1. Introduction

With the rapid development of nanophotonics techniques, nanostructures with the excitation of surface plasmon polaritons (SPPs) have attracted increasing attention in the past decade. SPPs are essentially electromagnetic waves which travel along a metal-dielectric interface. By introducing periodic holes or slits into the metallic layer, the properties of the excited SPPs could be tailored, which enables the whole nanostructure to own the unique capabilities in control of light [1–7].

Recently, much attention has been drawn to improve the transmission through a seamless metallic film, due to the application of transparent conducting metal in optoelectronic devices [8–11]. Different from the extraordinary optical transmission discovered by Ebbesen et al. [1], considerable light transmission is desired here without any nanoholes or nanoslits directly perforated on a metallic layer to fully retain electronic and mechanical features of metal. Although a continuous metallic film itself is of high conductivity, it is opaque for visible and infrared light. Even if the thickness is only several tens of nanometers, most of incident energy would be reflected and the transmitted is very weak. It has been reported that transparency of a continuous metallic film could be achieved with the assistance of SPPs resonance. For instance, broadband transparency for a seamless Ag film was realized in a multiple-layered nanostructure with two perfect blackbodies [12, 13]. By employing subwavelength metal cylinder [14] or sphere [15] arrays on both surfaces, near-unity transparency through a continuous Au film could be obtained. However, all these structures are difficult to be fabricated because of highly symmetric structures on both sides of the metal film.

Experimental enhanced transmission with polarization-sensitivity from insulator-metallic grating architecture has been demonstrated by Nazarova et al. in [16]. Diffraction gratings coated with Al [16], then Au [17, 18] and Ag [18] were studied and extraordinary transmission through the continuous metal film was observed. In our previous work, a seamless Ag film covered by a dielectric layer and metallic grating has been theoretically demonstrated to achieve enhanced transmission in both visible and near-infrared regions [19]. Here, enhanced transmission with polarization-sensitivity and wide-angle operating is experimentally realized based on this grating-insulator-metal (GIM) architecture. The transmittance of the proposed GIM structure reaches 40% at 930nm, showing 3.7 dB and 9.1 dB increase compared with a bare gold film and a flat metal-insulator-metal (MIM) stack, respectively. The enhanced light transmission primarily benefits from the strong near-field coupling of the magnetic resonances between the metal array and the metal layer. Further theoretical calculation shows that the transmission peak can be tuned by varying the structure parameters.

2. Simulation and experiment

The proposed nanostructure with enhanced transmission consists of a dielectric layer deposited on a seamless gold film with a periodic gold grating on the top, which is termed as a GIM structure in this paper, as illustrated in Fig. 1(a). To reveal and verify the enhanced transmission effect of the GIM structure, a traditional MIM stack and a gold layer with same thickness covered by a dielectric layer (IM) are also prepared as references [Fig. 1(b)]. The thickness of the gold grating, Al₂O₃ dielectric layer and the bottom gold film are denoted as t, d and h, respectively. The width and period of the grating strip along the grating vector direction are represented as w and a. Right angles are modified to fillets (radii are set as r = t/2) to approximate the cross-section of the fabricated nanostructures. The whole structure is on a glass substrate and illuminated by normal incident light, which is linearly polarized and either perpendicular (denoted as TE) or parallel (denoted as TM) to the grating vector. In the experiment, a continuous three-layer MIM stack is first prepared by electron beam evaporation and then grooves are structured into the top gold layer using focused ion beam (FIB) milling system to form the metallic gratings [Figs. 1(c) and 1(d)]. The period number for the metallic grating is 200. For the theoretical calculation, we use the commercial software COMSOL MULTIPHYSICS based on finite element method to model the GIM nanostructure. The permittivity of gold is extracted from the experimental data by Johnson and Christy [20]. The refractive indices of Al₂O₃ and the substrate are set as 1.75 and 1.45, respectively. The geometric parameters are set as follows: t = 10 nm, d = 17 nm, h = 20 nm, a = 100 nm, and w = 65 nm. One unit cell with periodic boundary conditions are used to simulate infinite gratings. In reality, no grating has infinite periods. However, as validated by our simulations, when the total period number of the grating is above 20, the transmission results are almost the same as those for the grating with infinite number of periods.

 

Fig. 1 Schematic of (a) GIM, (b) MIM and IM samples. (c) and (d) show the top view and side view of the GIM sample etched by FIB, respectively. Scale bars are 300 nm.

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Figure 2 shows both theoretical and experimental results of the enhanced transmission of the proposed GIM structure with the other two references. For the TM wave, the experimental maximum transmittance reaches 40% at 930 nm with an obvious enhancement compared with the referenced IM and MIM structures. While for the TE wave, no enhanced transmission can be obtained and the transmittance of the GIM structure is even lower than IM. In the situation of TE wave, the electric field is always perpendicular to the grating vector and thereby the SPP cannot be excited. For the symmetrical geometry such as the IM and MIM structures, the transmission is almost the same under TM and TE polarizations. Here, the simulated and experimental results coincide with each other very well. The remaining slight deviations can be attributed to diverse profiles of the gratings at different areas in the experiment, while identical fillets are used in the simulation. Additionally, surface roughness and thickness fluctuation of gold films also contribute to the deviations.

 

Fig. 2 Transmission of the designed structure as a function of wavelength in comparison of experiment and simulation results under (a) TM and (b) TE mode.

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The transmission enhancement ratio of the GIM structure to the referenced IM and MIM structures are depicted in Fig. 3(a). At 930 nm, 3.7 dB and 9.1 dB increase in transmittance for the GIM structure compared to IM structure and MIM structure can be obtained, respectively. In order to reveal the underlying physics behind enhanced optical transmission, the electromagnetic field distributions for the resonant modes are investigated. Figures 3(b) and 3(c) illustrate the normalized magnetic field maps inside the devices at 930 nm. Enhanced transmission can be clearly seen especially through the comparison in Fig. 3(c). For the MIM nanostructure composed of three continuous films, transmitted energy becomes very low after incident light attenuated twice. However, SPPs can be excited effectively when metallic gratings are introduced, which induces strong magnetic resonance between two metallic layers in the GIM nanostructure. A destructive interference between ground plate reflection and magnetic dipole radiation is generated [21], thereby cancelling reflection of the whole structure; meanwhile, the significant electromagnetic response inevitably causes a few resistive losses. It is evidenced in simulations that reflection is reduced to only 9% and about 44% of incident power is absorbed by the GIM nanostructure at resonant wavelength. In other words, a GIM architecture nanostructure has the ability to operate as an anti-reflection film and the thickness of bottom metal layer determinates energy distribution, resulting in either near total absorption or efficient transmission. The discussion in this article belongs to the latter as the bottom Au film is not thick.

 

Fig. 3 (a) Transmission enhancement ratios of GIM to MIM and IM. The vertical black dashed line indicates the position of resonant wavelength. (b) The normalized magnetic field maps for GIM and MIM devices at 930 nm. (c) Magnetic field maps where the maximum of the color bar is half of that in (b) so that the resonance in GIM is saturated and the transmission can be clearly seen.

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The enhanced optical transmission of the proposed GIM nanostructure is also robust for oblique incidence, which has been discussed in detail in [19]. In fact, the transmission peak always exists if incident electric fields possess the component parallel to the grating vector. Figure 4 shows one situation where the incident plane is yz plane, and the magnetic field is always parallel to the x axis. With the increasing of incident angle θ, the transmittance declines slightly in the experiment, meanwhile, the resonance wavelength is almost the same. The transmittance remains above 30% even when θ is as large as 40 degree.

 

Fig. 4 Transmission spectra of GIM according to incident angle θ in a case of oblique incidence where the magnetic field is always parallel to the x axis.

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Further studies about operating properties of the GIM device are theoretically discussed in Fig. 5. The resonance peaks could be easily tuned in the range of specific wavelengths by varying geometric parameters. As illustrated in Fig. 5(a), a blue-shift of peak wavelengths and rise of transmittances are simultaneously induced when the thickness of continuous gold film becomes thick from 5 nm to 20 nm. Figure 5(b) indicates that increasing the spacer thickness d also leads to the blue-shift of the transmission spectrum. The influence of grating height h on transmission is presented in Fig. 5(c). By increasing h from 10 nm to 25 nm, a remarkable red-shift can be seen with the growth of transmittance. Moreover, it can be obtained from above that the transmission effect is sensitive to the thickness of metallic layer or grating and relatively insensitive to the spacer thickness.

 

Fig. 5 Variation of transmission spectra toward different geometric parameters.

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3. Summary

In summary, the enhanced optical transmission through a seamless gold film with grating-based coating is investigated. The proposed GIM nanostructure shows enhanced transmission compared with a bare gold film and a traditional MIM stack. The enhanced transmission could be tuned by varying the geometric parameters. It also remains robust at large oblique incident angle. Such a device retains high conductivity of metal and has high transmission performance. These desirable features exhibit potential applications in optoelectronic fields.