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Abstract
In this work, a method for designing an ultra-narrowband absorber platform is presented with asymmetric silicon-based dimer-resonators grating. Within the infrared range of 3000 ∼ 4000 nm, two narrowband absorption peaks with absorptivity greater than 99% are produced by the absorber. Moreover, during the optical sensing, such an absorber platform shows high-performance sensitivity factors for the absorption wavelengths at λ1 = 3468 nm (S = 3193 nm/RIU, FOM = 532) and at λ2 = 3562 nm (S = 3120 nm/RIU, FOM = 390). Strong scattering coupling and the magnetic resonances supported in this silicon based grating produce the high absorption. Otherwise, additional methods such as the polarization and incident angles are used to further tune the absorption responses in the intensity and wavelengths, indicating the feasibility for artificial manipulations. The achieved ultra-sharp perfect absorption and the related sensitive response hold the silicon based resonant scheme with wide applications in bio-sensing, spectral filtering and other fields.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, metamaterial absorbers that achieve near-perfect absorption by designing nanostructures have become a hot topic [1–3]. Resonant modes of nanostructures have attracted wide attention because they can significantly affect the absorption characteristics [4–7]. According to different absorption bandwidths, absorbers can generally be divided into two main categories: broadband absorbers and narrowband absorbers. Among them, broadband absorbers are mainly used for light trapping and solar energy techniques [8–10], while narrowband absorbers are mainly used for sensing, filtering, selective thermal radiation [11–18]. Among them, refractive index sensor has a good research prospect because of its advantages such as simple structure, high sensitivity, simple manufacturing process [19–24].
At present, most studies focus on all-metal structures or classical metal-insulator-metal (MIM) structures [25–30]. However, these structures generally have two disadvantages: first, due to the inherent loss of the metal, it is somewhat difficult to achieve perfect absorption; second, the manufacturing process of these structures is difficult, which will lead to a slightly higher manufacturing cost [31]. In the case of an all-metal absorber. Although they are now also used in a variety of areas, there are still significant challenges in obtaining multi-band perfect absorption in the visible light range using a single metal structure. Moreover, it has been difficult to control the number of absorption bands freely by using all metal structures. The all-metal structures based on surface lattice resonance need high design requirements and precise geometric parameter control ability. As a result, the development of these absorbers and their further application are still quite difficult. The absorber based on dielectric grating structure can solve these problems to a large extent [32–34]. Compared with those metal structures, the dielectric grating absorbers have the following advantages: reduced manufacturing difficulty, cost savings, avoidance of complex photolithography, reduced structural losses, two or more perfect absorption bands and lower design requirements [35]. Different from the common structural design that places the medium between the metal and the grating, we choose to place the medium above the metal, which can make the structure simpler than the previous ones [36–40]. For example, narrow band IR absorbers based on gap plasmonics and absorbers based on ultra-narrow nanowire grids designed by Liao [41,42]. This structure can reduce the complexity of the structure while achieving a very narrow bandwidth.
In this paper, we study a narrowband perfect absorber based on an asymmetric silicon grating. The absorber can produce two perfect absorption peaks at 3468 nm (λ1) and 3562 nm (λ2) wavelengths, and its absorption efficiency is both above 99%. Due to the narrow bandwidth of about 6 nm (λ1) and 8 nm (λ2), the absorber based sensor platform shows high refractive index sensitive performance for the sensitivity S (λ1: S = 3192 nm/RIU, λ2: S = 3120 nm/RIU) and quality factor of figure of merit FOM (λ1: FOM = 532, λ2: FOM = 390).
2. Results and discussions
The proposed absorber is based on the asymmetric silicon grating as shown in Fig. 1. This model adopts the two-dimensional simulation mode. The structure consists of a silica substrate, a gold layer in the middle, and two silicon columns of different heights on the top. Among them, h1 and h2 are the heights of two silicon columns with different heights, h3 is the thickness of the gold layer in the middle, h4 is the thickness of the silica substrate, l1 is the width of the silicon column, l2 is the interval of the two silicon columns, and P is the period of the unit structure. Geometric parameters: h1 = 510 nm, h2 = 410 nm, h3 = 90 nm, h4 = 2500 nm, l1 = 260 nm, l2 = 390 nm, P = 3250 nm. Two-dimensional finite-difference time-domain method is used in the simulation of this structure, and the incident light is plane wave and perpendicular to the grating surface.
Fig. 1. The structure of an asymmetric silicon grating absorber. The inset is a two-dimensional cross section of the structure.
Figure 2(a) shows the absorption spectrum of symmetric and asymmetric grating structures under normal illumination. There is only one absorption peak in the symmetric grating structure, and there is a significant difference between the peak and the two peaks generated by the asymmetric grating structure. In an asymmetric grating structure. It is observed that two sharp anti-reflection dips and the related absorption peaks are occurred in the infrared regime. Two perfect absorption peaks at 3468 nm (λ1) and 3562 nm (λ2) wavelengths with its absorption efficiency both above 99% are achieved in such silicon based system. The spectral bandwidth for these absorption peaks is about 6 nm (λ1) and 8 nm (λ2), respectively. In order verify the absorption mechanism of the absorber, we introduce the impedance matching theory and add discussion on it. When the light source is normally incident, the absorbance and relative impedance can be calculated in the following way [43]:
Fig. 2. (a) The absorption spectra of the absorber in symmetric grating and asymmetric grating structures. (b) Real part and imaginary part of normalized impedance Z.
Z and Z0 are the effective impedance values respectively in the absorber and t free space. The Zr = Z / Z0 for the relative impedance between the absorber and free space. When absorbing wave impedance and the free space impedance matching, the absorber can absorb incident light most strongly. Figure 2(b) displays the relative impedance as the conductivity increases, the real part gradually approaches 1 and the imaginary part approaches 0. This shows that the impedance of our structure and the external space is gradually matching when the absorption reaches its peak, the real part of the impedance is close to 1 and the imaginary part is close to 0.
In order study the influence of geometric parameters on structure performance. We calculate the changes of reflectance under different structural parameters. Figure 3(a) shows the effect of the height of the upper silicon column on the reflectance spectrum of the absorber. The difference in height between the two silicon columns remains the same. With the increase of the height of the two silicon columns, the two absorption peaks showed the following changes: the absorption curves showed red shift, the absorption peaks first increased and then decreased, and the distance between the two peaks gradually increased. When h1 = 510 nm and h2 = 410 nm, the two absorption peaks reach the maximum value, and them are greater than 99%. Figure 3(b) shows the effect of the width of the two silicon columns on the reflectance spectrum of the absorber. With the increase of silicon column width, the two absorption peaks will also redshift, and the distance of each redshift is basically the same. Both absorption peaks increase first and then decrease, and the full width of the half-height increases gradually with the increase of the width of the silicon column. This is because as the thickness of the silicon column increases, the spacing between the two silicon columns gradually decreases, resulting in a more powerful electric field within a smaller spacing, resulting in a larger full-width at half-height. When w = 260 nm, the two absorption peaks reach the maximum value. Figure 3(c) The effect of the period size of this structure on the reflectance spectrum of the absorber. With the increase of the structure period, both absorption peaks showed red shift and the peak values first increase and then decrease. The FWHM also decreases slightly, the principle is the same as that caused by the width of the silicon column. However, the change of half height and full width caused by cycle change is not as great as that caused by silicon column width. The absorptivity reached the maximum when the period P = 3250 nm.
Fig. 3. Reflectance spectra in the wavelength range of 3000-4000 nm for the absorber under different h2 (a), w (b) and P (c).
In order better explain the physical mechanism of the absorption in such an asymmetric grating structure, the electric and magnetic field distributions on the surface of the structure are simulated [44–46]. As shown in Fig. 4(a-b), the electric field at the wavelength of λ1 is mainly concentrated at the two top angles of the silicon column on the right, while the electric field at the wavelength of λ2 is mainly concentrated at the two top angles of the silicon column on the left. As shown in Fig. 4(c-d), the magnetic field at the λ1 wavelength is mainly concentrated around the silicon column on the left, while the magnetic field at the λ2 wavelength is mainly concentrated around the silicon column on the right. This is because the incident light interacts with free electrons on the surface of the structure to produce surface plasmon waves. Surface plasmon resonance is induced by the coupling of surface plasmon wave and electromagnetic field. Through the distribution of electric field, we can see that a strong displacement current is formed in the interval of two silicon columns, and the magnetic field distribution can also prove this conclusion laterally. They both follow the right-hand spiral rule.
Fig. 4. Normalized electric and magnetic field intensities of the surface of the asymmetric silicon grating absorber. (a) and (c) Electric and magnetic field distributions at the wavelength of λ1 = 3468 nm. (b) and (d) Electric and magnetic field distributions at the wavelength of λ2 = 3562 nm.
The asymmetric silicon grating absorber also has excellent sensing performance. The sensing performance such as the sensitivity and quality factor (figure of merit, FOM) of the absorber are obtained by the following two formulas [47]:
where $\delta \lambda$ is the change of resonance wavelength caused by the change of ambient refractive index, $\delta n$ is the change of refractive index of the surrounding environment, FWHM is the full width of the band at the half height of the absorption peak. As shown in Fig. 5(a), we simulate and calculate the reflectance spectrum of the absorber at normal incidence in different ambient refractive index. It is observed that the resonant wavelengths of the two absorption peaks move to the longer wavelength ranges with the increase of the ambient refractive index. Although with the increase of refractive index n, the absorption rate will decrease slightly, but the overall absorption rate keeps at a relatively high level. At the same time, the FWHM remains basically unchanged. Figure 5(b) shows the relationship between the wavelength positions and the ambient refractive index n. Linear fitting of the data shows that the sensitivity S = 3192 nm/RIU (S* = 3120 nm/RIU) and the quality factor FOM = 532 (FOM* = 390) are obtained as shown in Fig. 5(b). This confirms that the structure has high sensitivity and quasi-linear response. This structure can be widely used in refractive index sensing, spectral sensing, detection and other fields [48–54].Fig. 5. (a) Spectra of sensors in different environmental media. (b) The absorption peak varies with the refractive index n of the environmental media between 1.0 and 1.1 with a step of 0.02.
Figure 6(a) shows the relationship between absorption and polarization angle. The polarization angle of incident light gradually changes from transverse 0° to 90° with an interval of 5°. The absorption gradually decreases with the increase of the polarization angle. This means that the absorbance at the two peaks gradually decreases as the polarization angle increases. This is because the asymmetric grating belongs to the asymmetric structure, and the polarization angle will have a great influence on the resonant coupling effects of the structure. It is also observed that if the polarization angle is less than 40°, the structure will still have a good absorption rate, which indicates the structure also has a relatively good polarization tolerance.
Fig. 6. (a) Plot of absorption rate versus polarization angle. (b) Plot of the relationship between absorption rate and incidence angle. (c) Absorption curves at three different angles. (d) and (e) are the electric field diagrams at the two peak values of the incident angle of 3.75°. (f) and (g) are the electric field diagrams at the two peak values of the incident angle of 7.5°.
In order study the effect of the incidence angle on the absorption spectrum, we calculate the absorption spectrum at different incidence angles (Fig. 6(b)). Since the absorber uses an asymmetric silicon grating structure, we simultaneously calculate the effects of positive and negative incidence angles on the absorbance. It can be seen from Fig. 6(b), the impact of positive and negative incident angles on absorbance is basically the same. With the change of incident angle, the resonant absorption peak changes obviously. In addition, when the incidence angle increases (decreases), the absorption bandwidth will increase. This is because the electric field is no longer concentrated in only one silicon column but distributed in the top of both silicon columns with the increase of the incidence angle. The larger area of electric field distribution leads to a larger absorption bandwidth. An increase in the angle of incidence also leads to a decrease in the absorption rate since the strength of the electric field decreases with the increase of the angle of incidence. As shown in Fig. 6(c), when the incidence angle is 3.75°, the two absorption peaks are greater than 0.4. Figures 6(d-e) show the electric field diagrams at the two peak values of the incident angle of 3.75°. Figures 6(f-g) show the electric field diagrams under the incident angle of 7.5°. We can see that as the incidence angle increases, the electromagnetic field enhancement exists not only near one silicon column, but also covering both silicon columns. This results in two distinct absorption peaks. Simulation results show that the resonant peak can be changed by adjusting the incidence angle [55,56].
3. Conclusion
In summary, we present an ultra-narrowband absorber platform via using asymmetric silicon dimer-resonators based grating. In the wavelength range of 3000 nm to 4000 nm, two narrowband absorption peaks with absorptivity greater than 99% are achieved by the strong optical electromagnetic resonances. Additionally, such absorber platform has been used for optical sensing, showing high-performance sensitivity factors for the wavelengths at λ1 = 3468 nm (S = 3193 nm/RIU, FOM = 532) and at λ2 = 3562 nm (S = 3120 nm/RIU, FOM = 390). Otherwise, the polarization and incident angles are observed with the capability to further tune the absorption responses in the intensity and wavelengths, indicating the feasibility for artificial manipulations. Such designed ultra-sharp perfect absorption and the related sensitive response hold the silicon based resonant scheme with wide applications in bio-sensing, subtractive filtering and others.
Funding
National Natural Science Foundation of China (62275112, 62065007, 11804134); Natural Science Foundation of Jiangxi Province (JXSQ2019201058).
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 maybe obtained from the authors upon reasonable request.
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