra-thin and durable optoelectronic elements.
Keywords: Metamaterials; Absorbers; Perfect absorption; Surface plasmons
1
Introduction Metamaterial devices have received great attention because of their marvelous electromagnetic performances e.g. extraordinary light absorption/transmission [1-4], Fano resonances [5,6], strong electromagnetic localization properties [7,8] and cloaking [9,10] and thus can be widely applied in ultra-sensitive sensing [11-18], perfect absorbers [19-33], surface-enhanced Raman scattering [34-36] and filters [37,38]. During the last decade, the trends to achieve high light absorption in metamaterials and plasmonic nanostructures have increased tremendously. All kind of metamaterial absorbers have been reported such as holes [26], nanopatch resonators [24], nanocones [25], multilayer structures [20,22], and grapheme-based structures [28,39,40]. In generally, there are two types of absorbers: the wide- and narrow-band absorbers [19-33,41-44]. Nevertheless, the broadband absorbers are greatly desired in many cases, e.g., thermal emitters [30], photovoltaics (PVs) [41], solar cells [42,43], and so on. Due to the reduction of non-renewable energy, the huge interest is now focused on the development of solar energy harvesting. Plenty of complex structures have been designed to build the broadband perfect absorbers. For example, based on a SiNx-α-Si-Ag three-layer film structure, a bandwidth of 300 nm with the average light absorption of 92% was obtained in the visible range due to the excitation of asymmetric multi-cavity resonances [22]. By placing a periodic cube array on the Al film, a 90% absorption bandwidth of 712.4 nm was achieved [23].Using a trapezoid plamonic nano-resonators, an average absorption of 85% was demonstrated in the range of 400-700 nm [45]. By incorporating varied geometries in one unit, strong absorption in the range of 450-900 nm was realized due to the simultaneous excitation of different resonances [46]. Although the broadband light absorption phenomena were demonstrated in these platforms, these methods may suffer from the problems such as the relatively finite absorption bandwidths (< 1000 nm), sophisticated geometries or the large requirement of noble metals. In this work, we present a broadband perfect absorber with simple geometrical configuration by combining the semiconductor gallium arsenide (GaAs) material with the refractory metal tungsten (W). The absorber consists of a thick W film substrate, an ultrathin GaAs intermediate layer and a one-dimensional GaAs grating. A broad absorption band with the bandwidth of 1308 nm in the range of 506 -1814 nm is achieved the average absorption over 97% and the maximum absorption high up to 100%. High short-circuit current density more than 56 mA/cm2 is obtained under the AM1.5 solar illumination and near-unity solar energy is trapped by this absorber due to its resonant response occurred in the ultra-broadband frequency range by the multiple resonant coupling. Therefore, the absorber proposed here can be widely applied in thin film solar cells, solar energy harvesting, magnetic recording, ultra-thin and durable optoelectronic elements.
Materials and method
2
Fig. 1. (a) Schematic diagram of the metamaterial absorber. (b) Absorption spectra of the metamaterial absorber (black), W-GaAs two-layer film (red) and the structure with a GaAs grating on the tungsten film (blue). Here, P = 500 nm, d = 240 nm, h1 =30 nm, and h2 = 120 nm. Fig. 1(a) shows the proposed broadband absorber consisting of a one-dimensional GaAs grating and a flat W substrate separated by an ultra-thin GaAs spacer layer. The width and the period of GaAs grating are denoted by d and P, respectively. The thicknesses of grating and spacer layer are marked with h2 and h1, respectively. The thickness of W film is 100 nm, enough to prevent the light transmission. This absorber is comprised by only GaAs and W materials, not noble metals. Most solar energy absorbers reported previously were based on the noble metal-dielectric
composite structures [24,27,32]. In this proposed absorber, the used semiconductor GaAs is with a good electrode section. Simultaneously, GaAs material with high dielectric constant has similar properties of noble materials [19]. Therefore, it is expected that the GaAs grating is responsible for the absorption in the shorter wavelength range. Moreover, the absorber is with geometrical configuration very simpler than those reported before [23-27,45,46]. The fabrication process of this
absorber is compatible with the popular nano-fabrication technologies. Firstly, homogeneous W and GaAs flat films with certain thickness can be deposited on a clean SiO2 substrate in turn via chemical vapor deposition [47] or physical sputtering deposition [48]. Then, a layer of photoresist would be coated on the GaAs film and etched by the electron beam lithography [24] to form one-dimensionally periodic grating. After that, GaAs material should be sputtered in the slits and on the photoresist antennas. Finally, the photoresist grating coated with GaAs materials would be removed using the lift-off method to obtain the final structure [32]. Numerical calculations are performed by the three-dimensional finite-difference time-domain ways (FDTD) [49]. Since the structure can be regarded as infinite long in the y direction, a 2D simulation model is applied to save compute space. The periodic boundary conditions are applied in the x direction and perfectly matched layers are used in the z direction. The dielectric constants of W and GaAs are taken from Palik [50]. A wide frequency plane wave with the linear polarization along the x axis is irradiated from the top of metamaterials. The transmission (T) in this absorber is equal to zero due to the opaque metal film used at the bottom. The absorption (A) of this absorber can be calculated by A = 1- R, where R denotes the reflection.
Results and discussion Fig. 1(b) shows the absorption spectrum of the optimized absorber at normal incidence 3
(marked with the absorber, black solid line). Here, the optimized period (P), width (d) and thickness (h2) of grating are 500 nm, 240 nm, and 120 nm, respectively. The thicknesses h1 of the GaAs film is 30 nm. In this work, the parameters remain invariable unless otherwise specified. For comparison, the absorption spectra of the two-layer film structure consisting of W and GaAs (marked with W-GaAs two-layer film, red dash dot line) and the structure consisting of a GaAs grating standing on the W film (marked with W-film+GaAs grating, blue short dot line) are also shown here. These three structures are with the same structural parameters. Considering the 90% absorption, only a narrow absorption band within the range of 681-932 nm is found in the two-layer film structure. When a GaAs grating is placed on the W film, a wide shoulder peak is achieved in the range of 590 -1726 nm with an absorption dip at 1074 nm (A = 79%). Interestingly, by integrating the GaAs grating and the two-layer film stacks of W and GaAs, an ultra-broad band with the absorption over 90% is observed in the wavelength region from 506 nm to 1814 nm, indicating a 90% bandwidth of 1308 nm. This absorption bandwidth is much wider than those of the metamaterial absorbers reported previously [22, 23]. The average absorption in this range is up to 97% with the maximum absorption reaching 100% in the ranges of 623-724 nm and 1666-1717 nm. The minimum absorption is 92% at λ =1250 nm. Therefore, the GaAs grating is responsible for the strong absorption.
Fig. 2. Electric field |E|2, magnetic field |H|2 distributions and current density J at the wavelengths of 641 nm (a-c), 1250 nm (b-f) and 1700 nm (g-i) in the xoz plane, respectively. To reveal the physical mechanism of the optical property observed above, the electric field intensity (|E|2), magnetic field intensity (|H|2) and current density (J) distributions in the xoz plane at λ = 641 nm, 1250 nm and 1700 nm are calculated and presented in Fig. 2. At λ = 641 nm, the electric field energy concentrating on both top corners of the grating strips demonstrates the obvious electric dipole behaviors [Fig. 2(a)]. Very weak magnetic field energy is found in the inner sides of the grating and at the interface of W film and GaAs film [Fig. 2(b)], indicating the dominant role of electric field distribution on this resonant band. The current mainly distributes in 4
the thin GaAs film and grating [Fig. 2(c)], confirming the effectiveness of GaAs material for this absorption enhancement [23]. At λ =1250 nm, the electric field energy locates at the four corners of the grating strips [Fig. 2(d)], indicating the excitation of localized surface plasmons (LSPs) [51,52]. Strong electric field energy is also observed at the interface of GaAs spacer layer and W film [Fig. 2(d)] and magnetic field energy mainly distributes in the GaAs film, especially at the interface of W film and GaAs film as shown in Fig. 2(e), which demonstrate the excitation of propagating surface plasmons (PSPs) [53,54] and the hybridized coupling effect of LSPs and PSPs [12,20,23]. In Fig. 2(f), the current mainly locates at the interface of W film and GaAs film. At λ = 1700 nm, the electric field energy mainly focuses on two sides of grating strips as well as in the GaAs spacer layer [Fig. 2(g)]. The magnetic field is mainly confined in the area below the grating strips [Fig. 2(h)] and the current is located in the bottom of grating strips, the area in the spacer layer below the grating strips and the top of the W film [Fig. 2(i)]. These indicate the excitation of LSPs and PSPs [12,20,21,23,27], the strong coupling of LSPs of grating strips and PSPs on the bottom W film and the current flowing along the lower metal layer [23].
Fig. 3. Absorption spectra of the structures by changing the materials of the bottom metal film (a) and the top layer grating (b). We further explore the effects of materials and structural parameters on its absorption performance. Fig. 3(a) shows the absorption spectra of the structure by replacing the bottom metals with several commonly used metals but keeping the structural parameters invariable. When replacing the bottom W film with refractory metals Cr or Ti, the broadband absorption performance is hardly changed. The lossy metal of Ti, Cr and W with an appropriate thickness satisfies the impedance match conditions, and thus can contribute to the acquirement of broadband absorption [20]. When the W film is replaced by Al, the absorption spectrum exhibits three wide resonant bands at around 550 nm, 786 nm and 1420 nm. While for the case replaced by Cu or Au, the absorption spectrum exhibits two narrow resonances at 801 nm and 865 nm and two wide resonances in the range of 400-704 nm and 1200-1700 nm. Strong absorption only occurs in the range of 500-1000 nm in the structure with the Al, Cu or Au film substrate. The narrow bandwidth is due to the small metal loss [23]. Fig. 3(b) displays the absorption spectra of the structures with the top GaAs, TiO2, Al2O3 or SiO2 grating. Obviously, for the grating consisting of the materials with higher dielectric constants, the absorption intensity and bandwidth are larger than those of the structures with the lower dielectric constants. Significant decreases in absorption and bandwidth 5
are thus observed in the structures with the decreased dielectric constants due to the greatly reduced excitation and limitation of the fields [19].
Fig. 4. Effects of the structural parameters on the absorption performance. (a) Period P of grating. (b) Width d of grating strips. (c) Thickness h1 of GaAs film. (d) Thickness h2 of grating. The influences of period (P), width (d) and thickness (h2) of grating and thickness (h1) of GaAs film on the absorption properties are shown in Fig. 4. As depicted in Fig. 4(a), with the decreased P, the structure behaves more like a continuous film stack [20], which leads to a wider absorption bandwidth. Contrarily, the increase of P, accompanied by intensive PSPs [14], leads to a narrower absorption band with reduced absorption. Thus, the tradeoff of P to achieve an ideal absorption is around 500 nm. The increased grating width d leads to the increased absorption bandwidth and the firstly increased and then decreased absorption intensity [Fig. 4(b)], which mainly originate from the excited PSPs and its coupling with other resonances [20,23]. The relatively larger filling factor (i.e., d/P) also leads to a wider absorption bandwidth [42]. The optimized d is equal to 240 nm. In Fig. 4(c), a gradually red-shift is observed as h1 increases. The minimum absorption firstly increases and then decreases. These are mainly due to the increased effective cavity length [20,21,23,55,56]. The ideal spectral property can be achieved when h1 is around 30-40 nm. Fig. 4(d) presents the influence of the grating thickness h2 on the absorption. A thinner grating promises a less lossy cavity and a thicker one indicates a weaker coupling [23]. As a result, the optimal performance can be obtained with h2 =120-140 nm. The parameters of grating array (p, d, h2) generally affect the plasmon resonant effects and the thickness of the intermediate layer (h1) influences the cavity modes. In Table 1, we display the light absorption features of other perfect absorbers [20-27,42]. Obviously, the bandwidth with 90% absorption obtained in our structure is larger than the absorbers based on the three-layer film structure (< 300 nm) [22], multilayer film structures with meta-surfaces (575 nm, 1110 nm, 1100 nm or 400 nm) [21,24,25,42], and complex hole structure (360 nm) [26]. Moreover, these structures [21,24-26], except for the three-layer film structure 6
[22], are all more complicated than ours. By placing a three-layer one-dimensional grating on the three-layer film stacks, an ultra-broad band (2969 nm) with 90% absorption was carried out [20] in the visible and infrared regions. Noting that the six-layers structural characteristic increases not only the cost but also the size of the sample, which is not conducive to its integrated development. Using the non-close-packed composite colloids on the Au film, an absorption bandwidth of 2851 nm was achieved by Liu et al [27] in the infrared range, not including the visible region. These hold the proposed scheme with potential application in the solar absorbers. Table 1. Comparison of structure and absorption characteristics Structures
Bandwidth
Region
References
(absorption >90%) Composite six-layer stacks
2969 nm
570-3539 nm
Ref. [20]
Composite nano-resonators on two-layer films
1110 nm
354-1426 nm
Ref. [21]
Three-layer film structure
< 300 nm
420-680 nm
Ref. [22]
Composite cube array on the Al film
712.4 nm
354-1066 nm
Ref. [23]
Three-layer film structure with a complex array
575 nm
300-875 nm
Ref. [24]
Three-layer film structure with a nanocone array
1100 nm
400-1500 nm
Ref. [25]
Complex circular hole structure
360 nm
400-760 nm
Ref. [26]
Non-close-packed composite colloids on the Au film
2851 nm
1500-4200 nm
Ref. [27]
Nanowire array on the GaAs film
400 nm
300-700 nm
Ref. [42]
Fig.5 (a) Calculated incidence angle resolved spectrum response of the absorber. (b) Short-circuit current density with the GaAs grating thickness h2. (c) Absorbed and missed energy of this absorber in the full spectral range of solar radiance AM 1.5. Absorption under different oblique angles of incident light is very important for the application of absorbers. As show in Fig.5(a). When the incidence angle increases from 0° to 55°, the perfect absorption bandwidth is always greater than 1200 nm, suggesting a promising application in solar cells and PVs. In Fig. 5(b), the effect of the thickness h2 of GaAs grating on the short-circuit current density is investigated. Assuming that each absorbed photon generates one electron-hole pair and all photo-generated carriers are collected, the short-circuit current 3000 𝑛𝑚𝑞𝜆 ℎ𝑐Φ𝐴𝑀1.5(𝜆)𝐴(𝜆)
density (Jsc) under AM1.5 solar illumination can be calculated by 𝐽𝑠𝑐 = ∫400 𝑛𝑚
[22]. Here, assuming the internal quantum efficiency is 100%, h is the Planck constant , q is the charge of the electron, λ is the wavelength of incident light, c is the speed of light, Φ𝐴𝑀1.5(𝜆) is the solar radiance AM 1.5 and A(λ) is the absorption of this absorber. When the thickness h2 of 7
grating is tuned from 80 nm to 160 nm with a step of 20 nm, the change in short-circuit current density is not regular. It is because that the short-circuit current density Jsc mainly depends on the number of resonant modes generated in the 400-3000 nm wavelength range [22,43]. The best Jsc (56.4736 mA/cm2) is obtained as h2 = 140 nm. For h2 = 120 nm, Jsc is equal to 56.3547 mA/cm2, which is much larger than the short-circuit current of 30.3 mA/cm2 reported by Meng et al [43]. The solar absorption response of the absorber is usually studied to evaluate the solar energy collection. The ideal absorber possesses a near-unity absorption band in a wide wavelength range. In this work, we carry out the solar absorption investigation by putting the absorber under the illumination of the AM 1.5 source. Fig. 5(c) shows the absorbed and missed solar energy of this absorber. The efficiency of light capture is very high although some of energy is missed. Near-unity solar energy is trapped by this absorber due to its resonant response occurred in the ultra-broadband frequency range by the multiple resonances coupling. Therefore, the absorber can hold potential applications in magnetic recording, ultra-thin and durable optoelectronic elements.
Conclusion We have proposed an ultra-broadband metamaterial absorber configured by a periodic GaAs grating array, an ultra-thin GaAs interlayer and a thick W film. This absorber exhibits the perfect broadband absorption with 90% absorption bandwidth over 1300 nm from visible to near-infrared region. The average absorption in this range is over 97%. Superior surface plasmon resonances combined with the cavity modes lead to this broadband perfect absorption. The absorber also has a high short-circuit current density over 56 mA/cm2 so that it can be widely applied in electricity. Near-unity solar energy is trapped by this absorber due to the multiple resonances coupling. Moreover, this absorber requires no noble metal and is compatible with the popular nano-fabrication technologies. These offer new perspectives for achieving thin film solar cells, solar energy harvesting, magnetic recording, ultra-thin and durable optoelectronic elements. Funding We gratefully acknowledge support from National Natural Science Foundation of China (51761015, 11564017, 11804134 and 11464019) and Provincial Natural Science Foundation of Jiangxi (20181BAB201015, 20182BCB22002 and 2018ACB21005). Declaration of Competing Interest The authors declare no conflict of interest. References [1] [2] [3] [4] [5] [6]
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Highlights 1. The broadband metamaterial absorber consists of a periodic gallium arsenide (GaAs) grating array standing on a tungsten film separated by an ultra-thin GaAs spacer layer. 2. The perfect absorption broadband is achieved with an average absorption of 97% in the range from 506 nm to 1814 nm and the bandwidth with the absorption over 90% is larger than 1300 nm. 3. High short-circuit current density over 56 mA/cm2 and near-unity solar energy trapping are obtained in this perfect absorber. [57] 10
Conflict of Interest The authors declare no conflict of interest.
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Author Statement The authors claim that none of the materials in the paper has been published or is under consideration for publication elsewhere. [59]
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