## 1 Introduction  
In the past decade, the increasing cost of fossil fuels and the global warming problem have increased the urgency of the need to develop renewable green energy (such as solar, wind, tidal, etc.) [1]. Because of having the properties of security, universality, and other advantages, the transformation solar energy to useful energies has been a field of intensive researches in recent years. Functionally, there are two major conversion mechanisms of solar energy: photovoltaic and solar-thermal applications [2, 3]. In solar-thermal conversion devices, many researches have aimed to convert the incident solar light to heat for further application. The key component is using a solar absorber, which can provide ideal absorption of sunlight over the entire solar spectrum and convert solar energy to other energy that we can utilize [4]. Many solar-thermal applications can be directly applied in many industrial processes, such as water purification, desalination, distillation, and so on [5; 6]. Besides, for thermophotovoltaic (TPV) devices, the thermal energy can also be converted to electricity energy [7]. It effectively traps the incident light in the nanospaces and dissipates it by the ohmic losses of the metal that can give rise to the high absorption rate of the incident light. For that plasmonic metamaterial absorbers have attracted considerable attention for the harvesting of solar energy [8]. [PERSON] et al. proposed a dual functional asymmetric plasmonic absorber for solar water purification and pollution detection [5]. [PERSON] et al. reported a plasmonic absorbers which can enable a strong absorption rate (72-91%), over 400-900 nm, and a significantly (20-fold) enhanced photocurrent as compared to the bare TiO2 film for enhanced photocurrent of visible-light photocatalysis [7]. [PERSON] et al. proposed a 90-nm-thick graphene sheet absorber, which can be heated to 160 \({}^{\circ}\)C in natural sunlight [9]. When light interacts with nanostructured absorbers, the energy is confined in a nanospace, and this confinement can lead to heat generation. Thus, many various nanostructures based on metals have been proposed for the absorption enhancement. For instance, [PERSON] et al. investigated a broadband visible-light absorber via hybridization of propagating surface plasmon (PSP), and the average absorption rate of the designed absorber could reach up to nearly 90% in the visible range of 400-750 nm [10]. [PERSON] et al. demonstrated experimentally plasmonic super absorbers that are capable of absorbing light from 400 nm to 700 nm, with a measured average absorption rate of 71% [11]. [PERSON] et al. theoretically proposed absorbers based on concentric multi-split-ring arrays with which the average absorption rate reaches 97.2% over the wavelength range from 585 to 800 nm [12]. [PERSON] et al. experimentally demonstrated an omnidirectional broadband-light absorber, using large-area ultrathin lossy metallic film coatings [13]. However, it is obvious that simply designing structures is not sufficient for building a solar device, because the narrowband plasmonic resonance of metal nanoarrays only allows the utilization of a small fraction of light in the solar spectrum (300-2500 nm). It is therefore highly desirable to design solar absorbers with a high solar absorption rate, to significantly enhance the conversion efficiency from sunlight to heat. In order to enhance absorption rate of sunlight, researchers have investigated many different kinds of complex models. [PERSON] et al. proposed a patterned gold (Au)-carbon (C)-gold (Au) structure to realize a metasurface absorber with a selective high absorption, ranging from 400 to 1200 nm [14]. [PERSON] et al. theoretically investigated a selective solar absorber consisting of tungsten nanoparticle arrays embedded in a silica layer deposited on alternating tungsten/silica layers with an extremely high absorption efficiency, above 99%, within the range of 435-1520 nm [15]. [PERSON] et al.