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Abstract
In this paper, a solar thermal absorber based on a dielectric filled two-dimensional nickel grating is designed and numerically investigated for wide-angle and polarization-independent broadband absorption. The absorption of the proposed two-dimensional meta-surface absorber reaches nearly 100% in the whole visible region (400-800 nm). The physical mechanisms responsible for the high absorption including the impedance matching with the free space, the cavity resonances and the surface plasmonic resonances have been elucidated in detail. The strong resonances effectively trap the incident light in the nano-cavities and then dissipate it by the ohmic losses of the metal, giving rise to the high absorption of the proposed absorber. The meta-surface absorber may find applications in solar cells, photovoltaics, thermos-photovoltaics, thermal emitters, plasmonic sensors, and solar-energy harvesting.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The increasing cost of fossil fuels and the suffering from global warming have increased the urgency of the finding of highly efficient renewable energy sources. As one of the renewable energy sources, solar energy has attracted much attention. Due to the abundance, sustainability, and cleanness of the solar energy, solar energy can then be utilized for heating, cooling, electricity generation and thermo-chemical processes. Solar thermos-photovoltaic systems (STPV) due to their efficient methods of converting solar energy into electricity have attracted much attention. The efficiency of a STPV system largely depends on the performance of its solar selective absorber. For STPV applications, it is essential that the materials can withstand high temperatures (at least up to 1500 K) [1,2]. Recently, metasurface-based solar absorbers have attracted increasing interest owing to their high optical performance, and the successful control of their absorptivity with geometrically controlled electromagnetic resonances. Most of the metasurface-based solar absorbers based on noble metals suffer from problems of scarcity, high price and low melting points (Au (∼1063°C) and Ag (∼961°C)), thus limiting their performances for specific applications (e.g., STPV). Thus, it is necessary to investigate the absorber based on non-noble metal with high melting points, such as Nickel (Ni) (∼1453°C), Tungsten (W) (∼3422°C), and Chromium (Cr) (∼1857°C).
Various nanostructures based on non-noble metals have been proposed for the absorption enhancement, such as metal-insulator-metal (MIM). For MIM structures, the incident light is mainly confined in the sandwiched thin insulating layer. Matsuno et al. have proposed a solar selective absorber with Tungsten–SiO2-based film-coupled fishnet gratings [3]. Wu et al. have proposed an UV-visible broadband wide-angle polarization independent absorber based on multiple metal grooves with different depths combining in one single period. The average absorption efficiency is over 80% within the incident angle of 40° [4]. Lee et al. has proposed a wavelength-selective solar thermal absorber composed of nickel substrate/ a thin SiO2 film /a two-dimensional (2D) nickel grating, the total absorptance are 0.86 and 0.82 at the TM- and TE-wave incidence, respectively [5]. Rana et al. have proposed a tungsten-based ultrathin absorber with square ring structure and cross-shape structure for visible regime, and the maximum average absorbance is 99.3% [6]. Luo et al. have presented a wide-angle broadband absorber composed of nickel substrate/1D nickel grating (where grooves are filled with dielectric material). However, the average absorption of the proposed absorber is only about 90% in the visible region for both the TM polarization and the TE polarization [7].
The present study here proposes a novel design of perfect absorbers, specifically aiming to the applications under circumstance of high temperature operations. In order to achieve a polarization-independent electromagnetic response, we design the structure to be symmetric in both x- and y-directions. The designed absorber is composed of the upmost dielectric film/2D metallic grating filled with dielectric material/metallic substrate. The absorption spectrum of the proposed 2D meta-surface absorber reaches absorptance values near 100% in the whole visible region (400-800 nm). The meta-surface absorber may find many applications in solar cells, photovoltaics, thermos-photovoltaics, thermal emitters, plasmonic sensors, and solar-energy harvesting.
2. Materials, structures and simulation methods
The three-dimensional (3D) configuration and the cross-sectional views of the proposed metasurface absorber are shown in Fig. 1 (a) and (b). The proposed meta-surface absorber is composed of three layers structures. The background substrate is a metallic film. The middle layer is a 2D subwavelength metallic grating, where the nano-cavities are filled with a dielectric material. The topmost is a dielectric film. In this study, the non-noble metal nickel (Ni) is chosen as the metallic material, due to its autologous nature, strongly attenuating high melting point (∼1453°C) and low emissivity at long wavelength [8,9]. The permittivity of nickel is taken from [10]. Additionally, silicon dioxide (SiO2) is employed as the dielectric material. And the refraction index of SiO2 is set to be 1.5. In the design process, structure parameters are optimized by the finite-difference time-domain (FDTD) method [11,12]. The structure is characterized by the thickness of the substrate ${t_{1}}$, the thickness of the 2D metallic grating ${t_2}$, the thickness of the upmost dielectric film ${t_{3}}$, the width of the ridge of the metal grating ${w_{1}}$, the width of the nano-cavities ${w_2}$ and the period $P$, respectively. The thickness of the bottom Ni substrate is set to be 200 nm (${t_1} = 200nm$), which is thick enough (much greater than the penetration depth of Ni ∼20 nm) to block light transmission. The absorption can be calculated by $A = 1 - R - T$, where R is the reflection and T is the transmission (here T = 0). The optimized parameters are: ${t_2} = 300nm$, ${t_3} = 60nm$, ${w_1} = 60nm$, ${w_2} = 200nm$, and $P = 260nm$ (The filling ratio of the Ni is 0.23). Since the 2D periodic structure exhibit a fourfold rotational symmetry about the propagation axis, the absorption spectra are the same for both the TM polarization (magnetic field in the y-direction) and TE polarization (electric filed in the y-direction) polarized light. Therefore, only TM polarization is taken as an example to investigate. The numerical simulated absorption spectrum of the proposed meta-surface absorber for normal incidence TM-polarized light is shown in Fig. 1 (c). Apparently, the absorption values stay nearly 100% in the whole visible regime (400 nm- 800 nm).
Fig. 1. (a) Schematic diagram and (b) 2D topography of two by two arrays of the proposed meta-surface perfect absorber. The corresponding cross-section configuration of the proposed meta-surface perfect absorber with the geometric dimension of ${t_1} = 200nm$, ${t_2} = 300nm$, ${t_3} = 60nm$, ${w_1} = 60nm$, ${w_2} = 200nm$, and $P = 260nm$, respectively. (c) The numerical simulated absorption spectrum of the proposed meta-surface absorber for normal incidence. The inset is the calculated real and image parts of the effective impedance for the proposed meta-surface perfect absorber.
To gain further insights into the physical origin of near perfect absorptivity, the magnitude of the simulated S parameters for TM wave and the corresponding real part and image part of the effective impedance through the inversion of the S parameters are calculated and displayed in the inset of Fig. 1(c). Here reflection $R = {|{{S_{11}}} |^2}$ and transmission $T = {|{{S_{21}}} |^2}$ (here $T = {|{{S_{21}}} |^2} = 0$). The effective impedance is calculated by using S-parameter retrieval algorithm as [13]:
As the solar radiation is randomly polarized and the sun illuminates at wide angle, independence on the polarization state and the incident angle of light are also crucial for a perfect absorber to maximize the absorption of solar energy. The effects of polarized states and the oblique incidence on the spectral are further analyzed. Figure 2(a) shows the calculated color map of absorption as a function of wavelength and the polarization of the incident light. It is evident from Fig. 2(a) that the proposed absorber is polarization independent. In Fig. 2(b), the incident radiation is TM polarization with incident angles from 0°to 80°with a 10°step. The incident angle is defined between the z-axis and the wave vector ($k$) of the incident wave, where k lies in the x-z plane. As is shown in Fig. 2(b), the meta-surface absorber performance degrades slightly with the increasing angle of incidence for oblique incident TM polarization, and still has an absorbance of 90% with the incident angle of 70°.
Fig. 2. (a) Absorption spectrum with different angle of polarization at normal incidence. (b) Absorption spectrum with different incident angle of the TM polarization.
The performance of the SiO2 in the structure is also investigated, as is shown in Fig. 3(a). For the absorber composed of 2D Ni grating /Ni film, the absorption is above 0.9 in the range of 400-500 nm and decreases dramatically with the increasing of wavelength for wavelength above 500 nm. By filling the nano-cavities formed by the 2D Ni grating with SiO2 (i.e. 2D Ni grating (filled with SiO2) /Ni film), the absorption is enhanced (above 0.8) in the whole visible range, and two absorption peaks are appeared at 510 nm and 700 nm. By employing the additional SiO2 film above the 2D Ni grating, i.e. SiO2 film/2D Ni grating (filled with SiO2) /Ni film, the spectral absorption increases obviously as is shown in Fig. 3(a)–(c), and shows nearly perfect absorption in the whole visible region with the optimized thickness, i.e. 60 nm. Figure 3(b) and (c) shows the absorption spectrum of the proposed absorber with the thickness of the topmost SiO2 film varies from 0 to 90 nm. As is showed in Fig. 3(b) and (c), the optimized thickness of the topmost SiO2 film is 60 nm. Owing to its fairly low relative permittivity at optical range, the topmost SiO2 film could be act as an anti-reflection layer, thus making the proposed structure a nearly perfect absorber in the whole visible regime. Besides, the SiO2 provide fairly high melting point which is a desired property for the topmost dielectric layer as well.
Fig. 3. (a) Comparison absorption spectra of the proposed absorber, the absorber composed of 2D Ni grating (filled with SiO2) /Ni film, and the absorber composed of 2D Ni grating /Ni film. The absorption spectra of the proposed absorber with the thickness of the topmost SiO2 film varies from (b) 0 to 50 nm, (c) 50 to 90 nm.
For comparison, we also study the performance of the absorber with one-dimensional (1D) grating. As is depicted in Fig. 4, only two broad absorption peaks could be obtained in the whole visible range for the absorbers with 1D grating in both the x-direction and y-direction. For the absorber with 1D grating in x-direction, the absorption peaks appear at about 450 nm and 760 nm. And the absorption peaks are about 400 nm and 560 nm for the absorber with 1D grating in y-direction. By employing two 2D grating, perfect broadband absorption could be achieved in the whole visible range.
Fig. 4. Comparison absorption spectra of the proposed absorber with 2D grating, the absorber with 1D grating in x-direction and the absorber with 1D grating in y-direction.
3. Theoretical analysis and discussions
In order to understand the physical mechanism that gives rise to the broadband absorption, the electric field (|E|2) distributions and the magnetic field (|H|2) distributions in the x-z plane and y-z plane with four different TM waves (450 nm, 550 nm, 650 nm and 750 nm) at normal incidence are calculated and depicted, as is shown in Fig. 5.
Fig. 5. Distributions of the electric field (|E|2) (color maps) and the energy flow (arrow maps) of the structure in (a) x-z plane and (b) y-z plane. Distributions of the magnetic field (|H|2) in (c) x-z plane and (d) y-z plane. The incident light is TM wave at 450 nm, 550 nm, 650 nm, and 750 nm, respectively.
In the x-z plane, the strong localized electric field intensity is concentrated at the sidewalls and the corners of the 2D nickel gating, as depicted in Fig. 5(a). This occurs at the interface between a negative and a positive real dielectric constant materials (Ni and SiO2), indicating the excitation of SPPs. The 2D grating structure provides the additional momentum, resulting in phase matching between the incident field and the SPPs, leading to the excitation of SPPs [16–20]. Moreover, the array periodicity of the 2D nickel gating provides addition dimension for improving the absorption performance through phase-coherent coupling of the localized electromagnetic fields between adjacent nano-cavities or through multiple scattering or grating-diffraction effects.
While in the y-z plane, the strong electric filed is concentrated in the nano-cavities formed by the sidewalls of the 2D metallic grating ridges, as depicted in Fig. 5(b). This suggests that the cavity modes are supported inside the SiO2-slots, resulting in the strongly broadband absorption in the visible regime [4,7,21,22]. For wavelength 450 nm, the localized electric field is confined at about z = 250 nm with a much weaker electric field center at about z = 60 nm. For wavelength increases to 650 nm, the electric fields is localized at z = 100∼300 nm, and become much stronger. For the wavelength 750 nm, the strong localized electric fields are beginning to leak out further into the topmost SiO2 film, leading to much stronger coupling effects. The coupling effects give rise to the omnidirectional absorption.
The geometry of the 2D metallic nano-array whose cavities are filled with dielectric creates cavity modes, thus concentrating the energy of the incident beam within the nano-cavities. For the incident light with free-space wavelength greater than the cut-off wavelength of the cavity, the incident light has no supported cavity mode to couple into, and the light is thus reflected. For incident light with free-space wavelength less than the cut-off wavelength of the cavity, the incident light couples into the cavity modes and thus absorption is enhanced due to the increased interaction time with the metallic walls. The 2D photonic crystals with cavities in the plane perpendicular to the vertical direction (z), as shown in Fig. 1 (a), can be analytically calculated by [21]
The plots of the Poynting vectors (S) (depicted in Figs. 5(a) and (b) by black arrows) show that, the incident flow bends when reaching the Ni nano-array surface, and propagates along the Ni/SiO2 interface toward the dielectric nano-cavity, which can be attributed to the magnetoelectic interference of the incident wave with the scattered evanescent field [23]. Most of the incident energy is trapped in the nano-cavities and finally absorbed by the Ni sidewalls, due to the funneling effect of the cavity modes, which plays a key role in the absorptive behavior in the structure [23,24]. The strong resonances effectively trap the incident light in the nano-cavities and provide sufficient time to dissipate it by the ohmic losses of the metal.
We can also see that the strong magnetic field (|H|2) is concentrated within the nano-cavities and along the top of the Ni grating, as is shown in Fig. 5(c) and (d). For wavelength 450 nm, and 550 nm, the magnetic field distributions show strong confinement within the nano-cavities and along the top of the Ni grating. These field patterns characterize the existence of the cavity resonance. While the magnetic field is concentrate at the corners of the sidewalls of the Ni grating and along the top of the Ni grating for wavelengths 650 nm and 750 nm. This allows the slot modes that exist within the nano-cavities to couple to one another, leading to a supported resonant surface wave along the top of the Ni grating [16].
The energy loss due to the absorption can be calculated by using [25]
where $\omega$ is the frequency of the input wave, $\varepsilon ^{\prime\prime}$ is the imagery part of the permittivity, and ${|E |^2}$ is the magnitude of electric field. The calculated absorption contributions of the proposed absorber shown in Fig. 6(a) verify that above 90% of the incident energy is absorbed by the middle 2D Ni grating. As is known, the imaginary part of nickel permittivity is larger than its real part, which is opposite to the noble metals, such as gold and silver. The large imaginary part of the nickel permittivity leads to the significantly high absorption losses in nickel. The calculated absorption contributions of the three different parts ($z = 0 \sim 100nm$, $z = 100 \sim 200nm$, and $z = 200 \sim 300nm$) of the 2D Ni grating is depicted in Fig. 6(b). As is shown in Fig. 6(b), ${50\%\sim 75\%}$ of the incident energy is absorbed in the top part of the Ni grating; ${20\%\sim 30\%}$ of the incident energy is absorbed in the middle part of the Ni grating, and less than 20% of the incident energy is absorbed in the bottom part of the Ni grating. And as the incident wavelength increases from 400 nm to 650 nm, the contributions of the top part of the Ni grating to the absorbance decreases from 60% to 50%, and then increases from 50% to 75% when the incident wavelength increases from 650 nm to 800 nm.As a high-performance solar absorber, it should have unity absorptance in the ultraviolet-visible and near infrared region (0.3∼4 $\mu$m), where almost 99% of the solar irradiation is distributed. On the other hand, low thermal emission in the mid-infrared range (0.3∼20 $\mu$m) is highly desirable to minimize the heat loss from self-emission. The absorption spectrum (${A_\lambda }$) of the proposed meta-surface absorber in the ultraviolet-visible and near infrared region (0.3–4 $\mu$m) is shown in Fig. 7(a). The total solar absorptance (${{\rm A}_{total}}$) of the proposal absorber at the normal incidence is calculated by [5]
Fig. 6. (a) The contributions of the top SiO2 film, the 2D Ni grating (filled with SiO2) and bottom Ni film to the absorbance. (b) The contribution of three different parts of the 2D Ni grating (filled with SiO2) to the absorbance.
Fig. 7. (a) AM1.5 reference solar spectrum, and the absorption spectrum of the proposed meta-surface absorber in the ultra-visible and near infrared region (0.3–4 $\mu$m). (b)The normalized emission at 100°C for the blackbody and the proposed structure in the longer wavelength region from 0.3 $\mu$m to 20 $\mu$m.
The emission spectrum can be related to the absorption via Kirchoff’s law, where an object in thermal equilibrium emits as much radiation as is absorbed. The thermal emission spectrum of the proposed structure can be obtained by multiplying the absorptivity by the black body emission spectrum. The Planck’s law distribution of blackbody radiation (${I_B}(\lambda ,T)$) can be computed by [27,28]
Actually, the proposed solar thermal absorber could be fabricated by combining the electron beam lithography with vacuum coating equipment [29,30]. Firstly, the bottom metal film could be deposited on the silica substrate by the electron-beam (E-beam) evaporation. Secondly, an inorganic negative-tone hydrogen silsesquioxane (HSQ) electron-beam resist will be spin-coated on the bottom metal film. Thirdly, HSQ will transform into the transparent dielectric SiOx after electron-beam exposure. After the development, the 2D array of SiOx could be obtained. Fourthly, after depositing the Ni film, the surface of the 2D SiOx array and the surface without SiOx patterns are then covered by the Ni film. The 2D Ni grating (which is filled with SiOx) could be obtained after polishing the Ni above the SiOx patterns by ion beam polishing. Finally, the SiO2 dielectric overlayer is deposited to the 2D Ni grating (which is filled with SiOx) to finally obtain the designed structure (the upmost SiO2 film /2D Ni grating (filled with SiOx) /Ni film). Note that, the optical constants of SiOx and SiO2 are almost the same. Therefore, SiO2 is used instead of SiOx in the simulation. The cross-sectional morphology of the fabricated absorber could be observed by a field-emission scanning electron microscope. The reflection spectra of the fabricated structures at the normal incidence can be measured by a reflection spectrometer.
4. Conclusion
In conclusion, we have numerically designed a perfect absorber consisting of the upmost SiO2 film/ 2D Ni grating (filled with SiO2) /Ni film. Nearly unity absorption in the whole visible range with 87.3% of the total solar absorptance and less than 5% of the total normal emittance at the temperature of ${100^o}C$ can be achieved. In addition, the proposed absorber shows polarization-independent and exhibits broadband absorption performance even at large incidence (70°). The calculated effective impedance of the structure indicates that the proposed absorber is impedance matched with the free space, leading to high absorption over the whole visible region. The strong electric filed concentrated at the sidewalls of the 2D nickel gating and in the nano-cavities formed by the metallic walls indicates excitation of the surface plasmons resonance and the cavity resonance. The strong resonances effectively trap the incident light in the nano-cavities and provide sufficient time to dissipate it by the ohmic losses of the metal, giving rise to the efficient absorption of the proposed absorber. The proposed structure could be fabricated by combining the electron beam lithography with vacuum coating equipment. The design of the solar thermal absorber here would provide a theoretical support for the ongoing experiment and also will be beneficial to enhance the performance of solar energy harvesting and conversion system.
Funding
Natural Science Foundation of Guangxi Province (2016GXNSFBA380204, 2016GXNSFAA380071, 2018GXNSFAA138180).
Acknowledgements
The authors acknowledge the support from the Natural Science Foundation of Guangxi Province (2016GXNSFBA380204, 2016GXNSFAA380071, 2018GXNSFAA138180), and Ph.D. research startup foundation of GuiLin University of Technology (Grant No. 002401003523).
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