1. Introduction

Perfect absorbers based on high-refractive index dielectric nanostructure metasurface have attracted a lot of attention due to their outstanding perspectives such as low nonradiative loss and resonant enhancement of both electric and magnetic field, which are promising in the nanophotonic area of optical sensing [1], filtering [2] and energy harvesting [3–5]. Since the Mie resonances of a nanoparticle are firstly analyzed in detail by Lewin [6], the scattering behavior of dielectric nanostructure metasurfaces supporting Mie type resonances have been widely researched [7–13]. Recently, the silicon nanopillar metasurface set on a total internal reflection configuration has been experimentally demonstrated by Länk’s group, which achieves a polarization-independent near-perfect absorption due to interference between coherent electric and magnetic dipole scattering from the silicon nanopillars array and the total internal reflected field [14]. Besides, amorphous silicon(a-Si) is considered as an outstanding alternative approach to gain high absorption, and Yang’s group has proposed and fabricated an a-Si nano antenna metasurface, which realizes a nonradiating narrow band perfect absorption with a higher quality factor than that of the plasmonic absorber [15]. For the purpose of realizing extended high absorption devices, it is highly desired to construct a nanostructure metasurface with simple geometries.

In this paper, a dielectric near-perfect absorber with a broadened absorption band based on an amorphous silicon T-shaped nanostructure metasurface is numerically studied and experimentally analyzed. By investigating interference of periodically adjustable electric dipole(ED) and magnetic dipole(MD) Mie resonances, the transmission and reflection of a-Si metasurface are simultaneously suppressed. The absorption spectrum of a-Si metasurface under normal incidence is simulated by the finite-difference time-domain (FDTD) solutions, in which enhanced absorption approaches the maximum of 95% with a broadened bandwidth of 40 nm by employing different size rectangles in a T-shaped nanostructure. The a-Si T-shaped nanostructure metasurface was fabricated by electron-beam lithography process and silicon etching system, of which the experimental near-perfect absorption measured by a spectrometer arrives 80% with a top-hat shape in the spectral range from 580 nm to 620 nm. The fabricated broadened band near-perfect absorber without coupled prism is a potential candidate for nanophotonic applications in area of optical isolation, optical trapping and energy harvesting in visible region.

2. Structure and theoretical background

The schematic of the near-perfect absorber based on dielectric metasurface is shown in Fig. 1(a), which is an a-Si T-shaped nanostructure periodical array set on the SiO$_2$ substrate. The unit cell of the periodical array, which contains different size rectangles in an a-Si T-shaped nanostructure, is shown in Fig. 1(b). The widths of the a-Si T-shaped nanostructure are $w_1$ = 45 nm, $w_2$ = 70 nm, $w_3$ = 45 nm, and the heights are $l_1$ = 110 nm, $l_2$ = 60 nm. The periods of the unit cell are $D_x$ and $D_y$ in the x and y direction, respectively. The thickness of the a-Si film is $h$ = 110 nm. The a-Si T-shaped nanostructure periodical array is set on the SiO$_2$ substrate with refractive index $n_{SiO_2}$ = 1.528 and is submerged in immersion oil with the refractive index $n_{Oil}$ = 1.485. The refractive index $n$ and extinction coefficient $\kappa$ of the a-Si film are measured by spectroscopic ellipsometry apparatus, which is shown in Fig. 1(c).

 

Fig. 1. (a) Schematic of the a-Si near-perfect absorber. (b) Top view of the unit cell. (c) The refractive index $n$ and extinction coefficient $\kappa$ of the a-Si film.

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The scattering behavior of the a-Si T-shaped metasurface near-perfect absorber is analyzed based on the discrete dipole approximation(DDA). According to DDA, the electric dipole(ED) moment and magnetic dipole(MD) moment are numerically calculated from the light-induced displacement current simulated by FDTD in the a-Si T-shaped nanostructure which is in the periodical array set on the SiO$_2$ substrate and submerged in the immersion oil. Under the illumination of x-polarized incident light, only the ED moment $p_x$ and the MD moment $m_y$ contribute to the light scattering in the far-field approach due to the periodicity of the metasurface [16]. The reflection(r) and transmission(t) coefficients of the near-perfect absorber are derived from the light scattering of the light-induced point source dipole moments of the a-Si T-shaped nanostructures in the far-field approach:

3. Simulation and numerical analysis

The ED and MD resonances of the a-Si T-shaped nanostructure metasurface are numerically analyzed based on the discrete dipole approximation with different periods. $\alpha _{eff}^E$ and $\alpha _{eff}^M$ are numerically calculated in the spectral range from 500 nm to 700 nm, which is shown in Fig. 2. When $Re(\alpha _{eff}^E)$ is around zero shown by the brown dashed curve in Fig. 2(a), $Im(\alpha _{eff}^E)$ has two adjacent peaks with low degeneracy where the ED resonances are excited shown by the brown solid curve in Fig. 2(a). Similarly, $Im(\alpha _{eff}^M)$ has a single degenerate peak where the excited MD resonance of the a-Si metasurface is achieved shown by the black solid curve in Fig. 2(b). Both degenerate ED and MD resonance peaks have broadened bandwidths by employing different size rectangles in an a-Si T-shaped nanostructure. With the increasing period in the y direction $D_y$, the ED resonance peaks red shift, while the MD resonance peak keeps almost unchanged, as shown in Fig. 2(a) and Fig. 2(b), respectively. The MD resonance peak red shifts with the increment of the period in the x direction $D_x$, while the ED resonance peaks remain almost unchanged, as shown in Fig. 2(d) and Fig. 2(c), respectively. When the periods of the a-Si metasurface are $D_x$ = 300 nm and $D_y$ = 350 nm, $Re(\alpha _{eff}^E)$ and $Re(\alpha _{eff}^M)$ both approach zero at the wavelength around 600 nm, as shown by the brown dashed curve in Fig. 2(a) and the black dashed curve in Fig. 2(b), meanwhile the maximums of both $Im(\alpha _{eff}^E)$ and $Im(\alpha _{eff}^M)$ are approximately equal to $S_L/k_0$, which satisfies Eq. (2). The spectral positions of ED and MD resonance peaks are independently tuned by the period of the a-Si metasurface in different directions, which is due to the interaction between the T-shaped nanostructures.

 

Fig. 2. $\alpha _{eff}^E$ and $\alpha _{eff}^M$ of the a-Si metasurface with different periods. (a) and (b) correspond to $\alpha _{eff}^E$ and $\alpha _{eff}^M$ with varied $D_y$, respectively; (c) and (d) correspond to $\alpha _{eff}^E$ and $\alpha _{eff}^M$ with varied $D_x$, respectively.

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The optical spectra of the a-Si T-shaped nanostructure metasurface are simulated by FDTD solutions with different periods. The transmission(T), reflectance(R), and absorption(A) spectra at optimized T-shaped nanostructure parameters with continuously changing period $D_y$ are presented in Fig. 3(a-c), respectively. The transmission spectrum has a red shifted broadband suppression at wavelength longer than 540 nm and an broadened fixed suppression at wavelength from 580 nm to 620 nm, as shown in Fig. 3(a), corresponding to the excitation of ED resonance and MD resonance, respectively. When $D_x\approx$ 300 nm and $D_y\approx$ 350 nm, the ED resonance overlaps with the MD resonance in the spectral range from 580 nm to 620 nm where the reflection spectrum is suppressed and the absorption spectrum rises up to $95.0\%$ because of the interference of ED and MD resonances, as shown in Fig. 3(b) and Fig. 3(c), respectively. Furthermore, the electric field distributions and displacement currents are simulated by the FDTD solutions inside the a-Si T-shaped nanostructure in the x-z plane. The simulated electric field distributions at the overlapped wavelength of ED and MD resonances are enhanced in both size a-Si rectangles, as shown in Fig. 4(a), which are stronger than those at the non-overlapped wavelengths in Figs. 4(b)-(c). The enhanced displacement currents at overlapped spectral position shown in Fig. 4(a) result in the localization of electric fields, which are linear superposed by the horizontal and circular displacement currents indicative of ED and MD resonances as shown in Figs. 4(b) and 4(c), respectively. By tuning the periods and optimizing the geometrical parameters of the a-Si T-shaped nanostructure metasurface, the ED and MD resonances overlap and interfere with each other, leading to the enhanced electric field localization, resulting in the suppression of both transmission and reflection as well as the absorption enhancement of the a-Si metasurface.

 

Fig. 3. The simulated (a) transmission, (b) reflection, and (c) absorption spectra of the a-Si metasurface with different periods $D_y$. $D_x$ is fixed at 300 nm.

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Fig. 4. The electric field distributions with displacement currents in the xz-plane of the upper and lower rectangles in a T-shaped nanostructure: (a) overlapping of ED and MD resonances; (b) ED resonance and (c) MD resonance.

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4. Experiment

The 110 nm-thick a-Si film was deposited on the SiO$_2$ substrate by PECVD system. The a-Si T-shaped nanostructure metasurface were fabricated by electron-beam lithography process and silicon etching system. The scanning electron microscopy(SEM) images of the fabricated a-Si T-shaped nanostructure metasurface are shown in Fig. 5(a). The geometrical parameters of the a-Si T-shaped nanostructure are $w_1=w_3\approx$ 44nm, $w_2\approx$ 72 nm, $l_1\approx$ 110 nm and $l_2\approx$ 60 nm. The periods of the fabricated a-Si T-shaped nanostructure array are $D_x\approx$ 299 nm, $D_y\approx$ 349 nm. The total pattern size of the fabricated metasurface is $\sim$ 180$\mu$m $\times$210$\mu$m.

 

Fig. 5. (a) The scanning electron microscopy image of the fabricated a-Si T-shaped nanostructure absorber. (b) The measured reflection, transmission and absorption spectra are presented by the blue, yellow and red curves, respectively.

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The a-Si T-shaped nanostructure metasurface was submerged in immersion oil(n = 1.485). The transmission(T), reflection(R) and absorption(A) spectra of the fabricated a-Si metasurface are measured by a convergent spectrometer, as shown by the yellow, blue and red dashed curves in Fig. 5(b), respectively. The absorption of the fabricated metasurface reached the maximum around 80% in the spectral region from 580 nm to 620 nm. By employing different size rectangles in a T-shaped nanostructure, the near perfect absorption reveals a broadened band of 40 nm with a top-hat shape. The reflection and transmission spectra of the fabricated metasurface are both suppressed because of the Kerker effect and enhanced electric fields localization, respectively, but both little higher than those in optimized simulation in Fig. 3. Correspondingly, the near perfect absorption in experiment is less than that in optimized simulation, which is caused by the convergent incident light and the fabrication deviations.

Although the absorption bandwidth of the near perfect absorber in our experiment is 40 nm, it will be further extended by integrating more nanostructures in the unit cell of the metasurface. With the increment of the size of the T-shaped nanostructures, the spectral positions of the ED and MD Mie resonances of the a-Si metasurface are red shifted, but the perfect absorption peak is hard to be achieved due to the low extinction coefficient of the deposited a-Si film at the wavelengths longer than 700 nm. By utilizing other dielectric such as germanium, the perfect absorption can be realized in the short-wave near-infrared-region (700nm-1100nm) where the extinction coefficient is higher than that of silicon.

5. Conclusion

In summary, a broadened band near-perfect absorber implemented by a periodical a-Si T-shaped nanostructure metasurface on the SiO$_2$ substrate is studied numerically and experimentally. Based on the discrete dipole approximation, the electric dipole and magnetic dipole moments of the T-shaped nanostructureu are calculated by the simulated displacement current. Moreover, simultaneous suppressed reflection and transmission are obtained by exploring the interference of the periodically adjustable ED and MD Mie resonances. The simulated absorption spectrum of the a-Si absorber approaches the maximum of 95% with a broadened bandwidth of 40 nm by utilizing T-shaped nanostructure. The near-perfect absorption of the fabricated a-Si metasurface achieves 80% with a top-hat shape in the spectral range from 580 nm to 620 nm, which will be improved by the precise nanofabrication process. The a-Si T-shaped nanostructure near-perfect absorber has the characteristics of the broadened absorption band, response in the visible region and no necessity of coupling prism, which offers avenues of manipulating light at nanoscale of optical isolation and optical trapping.