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Abstract
An ultra-broadband near-perfect absorber based on one-dimensional meta-surface utilizing refractory materials is proposed and demonstrated numerically. High absorptivity from UV to the near-infrared region (300-1200 nm) is attained for both transverse electric (TE) and transverse magnetic (TM) polarizations. For TE polarization, an average absorption of 96.0% with peak absorption up to 99.4% is attained. Simultaneously, an average absorption of 91.0% with peak absorption about 99.8% is achieved for TM polarization. Moreover, the high absorptivity can be maintained with incident angles up to 45°. The excellent performances are attributed to the trapping effect of the multiple resonance modes supported by the multi-layered structure. The ultra-broadband near-perfect absorber presented in this paper will provide a new method for realizing ultra-broadband polarization-independent absorption with a one-dimensional meta-surface, and has potential application prospects in color-printing, solar-energy harvesting, and other fields.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Meta-surface absorbers, which are made of artificial periodic sub-wavelength metal-dielectric structures with wide profile diversity and dimensionality, have attracted considerable interest because of their extensive applications, such as solar cells, plasmonic sensors, photodetectors, and thermal emitters [1–31]. In previous studies, most of the polarization-independent broadband absorbers are implemented by relatively complicated two-dimensional (2D) periodic structures [1–5,7–11]. For example, Yan Peng et al. designed a 2D double-layered doped-silicon grating structure to realize polarization-independent and ultra-broad absorption over the Terahertz (THz) range [7]. Sunwo Han et al. numerically investigated and analyzed the electromagnetic resonances on a 2D tandem grating and its application for broadband absorption in the visible spectrum [11]. These absorbers can be fabricated using ion or electrochemical etching of metal and the complex nanofabrication requirements limit practical device applications. In recent years, polarization-independent broadband absorber incorporating 1D metal-dielectric grating has been demonstrated [12–15]. Chia-Hung Lin investigated an absorber consisting of a lossy dielectric grating on top of a low-loss dielectric layer and a substrate of the same lossy dielectric placed at the bottom. Enhanced absorbance (>80.0%) occurs over a wide range of wavelength (500-700 nm) for TE and TM polarizations [13]. Jeremy N. Munday et al. described an ultrathin solar cell architecture formed from 1D metal-dielectric grating and antireflection coatings. Highly optical absorption can be obtained in the wavelength range of 400-1100 nm for two polarizations [15]. Jun Wu presented a polarization-insensitive broadband infrared absorber with a tapered metal-dielectric multi-layered grating structure [12]. Optical absorption of above 90.0% is observed over a wide wavelength range of 2500-5500 nm. Minghui Luo et al. proposed a broadband absorber based on a metallic substrate and a 1D subwavelength metallic grating with the grooves filled with dielectric material in the visible regime [22]. Yuyin Li proposed an ultra-broadband perfect absorber composed of the insulator-metal-insulator grating on the metal-insulator-metal (MIM) film stacks utilizing refractory materials, achieving over 90% absorption in the wavelength range from 570 nm to 3539 nm with an average absorption of 97% [28]. Although some progress has been made, further research on broadband absorbers using refractory materials is needed.
In this paper, an ultra-broadband near-perfect absorber incorporating 1D meta-surface with refractory materials is presented. Strong absorption over an ultra-broad range of wavelength (300-1200 nm) for both TE and TM polarizations is attained. Particularly, near-perfect absorbances (over 99.0%) are achieved at λ=385 nm, 980 nm for TE polarization, and at λ=430 nm for TM polarization. The high absorptions are attributed to the trapping effect of the multiple resonance modes supported by the multi-layered structure. Compared with the previously reported ultra-broadband metamaterial absorbers, the structure presented in this work can be fabricated more easily without the need of ion or electrochemical etching of metal, which can be potential candidate for a number of applications, including solar-energy harvesting, optical data storage and thermally controlled photonic devices.
2. Simulation and discussion
Figure 1 shows the schematic of the proposed absorber with refractory materials, consisting of the bottom metal layer of tungsten (W), the dielectric layer (SiO2), the dielectric grating (SiO2), the metal cover layer (W), and dielectric cover layer (SiO2). The heights of the dielectric layer, the dielectric grating, the W cover layer and the SiO2 cover layer are h1, h2, h3 and h4, respectively. The dielectric constants of W are fitted by the Drude-Lorentz model [32]. The refractive index of the dielectric layer is the same as that of the dielectric grating (n = 1.5). The bottom W layer with the thickness larger than the skin depth is employed to hinder the transmission. The TE- and TM-polarized lights are incident from the top air side at an angle of θ. The simulated absorption (A) is performed using the rigorous coupled analysis (RCWA) method [33–34].
Figure 2 shows the absorption spectra from UV to near-infrared region (300-1200 nm) at normal incidence, where p = 300 nm, d = 90 nm, h1=55 nm, h2=110 nm, h3=5 nm, and h4=45 nm. As shown in Fig. 2, high absorption can be obtained in the wavelength range of 300-1200 nm for both TE and TM polarizations. For TE polarization, the average absorption is about 96.0% from 300 nm to 1200 nm and the absorption peaks (A > 99.0%) appear at the wavelengths of λ=385 nm and λ=980 nm. For TM polarization, the average absorption is greater than 91.0% from 300 nm to 1200 nm and the absorption peak (A > 99.0%) appears at the wavelength of λ=430 nm.
The absorbance spectra as a function of the incident angle for TE and TM polarizations are shown in Fig. 3, where the incident angle is varied from 0° to 75°. It is noted that the absorption bandwidth (A > 90.0%) decreases with the increase of the incident angle. As shown in Fig. 3(a), for TE polarization, the absorption is still over 85.0% when the incident angle adds up to 55°. For TM polarization, the absorption is still over 80.0% when the incident angle adds up to 45° over the wavelength range (300-1200 nm), as presented in Fig. 3(b). The above simulation results show that the proposed absorber has robust angle tolerance, which is crucial and needful for harvesting solar energy.
Fig. 3. Absorbance spectra as a function of the incident angle for TE polarization (a) and TM polarization (b).
To understand he physical mechanism of the ultra-broadband absorption of the proposed absorber, the electromagnetic field distributions at several selected wavelengths for both TE and TM polarizations at normal incidence are investigated. For TE polarization, the non-zero electric field component (Ey) with a coordinate system Oxyz oriented at several wavelengths (385 nm with A = 99.1%, 600 nm with A = 92.0%) are depicted in Fig. 4. From these figures, it is found that the electric field is mainly confined within the multi-layered structure, suggesting that the guide-mode resonances exist [28,35]. In Fig. 4(a), the electric field in the grating groove formed by dielectric cover layer (SiO2) is very strong, indicating that the guided-mode resonance excited by the dielectric cover layer plays a dominant role at this time, which is the main reason for the efficient absorption at 385 nm wavelength. At the wavelength of 600 nm, the field localization of the guided-mode resonance excited by the SiO2 cover layer is slightly weakened, and the field localization of the guided-mode resonance in the dielectric grating is enhanced. Therefore, the synergy of the guide-mode resonances supported by the multi-layered structure enable the meta-surface to efficiently absorb TE-polarized light.
Fig. 4. Calculated electric field distributions along y-axis for TE polarization at normal incidence: (a) at λ=385 nm; (b) at 600 nm.
For TM polarization, the pattern of the magnetic field distributions along y-axis (Hy) at several wavelengths (430 nm with A 99.8%, 600 nm with A 90.2%) for TM polarization are shown in Fig. 5, respectively. It is clearly seen that the magnetic field is strongly confined at the interface between the bottom W layer and the dielectric layer, and the top surface of W grating, depicting the typical feature of surface plasmon waves [28,35]. It is also noted that the field in the ridge of the grating is slightly weaker, which is the effect of the cavity mode [28,35]. Since the height of the dielectric grating (h2) is only 110 nm, the characteristics of the cavity modes are not particularly obvious. As the height of the dielectric grating increases, the characteristics of the cavity modes will be obvious. Therefore, it can be seen from the magnetic field distribution that, due to surface plasmon waves and the cavity mode resonance, the proposed meta-surface exhibits strong absorption in the ultra-wide wavelength region.
Fig. 5. Calculated magnetic field distributions along y-axis for TM polarization at normal incidence: (a) at λ=430 nm; (b) at 600 nm.
The dependence of absorbance on the period (p) of the dielectric grating for TE and TM polarization is shown in Fig. 6(a) and Fig. 6(b), respectively. It is seen that the absorption curves for the two polarizations are blocked by the line p=λ (denoted by the black dashed line), above which the nonzero orders of the diffraction emerge. As can be seen from Fig. 6(b), for TM polarization, the period needs to be greater than 200 nm in order to achieve high broadband absorption (A > 80%). The dependence of absorbance on the height of the dielectric grating h2 under TE and TM polarization are shown in Fig. 6(c) and (d). It is indicated that the broadband character is also controlled by h2. From the production feasibility point of view, the best parameter for wide bandwidth is h2=110 nm.
Fig. 6. Dependence of absorbance on the dielectric grating period p and the height of the dielectric grating h2: (a) and (c) for TE polarization; (b) and (d) for TM polarization.
Figure 7 shows the dependence of absorbance on the thickness of the middle W cover layer h3 and the thickness of the top SiO2 cover layer h4 for TE and TM polarization, respectively. It is obvious that the thickness of the middle W cover layer is extremely significant for broadband character for the two polarizations, as presented in Fig. 7(a) and (c). Without middle W cover layer, it is impossible to achieve ultra-broadband absorption for TE- and TM-polarized light concurrently. As can be seen from the magnetic field distributions in Fig. 5, the middle W layer, which is covered on the dielectric grating, forming the W grating, can interact with SiO2 cover layer to make the light field local and excite surface plasmon waves at the interface of bottom W layer and the dielectric layer and the top surface of W grating. If the thickness of the middle W cover layer is too thick, the reflection of W will be obvious, leading to a decrease in absorption efficiency. In terms of performance and cost savings, h3=5 nm is selected to obtain better broadband character for both TE and TM polarization. Figure 7(b) and Fig. 7 (d) show that the change of the h4 will also affect the broadband characteristics for the two polarizations. In order to obtain broadband absorption for both TE and TM polarization, an optimal parameter is h4=55 nm. So well-defined choices of the grating period, ridge width, height and the thickness of the metal coverage layer allow us to control the trapping effect of the multi-layered structure, thereby determining the efficiency and the bandwidth of the absorption of the absorber.
Fig. 7. Dependence of absorbance on the thickness of the middle W cover layer h3 and the thickness of the top SiO2 cover layer h4: (a) and (c) for TE polarization; (b) and (d) for TM polarization.
3. Conclusion
In this work, an ultra-broadband polarization-independent near-perfect absorber incorporating 1D multi-layered gratings structure with refractory materials is numerically and theoretically analyzed. The average absorption of TE polarization is 96.0%, and the peak absorption can reach 99.4%. At the same time, the average absorption of TM polarization is 91.0% and the peak absorption is 99.8%. This excellent absorption performances are due to the trapping effect of the multiple resonance modes supported by the multi-layered structure. The results of this paper can provide reference for the design of polarization-independent absorbers with 1D meta-surface with refractory materials, and are expected to find potential applications in structural color, solar-energy harvesting, and other optical devices.
Funding
the Science and Technology Project of Suzhou (ZXG201427); National Natural Science Foundation of China (61405133, 61505134, 61575133, 61775076); Natural Science Foundation of Jiangsu Province (BK20140357); Natural Science Research of Jiangsu Higher Education Institutions of China (14KJB140014); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Disclosures
The authors declare no conflicts of interest.
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