Sub-wavelength hole array (SHA) combined with thick metal layer (TML) is shown to have simultaneous suppressed transmission and reflection, resulting near-perfect absorption. Unlike the simultaneous electric and magnetic resonances in electric ring resonator and cut wire [PRL, 100, 207402 (2008)], such behavior results from strong anti-symmetric surface plasmons coupling supported by SHA and TML. The polarization-free characteristic permits to construct an ideal absorber for some practical applications in turbid backgrounds.
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Recent decade, Sub-wavelength hole array (SHA) has attracted much attention due to its exotic optical phenomena, such as extraordinary optical transmission , mimicking surface plasmons , and negative refractive index in dual-layers SHA [3,4]. In this paper, we demonstrate another interesting phenomenon of SHA when combined with thick metal layer (TML). It is the realization of simultaneous suppressed reflection and transmission, finally resulting near-perfect absorption in normal direction. Perfect absorption is proposed by Landry et al. [5–7]. In their experiments, there is a notable obstacle in practical application which is anisotropy introduced by electric ring resonator (ERR) . SHA combined with TML gives us permission to realize a more general design to overcome the obstruction and push perfect absorber (PA) into visible range. The proposed structure can be utilized as a convenient and effective PA due to the isotropy and sub-wavelength properties. Further, the single-layer SHA design largely simplifies the fabrication progress, and can be expanded to infrared and terahertz frequencies by geometrically scaling.
2. Structure design and simulations
Figure 1(a) shows the proposed structure consisting of a single-layer SHA and TML. The yellow and black parts is gold and SiO2, respectively. The unit cell shown by the dash lines has dimensions, in nanometers, of: L = 240, a = 100, te = 20, td = 40 and tm = 60. We perform the simulation using Finite Difference Time Domain (FDTD) method. In simulation, gold is described by Drude model to fit its realistic characteristic at visible frequencies [9,10]. Figure 1(b) shows the simulation spectra. Transmission is suppressed in overall range. The local maximum transmission is smaller than 0.4%. Minimum reflection is simultaneously observed at 642.7nm and 486.4nm, resulting two sharp peaks of absorption.
Although high absorption has been realized using an array of Tungsten wires on top of a Silicon-Nitride substrate in terahertz frequency , it is the first time that near-perfect absorption is observed in SHA combined with TML at visible frequencies. The parameters of the composite structure, including thickness of center dielectric layer (td), period (L) and thickness of SHA (te), are explored to figure out the physical origin of the near-perfect absorption.
Figure 2 and 3 show the simulated results. In order to simplify discussions, only near-perfect absorption at the first order is investigated. The second order near-perfect absorption has the same phenomena. First, the absorption peak moves to longer wavelength when period of SHA increases in Fig. 2(a). The dependence of period has excluded the possibility that near-perfect absorption in SHA combined with TML can be explained using simultaneous electric and magnetic resonances in unit cell as suggested in [5–7]. Second, the absorption peak shifts to shorter wavelength and is gradually close to λ0 = 590nm when the thickness of dielectric layer increases in Fig. 2(b). The maximum absorption happens at td = 40nm. This provides obvious evidence for that near-perfect absorption is not caused by only SHA, but by coupling supported by SHA and TML.
In Fig. 3(a), the absorption peak disappears gradually when the thickness of SHA increases. This can be explained using the physical model shown in Fig. 3(b). Due to the sub-wavelength characteristic of the metal holes, the field into SHA has exponential attenuation. When the thickness of SHA is much smaller the attenuation length as shown by the top figure in Fig. 3(b), most energy can enter the coupling region between SHA and TML. Correspondingly, little energy is reflected, such as the result when thickness of SHA is 40nm as shown in Fig. 3(a). Although partial energy is reflected with increasing thickness of SHA, there is still partial coupled energy as shown in Fig. 3(a). Finally, when SHA has enough thickness, such as 200nm as shown by the bottom figure in Fig. 3(b), energy cannot enter coupling region, which results that most energy is reflected back.
As proved above, parametric explorations have clarified that not simultaneous electric and magnetic resonances in unit cell, but coupling supported by SHA and TML dominates the behavior of SHA combined with TML. Now, we present the physical model of the observed near-perfect absorption. As a beginning, the current density of SHA combined with TML at 642.7nm is investigated. The results are shown in Fig. 4(a) . Apparent coupling supported by SHA and TML is observed. Second, the current density indicates that opposite electric charges are concentrated at the ends of holes, and neighboring TML has corresponding reversed distribution of charges as shown in Fig. 4(b). It is much analogous with anti-symmetric surface plasmons coupling in two neighboring metal layers . Thus, as a comparison, we calculate dispersion relationship of a “sandwich” structure which consists of two pure metal layers with thickness of 20nm and 60nm separated by a dielectric layer with thickness of 40nm. Figure 5 shows the dispersion diagram. The middle light line represents the anti-symmetric surface plasmons coupling mode of the “sandwich” structure. The red and green dash line shows the dispersion curve of first and second order near-perfect absorption, respectively. Apparently, the dispersion curve is spilt when periodic arrayed square holes are introduced into the top metal layer. The first order dispersion curve shifts to longer wavelength and the second order dispersion curve shifts to shorter wavelength. Correspondingly, two different distributions of magnetic density are shown by the inset figures in Fig. 5. So, it can be concluded that SHA combined with TML can effectively couples electromagnetic field into the center dielectric region. Distinguishing with perfect absorption based on ERR proposed in [5–7], SHA can be seen as a structured surface and supports suitable momentum matching to realize plasmons coupling, further to obtain perfect absorption when combined with TML.
To date, the physical mechanism of perfect absorption in this case is proved to be plasmons coupling supported by SHA and TML. Thus, the impressive results shown in Fig. 2 can be understood. As shown in Fig. 4(b), the distribution of electric charges in SHA and TML clearly demonstrates the coupling of the composite structure with normally incident light. Obviously, the increasing L will increase the distance of the two regions with opposite charges when plasmons coupling exists in SHA and TML. As a result, the wavelength of plasmons coupling is increased. Finally, the red shift of absorption peak with increasing L is observed as shown in Fig. 2(a). In Fig. 2(b), λ0 = 590nm is the electric resonance wavelength of SHA. The plasmons coupling will gradually degenerate into the pure electric resonance of SHA with the increasing distance (td) of SHA and TML. Simultaneously, there exists an optimal td between SHA and TML to maximize the absorption. This phenomenon can be explained as follows: first, the absorption rate will decrease with the degenerating of plasmons coupling between SHA and TML when td increases. Second, the decreasing of td will push R of TML to dominate the electromagnetic behavior of SHA combined with TML in the condition of normal incidence. SHA combined with TML will degenerate to be a simple metal layer with two-dimensional grooves when td is zero.
In conclusion, we show near-unity absorption of SHA combined with TML in normal direction. Anti-symmetric surface plasmons coupling formed by SHA and TML is proved to dominate the multi-orders near-perfect absorption. Although SHA is investigated in this case, the design idea: realizing perfect absorption based on structured surface combined with thick metal layer, can also be extended to other cases, such as ERR combined with metal black plate . The most fascinating potential application of SHA combined with TML is the introducing of functional material into center dielectric region to realize imaging and detecting. Benefiting from the narrow absorption band shown in Fig. 1 and the isotropy characteristic which indicates polarization-free, it is ideal to construct a low-noise receiver for some application under the complicated backgrounds, such as short-range battlefield communications and detecting organism or concealed explosives [13,14].
This work was supported by 973 Program of China (No.2006CB302900) and National Natural Science Foundation of China (No.60507014, No.60528003 and No.60778018).
References and links
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