1. Introduction

Perfect ultra-broadband absorbers have attracted much interest in numerous applications, such as detection [1–3], imaging [4–7], and photovoltaics [8,9]. Subwavelength-thickness metamaterial perfect absorbers (MPAs), consisting of metal and dielectric elements, have attracted considerable attention in recent years. Landy et al. first proposed a two-dimensional metamaterial electric ring resonator and achieved perfect absorption in the microwave region [10]. Therefore, significant effort has been applied in recent years to achieving perfect broadband absorbers. Multilayered hyperbolic metamaterials (HMM) have been proposed as solar absorbers, and their absorptivity is higher than 90% in the visible to near-infrared (NIR) range [11,12]. Multiple tungsten cross resonators have been demonstrated as metamaterial absorbers in the mid-infrared (MIR) range, where a near-perfect absorption with a single absorption peak is tuned from 3.5 to 5.5 µm by adjusting the geometrical parameters of cross resonators in single-sized unit cells [13]. Dielectric metasurfaces have been experimentally developed, and their absorption is approximately 87% from 250 to 2250 nm [14]. Semiconductor–metal stack devices exhibit an average absorption of approximately 98% over a wide range of wavelengths, from 400 to 2000 nm [15]. In addition, other methods were successfully proposed for realizing ultra-broadband perfect absorption by using the interaction of metallic resonators [16,17] in planar or vertical arrangements [18]. However, the aforementioned broadband absorbers require either accurate control or complicated manufacturing processes, which results in high costs and hinders the fabrication of wafer-scale high-absorption broadband absorbers. Alternatively, film-stack metasurfaces, including dielectric–metal–dielectric (DMD) stack metasurfaces and metal-dielectric-metal (MDM) stack metasurfaces, have emerged and become a novel broadband absorption configuration [15,19–23]. Materials such as TiN, ITO, Mo, W, Pd, and Cr are typically selected to facilitate perfect absorption in metamaterial-based absorbers [24,25]. Generally, plasmonic metal materials (Au, Ag, Al) have the potential to achieve narrowband, single-band, or multiband perfect absorption owing to their intrinsically narrow bandwidth. However, achieving perfect absorbers for lithography-free plasmonic metal materials is still a challenge.

In this study, we experimentally manufactured wafer-scale, lithography-free ultra-broadband absorbers consisting of metal and dielectric (MD) stack metasurfaces. The plasmonic material Al is selected to be the metal layer because it is not only low-cost but can also support surface plasmon resonance (SPR) over a wide spectrum from the deep ultraviolet (DUV) to near infrared (NIR) regions. The lossy material SiO₂ was used as the dielectric layer to achieve optimal coupling of the PSPR- and LSPR-dominated absorption. The wafer-scale Al-SiO₂ stack metasurfaces facilitated pronounced broadband absorption (98%) ranging from the DUV to NIR. This study systematically establishes strategies for guiding the design of plasmonic metal-based ultrathin MD broadband absorbers with excellent absorption. This will pave the way for enhancing the optical performance of robust optoelectronic devices [23].

2. Fabricating the Al-SiO₂ stack metasurfaces

In this study, an ultrathin metal-dielectric (MD) lithography-free absorber consisting of an aluminum-silica (Al-SiO₂) multilayer metasurface was fabricated. First, quartz glass was selected as the substrate. The quartz glass was continuously sonicated in deionized water, acetone, and ethanol for 15 min and baked in an oven at 120 °C for 40 min. Subsequently, an 8-nm Al (99.999%) film and 50-nm SiO₂ (99.99%) film were deposited onto the prepared substrates using an electron beam film deposition system (DZS-500). The depositing pressure was 7×10⁻⁴ Pa, and the depositing rate adopted was 0.5 and 1.5 Å/s for the Al and SiO₂ film, respectively. Finally, Al and SiO₂ films were alternately deposited using the same technology. A schematic of the fabrication process of the ultrathin broadband absorber is shown in Fig. 1(a). Figure 1(a) shows the MD as a one-layer unit of the absorber. Figure 1(b) shows a cross-sectional scanning electron microscopy (SEM) image of the seven-layer broadband absorber. An Au film with a thickness of a few nanometers (the red-dotted line in Fig. 1(b)) is sprayed on the top of the absorber to clearly observe the cross-sectional image of the absorber. It is clear that the brighter color represents the ultrathin Al metasurface and darker color represents the SiO₂ dielectric metasurface in Fig. 1(b) (the inset represents the cross-sectional image of the seven-layer absorber). In contrast to the MDM-based cavity configuration, transparent SiO₂ was selected as the substrate, and the Al metasurface in the MD stack configuration was only a few nanometers.

 

Fig. 1. (a) Schematic illustration of fabricating an ultrathin broadband absorber consisting of Al and SiO₂ metasurfaces. (b) Cross-sectional scanning electron microscopy (SEM) image of seven-layer stack metasurface. The inset represents the diagram of cross-sectional image of seven-layer absorber and the red-dotted line denotes few nanometers Au film for increasing the conductivity.

Download Full Size | PDF

3. Results and discussion

The thickness and roughness of the ultrathin Al metasurface are key factors in achieving a perfect absorber. Therefore, the topography of the ultrathin Al metasurface was determined using atomic force microscopy (AFM). The results shown in Figs. 2(a) and 2(b) demonstrate that the 8-nm Al metasurface displays the distribution of flip-chip particles. The height and diameter distributions show that the height and diameter of the ultrathin flip-chip Al metasurface were dominantly 8 and 188 nm, respectively, as shown in Figs. 2(c) and 2(d). The discontinued flip-chip Al metasurface can also support broadband absorption, with an absorption rate of approximately 20% (shown in Fig. 2(e)). Individual nanoparticle close to 200 nm can excite localized surface plasmon resonance (LSPR) [26]. Discontinued and random nanoparticles will interact with each other, which is a key factor for realizing perfect broadband absorption in ultrathin Al-SiO₂ stack metasurfaces [27–29].

 

Fig. 2. Atomic force microscope (AFM) images of an ultrathin Al metasurface. (a) 2D and (b) 3D images of ultrathin flip-chip Al metasurface with a size of 5 × 5 µm². The corresponding (c) height and (d) diameter distribution histograms of the Al nanoparticles. (e) The absorption of 8-nm flip-chip Al metasurface ranging from 350 nm to 1400 nm.

Download Full Size | PDF

To realize perfect broadband absorption, a low-loss SiO₂ antireflection coating with a thickness of 50 nm was added to the top of the flip-chip Al metasurface. The results shown in Fig. 3(a) demonstrate that the introduction of the SiO₂ metasurface can elevate the absorptivity and realize ultrabroadband absorption, while the absorption is still lower than 40% for the unit cell MD metasurface. To achieve near-perfect absorption, multilayer Al-SiO₂ metasurfaces were fabricated using simple film deposition technology. For a three-layer Al-SiO₂ metasurface (with a total thickness of 174 nm), the absorption can exceed 80% from the 350 to 1400 nm broadband range. As the layer number of the Al-SiO₂ metasurface increases, the absorption can exceed 98% within the DUV to NIR range. Sample pictures corresponding to the layer number of the Al-SiO₂ stack metasurfaces are shown in Fig. 3(b). The finite-difference time-domain (FDTD) method was used to calculate the absorption spectrum of the Al-SiO₂ stack metasurfaces. The contributions from the random resonant metallic films are also significant [30,31]. To make simulation model close to experimental films, the data points were extracted from the 3D-AFM images (NanoScope Analysis) to establish the Al-SiO₂ stack metasurfaces. The mesh size was set at 1 nm. Periodic boundary conditions were adopted in the x and y directions. A perfect matching layer (PML) was used in the z-direction. The simulated results indicate that the absorption can be sustainably increased by increasing the layer number of the Al-SiO₂ stack metasurfaces, as shown in Fig. 3(c). The simulation results are consistent with the experimental results.

 

Fig. 3. (a) Experimental results for the absorption spectra of 1 to 11-layer Al-SiO₂ stack metasurfaces. (b) Photograph corresponding to the layer number of Al-SiO₂ metasurfaces from 1 layer to 11 layers (Scale bar 10 mm). (c) Simulation results for the absorption spectra of 1 to 11-layer Al-SiO₂ stack metasurfaces.

Download Full Size | PDF

To clarify the mechanism by which the Al-SiO₂ stack metasurfaces realize perfect absorption, interference theory [32] was used to theoretically describe perfect absorption. A Fabry-Perot cavity is formed by the Al-SiO₂ stack metasurfaces and can confine the light intensity to achieve perfect absorption. The two layers of the ultrathin Al metasurface could be decoupled, and multiple reflections occurred between them. As shown in Fig. 4(a), the nature light, entering the low-loss SiO₂ antireflection coating, is partially reflected to air with a reflection coefficient ${\tilde{r}_{12}} = {r_{12}}\exp ({i{\varphi_{12}}} )$ and partially transmitted into the ultrathin Al metasurface with a transmission coefficient ${\tilde{t}_{12}} = {t_{12}}\exp ({i{\varphi_{12}}} )$. The transmitted light into the ultrathin Al metasurface continues to propagate until it reaches the next Al metasurface with a complex propagation phase $\tilde{\beta } = {\beta _r} + i{\beta _i} = \sqrt {{{\tilde{\varepsilon }}_{Al}}} {k_0}d$, where ${k_0}$ represents the free-space wavenumber, ${\beta _r}$ represents the propagation phase, and ${\beta _i}$ represents the absorption in the Al flip-chip metasurface. Finally, the overall reflection can be expressed through the superposition of multiple reflections [32]:

 

Fig. 4. (a) Schematic diagram of the transmission and reflection for a seven-layer Al-SiO₂ stack metasurface; (b) Simulation results about electric field distributions of 2 to 10-layer Al-SiO₂ stack metasurfaces.

Download Full Size | PDF

As is well-known, Au thin film-based absorbers do contribute a better performance [33]. To verify the superiority of the Al-SiO₂ stack metasurfaces, we experimentally measured the absorption spectra of Ag-SiO₂ and Au-SiO₂ stack metasurfaces. Figure 5(a) demonstrates that Al-SiO₂ stack metasurfaces can display absorption above 80% from the near-UV to NIR regions, even reaching 95% in the UV and visible regions. However, Au-SiO₂ stack metasurfaces have weak absorption lower than 40% in the broadband regions. Although Ag-SiO₂ stack metasurfaces have high absorption in the image, the absorption shown in the absorption spectrum is lower than 80% above 600 nm. Corresponding photos of seven-layer different metal-based MD stack metasurface are shown in Fig. 5(b). Because Al materials possess a higher bulk plasma frequency [34] than Au and Ag, the Al film has a stronger capability of decoupling in the Al-SiO₂ stack metasurfaces.

 

Fig. 5. (a) Experimental absorption spectra of seven-layer different metal-based MD stack metasurface; (b)-(d) Corresponding photos of seven-layer different metal-based MD stack metasurface (Scale bar 10 mm).

Download Full Size | PDF

4. Conclusion

In summary, wafer-scale MD stack metasurfaces consisting of alternating Al and SiO₂ metasurfaces have been demonstrated as perfect ultra-broadband absorbers. The Al-SiO₂ stack metasurfaces with eight layers display an average absorption of over 98% from 350 to 1400 nm. The results demonstrate that Al has a great advantage in achieving perfect ultrabroadband absorption through the designing of multilayer Al-SiO₂ stack metasurfaces. In addition, engineering the layer number of the Al-SiO₂ stack metasurfaces can manipulate the absorption from 40% to 98%, within the 350 to 1400 nm wavelength region. Importantly, wafer-scale ultra-broadband perfect absorbers can be obtained using simple film-deposition technology. A perfect ultra-broadband absorber based on Al-SiO₂ stack metasurfaces will be beneficial as a substrate for detection, imaging, and photovoltaics.