Spectrally Selective Ultra‐Broadband Solar Absorber Based on Pyramidal Structure

Here, a spectrally selective solar absorber is explored and an ultra‐broadband solar absorber is proposed based on pyramidal structure. The finite‐difference in time domain (FDTD) software is used to model the spectral characteristics and magnetic absorption patterns of this absorber. The emissivity of the absorber is less than 20% in the far‐infrared band over 6000 nm, showing good selectivity, and the total solar thermal conversion efficiency is very close to that of an ideal truncated selective solar absorber by analyzing the performance of our proposed absorber‐related indexes. By studying the high absorption band of the absorber, the selectivity can be better investigated in depth. Here, 200–4000 nm is chosed as the depth study band. The absorber possesses an ultra‐wide bandwidth of 3554 nm and an average absorption of over 97.4%, and in the 200–3754 nm band, the absorber has an ultra‐high absorption rate of more than 98.3%, and its thermal emitter has a high emission efficiency of 94% at a temperature of 1000 K. Notably, the weighted average absorption in the 280–4000 nm band at AM1.5 is as high as 98.86%, with a loss of only 1.14%. The ultra‐broadband absorption property of this solar absorber is mainly a joint effect of surface plasmon resonance coupling.


Introduction
[3][4] In response to the global energy crisis and environmental deterioration, governments throughout the world are focusing on the continual development and effective use of renewable energy sources such as solar energy.7] High-temperature solar thermal system performance is often described using the solar thermal energy conversion rate.Collect as much solar energy as is practical while reducing energy loss from thermal radiation to increase the efficiency of solar thermal conversion.As a result, developing an ideal selective absorber is the key to increasing the thermal convert effectiveness of high-temperature solar systems, and spectrally selective solar absorbers may maintain an excellent absorption rate in the wavelength region where solar radiation is more competent, while maintaining a low emission rate for electromagnetic waves Here, a spectrally selective solar absorber is explored and an ultra-broadband solar absorber is proposed based on pyramidal structure.The finite-difference in time domain (FDTD) software is used to model the spectral characteristics and magnetic absorption patterns of this absorber.The emissivity of the absorber is less than 20% in the far-infrared band over 6000 nm, showing good selectivity, and the total solar thermal conversion efficiency is very close to that of an ideal truncated selective solar absorber by analyzing the performance of our proposed absorber-related indexes.By studying the high absorption band of the absorber, the selectivity can be better investigated in depth.Here, 200-4000 nm is chosed as the depth study band.The absorber possesses an ultra-wide bandwidth of 3554 nm and an average absorption of over 97.4%, and in the 200-3754 nm band, the absorber has an ultra-high absorption rate of more than 98.3%, and its thermal emitter has a high emission efficiency of 94% at a temperature of 1000 K. Notably, the weighted average absorption in the 280-4000 nm band at AM1.5 is as high as 98.86%, with a loss of only 1.14%.The ultra-broadband absorption property of this solar absorber is mainly a joint effect of surface plasmon resonance coupling.
above the cut-off wavelength, which is highly advantageous in both photothermal conversion systems and thermophotovoltaic systems. [8,9]Such selective absorbers are particularly important for solar thermal conversion systems operating at ultra-high temperature conditions (T > 1000 K).The perfect solar absorber should not only be spectrally selective but also have great thermal stability, a low thermal mass, and the ability to be mass produced. [10]22] The ideal optical blackbody possesses high optical absorbance, wide absorption band, angle of incidence, and polarization angle insensitivity properties.It has been much easier to construct perfect absorbers with exceptional absorption qualities in recent years thanks to the advancement of iso-excited element optics.The perfect absorber realizes the broad spectrum absorption of solar energy, and the absorption in the spectrum from 0.5 to 1.8 μm reaches 90% or more, which greatly improves the utilization of solar energy.The addition of composite films made of metal dielectric layers is one way to realize broadband absorption, starting with simple momentum iterative method (MIM) three-layer absorber structures, and Landy first proposed a metal-insulator-metal framework for narrowband ideal absorbers in 2008. [23]The first was a simple MIM three-layer absorber structure, but due to the lack of capture and confinement mechanism, the absorption performance was not very satisfactory, and then slowly expanded to a variety of forms of structures, which is an effective way to increase the absorption bandwidth.This is an effective way to increase the absorption bandwidth.Researchers have used different materials to design shapes to achieve perfect absorption in specific wavelength bands.Cong et al. [24] and Hoa et al. [25] developed a metal/dielectric multiple-layer piled receiver using Au/Si material, with wavelengths of absorption of 400-750 nm for the 5-layer stacking structure and 480-1480 nm for the 10-layer stacking structure.However, the number of layers of these structures is high and the absorption band is still not wide.Even though precious metals can be used to achieve broadband absorption, the cost is significant.Wu et al. [26] designed a hyperbolic metamaterial absorber using W/SiO 2 , consisting of a nanoporous hyperbolic metamaterial (HMM) structure stacked with W and SiO 2 multilayers.It has a more complicated structure but can widen the range of absorption to 260-1580 nm with an average absorption rate of 98.9%.The 420-1950 nm wavelength band was completely absorbed.Yi et al.'s [27] four-layer W/SiO 2 disk arrangement achieves complete absorption with a rate of over 90 % in the 420-1950 nm region.The Ti/W/SiO 2 -based three-layer supermaterial absorber achieved a high absorption of 90% in the 166-1936 nm range.The above absorbers can expand the absorption bandwidth to a certain extent, but the absorption bandwidth is still not wide enough and the structure is complicated, which is not favorable for absorbing and using solar energy.Therefore, it is important to study a high absorption, [28][29][30] ultra-broadband [31][32][33] absorber with simple structure.
Pyramidal microstructures are stacked on a MIM multilayer film structure to create the absorber that is proposed in this article.The absorber exhibits more than 90% absorption in the 200-3754 nm wavelength range, 94% thermal radiation efficiency (T = 1000 K), and as high as 98.86% weighted average absorption in the 280-4000 nm band under AM1.5.And this special novel combined structure makes the ultra-wide bandwidth, ultra-high absorbance absorber possible.Using the finite-difference in time domain (FDTD) approach, we investigated the absorption characteristics of the ultra-wideband solar absorber.The physical process of ultra-wideband excessive absorption is investigated by examining the electromagnetic field distribution as well as various geometrical characteristics.Compared to other absorbers, our proposed absorber has an excellent potential for use in applications such as heat harvesting, thermoelectronic components, and so on.

Design and Method of Absorber Structure
Our designed spectrally selective solar absorber is composed of a pyramidal microstructure stacked on a MIM multilayer membrane structure, as shown in Figure 1a,b depicts a twodimensional unit cycle of the structure.A unit cycle is formed by the combination of a W pyramidal structure stacked on a Ti-Al 2 O 3 -Ti (MIM) multilayer membrane structure.The bottom layer of this absorber is a metal substrate Ti, and when the metal substrate is thick enough, the structure can be considered opaque, that is, when the electromagnetic wave is incident, the transmission T can take the value of 0. Between the metal Ti at the bottom of the pyramid and the substrate Ti, there is a layer of Al 2 O 3 dielectric layer, thus forming the MIM structure.This not only lays the foundation for the broadband absorption of the absorber, but also improves the absorption efficiency to a certain extent in terms of absorption performance.Furthermore, because of the high degree of symmetry in our proposed absorber, the structure's polarization reliance on incident light is reduced.This solar absorber's structure can be summed up as follows: a substrate that is thick enough to prevent transmission, MIM structure provides the possibility of multiple resonance generation, broadens the bandwidth of the main band of multiple resonances, and enhances the reliability of the structure. [34]Different structural dimensions also form resonant bands in different frequency bands, which are coupled to each other to form broadband near-perfect absorption.The shape of the top pyramid, which gradually increases from top to bottom, facilitates the entry of electromagnetic waves, and this structure makes a great contribution to the improvement of absorption performance.
[37] After that, it is investigated how physical parameters affect absorption performance, and the physical process of ultra-wideband absorption is investigated in combination with electromagnetic theory in order to give theoretical direction for the design of ultra-wideband absorbers.The conditions for the boundary in the x, y, and z directions are periodic boundaries, and the boundary condition in the z direction is the ideal matching layer (PML). [38,39]The dielectric constants of Ti, W, and Al 2 O 3 are referred to the experimental values of Palik. [40]For the simulation verification, the period P = 200 nm in the transverse direction, the base metal material is titanium (Ti) with thickness H1 = 270 nm, the dielectric material is Al 2 O 3, with thickness H2 = 30 nm, the metal thin film layer H3 = 120 nm, the period P1 = 140 nm, and the top pyramidal (W) layer with the same period with height H4 = 650 nm. because the structured thick base layer ensures that the light transmittance is zero, i.e., T = 0, so the absorbance A = 1-R, [41] and R represents the reflectance of the structure. [42,43]The formula for calculating the absorber's average absorbance is [44] A aver ¼ Where A aver denotes the average absorbance, and the incident wavelength's maximum and minimum values are designated as max and min, respectively.Based on the equation beforehand, the average absorbance of the metamaterial solar absorber in the 200-4000 nm region is 97.4%, and in the 200-3775 nm band, where the absorbance exceeds 90%, is 98.3%.

Simulation Result Analysis of Absorber
In the wavelength region where the sun radiative radiation is greatest, the ideal selective absorber spectrally selective solar absorber maintains a high absorption rate, while maintaining low emission for electromagnetic waves above the cut-off wavelength.As displayed in Figure 2a, for this absorber in the 200-3774 nm wavelength region, our suggested absorber has greater than 90% absorption.When the wavelength exceeds 3774 nm, the absorbance of this absorber decreases rapidly, and there is a brief rebound of absorbance at 4380 nm, but then it shows a steep decrease in the absorption curve.The illustration in the upper right corner of Figure 2a shows that the solar absorber's absorption rate is already below 20% in the wavelength range beyond 5836 nm and subsequently decreases to roughly 7%, so we set the cut-off wavelength of the selective absorber to 5836 nm.The proposed solar absorber's absorption curve closely resembles that of the ideal selective solar absorber.The recommended solar absorber has good selectivity, as can be seen.The proposed solar absorber's absorption spectrogram in the 200-4000 nm range is shown in Figure 2b in a local zoom, and it can be seen that the absorption bandwidth-the band range where the absorption is larger than 90%-reaches 3554 nm throughout the entire band range.It is worth noting that our suggested absorber achieves greater than 99% absorption in three separate wavelength bands.The first range is from 200-580 nm, which is the long 380 nm range, the second range is from 1191-1591 nm, which is about 400 nm range, and the third range is 2732-3432 nm, which is about 700 nm long range absorption.Since the absorption performance in the high absorption band (200-4000 nm), it directly affects the change in cut-off wavelength, it is necessary to explore the physical mechanisms and parameter effects in this range, which directly affect the overall absorption efficiency and spectral selectivity of this solar absorber.In the next section, we shall talk about the underlying physical principles of the high absorption ultrabroadband in this band.

Electric Magnetic Field Strength Distribution
We examined the electromagnetic field distribution thoroughly for the purpose to further comprehend the precise physical mechanical action phenomena behind the ultra-broadband, high absorption.The results are displayed in First, let's analyze the electric field distribution at each high absorption wave, as shown in Figure 3a-f.The electric field is largely centered on the two sides of the pyramidal structure's surface when the incoming light wavelength is 200 nm, as illustrated in Figure 3a.When the incoming wavelength rises to 580 nm, Figure 3b demonstrates that the electric field is largely concentrated over the surface of the upper pyramidal structure W and the first Ti film, and it can be seen from the intensity that the intensity is also increasing, which is due to the surface of the pyramidal structure W producing propagating surface polarization excitations. [45]When the wavelength increases again, to 1191 nm, as in Figure 3c we find that the electric field around the pyramid is greatly weakened, and at this time the electric field distribution starts to move downward, and the Al 2 O 3 and the first Ti film have steadily decreasing electric field intensities, and the surface equipolar excitations have acted between the first Ti film structure and Al 2 O 3 film. [46]As the wavelength increases, as in Figure 3d-f shows, the electric field is now primarily concentrated between the first Ti film and Al 2 O 3 film, and it is almost entirely produced by the action of the surface equipolar excitations between the first Ti film and the Al 2 O 3 film.The pyramid's top and sides' electric field nearly totally vanishes.Generally speaking, the transmission of the electric field is mostly transferred gradually from the top of the pyramid to the bottom, i.e., the pyramid structure W is primarily affected by short wavelength absorption, and the first Ti metal film and Al 2 O 3 dielectric film are primarily affected by long wavelength absorption.
To learn more about how this solar absorber absorbs energy, as shown in Figure 3, we individually estimated its magnetic field distribution in each band (g)-(l).We estimated the magnetic field distribution in the XOZ plane for the analysis to reach the broadband mechanism in the same manner as the electric field distribution was analyzed.The electric field created by external equipolar excitations is largely distributed on the top and both surfaces of the pyramidal construction at 200 nm.When in the visible-light band 580 nm, the external interaction of the pyramidal W with Ti is strengthened.When the wavelength is 1191 nm, the magnetic field is generated by the joint interaction of the surface of W with Ti and Al 2 O 3 film; by 1591 nm, the near 1591 nm, the electric field near the apex of the pyramid begins to diminish.And at this time, the role of Ti film and Al 2 O 3 film is the main role of magnetic field, and the distribution of magnetic field intensity can also be seen from the mark.With the increase of wavelength, at 2732 nm, the magnetic field intensity between the metal film Ti and the dielectric film Al 2 O 3 is greatly enhanced; at 3432 nm, the magnetic field around the pyramid structure has almost disappeared, and at this time, the magnetic field is concentrated in Ti metal film and the dielectric film Al 2 O 3 .According to Figure 3g-l, It is evident that the magnetic field changes depending on the waveband, from UV to midinfrared, ranging from the distinct effect of the pyramidal structure alone to the magnetic field created by the connection between the pyramidal framework and the film, with the film serving as the primary action.In general, when the incident wavelength increases, the magnetic field is excited very strongly from top to bottom in a specific part of the pyramid structure, and the magnetic field distribution moves from top to bottom. [47,48]In other words, the influence of the pyramid and the metal film dominates the absorption in the short wavelength band, whereas the effect of the metal film and the dielectric film dominates the absorption in the long wavelength band.These phenomena are mainly related to the excitation of surface equipolar excitations and magnetic polaritons in the metal and dielectric layers, which are coupled to each other in different dielectrics and contribute to the resonant absorption in the structure, which explains our suggested solar absorber's excellent absorption and broadband features. [49]gure 3. a-f ) Electric field distribution maps for each wavelength (XOZ plane).g-l) Magnetic field distribution maps for each wavelength (XOZ plane).

Analysis of the Impact of Several Factors on Absorption Effectiveness
Other things being equal, in response to the absorbance, the thickness of the dielectric layer (Al 2 O 3 ) varies from 20 to 40 nm in Figure 4a.Accordingly, the absorbance is identical in the UV and visible regions, and it decreases with increasing thickness in the wavelength range from 1400 to 2300 nm.After 2300 nm, the absorbance rises with increasing thickness until 3000 nm, at which time the dielectric layer with a thickness of 30 nm exhibits a higher near-infrared band absorbance than 40 nm.As a result, 30 nm is the dielectric layer H2's optimal thickness.The base metal (Ti) (H3) of the pyramid varies in thickness from the same 100 to 140 nm, rising by 20 nm each time.It is seen in Figure 4b as the metal's thickness grows, the rate of absorption rises.However, in the 1600-2800 nm band, the increase in thickness causes the absorption to become worse, at which point the absorber with a thickness of 120 nm performs best.Although the bandwidth is widest when the metal Ti thickness H3 = 140 nm, the mid-infrared band's poor absorption makes up for its good effect.
Therefore, to balance the bandwidth and absorption, on balance, we take the optimal parameter H3 = 120 nm.For the pyramid height (H4), we investigate the absorption at different heights (500-700 nm), increasing by 50 nm each time.As shown in Figure 4c, the other parameters are kept constant, the absorption at short wavelengths does not change much, while the absorption peak at long wavelengths shows a redshift of the pyramid height in the middle wavelength band is proportional to the absorption.When implementing process manufacturing, the absorption bandwidth and average absorption must be balanced, and the absorber's overall height must be taken into account, we determined H4 = 650 nm.In short, the increase of H2 contributes to both the absorption at short and long wavelengths and has an impact on the improvement of the bandwidth.H4 primarily controls the absorption in the medium wavelength region, while H3 is more closely associated to the absorption at long wavelengths.This agrees with the outcomes in Figure 4.The thickness parameters of this structure are H2 = 30 nm, H3 = 120 nm, and H4 = 650 nm in order to equalize the absorption band and average absorption.
To investigate the effect of geometry, we also discuss the variation of the results produced by different periods.As seen in Figure 5a, while the absorption after the middle band from 1300 nm dramatically declines and the absorption peak in the broad wavelength band exhibits a red shift, the absorption in the short wavelength band steadily increases when the period P1 of the pyramid base metal Ti increases from 120 to 160 nm.It is worth noting, however, that ultraviolet radiation accounts for only about 8 % of solar radiation, while the visible and infrared regions account for 42% and 49%, respectively.Therefore, a slight increase in absorption in the short wavelength range may not have a significant impact on the overall absorption, whereas a decrease in absorption above 1500 nm is more critical.Figure 5b shows the absorber's absorption spectra as its overall period P shifts from 180 nm to 220 nm.In the band before 1500 nm, the absorption is inversely proportional to the period size, but between 1500 and 2700 nm mid-wave band, the absorption gets better as P increases and the absorption peak in the long band appears blue-shifted.In conclusion, the parameter changes of both periods are highly related to the medium and long wavelength bands. [50]In order to equalize the absorption bandwidth and average absorption, P1 = 140 nm and P2 = 200 nm, respectively, were chosen as the structure's periods.
Next we use different materials to further analyze the absorption effects.The absorption spectra obtained by substituting a number of different standard metallic absorber material for the pyramidal material W are shown in Figure 6a.The lossless metals Ti, Cr, and W combined with Al 2 O 3 material, when available in the right thickness, satisfy the impedance matching conditions that are available to achieve strong absorption. [51]The broadband absorption properties can be almost completely maintained when we utilize Ti or Cr, but the bandwidth is slightly less than that of the material W. When the W pyramid is replaced by Al, although resonance bands occur in the short, medium, and long wavelength bands, the absorption impact is considerably decreased.Figure 6b shows the absorption spectra when different dielectric layer materials are used for the structure, and we compare them with the common dielectric layer materials SiO 2 and Al 2 O 3 .It has been discovered that the layer of dielectric is a material with a greater dielectric constant, the absorber has a better absorption performance, considering both the absorption rate and bandwidth, and the higher dielectric constant has a better absorption effect. [52]As the dielectric constant decreases, the absorption and bandwidth will be significantly reduced because the excitation and limitation of the field is greatly reduced.So finally we determine the absorber dielectric layer material as Al 2 O 3 , and the top pyramidal material as W.
We modelled the absorption efficiency under various topologies to better understand the physical workings of the ultrabroadband ideal solar absorber.Figure 7a demonstrates that the absorption rate and bandwidth of case 1 are greatly reduced after removing the top pyramidal structure, which indicates that the pyramidal structure contributes greatly to not only the absorption rate but also the realization of ultra-broadband.Case 2 is the structure with the bottom Ti metal film removed from the pyramid, and from the figure, we can see that this structure absorbs well in the visible and mid-infrared band around 2500 nm, but is slightly inferior in the near-infrared band around 900-2200 nm, and the bandwidth is also greatly shortened.Case 3 is the top microstructure replaced by a rectangular structure, and it can be seen from figure (a) that there is almost no band with absorption rate greater than 90%, and the overall effect is very poor.Case 4 also has only a small number of bands in the absorption > 90% or more, and the mid-infrared band is even lower.So we chose case 5, which is our proposed perfect absorber.Taken together, the top pyramid microstructure contributes to the overall absorption rate, especially in the mid-infrared band.The Ti metal film at the bottom of the pyramid mainly contributes to the improvement of absorption rate and expansion of bandwidth in the near-infrared band. [53,54]

Examining Radiated and Absorbed Energy
We obtained the solar radiation spectrum by placing the absorber at the optimal parameters in the AM1.5 spectrum.The solar absorber has a virtually perfect absorption characteristic across the whole range of solar radiation distribution of energy in the visible, near-infrared, and mid-infrared spectrums, as shown in Figure 8a,b.In Figure 8a, the red line reflects the suggested absorber's absorbance at AM1.5, while the black line depicts the solar spectrum at this wavelength.It is evident that the black line, red and black lines, and the black line nearly coincide roughly throughout the nanowavelength band, shows that the absorber has an optimum absorption and a very high efficiency, with the average of the weighted absorption efficiency reaching 98.86% in the range from 280 to 4000 nm.In terms of how much energy is absorbed and dissipated, Figure 8b provides a more visual illustration of the prior result.In this diagram, the red portion denotes energy that is absorbed, while the green portion denotes energy that is lost.Because several resonance states occur simultaneously, we can observe that only a little amount of energy is lost and that the majority of the energy that is emitted is collected, particularly beyond 2000 nm with nearly no loss, which is negligible in comparison to the area of the absorbed component.It is clear from these two factors that the absorber we suggest works well in practice: we designed a structure with a high absorption effect and achieved a deep optimization.In addition, when the temperature rises within a specific range, the refractory material employed in the absorber aids in maintaining the thermal resistance of the structure. [55,56]As shown in Figure 8c, our proposed solar absorber possesses a very excellent thermal radiation efficiency at 1000 K.In this solar absorber system, when contrasted with the ideal blackbody method, the thermal emitter exhibits nearly perfect intensity of emission in the wavelength region up to 4000 nm at 1000 K. Our suggested absorber may be used in photovoltaic systems in addition to thermal emission from light sources because the thermal emitter's 94% emission efficiency in the 200-4000 nm wavelength range.

Analysis of Angle Sensitivity
[59][60] Therefore, additional simulations were performed to investigate the effect of angle and polarization on the solar absorber.Due to the structure's complete symmetry in planar space, it can be seen in Figure 9a that the absorption efficiencies in the Transverse Magnetic (TM) and Transverse Electric (TE) wave modes are very similar.The changes in absorption efficiency are small within 30°of the incident angle based on color distribution, however, altering the angle from 30°to 60°g reatly increases the absorption efficiency from visible wavelengths to near-infrared light, which is about 2500 nm, and the absorption spectrum appears.In addition, in TM and TE modes, we also compute the variations in absorption efficiency caused by changes in incidence angle.The spectrum showed a little widening.3][64]

Conclusion
In this study, we develop a pyramidal-based design for an ultra-wideband selective solar absorber.The materials used in the article all have high melting points, enabling the structure to function unaffectedly at excessive temperatures.The FDTD method is used to analyze the absorption characteristics of the solar absorber by changing the geometrical parameters, and the absorption bandwidth (A > 90%) over the full wavelength range is increased to 3554 nm.In three distinct wavelength bands, our recommended absorber achieves an ultra-high absorption of exceeding 99%, which is significant, greatly extending the original simple structure's absorption bandwidth and making it superior to the majority of previously proposed most absorbers.In addition, the absorber has a good selectivity as the emissivity is below 20% in all mid-infrared bands beyond 5836 nm.And it is worth mentioning that its weighted average rate is as high as 98.86% at AM1.5 conditions, and the solar energy loss is only 1.14%.And it possesses good thermal emission efficiency (94%) at 1000 K temperature condition, and the outcomes demonstrate that the structure has good heat radiation and absorption capabilities.The electrical field distributions of the selected structure further demonstrates that the broad band high absorption is mostly caused by surface plasma resonance at the metal-dielectric interface, which is also a significant contributor to the wide band high absorption.This vertical construction has a high degree of symmetry and the absorber possesses polarization insensitivity and oblique incidence broad angle absorption properties, which considerably enhances the solar absorber's performance while simplifying the structure.The perfect absorption in the visible and infrared spectrum offers fresh perspectives on the effective creation and application of renewable energy.For the construction of electromagnetic metamaterial devices such ideal absorbers, thermal emitters, and electromagnetic shielding, this work has a definite reference value.

Figure 3 .
The varying electric and magnetic field distributions at 200, 580, 1191, 1591, 2732, and 3432 nm, which are the six wavelengths with absorption rates greater than 99%.The three high absorption bands' upper and lower boundaries when analyzed can better derive the mechanism behind the high absorption.The selected plane is the XOZ plane, the black dashed lines indicate the top pyramidal W and the first Ti film layer, and the white dashed lines indicate the dielectric layer Al 2 O 3 .

Figure 5 .
Figure 5. a) Absorption spectra of the pyramidal period P1 when taking different values.b) Absorption spectra of the absorber for different values of the overall P.

Figure 6 .
Figure 6.a) The pyramid structure's absorption spectra after being filled with various materials.b) Absorption spectra when different materials are taken for the dielectric layer.

Figure 7 .
Figure 7. a) Various structures' absorption spectra.b) Three-dimensional schematic diagrams of different structures.

Figure 8 .
Figure 8. a) Solar absorption spectrum.b) Analysis of energy loss and absorption.c) Solar absorber energy emission diagram at 1000 K temperature.

Figure 9 .
Figure 9. a,b) Shows the electric field plots of our designed structure from 0°to 60°under the action of TE and TM waves.