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Abstract
Thin devices with large areas have strong and omnidirectional absorption over a wide bandwidth and are in demand for applications such as energy harvesting, structural color, and vehicle LiDAR (laser detection and ranging). Despite persistent efforts in the design and fabrication of such devices, the simultaneous realization of all these desired properties remains a challenge. In this study, a 190-nm-thick metasurface with an area of 3 cm2, incorporating dielectric cylinder arrays, a chromium layer, a silicon nitride (SiNx) layer, and an aluminum layer is theoretically and experimentally demonstrated. The developed device achieves an average absorptivity of ∼99% (97% in the experiment) in the entire visible spectrum 400–700 nm. Moreover, it exhibits strong absorption over a wide range of incident angles (∼91% and 90% at 60° in the calculation and experiment, respectively). Importantly, the feasibility of applying the developed metasurface absorber to solar thermophotovoltaics and vehicle LiDAR (laser detection and ranging) has been explored. Moreover, the photoresist can be replaced by other glues and easily scaled up to a large area using the roll-to-roll nanoimprinting process. With the excellent spectral properties and performance, this device is promising for large-area applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
An ideal absorber, which is a specially engineered material with strong and omnidirectional absorption over a wide bandwidth [1–40], has various applications in energy harvesting [2,3], structural color [4,5], concentrating solar power, and photodetectors [6–8]. Stacked metal/dielectric absorbers, where absorptive metals such as nickel [9], germanium [10], platinum [11], silver [3], and titanium nitride (TiN) [12] are utilized, have been widely investigated. These absorbers are suitable for large-area fabrication. However, achieving a wider absorptive bandwidth with these absorbers requires an increase in the number of coating layers, which requires more time for the coating processes. In addition to the aforementioned approaches, a metasurface absorber is another promising method. It generally comprises only two components: one- or two-layer absorptive materials and engineered nanostructures, such as nanoholes [13–15], gratings [16], nanowires [17,18], nanoparticles [19–22], nanopatches [23,24], and nanocylinders [25–29], to enhance absorption. Based on this, Huang et al. carefully designed W gratings with suitable periods, heights, and widths to excite different resonances, thereby realizing strong and wide-angular absorption for broadband light in the range of 200–900 nm [30]. However, as scaling up to a large area relies on metal gratings, it remains challenging.
In this study, to achieve strong and omnidirectional absorption, we numerically and experimentally developed a photoresist array-based metasurface absorber that can cover the entire visible spectrum (400–700 nm) with an thin thickness (160 nm) on the top surface of a quartz substrate. This thin absorber comprises dielectric gratings, a Cr layer and a SiNx layer on the top surface of the substrate, and a reflective Al layer on the bottom surface of the substrate to enhance absorption. Remarkably, the photoresist can be replaced by other glues; therefore, this dielectric-grating-based absorber can be easily scaled up to a large area using the roll-to-roll nanoimprinting process. In the range of 400–700 nm, the developed device shows an absorptivity of ∼99% (97% in the experiment) at normal incidence, and a broadband strong absorption for incident angles up to 60° (91% and 90% in the simulation and experiment, respectively). The outstanding performance of this device is realized by the combination of surface plasmonic mode resonance and Mie resonances. Furthermore, this device shows an ultra-low hemispherical thermal emittance of ∼3.3% for the long infrared radiation range (5–13 µ;m), and much stronger absorption can be achieved in the near-infrared range by adjusting the height of the cylinder arrays, which is important for applications such as concentrating solar power devices and vehicle LiDAR.
2. Structure design
The developed metasurface absorber consists of two-dimensional (2D) photoresist cylinder arrays embedded inside Cr and SiNx layers on the top surface of the substrate, and a reflective Al layer on the bottom surface, as shown in Fig. 1. The optimized structural parameters of the cylinder arrays are listed as follows: the period (p) is 290 nm, width is 60 nm, and height (h) is 90 nm. The thicknesses of the Cr, SiNx, and Al layers are 30, 40, and 30 nm, respectively. The performance of the developed device is calculated using the 3D finite-difference time-domain (FDTD) method [31] and the index information associated with these materials are derived from Palik [32].
Fig. 1. Schematic of the metasurface absorber. This absorber comprises dielectric gratings, a Cr layer and a SiNx layer on the top surface of the substrate, and an Al layer on the bottom surface.
3. Results and discussion
Figure 2(a)–(c) shows the numerically calculated absorptive spectrum as a function of incident wavelengths and angles for TE- (transverse electric), TM- (transverse magnetic), and un-polarized light, respectively. For TE polarization, the peak absorption is up to 99%, and an average absorption exceeding 96% is achieved in the wavelength range of 400–700 nm over the incident angle range of 0–60°. For TM polarization, with the incident angles ranging from 0° to 60°, the peak and averaged absorption are 99% and 97%, respectively. For un-polarized light, an average absorption of 96.5% is obtained for the entire visible spectrum in all circumstances. Notably, the absorption of photoresist cylinder arrays and SiNx layers is ∼0 for both polarizations, as that of the imaginary parts of these materials is ∼0.
Fig. 2. Calculated absorptive spectrum of the developed absorber. (a)–(c), Absorptive spectrum as a function of incident wavelengths and angles for TE- (a), TM- (b) and un- (c) polarized light.
The metasurface absorber was fabricated in three steps, as depicted in Fig. 3. First, the photoresist cylinder (RZJ-340) arrays were fabricated by photolithography, Secondly, 30-nm-thick Cr and 40-nm-thick SiNx layers were deposited on the cylinder arrays via magnetron sputtering and inductively coupled plasma chemical vapor deposition, respectively. Finally, a 30-nm-thick Al layer was deposited on the bottom surface of the substrate via magnetron sputtering to reflect the transmitted light. The scanning electron microscopy image and optical image of the fabricated absorber with an area of ∼1×3 cm2 under indoor ambient light illumination are also shown in Fig. 3. It can be observed that the photoresist array-based metasurface absorber appears completely black. Remarkably, the photoresist can be replaced by other glues and easily scaled up to a large area using the roll-to-roll nanoimprinting process. Therefore, the proposed metasurface absorber has advantages in terms of fabrication and practicability compared to conventional metallic grating absorbers.
Fig. 3. Schematic of the fabrication process, including fabrication of the photoresist cylinder arrays and deposition of the Cr, SiNx, and Al layers; and image of the fabricated metasurface sample.
Subsequently, we measured the reflection (R) and transmission (T) of the fabricated metasurface absorber using a spectrophotometer (LAMBDA 750, PerkinElmer) with incident angles ranging from 8° to 60° for unpolarized light, and the absorption (A) was calculated as A = 1-R-T. Figure 4(a) shows that a strong absorption of over 94% is obtained in all circumstances, indicating good angular tolerance. As depicted in Fig. 4(b), at an incident angle of 8°, the peak and average absorption of the fabricated metasurface absorber are 98% and 96%, respectively. Therefore, the fabricated absorber exhibits consistency with the design and the calculations (Fig. 2(c)).
Fig. 4. (a) Measured absorptive spectrum of the device for un-polarized light over the wavelength of 400–700 nm with incident angles varied from 8° to 60°. (b) Measured absorptive spectrum at an incident angle of 8° of this device (black), without the Al layer (red), and the SiNx coated Cr layers (blue).
To understand the absorptive mechanism behind the metasurface absorber, we measured the absorption of the reduced absorber with the same configuration. As depicted in Fig. 4(b), the SiNx-coated Cr layers achieved a low absorption of ∼67% over the wavelength of 400–700 nm. As expected, the photoresist cylinder arrays enhanced the averaged absorptivity up to 90%, especially at short wavelengths. Finally, due to the Al layer on the bottom surface of the substrate, an average absorptivity of ∼98% was achieved over the entire visible spectrum, and the absorption increased from 83% to 95% at a wavelength of 700 nm, resulting in a strong broadband absorption.
To further explore the absorptive mechanism underlying the metasurface absorber, we calculated and mapped the magnetic field distribution of this device at the reflective dips. As depicted in Fig. 5(a), at a short wavelength of 460 nm, the magnetic power is mainly around the top surface and sidewalls of the Cr-coated gratings, resulting from the surface plasmonic mode resonance and Mie resonances [27]. However, little power was distributed inside the substrate induced by the bottom Al layers. At a wavelength of 532 nm, much more light is absorbed by the Al layer than that at the wavelength of 460nm. the magnetic power inside the substrate disappeared (Fig. 5(b)), and the surface plasmonic mode resonance and Mie resonances (magnetic dipole) were weaker than those at 460 nm. The simulation shown in Fig. 5(c) shows that only the reflected specular orders appear in the wavelength range of 400–700 nm. The combination of the strong reflection of the Al layer, effective absorption of the Cr layer, excitation of the surface plasmonic mode resonance and Mie resonances, and limited light diffractions resulted in a strong absorption of over 96% over the entire visible spectrum. Therefore, strong broadband and wide-angular absorption can be achieved by a seamless combination of the aforementioned mechanisms.
Fig. 5. Calculated magnetic field distribution of this device at reflective dips. λ = 460 nm (a), 532 nm (b). (c) Calculated diffractive intensity of the (0, 0) order normalized to the total intensity at wavelengths of 400, 500, 600, and 700 nm.
4. Potential applications
To demonstrate the feasibility of applying the developed absorber to solar thermophotovoltaics, the absorptivity of the absorber in the wavelength range of 250–13000 nm is calculated. As shown in Fig. 6(a), an average absorptivity of 85.6% is achieved for the wavelength range of 250–2300 nm. Remarkably, the average absorption in the wavelength range 5000–13000 nm is only 3.3% as depicted in Fig. 6(b). This means that this metasurface absorbs ∼ 85.6% of the incoming solar energy, and only ∼3.3% of the energy is wasted by heat dispersion through infrared radiation. Therefore, the selective absorber developed in this study is suitable for solar thermophotovoltaic applications.
Fig. 6. Calculated absorptivity of the device for un-polarized light at normal incidence over the wavelength of (a) 250–2300 nm, (b)250-13000 nm.
Subsequently, to demonstrate the feasibility of applying this device to LiDAR to suppress the disorderly light, the absorptivity of this device in the wavelength range of 900–1550 nm is calculated and depicted in Fig. 7(a). An average absorption of 73% is achieved and therefore, more than half the disorderly light was suppressed. To further enhance the absorptive ability of this device, we calculated the absorptive spectrum as a function of the height of the cylinder arrays. As mapped in Fig. 7(b), when the height varied from 90 to 300 nm, the maximum averaged absorption is 87% with a height of 183 nm. Moreover, peak absorptions of 91% (h = 136 nm) and 89% (h = 253 nm) are obtained at wavelengths of 940 and 1550 nm, respectively. Thus, the developed device can effectively suppress disorderly light for applications such as vehicle LiDAR.
Fig. 7. (a) Calculated absorptivity of the device for un-polarized light at normal incidence over the wavelength range of 900–1550 nm; (b) calculated absorptive spectrum of the device for unpolarized lights over the wavelength range of 900–1550 nm with the height of cylinder arrays varied from 90 to 290 nm.
Finally, a comparison of this device with three other state-of-the-art optical absorbers is presented in Table 1. The proposed metasurface absorber exhibits comparable performance, has advantages in terms of thickness, and is suitable for large-scale production. Remarkably, a much broader bandwidth can be achieved by adjusting the height of the cylinder arrays, as shown in Fig. 7(b).
Table 1. Comparison of the developed device with three other types of state-of-the-art, optical absorbers.
5. Conclusion
In this study, we developed a metasurface absorber with a broadband absorptivity of ∼94% for unpolarized light in the wavelength range of 400–700 nm in all circumstances, and a low hemispherical thermal emittance of ∼3.3% in the wavelength range of 5000–13000 nm. Furthermore, this absorber, comprising a sample with an area of ∼10×30 mm2, incorporates a combination of various resonances, which enables strong and wide-angular absorption within the visible spectrum. Moreover, much stronger absorption can be achieved in the near-infrared range by adjusting the height of the cylinder arrays. Remarkably, the photoresist can be replaced by other glues and easily scaled up to a large area using the roll-to-roll nanoimprinting process. Therefore, the photoresist cylinder array-based absorber has advantages in terms of fabrication and practicability compared to the conventional metallic grating structures. Therefore, the proposed device shows great potential for large-area applications, such as solar thermophotovoltaics, energy harvesting, and vehicle LiDAR.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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