Plasmonics DOI 10.1007/s11468-016-0459-z

Polarization-Independent Near-Perfect Absorber in the Visible Regime Based on One-Dimensional Meta-Surface Yun Zhou 1,2 & Minghui Luo 1,2

&

Linsen Chen 1,2

Received: 3 August 2016 / Accepted: 25 November 2016 # Springer Science+Business Media New York 2016

Abstract We present a polarization-independent near-perfect absorber in the visible regime based on one-dimensional meta-surface. In this absorber, the dielectric grating layer with high refractive index is essential, which supports strongly localized electromagnetic modes concentrated in the doublemetal regions. The simulation results show that absorption peaks are over 99% at the wavelength of 0.45 μm for both TM and TE polarizations. The near-perfect absorptions are attributed to the lateral plasmon cavity resonance and longitudinal Fabry-Perot-like cavity resonance for TM and TE polarization, respectively. Furthermore, theses absorption peaks can maintain almost unchanged within wide ranges of incident and azimuthal angles. Comparing with those meta-material absorbers with complex two-dimensional array and onedimensional stacked array, this one-dimensional absorber has a very simple geometrical structure which can be integrated into complex photonic devices expediently.

Keyword Absorption . Resonance . Surface plasmons . Cavity mode

* Minghui Luo [email protected] * Linsen Chen [email protected]; [email protected] 1

College of Physics, Optoelectronics and Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China

2

Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province and Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou 215006, China

Introduction Meta-surface absorbers, which are made of artificial periodic sub-wavelength structures, have attracted considerable interest because of their extensive applications, such as solar cells [1–3], plasmonic sensors [4, 5], photo-detectors [6, 7], and thermal emitters [8–11]. They are typically composed of the tri-layer structure: the top patterned metallic structure, the dielectric spacer layer, and the bottom metallic ground plane. Previous works involving the one-dimensional (1D) patterned metallic structure mainly focus on the absorption behavior on TM polarization due to the TM-polarized light can be very highly absorbed by exciting magnetic resonances or surface plasmon polaritons (SPPs) [12–15], while the TE-polarized light is completely reflected [16]. Thus, it is challenging to realize the polarization-independent perfect absorption by using only the 1D structure. To achieve the polarizationindependent behavior, relatively complex two-dimensional (2D) periodic structures [4, 17–21], which always possess fourfold rotational symmetry about the propagation axis or with two crossed 1D periodic structures are proposed. In recent years, polarization-independent near-perfect absorber incorporating 1D meta-structure has been demonstrated [22, 23]. Rui Feng et al. proposed a wide-angle and polarizationindependent absorber in the mid-infrared regime based on 1D stacked array [23]. However, the complex nanofabrication requirements of the above-mentioned complex 2D array and 1D stacked array may limit practical device applications. In this paper, we propose a polarization-independent nearperfect absorber in the visible regime based on 1D metasurface with three layers: the metal substrate, the dielectric grating, and the metal grating. Carefully choosing the geometries, this structure can highly absorb both TM- and TEpolarized light for the dielectric grating layer with high refractive index can support strongly localized cavity modes in the

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double-metal regions. What is more, it maintains high absorption within large incident and azimuthal angles. Comparing with those meta-material absorbers with complex 2D array structure and 1D stacked array, the proposed absorber not only achieves the better functions, but also is greatly simplified which can be easily integrated into complex photonic devices.

Results and Discussion Figure 1 shows the schematic of the proposed 1D absorber. The period of the dielectric grating is denoted by p = 0.162 μm. The ridge width of the grating is w = 0.038 μm. The thicknesses of the dielectric grating and the metal grating are d = 0.049 μm and h = 0.02 μm, respectively. The material of the metal is aluminum (Al). The dielectric constants of Al are fitted by the Drude-Lorentz model [24, 25]. In this structure, because the Al substrate is so thick (more than the skin depth) that there is no transmission through the system. The widely used gallium phosphide (GaP) is chosen to be the dielectric grating layer. Around the designed wavelength of 0.45 μm, the real part and image part of dielectric constant of GaP are 1.96 and 0.02, respectively. The TM- and TE-polarized lights are projected on the structure from the top air side at an angle of θ. The absorption is calculated by using the rigorous coupled wave analysis (RCWA) method [26]. Figure 2 shows the absorption spectra of the absorber for different polarization angles ranging from 0° (TM polarization) to 90° (TE polarization) at normal incidence. The absorption peaks for all polarization angles are at the same resonant wavelength of 0.45 μm, but with different spectral shape. For the TM polarization (polarization angle of 0°), the absorption spectra are a typical resonant peak with an asymmetric line shape and a full width half maximum (FWHM) of 0.08 μm. For the TE polarization (polarization angle of 90°), the absorption spectra possess a symmetric line shape with a narrower FWHM of 0.06 μm. The simulated absorptions have a maximum value of 99.17 and 99.46% for the TM and TE polarization, respectively, and the off-resonance absorptions are low. Although the proposed design is a 1D structure, it is very robust to the polarization angle around the designed wavelength. In order to understand the physical mechanism which gives rise to the near-perfect absorption for TM and TE polarizations, the electromagnetic field distributions at the absorption peaks are shown in Fig. 3 and Fig. 4, respectively. For TMpolarized light, the only three non-zero field components are Ex, Hy, and Ez with a coordinate system Oxyz oriented as in Fig. 3. The component Hy is essentially confined at the top and the bottom surface of the dielectric grating. The component Ex is strongly localized around the dielectric grating corners and the surface of the dielectric grating, as presented in Fig.3b. A standing wave pattern in the x-direction is clearly

Fig. 1 Schematic of the proposed absorber based on 1D meta-surface

visible in the contour -plots of Ez. The physical mechanism responsible for the near-perfect absorption of the TMpolarized light can be attributed to lateral plasmon cavity resonance. The excited surface plasmon wave propagates along the grating surface. Cavity modes in the double-metal region arise because of the strong impedance mismatch between the single-metal and double-metal regions which carry the TM0 mode [27]. For TE-polarized light, the only three non-zero field components are Ey, Hx, and Hz with a coordinate system Oxyz oriented as in Fig. 1. As shown in Fig. 4, the components Ey and Hz are essentially confined in the dielectric grating strip. A standing wave pattern in the z-direction is clearly visible in the contour-plots of Ey. The component Hx is located at the top and the bottom surface of the dielectric grating, as illustrated in Fig. 4b. For TE polarization, the surface plasmon wave cannot be excited. The physical mechanism responsible for the near-perfect absorption of the TE-polarized light can be attributed to longitudinal Fabry-Perot-like cavity effect [28]. The resonance wavelength at normal incidence is related to the longitudinal cavity length (i.e., grating thickness, d) approximately by λ = 4neffd/(2m − 1), where m is positive integer and neff is the effective refractive index of the metal-insulatormetal waveguide modes [29]. The electric field in Fig. 3c corresponds to the first excited resonance m = 1 under normal incidence (θ = 0°). Actually, the dielectric grating layer with

Fig. 2 Absorption spectra for different polarizations angles ranging from 0° (TM polarization) to 90° (TE polarization) at normal incidence

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Fig. 3 Calculated electromagnetic field distributions at the resonant wavelength of 0.45 μm at normal incidence for TM-polarized light

high refractive index increases the effective optical thickness of double-metal region, and finally, the TE-polarized wave is highly absorbed by exciting the cavity modes. Thus, as shown in Fig. 2, as the polarization angle increases, the absorption peaks are always very high, whereas the bandwidths become slightly narrow due to the absorption mechanism changing from the lateral plasmon cavity resonance to the longitudinal Fabry-Perot-like cavity resonance. Except the cavity modes, the nature of GaP also affects the absorption. As a semiconductor, the photon energy is well above its bandgap of 2.26 eV at the wavelength of 0.45 μm, which results in the strong absorption. Moreover, the calculated field distribution will be disturbed by the free carriers generated as a result of absorption. To further illustrate the feature of near-perfect absorption, the time-averaged power loss density dPloss/ dV = (1/2)ε0wIm{εAl(w)}|E|2generated by the structure for the two polarizations at the same resonant wavelength of 0.45 μm are also shown in Fig. 5 [12]. For TM polarization in Fig. 5a, most of the loss energy is found to be absorbed by the Al grating corners and the dielectric grating sidewalls. And a small portion of power is absorbed by the surface of the Al substrate. This feature is consistent with the plasmon cavity resonance effect (cf. Figure 3a and Fig. 3c) associated with the absorption peak. For TE polarization in Fig. 5b, the absorption occurs on the bottom surface of the Al grating and dielectric grating layer as well as on the top surface of the substrate, which gives a solid proof that the longitudinal Fabry-Perot-

like cavity resonance plays a key role in the absorptive behavior. Such strong resonances effectively trap light and provide sufficient time to dissipate it by the ohmic losses within metals. The absorption spectra on the normally incident TMpolarized light for different ridge widths (w), thickness (d), and refractive index (n) are shown in Fig. 6, respectively. As shown in Fig. 6a, two bright absorption bands can be observed, and the positions and bandwidths of absorption peaks change with the increase of w. The Hy field intensity distributions for the absorption peak at wavelength 0.45 μm corresponding to A2 with w = 0.105 μm has been shown in the inset of Fig. 6a. The Hy field intensity distributions of point A1 with w = 0.038 μm has been shown in Fig. 3a. It is clear that standing waves with different orders are excited in the dielectric grating layer, which further confirms the characteristics of the lateral plasmon cavity mode. The wider the ridge width w is, the higher order the cavity modes have. And it is noticed that the coupling between the resonators is weak in Fig. 3a. As w increases, cavity mode inside the double-metal region with different order is excited and the coupling between the resonators becomes stronger, as shown in the inset of Fig. 6a. The dependence of absorbance on the thickness (d) of the dielectric grating is shown in Fig. 6b. When d > 0.005 μm, the coupling between the two metal layers is very weak, so the resonance wavelength keeps unchanged. The dependence of absorbance on the refractive index (n) of the dielectric grating is shown in Fig. 6c. It can be seen from Fig. 6c that the near-

Fig. 4 Calculated electromagnetic field distributions at the resonant wavelength of 0.45 μm at normal incidence for TE-polarized light

Plasmonics Fig. 5 Contours of the timeaveraged power loss density dPloss/dV associated with the absorption peaks at λ = 0.45 μm a for TM polarization and b for TE polarization

perfect absorption emerge for the TM-polarized light when n is 3.2 and the position of the peak absorption can be effectively adjusted by n. The absorption spectra on the normally incident TEpolarized light for different thickness (d), ridge widths (w), and refractive index (n) are shown in Fig. 7, respectively. As shown in Fig. 7a, five bright absorption bands can be observed, and the positions and bandwidths of absorption peaks change with the increase of d. The Ey field intensity distributions for the absorption peak at wavelength 0.45 μm corresponding to B2 and B3 with d = 0.131 μm and d = 0.213 μm have been shown in the inset of Fig. 7a. It is clear that standing waves are excited in the dielectric grating strip, which confirms the characteristics of the longitudinal Fabry-Perot-like Fig. 6 Demonstration of geometric effect on the normally incident TM polarized light: a ridge width w, b the thickness d of the dielectric grating d and c the refractive index n. The insets of a show the Hy field intensity distribution for incident wavelength 0.45 μm at point A2 with w = 0.105 μm

cavity effect. The higher the thickness d is, the higher order the cavity modes have. The dependence of absorbance on the ridge width (w) of the dielectric grating is shown in Fig. 7b. When w > 0.05 μm, the resonance wavelength keeps almost unchanged. The dependence of absorbance on the refractive index (n) of the dielectric grating is shown in Fig. 7c. It can be seen that the near-perfect absorption emerge for the TEpolarized light when n is 3.4 and the position of the peak absorption can be effectively adjusted by n. It means that the polarization-independent near-perfect absorption can be achieved by choosing the high refractive index material of the dielectric grating layer. It is worth mentioning that the resonant wavelengths are independent from the period p, the periodicity of the system plays a crucial role in the efficient

Plasmonics Fig. 7 Demonstration of geometric effects on the normally incident TE polarized light: a the thickness d of the dielectric grating, b the ridge width w and c the refractive index n. The insets of a show the Ey field intensity distribution for incident wavelength 0.45 μm at point B2 and B3 with d = 0.131 μm and d = 0.213 μm

coupling of the incident photons into the cavities through the grating near field. Figure 8 shows the absorption spectra of TM- and TEpolarized light versus the incident angle θ. The geometric parameters are chosen as the same as those in Fig. 2. It is obvious that the peak position does not change, even though the absorption gradually decreases from about 99 to 75% and 72% for TM- and TE-polarized light with respect to the incident angle θ increasing from 0° to 60°, respectively. Thus, the absorber can work well within a wide incident angle for both TM and TE polarizations. The absorption spectra as a function of azimuthal angle at a fixed incident angle of 15° are presented in Fig. 9, which are more normal cases representing the illumination from all Fig. 8 The absorption spectra as a function of incident angle for the a TM and b TE polarizations

directions. The geometric parameters are also chosen as the same as those in Fig. 2. For the two polarizations, the absorption peaks remain very high for the azimuthal angle changes form 0° (the incident plane is perpendicular to the grating) to 90° (the incident plane is parallel to the grating). Thus, the absorber can work well within a wide range of azimuthal angle for both TM and TE polarization at small incident angle.

Conclusion In conclusions, we have proposed a polarization-independent near-perfect absorber based on 1D meta-structure. Absorption peaks of over 99% are simultaneously achieved at the

Plasmonics Fig. 9 The absorption spectra as a function of azimuthal angle at incident angle of 15° for the a TM and b TE polarizations

wavelength of 0.45 μm for both TM and TE polarizations. The physics mechanisms are attributed to the lateral plasmon cavity resonance and longitudinal Fabry-Perot-like cavity resonance for the TM and TE polarization, respectively. The absorber still has the high absorption above 72% as the incident angle is up to 60° for two polarizations. The angle-robust and polarization-independent light absorber based on 1D meta-structure can be potential candidate for a range of passive and active photonic applications, including solar energy harvesting, producing artificial colors, and other applications.

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Acknowledgements This work was supported by the Key Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (grant 14KJB140014, 14KJA510006), by the National Natural Science Foundation of China (NFSC) Major Research Program on Nanomanufacturing (Grant No. 91323303), the NFSC (Grant No. 61505134, 61575133, 91023044), the Natural Science Foundation of Jiangsu Province (Grant No. BK20140357, BK20140348), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20133201120027), the Science and Technology Project of Suzhou (Grant No. ZXG201427, ZXG2013040), the project funded by Soochow University (Grant No. SDY2012A18), and the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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