Full length articleHigh-efficient plasmonic solar absorber and thermal emitter from ultraviolet to near-infrared region
Introduction
Metamaterials are artificial materials with highly remarkable features like negative refractive index, perfect imaging, and invisible cloaking, which there is no similarity for them in nature [1], [2], [3], [4], [5]. Owing to these properties, metamaterial devices have attracted a lot of attention in various applications such as photo-detectors, sensors, imaging, filters, and absorbers [6], [7], [8], [9], [10], [11], [12]. As the metamaterial absorbers can absorb a wide range of sunlight with high efficiency and the least dependence on the incident light's polarization and angle, they play a crucial role in thermo-photovoltaic systems. It has been reported that metal–insulator-metal (MIM) absorbers can exhibit various absorption spectra like single wavelength, multi-band (using several resonators), and continuous broadband [13], [14]. For example, Wang et al. studied a large area broadband absorber composed of several stacked layers of Ag/SiO2 placed on a sphere with a diameter of 750 nm [15]. It can operate properly in the wavelength range of 350 nm to 850 nm. Liang et al. proposed a near-unity absorber based on an array of nano-pyramids, including 17 pairs of alternating metal/dielectric layers, working from 200 nm to 2500 nm [16]. Also, a numerical study on an ultra-broadband solar absorber made of 6 pairs of metal-dielectric layers, absorbing wavelengths from 400 nm to 2000 nm (absorption greater than 90%) is reported by Wu et al. [17]. However, most of these absorbers suffer from several limitations such as complex structure [15], [18], a large number of metal-dielectric layers in the stacking scheme [16], low absorption spectrum [15], and sensitivity to polarization and angle of the incident radiation [18], [19]. In recent years, due to the desirable plasmonic behavior of refractory metamaterials, they have drawn a lot of attention [20].
Titanium nitride (TiN) is a tough and CMOS-compatible material with a high melting point of 2930C° that makes it suitable for use in high-power and thermo-photovoltaic applications. Unlike traditional noble metals like Ag and Au, which suffer from shortcomings such as weak thermal and chemical stability, incompatibility with CMOS technology, and high fabrication and material cost [21], TiN benefits from advantages like having plasmonic resonances in the visible and NIR region and lower cost [22], [23], [24]. Chirumamilla et al. investigated 3D TiN nanopillars with diameters of 320 nm and heights of 1500 nm to achieve an unpolarized absorber with an average absorption of 94% in the spectral range of 300 nm to 2300 nm [25]. Similarly, Huo et al. reported a light absorber with an over 94% absorption spectrum ranging from 400 to 1400 nm. This absorber comprises TiN nano-cone arrays with diameters of 100 nm and heights of 350 nm with a total thickness of 850 nm. In another study, Li et al. achieved an absorption exceeding 87% with a TiN ring-shaped metasurface over the whole visible region [20]. In this work, we use TiN as the plasmonic metal instead of the noble metals in a MIM structure due to its intrinsic plasmonic resonant behavior. In the proposed absorber, we utilize two kinds of TiN resonators in a unit cell to broaden the absorption bandwidth (BW) from the ultraviolet (UV) to near-infrared (NIR). The disk-shaped and square ring TiN resonators are located on a stack of TiO2/TiN layers. The relative permittivity of both TiN and TiO2 are taken from the handbook of Palik [26].
The rest of the paper is organized as follows. Section 2 is devoted to a brief explanation of solar absorber theory. The schematic of the proposed absorber is provided in Section 3. In Section 4, the effects of geometrical parameters, polarization, and angle of the incident radiation and other metals instead of TiN are investigated. Finally, the conclusion is expressed in Section 5.
Section snippets
Theory of solar absorbers
The absorption of an absorber can be calculated by [27]:where T(ω), R(ω), and A(ω) are the frequency-dependent transmission, reflection, and absorption parameters, respectively. According to Eq. (1), to achieve an absorption near the unit, the transmission and reflection parameters must be close to zero. In other words, to obtain the maximum value of A(ω), the parameters T(ω) and R(ω) must be minimized. Regarding T(ω), it is possible to select the thickness of the opaque metal
Structure of the proposed solar absorber
The schematic of the proposed solar absorber is demonstrated in Fig. 1(a). A TiO2 dielectric layer with a height of h3 is located on a ground TiN substrate. A periodic array of TiN disk-square ring resonators are on the dielectric layer. The heights of the disk and square ring resonators are h1 and h2, respectively. The height of the opaque TiN substrate (h4) is supposed to be 100 nm, which is quite greater than the skin depth of the structure, results in blocking the transmission of light from
Simulation results and discussion
We utilize the three-dimensional finite-difference-time-domain (3D-FDTD) method to model the optical properties of the structure. Perfectly matched layers (PMLs) are employed as the absorbing boundary condition in the z-direction and the x- and y-directions, the periodic boundary condition is used. Also, a plane-wave source is applied to radiate to the structure in the z-direction.
Conclusion
We proposed and analyzed a broadband metamaterial solar absorber using two combined TiN disk-square ring resonators. The solar absorber traps a massive part of electromagnetic energy from 250 nm to 3000 nm with high average absorption of 94%. Regarding more than 90% absorption, a continuous bandwidth of 2200 nm expanded from 250 nm to 2450 nm is achieved. The absorber performs almost independently to both TE and TM polarizations. Also, it is insensitive to the angle of the incident light, so
CRediT authorship contribution statement
Samira Mehrabi: Conceptualization, Methodology, Software, Investigation, Writing – original draft. Mir Hamid Rezaei: Software, Investigation, Validation, Writing – review & editing, Supervision. Mohammad Reza Rastegari: Software, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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