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Silicon multi-resonant metasurface for full-spectrum perfect solar energy absorption
Solar Energy 199 (2020) 360–365
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Silicon multi-resonant metasurface for full-spectrum perfect solar energy absorption
T
Zhengqi Liua, , Haozong Zhonga, Houjiao Zhanga, Zhenping Huanga, Guiqiang Liua, Xiaoshan Liua, Guolan Fua, Chaojun Tangb ⁎
a
Jiangxi Key Laboratory of Nanomaterials and Sensors, Provincial Key Laboratory of Optoelectronic and Telecommunication, College of Physics and Communication Electronics, Jiangxi Normal University, Nanchang 330022, Jiangxi, China b College of Science, Zhejiang University of Technology, Hangzhou 310023, China
ARTICLE INFO
ABSTRACT
Keywords: Solar absorber Silicon Metasurface Full-spectrum
Energy efficiency of optoelectronic components is critically dependent on minimizing the undesired reflections. This paper is dedicated to a new type of silicon metasurface, possessing solar full-spectrum anti-reflection and absorption due to the multiple resonances. The proposed structure gains both in optical properties and simplified structural features. Experimental study is further carried out via using a discrete metal nanoparticles layer to build the metasurface with a series of air cavities in the silicon substrate, which therefore forms the model with different resonant modes at different wavelengths. Near-unity anti-reflection and absorption (efficiency > 97%) is achieved in the full solar radiation spectrum from 280 nm to 2500 nm. Based on the combination of multiresonant optical properties and the structural advantageous of large area and low cost under easy-design and fabrication process, such an optical metasurface can demonstrate new insight on achieving perfect solar antireflector and absorber, and the related optoelectronic devices.
1. Introduction Reflection is a natural phenomenon when the light passes the interface between different materials. Light reflection is essential for us to see and enjoy the colorful world. Nevertheless, in many applications, such as solar energy harvesting and solar cells and photodetectors, reflection is exactly unwanted loss process. The reduction of unwanted reflections at interfaces has long been pursued and still being a longstanding challenge. For photovoltaic and solar energy applications, the design of anti-reflection coatings is particularly challenging since it needs to be realized across a very broadband spectrum in the solar irradiation range. The minimization of reflection for a flat substrate was firstly demonstrated via using a thin transparent dielectric layer, as a Rayleigh’s film (Rayleigh, 1879), on the substrate to suppress the light reflection by destructive interference effect (Sexton, 1982). Nevertheless, the antireflection efficiency was still low. Further efforts have been then made via introducing the multiple or graded-index destructive coatings to achieve broadband anti-reflection (Yan et al., 2013; Burresi et al., 2015; Li et al., 2018; Yang et al., 2017). However, the spectral broadening was at the expense of the costs and the ease of the fabrication, etc. As alternative approaches, the scattering properties of optical ⁎
metallic and dielectric resonators have also been employed to reduce reflection from interfaces (Fan et al., 2017; Ghobadi et al., 2018; Pala et al., 2016). Optical Mie resonant response and localized cavity resonances by the high-index dielectric resonators have been demonstrated for numerous applications on light trapping and absorption (Kuznetsov et al., 2016; Du et al., 2018; Huang et al., 2017); Fano-like spectral manipulation (Limonov et al., 2017; Feng et al., 2017) and nonlinear optics (Zhang et al., 2018; Xiang et al., 2017; Liao et al., 2017). Recently, high-index dielectrics such as the semiconductors have been performed for plasmon-like metamaterials and metasurfaces (Yang et al., 2014; Liu et al., 2016; Bakker et al., 2015; Wang et al., 2017). For instance, the high-index dielectric resonators and metasurfaces have been exploited as novel approaches (Spinelli et al., 2012; Groep et al., 2015) to cancel reflection in a broad wavelength range. Based on a judicious design of a multi-body dielectric metasurface, an average reflection as low as 4% was obtained from a silicon (Si) wafer via using different Mie resonators (Pecora et al., 2018). Via using the combination of Mie resonances and Fabry-Perot resonances in the Si metasurfaces, broadband anti-reflection in the spectral range from 425 nm to 900 nm was experimentally achieved for optimized designs due to the multimodal interaction between these resonances within the pattern layer (Cordaro et al., 2019; Baryshnikova et al., 2019; Ospanova
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.solener.2020.02.053 Received 10 December 2019; Received in revised form 6 February 2020; Accepted 12 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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et al., 2018; Miroshnichenko et al., 2015). However, the spectral bandwidth and efficiency of these nanopatterned systems are limited by their resonance bandwidth, which only leads to the anti-reflection occurred at a small wavelength window and eventually hampers the achievement for solar full-spectrum anti-reflection. Moreover, the need of high-precise fabrication techniques such as the focused ion beam inevitably increases the cost and decreases the size scale, which would be the critical drawback for potential applications. In this paper, we demonstrate the design and experimental study on the novel broadband resonant mode sustaining Si anti-reflection metasurface in the whole solar radiation range (280–2500 nm). Near-unity anti-reflection and absorption (efficiency > 97%) is achieved in the solar spectrum. The proposed structure gains both in optical properties and simplified structural features. Therefore, this type metasurface is promising for efficient solar absorption (Pizzini, 2012; Fan, 2014; Huen and Daoud, 2017; Zhou et al., 2016) and the solar energy related components (Ho et al., 2018; Yu et al., 2018; Shan et al., 2019).
Fig. 1. Reflection of the silicon metasurfaces under a tuning of the structural parameters (D, t, P). The multiple reflection dips (1–5) are marked for the system with the size of D = 180 nm, t = 180 nm, P = 200 nm.
2. Model and structure Our metasurface model consists of a silicon wafer that is partially hollowed via a periodic array of air circular cavity. The structural parameters of the circular cavity (diameter, D, thickness, t) and the period P of the square array can be artificially tuned. We assume linearly polarized incident wave normally directed onto top of the metasurface. Numerical calculation is carried out via the three-dimensional finite-difference time-domain method (Taflove and Hagness, 2000; Liu et al., 2014), which has been widely used for the investigations on plasmonics and nano-optics (Spinelli et al., 2012; Ospanova et al., 2018). Periodic boundary conditions are used along the periodic directions. Perfectly matched layers are used at the outside areas of the structure and the reflective monitor. Experimental data of the dielectric permittivity for the silicon is used in the simulation (Palik, 1985). The light absorption A(ω) is equal to
A( ) = 1
R( )
T( )
Fig. 2. Schematic and normalized electric field distributions for the five dips in the xoy plane. The structural parameters (D, t, P) are with the size (180 nm, 180 nm, 200 nm).
(1)
schematic diagram presents the square lattice of the circular air cavity on the silicon wafer’s surface. Under the normal excitation with the polarized electric field along x-direction, electric field distributions for these resonant reflection dips are obtained. At λ1 = 290 nm, the field is observed to be confined in the air cavity, which confirms the excitation of the localized optical cavity resonance. At λ2 = 448 nm, the field is mainly distributed at the rear sides of the cavity, which indicates the electric dipole resonance. Otherwise, partial field is distributed at the four diagonal corner areas, suggesting the coexistence of the quadrupole resonance. This behavior is changed to be the optimized distribution when the wavelength is moving to 540 nm at λ3. At λ4 = 621 nm, the field distribution pattern is similar to be the slight combination process between the resonances at λ3 and λ5. At λ5, electric fields are distributed at the gap areas by the adjacent air cavities. It should be noted that the polarized electric field is at the orthogonal direction. Moreover, the fields at the former resonances (λ1-λ3) disappear. These features confirm the emergence of the destructive interference for the resonances and finally form the anapole resonance in this hollowed silicon wafer substrate. Although there are several reports on the existence of the anapole mode by the silicon resonators or antennas (Miroshnichenko et al., 2015), the report for the emerging of anapole resonance in the inverse cavity metasurface is not reported yet. The field distributions and the evolution are also slightly different. Owing to the excitation of the anapole mode, the total field scattering is cancelled and therefore introduces the near-zero reflection, which provides a broad anti-reflection spectrum. In particular, the anti-reflection is observed to with strong relationship to that of the structural size. Thereby, it is reasonable to expand the anti-reflection spectral bandwidth via combining a series of contributions from differently sized resonators.
Owing to the using of opaque metal substrate, the light transmission is completely canceled, suggesting a spectral T(ω) = 0. Hence, the absorption A(ω) can be obtained as the result of 1 - R(ω). The spectral absorption efficiency of the absorber can be estimated by calculating the solar absorption ηA according to the Eq. (2), where the IAM1.5(ω) is the incident solar power of the Air Mass 1.5 Global spectrum. The spectral range is broadened from the UV to the infrared range. The minimal (λmin) and maximal (λmax) wavelengths are 280 nm and 2500 nm, respectively. max
A
=
min
(1
R ( ))· IAM 1.5 ( )·d max
min
IAM1.5 ( )· d
(2)
Fig. 1 presents the reflection spectra of the silicon metasurfaces when the structural parameters (D, t, P) is tuned from (140, 140, 160) nm to (200, 200, 220) nm with an increase step of 20 nm. It is observed that the curves show obvious red-shift in the wavelength range. A broadband reflection inhibition range is obtained at the longer wavelength range. Moreover, accompanied by the tuning of the structural size scale, the anti-reflection window is simultaneously scaled. We note this anti-reflection dip as the fifth band. There are several sub-dips at the shorter wavelength range. As the inset picture shown in Fig. 1, other four reflection dips (1–4) are marked in the curve for the structure formed with the parameters of D = 180 nm, t = 180 nm, P = 200 nm. For these reflection dips, the reflection intensity is down to 5.7% at 1376 nm. The anti-reflection bandwidth reaches 964 nm with the reflection efficiency less than 10% in the spectrum, suggesting an intrinsic broadband anti-reflection. In order to get understand on the observed multiple reflection dips, field distributions have been simulated. As shown in Fig. 2, the 361
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Fig. 3. Schematic and experimental process for the solar anti-reflector and absorber.
3. Experimental results and discussion
thick Au film. Herein, the densely packed NPs coating on the Si surface can provide two main contributions: One is the automatically obtained pattern for the Si metasurface. The other is the strongly enhanced efficient etching process during the chemical reaction via using the hydrofluoric acid and the hydrogen peroxide solution. After a 7 min etching process and the rinsing treatment, the Si metasurface is achieved as shown in Fig. 4(b). The reflection and absorption is shown in Fig. 4(c). The spectral absorption A is defined as A = 1 – R – T, where the R and T are the reflection and transmission (Fang et al., 2012; Cui et al., 2014; Ng et al., 2017). Owing to the cancelled transmission by the thick Si wafer, the A can be directly obtained via A = 1 - R. A broadband anti-reflection is observed. The bandwidth can reach 1345 nm with the wavelength range from 280 nm to 1625 nm when we take the R less than 10% into account. With R < 4% (the refection efficiency by the transparent silica in theory), the anti-reflection range is crossing the range from 280 nm to 1207 nm, suggest the near-perfect anti-reflection bandwidth up to 927 nm. The inset picture shows the comparison for the common Si wafer and the Si metasurface based substrate. In contrast to the highly reflective response for the logo words of jxnu by the common Si wafer, a completely black Si wafer substrate is observed due to the existence of the metasurface. The black picture confirms the perfect absorption in the wide wavelength range, particularly for the visible light. Thereby, the introduced Si metasurface not only forms the ultra-wideband light anti-reflector but also indicates
Based on the observation for the theoretical model, we do further study on achieving broadband and even the solar full-spectrum antireflection metasurface via using simple fabrication process to build large-area structure at low cost. As can be seen from Fig. 3, the fabrication process can be devided into two main steps: i, the formation of a monolayer gold (Au) nanoparticles (NPs) on the Si wafer substrate, which can be realized via a moderate heat-treatment on the Au thin film (Tang et al., 2019; Huang et al., 2018); ii, the formation of partially hollowed Si metasurface, which can be obtained via a chemical etching assisted by the Au NPs and followed with a removing process of the Au NPs. Owing to the differently sized Au NPs, the obtained Si metasurface will then produce different air cavities in the top surface area. The size of the Au NPs is related to the Au film’s thickness and the heat-treatment process including the temperature and the time. The air cavity size is dependent on the Au NPs’ size and the etching treatment. Although it is difficult to obtain the Au NPs with the same size via this simple method, the dispersion on the size would be a great advantageous for the achievement of ultra-broadband anti-reflection. It is also the basic factor to form the automatically combined multi-resonant structure. Fig. 4(a) shows the Au NPs on a Si wafer substrate. The Au NPs are fabricated via a moderate heat-treatment (300 °C, 60 min) for the 2-nm-
Fig. 4. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection and absorption for the structure formed via a 2-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
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Fig. 5. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection and absorption for the structure formed via a 4-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
the perfect absorber. Following, we do detailed study on the structural and optical properties via using different thickness values for the Au film to form different structures. Fig. 5(a) and (b) show the SEM images for the Au NPs and the Si metasurface under the situation of using a 4-nm-thick Au film. It is observed that the NPs are much larger and packed with a big distance for adjacent ones. The formed Si metasurface can clearly show the emergence of air holes with different sizes. Fig. 5(c) shows the spectral response and indicates a further improving on the anti-reflection. For the R < 10% (4%), the bandwidth is up to 1655 nm (1080 nm). This suggests the better anti-reflection is achieved. Fig. 6 shows the structural and optical properties when a 7-nm-thick Au film is used during the fabrication process. The Au NPs are packed in the surface. It is observed that the size scale is continuously changed to be big when the thickness of the Au film is increased. It should be noted that some of small NPs also exist as shown in Fig. 6(a). Otherwise, the NPs also shows different geometry morphology features such as the disks, rectangule and oval-like particles. These NPs directly lead to the formation of the Si metasurface with different geometrical features. Fig. 6(b) shows the obtained metasurface. The pattern indicates the composite of a series different periodic cavities array. As the results shown in the theoretical model, the metasurface formed by one certain size cavity array can produce one broadband anti-reflection. As a result, for this multiple resonators combined metasurface, the spectral range will be expanded extremely (Zhou et al., 2016; Hedayati et al., 2011; Liu et al., 2015; Zhang et al., 2019). Fig. 6(c) presents the optical properties and shows an ultra-broadband anti-reflection window. It is observed that the reflection efficiency is all less than 10% in the whole spectral range. The spectral range is from the ultraviolet (280 nm) to the near-infrared region (1936 nm) with the reflection less than that of the transparent silica. The averaged absorption in the whole wavelength range from 280 nm to 2500 nm reaches 97%, indicating the solar full-spectrum absorber. As the solar irradiation spectrum shown in Fig. 6(c), the main solar energy is distributed at the visible and near-
infrared range, which matches with the anti-reflection and absorption of the Si metasurface very well. These features suggest the achievement of solar perfect anti-reflector and absorber. For the metasurface formed via using a 10-nm-thick Au film after the same treatment, much larger NPs and air cavities are obtained as shown in Fig. 7(a) and (b). The spectrum also shows a broadband antireflection. Nevertheless, the reflection intensity shows a slight increase in comparison with that of the former one in the near-infrared range. This would lead to the reduction on the solar anti-reflection and absorption. It mainly results from the much larger NPs or the related cavities and the bigger gap distance between the adjacent resonators, which eventually lead to the reduction on the optical field coupling and trapping. Finally, the optical properties of the Si metasurface formed by the 7nm-thick Au film under different etching processes are shown in Fig. 8. With increasing the chemical etching time from 2 min to 7 min, reflection shows a noticeable reduction due to the stronger light trapping by the deep cavity. In particular, the reflection at the longer wavelength range reduces quickly in the process. This is the reason for the structure with larger and deeper cavities will produce resonant absorption and confinement for the longer wavelength light wave (Cui et al., 2014; Hedayati et al., 2011; Liu et al., 2015). Nevertheless, for the light at the shorter wavelength range, strong optical field coupling and absorption can be realized via an ultra-thin high-index dielectric or the semiconductor due to the low-order and fundamental resonances (Chen et al., 2016; Liu et al., 2017; Hu et al., 2019). Otherwise, the maximal etching depth for the system is also less than 200 nm, suggesting a thin anti-reflective film. These features confirm the achievement for the solar full-spectrum anti-reflector and absorber via an ultra-thin metasurface structure, which could be more desirable for optoelectronic devices such as solar cells, photo-detectors and infrared detecting and filtering (Burresi et al., 2015; Callahan et al., 2013; Xu et al., 2019). In addition, it should be noted that the different optical resonances supported by the random silicon cavities can strongly broaden the Fig. 6. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection, absorption and solar irradiation for the structure formed via a 7-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
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Fig. 7. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection and absorption for the structure formed via a 10-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgements The work supported from National Natural Science Foundation of China (Grants 11664015, 11804134, 51761015, and 11564017), Natural Science Foundation of Jiangxi Province (2018ACB21005, 20182BCB22002 and 20181BAB201015). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.02.053.
Fig. 8. Reflection for the structure formed via a 7-nm-thick Au film by different etching processes.
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Silicon multi-resonant metasurface for full-spectrum perfect solar energy absorption
T
Zhengqi Liua, , Haozong Zhonga, Houjiao Zhanga, Zhenping Huanga, Guiqiang Liua, Xiaoshan Liua, Guolan Fua, Chaojun Tangb ⁎
a
Jiangxi Key Laboratory of Nanomaterials and Sensors, Provincial Key Laboratory of Optoelectronic and Telecommunication, College of Physics and Communication Electronics, Jiangxi Normal University, Nanchang 330022, Jiangxi, China b College of Science, Zhejiang University of Technology, Hangzhou 310023, China
ARTICLE INFO
ABSTRACT
Keywords: Solar absorber Silicon Metasurface Full-spectrum
Energy efficiency of optoelectronic components is critically dependent on minimizing the undesired reflections. This paper is dedicated to a new type of silicon metasurface, possessing solar full-spectrum anti-reflection and absorption due to the multiple resonances. The proposed structure gains both in optical properties and simplified structural features. Experimental study is further carried out via using a discrete metal nanoparticles layer to build the metasurface with a series of air cavities in the silicon substrate, which therefore forms the model with different resonant modes at different wavelengths. Near-unity anti-reflection and absorption (efficiency > 97%) is achieved in the full solar radiation spectrum from 280 nm to 2500 nm. Based on the combination of multiresonant optical properties and the structural advantageous of large area and low cost under easy-design and fabrication process, such an optical metasurface can demonstrate new insight on achieving perfect solar antireflector and absorber, and the related optoelectronic devices.
1. Introduction Reflection is a natural phenomenon when the light passes the interface between different materials. Light reflection is essential for us to see and enjoy the colorful world. Nevertheless, in many applications, such as solar energy harvesting and solar cells and photodetectors, reflection is exactly unwanted loss process. The reduction of unwanted reflections at interfaces has long been pursued and still being a longstanding challenge. For photovoltaic and solar energy applications, the design of anti-reflection coatings is particularly challenging since it needs to be realized across a very broadband spectrum in the solar irradiation range. The minimization of reflection for a flat substrate was firstly demonstrated via using a thin transparent dielectric layer, as a Rayleigh’s film (Rayleigh, 1879), on the substrate to suppress the light reflection by destructive interference effect (Sexton, 1982). Nevertheless, the antireflection efficiency was still low. Further efforts have been then made via introducing the multiple or graded-index destructive coatings to achieve broadband anti-reflection (Yan et al., 2013; Burresi et al., 2015; Li et al., 2018; Yang et al., 2017). However, the spectral broadening was at the expense of the costs and the ease of the fabrication, etc. As alternative approaches, the scattering properties of optical ⁎
metallic and dielectric resonators have also been employed to reduce reflection from interfaces (Fan et al., 2017; Ghobadi et al., 2018; Pala et al., 2016). Optical Mie resonant response and localized cavity resonances by the high-index dielectric resonators have been demonstrated for numerous applications on light trapping and absorption (Kuznetsov et al., 2016; Du et al., 2018; Huang et al., 2017); Fano-like spectral manipulation (Limonov et al., 2017; Feng et al., 2017) and nonlinear optics (Zhang et al., 2018; Xiang et al., 2017; Liao et al., 2017). Recently, high-index dielectrics such as the semiconductors have been performed for plasmon-like metamaterials and metasurfaces (Yang et al., 2014; Liu et al., 2016; Bakker et al., 2015; Wang et al., 2017). For instance, the high-index dielectric resonators and metasurfaces have been exploited as novel approaches (Spinelli et al., 2012; Groep et al., 2015) to cancel reflection in a broad wavelength range. Based on a judicious design of a multi-body dielectric metasurface, an average reflection as low as 4% was obtained from a silicon (Si) wafer via using different Mie resonators (Pecora et al., 2018). Via using the combination of Mie resonances and Fabry-Perot resonances in the Si metasurfaces, broadband anti-reflection in the spectral range from 425 nm to 900 nm was experimentally achieved for optimized designs due to the multimodal interaction between these resonances within the pattern layer (Cordaro et al., 2019; Baryshnikova et al., 2019; Ospanova
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.solener.2020.02.053 Received 10 December 2019; Received in revised form 6 February 2020; Accepted 12 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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et al., 2018; Miroshnichenko et al., 2015). However, the spectral bandwidth and efficiency of these nanopatterned systems are limited by their resonance bandwidth, which only leads to the anti-reflection occurred at a small wavelength window and eventually hampers the achievement for solar full-spectrum anti-reflection. Moreover, the need of high-precise fabrication techniques such as the focused ion beam inevitably increases the cost and decreases the size scale, which would be the critical drawback for potential applications. In this paper, we demonstrate the design and experimental study on the novel broadband resonant mode sustaining Si anti-reflection metasurface in the whole solar radiation range (280–2500 nm). Near-unity anti-reflection and absorption (efficiency > 97%) is achieved in the solar spectrum. The proposed structure gains both in optical properties and simplified structural features. Therefore, this type metasurface is promising for efficient solar absorption (Pizzini, 2012; Fan, 2014; Huen and Daoud, 2017; Zhou et al., 2016) and the solar energy related components (Ho et al., 2018; Yu et al., 2018; Shan et al., 2019).
Fig. 1. Reflection of the silicon metasurfaces under a tuning of the structural parameters (D, t, P). The multiple reflection dips (1–5) are marked for the system with the size of D = 180 nm, t = 180 nm, P = 200 nm.
2. Model and structure Our metasurface model consists of a silicon wafer that is partially hollowed via a periodic array of air circular cavity. The structural parameters of the circular cavity (diameter, D, thickness, t) and the period P of the square array can be artificially tuned. We assume linearly polarized incident wave normally directed onto top of the metasurface. Numerical calculation is carried out via the three-dimensional finite-difference time-domain method (Taflove and Hagness, 2000; Liu et al., 2014), which has been widely used for the investigations on plasmonics and nano-optics (Spinelli et al., 2012; Ospanova et al., 2018). Periodic boundary conditions are used along the periodic directions. Perfectly matched layers are used at the outside areas of the structure and the reflective monitor. Experimental data of the dielectric permittivity for the silicon is used in the simulation (Palik, 1985). The light absorption A(ω) is equal to
A( ) = 1
R( )
T( )
Fig. 2. Schematic and normalized electric field distributions for the five dips in the xoy plane. The structural parameters (D, t, P) are with the size (180 nm, 180 nm, 200 nm).
(1)
schematic diagram presents the square lattice of the circular air cavity on the silicon wafer’s surface. Under the normal excitation with the polarized electric field along x-direction, electric field distributions for these resonant reflection dips are obtained. At λ1 = 290 nm, the field is observed to be confined in the air cavity, which confirms the excitation of the localized optical cavity resonance. At λ2 = 448 nm, the field is mainly distributed at the rear sides of the cavity, which indicates the electric dipole resonance. Otherwise, partial field is distributed at the four diagonal corner areas, suggesting the coexistence of the quadrupole resonance. This behavior is changed to be the optimized distribution when the wavelength is moving to 540 nm at λ3. At λ4 = 621 nm, the field distribution pattern is similar to be the slight combination process between the resonances at λ3 and λ5. At λ5, electric fields are distributed at the gap areas by the adjacent air cavities. It should be noted that the polarized electric field is at the orthogonal direction. Moreover, the fields at the former resonances (λ1-λ3) disappear. These features confirm the emergence of the destructive interference for the resonances and finally form the anapole resonance in this hollowed silicon wafer substrate. Although there are several reports on the existence of the anapole mode by the silicon resonators or antennas (Miroshnichenko et al., 2015), the report for the emerging of anapole resonance in the inverse cavity metasurface is not reported yet. The field distributions and the evolution are also slightly different. Owing to the excitation of the anapole mode, the total field scattering is cancelled and therefore introduces the near-zero reflection, which provides a broad anti-reflection spectrum. In particular, the anti-reflection is observed to with strong relationship to that of the structural size. Thereby, it is reasonable to expand the anti-reflection spectral bandwidth via combining a series of contributions from differently sized resonators.
Owing to the using of opaque metal substrate, the light transmission is completely canceled, suggesting a spectral T(ω) = 0. Hence, the absorption A(ω) can be obtained as the result of 1 - R(ω). The spectral absorption efficiency of the absorber can be estimated by calculating the solar absorption ηA according to the Eq. (2), where the IAM1.5(ω) is the incident solar power of the Air Mass 1.5 Global spectrum. The spectral range is broadened from the UV to the infrared range. The minimal (λmin) and maximal (λmax) wavelengths are 280 nm and 2500 nm, respectively. max
A
=
min
(1
R ( ))· IAM 1.5 ( )·d max
min
IAM1.5 ( )· d
(2)
Fig. 1 presents the reflection spectra of the silicon metasurfaces when the structural parameters (D, t, P) is tuned from (140, 140, 160) nm to (200, 200, 220) nm with an increase step of 20 nm. It is observed that the curves show obvious red-shift in the wavelength range. A broadband reflection inhibition range is obtained at the longer wavelength range. Moreover, accompanied by the tuning of the structural size scale, the anti-reflection window is simultaneously scaled. We note this anti-reflection dip as the fifth band. There are several sub-dips at the shorter wavelength range. As the inset picture shown in Fig. 1, other four reflection dips (1–4) are marked in the curve for the structure formed with the parameters of D = 180 nm, t = 180 nm, P = 200 nm. For these reflection dips, the reflection intensity is down to 5.7% at 1376 nm. The anti-reflection bandwidth reaches 964 nm with the reflection efficiency less than 10% in the spectrum, suggesting an intrinsic broadband anti-reflection. In order to get understand on the observed multiple reflection dips, field distributions have been simulated. As shown in Fig. 2, the 361
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Fig. 3. Schematic and experimental process for the solar anti-reflector and absorber.
3. Experimental results and discussion
thick Au film. Herein, the densely packed NPs coating on the Si surface can provide two main contributions: One is the automatically obtained pattern for the Si metasurface. The other is the strongly enhanced efficient etching process during the chemical reaction via using the hydrofluoric acid and the hydrogen peroxide solution. After a 7 min etching process and the rinsing treatment, the Si metasurface is achieved as shown in Fig. 4(b). The reflection and absorption is shown in Fig. 4(c). The spectral absorption A is defined as A = 1 – R – T, where the R and T are the reflection and transmission (Fang et al., 2012; Cui et al., 2014; Ng et al., 2017). Owing to the cancelled transmission by the thick Si wafer, the A can be directly obtained via A = 1 - R. A broadband anti-reflection is observed. The bandwidth can reach 1345 nm with the wavelength range from 280 nm to 1625 nm when we take the R less than 10% into account. With R < 4% (the refection efficiency by the transparent silica in theory), the anti-reflection range is crossing the range from 280 nm to 1207 nm, suggest the near-perfect anti-reflection bandwidth up to 927 nm. The inset picture shows the comparison for the common Si wafer and the Si metasurface based substrate. In contrast to the highly reflective response for the logo words of jxnu by the common Si wafer, a completely black Si wafer substrate is observed due to the existence of the metasurface. The black picture confirms the perfect absorption in the wide wavelength range, particularly for the visible light. Thereby, the introduced Si metasurface not only forms the ultra-wideband light anti-reflector but also indicates
Based on the observation for the theoretical model, we do further study on achieving broadband and even the solar full-spectrum antireflection metasurface via using simple fabrication process to build large-area structure at low cost. As can be seen from Fig. 3, the fabrication process can be devided into two main steps: i, the formation of a monolayer gold (Au) nanoparticles (NPs) on the Si wafer substrate, which can be realized via a moderate heat-treatment on the Au thin film (Tang et al., 2019; Huang et al., 2018); ii, the formation of partially hollowed Si metasurface, which can be obtained via a chemical etching assisted by the Au NPs and followed with a removing process of the Au NPs. Owing to the differently sized Au NPs, the obtained Si metasurface will then produce different air cavities in the top surface area. The size of the Au NPs is related to the Au film’s thickness and the heat-treatment process including the temperature and the time. The air cavity size is dependent on the Au NPs’ size and the etching treatment. Although it is difficult to obtain the Au NPs with the same size via this simple method, the dispersion on the size would be a great advantageous for the achievement of ultra-broadband anti-reflection. It is also the basic factor to form the automatically combined multi-resonant structure. Fig. 4(a) shows the Au NPs on a Si wafer substrate. The Au NPs are fabricated via a moderate heat-treatment (300 °C, 60 min) for the 2-nm-
Fig. 4. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection and absorption for the structure formed via a 2-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
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Fig. 5. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection and absorption for the structure formed via a 4-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
the perfect absorber. Following, we do detailed study on the structural and optical properties via using different thickness values for the Au film to form different structures. Fig. 5(a) and (b) show the SEM images for the Au NPs and the Si metasurface under the situation of using a 4-nm-thick Au film. It is observed that the NPs are much larger and packed with a big distance for adjacent ones. The formed Si metasurface can clearly show the emergence of air holes with different sizes. Fig. 5(c) shows the spectral response and indicates a further improving on the anti-reflection. For the R < 10% (4%), the bandwidth is up to 1655 nm (1080 nm). This suggests the better anti-reflection is achieved. Fig. 6 shows the structural and optical properties when a 7-nm-thick Au film is used during the fabrication process. The Au NPs are packed in the surface. It is observed that the size scale is continuously changed to be big when the thickness of the Au film is increased. It should be noted that some of small NPs also exist as shown in Fig. 6(a). Otherwise, the NPs also shows different geometry morphology features such as the disks, rectangule and oval-like particles. These NPs directly lead to the formation of the Si metasurface with different geometrical features. Fig. 6(b) shows the obtained metasurface. The pattern indicates the composite of a series different periodic cavities array. As the results shown in the theoretical model, the metasurface formed by one certain size cavity array can produce one broadband anti-reflection. As a result, for this multiple resonators combined metasurface, the spectral range will be expanded extremely (Zhou et al., 2016; Hedayati et al., 2011; Liu et al., 2015; Zhang et al., 2019). Fig. 6(c) presents the optical properties and shows an ultra-broadband anti-reflection window. It is observed that the reflection efficiency is all less than 10% in the whole spectral range. The spectral range is from the ultraviolet (280 nm) to the near-infrared region (1936 nm) with the reflection less than that of the transparent silica. The averaged absorption in the whole wavelength range from 280 nm to 2500 nm reaches 97%, indicating the solar full-spectrum absorber. As the solar irradiation spectrum shown in Fig. 6(c), the main solar energy is distributed at the visible and near-
infrared range, which matches with the anti-reflection and absorption of the Si metasurface very well. These features suggest the achievement of solar perfect anti-reflector and absorber. For the metasurface formed via using a 10-nm-thick Au film after the same treatment, much larger NPs and air cavities are obtained as shown in Fig. 7(a) and (b). The spectrum also shows a broadband antireflection. Nevertheless, the reflection intensity shows a slight increase in comparison with that of the former one in the near-infrared range. This would lead to the reduction on the solar anti-reflection and absorption. It mainly results from the much larger NPs or the related cavities and the bigger gap distance between the adjacent resonators, which eventually lead to the reduction on the optical field coupling and trapping. Finally, the optical properties of the Si metasurface formed by the 7nm-thick Au film under different etching processes are shown in Fig. 8. With increasing the chemical etching time from 2 min to 7 min, reflection shows a noticeable reduction due to the stronger light trapping by the deep cavity. In particular, the reflection at the longer wavelength range reduces quickly in the process. This is the reason for the structure with larger and deeper cavities will produce resonant absorption and confinement for the longer wavelength light wave (Cui et al., 2014; Hedayati et al., 2011; Liu et al., 2015). Nevertheless, for the light at the shorter wavelength range, strong optical field coupling and absorption can be realized via an ultra-thin high-index dielectric or the semiconductor due to the low-order and fundamental resonances (Chen et al., 2016; Liu et al., 2017; Hu et al., 2019). Otherwise, the maximal etching depth for the system is also less than 200 nm, suggesting a thin anti-reflective film. These features confirm the achievement for the solar full-spectrum anti-reflector and absorber via an ultra-thin metasurface structure, which could be more desirable for optoelectronic devices such as solar cells, photo-detectors and infrared detecting and filtering (Burresi et al., 2015; Callahan et al., 2013; Xu et al., 2019). In addition, it should be noted that the different optical resonances supported by the random silicon cavities can strongly broaden the Fig. 6. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection, absorption and solar irradiation for the structure formed via a 7-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
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Fig. 7. (a),(b) SEM images for the Au NPs coating on the Si substrate and the obtained Si metasurface after the etching and Au NPs removing processes. (c) The reflection and absorption for the structure formed via a 10-nm-thick Au film and the following etch process. The inset shows the comparison view pictures for the planar Si substrate and the Si metasurface anti-reflector/absorber.
Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgements The work supported from National Natural Science Foundation of China (Grants 11664015, 11804134, 51761015, and 11564017), Natural Science Foundation of Jiangxi Province (2018ACB21005, 20182BCB22002 and 20181BAB201015). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.02.053.
Fig. 8. Reflection for the structure formed via a 7-nm-thick Au film by different etching processes.
References absorption bandwidth. The larger size dispersion for the silicon metasurface can also lead to the various resonant modes in this absorber platform. Otherwise, optical resonance supported by the periodic array (Liu et al., 2019; Qin et al., 2020; Wang et al., 2012) can also introduce additional contributions to the obtained light absorption in the wide wavelength range.
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4. Conclusion In conclusion, we innovatively proposed and demonstrated multiresonant modes assisted solar full-spectrum perfect anti-reflection and absorption. The averaged absorption reaches 97% in the wavelength range from 280 nm to 2500 nm. In the wide spectral range from the ultraviolet (280 nm) to the near-infrared region (1936 nm), the reflection is even less than that of the transparent silica. Moreover, the ultra-thin metasurface structure can be fabricated easily in large area at low cost since it is without the need of high-precise lithography methods such as the focused ion beam technique. Furthermore, the using of ultra-thin metal film and its discretizing process for differently sized NPs is an impressive approach for the formation of metasurface with multiple different resonators. These structural features directly hold the possibility for achieving ultra-broadband perfect anti-reflection. Our theoretical model opens a new way for broadband resonant absorption by the partially hollowed cavity structure. The black silicon metasurface paves the way for advanced optical devices on the base of all-dielectric semiconductor metamaterials and indicates potential applications in the related optoelectronic technologies for active photonics and nonlinear optics.
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