In this paper, we propose peelable thin films containing crescent shaped split-ring resonators (SRRs) that can be utilized for the fabrication of three-dimensional (3D) metamaterials. The SRRs were two-dimensionally fabricated by nanosphere lithography with closely packed polystyrene nanospheres in a large area prepared by electrostatic adsorption. The SRR monolayer film, which had macroscopic dimensions of over 10 × 10 mm2 (area) and a thickness of about 230 nm, was then obtained by using a separation technique. Once the SRR monolayer films were fabricated, multi-layered metamaterials were obtained via repetition of a simple stacking procedure. In this case, we fabricated a 4-layered SRR film with the same relative orientation for each layer. In order to investigate the optical characteristics of these multi-layered SRR films, transmission spectra for normal incident light were measured. The transmission spectra demonstrate that our multi-layered metamaterials operate in the near-infrared region.
© 2016 Optical Society of America
Metamaterials are artificial media that consist of an enormous number of unit cells, whose sizes are much smaller than the wavelength of electromagnetic waves. They are designed to possess novel electromagnetic properties that cannot be obtained using conventional materials found in nature.
Both the permeability and permittivity of a metamaterial can be manipulated so as to take on negative values, and thus the refractive index can be made negative as well . Such metamaterials can be adapted for use as perfect lenses  and in cloaking . In order to realize the above properties and associated devices, the bulk metamaterial must first be fabricated. There are certain types of unit cells that metamaterials consist of, e.g., the rod pair [4, 5], the double fishnet [6,7] and the split-ring resonator (SRR) . The SRR, in particular, is an attractive structure for inducing a strong magnetic resonance among the structures mentioned above. The SRR was first reported by Pendry et al. , and the structure has attracted much attention on account of its artificial magnetism. A single SRR structure induces magnetic resonance around the LC resonance frequency and changes the permeability of its bulk metamaterial. Recently, many studies have investigated metamaterials that operate in visible (VIS) region. However, most investigations were concerned with two-dimensional (2D) structures; that is because it becomes difficult to fabricate three-dimensional (3D) bulk metamaterials as one reduces the operating wavelength.
The study of metamaterials began in the microwave region as a result of the ease of fabrication for such structures; in this regime, 3D bulk metamaterials have already been observed to demonstrate negative refraction . In the THz region, 3D bulk metamaterials that exhibit a magnetic response were fabricated by stacking together plastic films (100 μm thick), on which the SRRs were fabricated in two dimensions (2D), as reported by Miyamaru et al. [10, 11]. On the other hand, 3D metamaterials operating in the infrared region have been fabricated by way of the serial exposure technique using electron beam lithography (EBL) .
In order to fabricate 3D metamaterials with SRRs operating in the VIS or near-infrared (NIR) region, it is necessary to reduce the size and interval of the SRRs to around 100 nm. Many research groups employ EBL to fabricate metamaterials operating in the optical region [13–16]. However, the EBL procedure is quite time-intensive when fabricating patterns across a large area, and is therefore an inefficient method for fabricating metamaterials that require a large number of structures. In our previous report, we demonstrated that a single narrow gap crescent-shape SRR fabricated by nanosphere lithography (NSL) could operate in the VIS-NIR region [17, 18]. According to the NSL method, only nanospheres are used for the template, and thus the fabrication procedure is comparatively easy, and boasts a high throughput. On the other hand, using this method, it is difficult to control the distance between nanospheres. It is not necessary to arrange the unit cells periodically because the characteristics of the metamaterial depend on the characteristics of the individual structure of which it composed. Thus, if the densely packed and isolated nanospheres can be easily prepared in a large area, then the NSL method becomes the most efficient method for 3D fabrication.
Recently, densely packed arrangements of crescent-shaped metal structures were fabricated by NSL, with the nanospheres being prepared by self-assembly. Furthermore, 3D fabrication was achieved via repeated SRR preparation, and embedding in the silica layer . In this fabrication process, the hexagonally close-packed nanospheres contact each other; it is therefore necessary to make space for SRR fabrication by reducing the size of nanospheres with reactive ion etching (RIE). In addition, SRR fabrication must be conducted on a rough surface, in which the SRRs are embedded.
In this paper, we develop a method of fabricating 3D metamaterials that operate in the optical region using a stacking technique. In our fabrication process, a large area of densely packed nanospheres is prepared using electrostatic adsorption . The isolated nanospheres are prepared directly through an immersion procedure, without using RIE to reduce the size of nanospheres. The narrow gap crescent-shape SRR fabrication procedure and the stacking procedure are conducted separately. In this way, all SRRs are always fabricated on a flat surface without any irregular influences, so that a stable shape and array can be obtained. A multi-layered SRR film is obtained by stacking the polymer thin films containing the SRRs. In order to investigate the optical characteristics of our multi-layer metamaterials, transmission spectra are measured.
2.1 Fabrication method
Figure 1(a) shows the formation of the sacrificial layer and the distribution of the polystyrene (PS) nanospheres by an electrostatic adsorption method . First, poly(4-hydroxystyrene) (PHS) in ethanol was spin-coated on a Pyrex glass substrate, which serves as a sacrificial layer for film separation . Next, the PHS surface was treated by RIE with O2 to obtain a negatively charged surface (RIE time: 8 s, Power: 150 W, O2 Gas flow: 80 sccm). The substrate on the PHS was alternately immersed in a poly(diallyldimethylammonium chloride) (PDDA) solution and a poly(styrenesulfonic acid) sodium salt (PSS) solution where each solution acted as a polycation and a polyanion, respectively. After repetition of the alternating cycle, the substrate was immersed in another PDDA solution. Thus, a pretreated substrate with a positively charged surface was obtained. Next, the pretreated substrate was immersed into an aqueous dispersion of a PS nanosphere with a diameter of 100 nm at a nanosphere concentration of 0.1wt%. In PS colloidal solution, because the surface of the PS nanospheres is negatively charged, they are mutually separated by an electrostatic repulsive force in demineralized water. In order to overcome this, PS nanospheres were adsorbed on the pretreated surface by an electrostatic attractive force by keeping a distance between each other. Thus, densely packed PS nanospheres were put on the substrate. Figure 1(b) shows the fabrication of SRRs by NSL. A 40-nm-thick gold layer was deposited at an oblique angle (about 35°). The excess gold was then removed by argon ion milling at an angle perpendicular to the surface (Milling time: 150 s, Power: 150 W, Ar gas flow: 40 sccm). Thus, narrow gap crescent-shape SRRs with PS nanospheres were successfully fabricated.
In the second step, SRR film separation was conducted according to the process illustrated in Figs. 1(c) and 1(d). The SRRs on the PHS layer were coated with a transparent polymer film by spin-coating, as shown in Fig. 1(c). PS was employed as a transparent polymer in order to embed the SRRs in a homogeneous material. In order to obtain a suitable surface for the next stacking layer, the roughness of the coated surface must be decreased at this stage. The film thickness is also important, as it determines the distance between SRRs in each layer. Next, the substrate was immersed in ethanol in order to dissolve the PHS layer. Subsequently, the substrate was immersed in water in order to separate the SRR film. Then, the SRR film floated on the surface of the water because of its hydrophobicity, as shown in Fig. 1(d).
In the third step, the SRR film was stacked by scooping the SRR film from the surface of the water, as shown in Fig. 1(e). Before stacking, the surface of the former PS film was treated by RIE with O2 to obtain a hydrophilic surface so that the scooped film was able to stick to the former film. To evaporate the residual water and to realize the adhesion of the film on the substrate and the stacked film, the substrate was placed on a hot plate and heated to about 150 °C, which is above the glass transition temperature of PS, for a minimum of 5 h.
According to this method, once some SRR films have been fabricated, the 3D metamaterial is realized through simple repetition of the stacking procedure.
2.2 Experimental results
Scanning electron microscopy (SEM) images of SRRs fabricated on the PHS layer before PS coating are shown in Fig. 2(a). The average outer diameter of each SRR was 132 nm. Variability in the outer diameter of SRRs is caused by the size distribution of nanospheres. The average distance of each SRR was about 80 nm, which was estimated using the density of SRRs. As described before, the PS spheres are mutually separated by an electrostatic repulsive force in demineralized water. In general, the spacing of the nanosphere narrows when colloidal solution has a high density. Therefore, the average spacing of SRR can be narrowed by increasing the density of the PS colloid in the relevant step shown in Fig. 1(a). However, the spacing of the nanosphere must be limited to a particular distance so as to not overlap with the sphere shadow during deposition in order to successfully fabricate SRR structures.
Figure 2(b) shows an SRR film maintaining its shape while separated on the surface of the water. The SRR film with an area greater than 10 × 10 mm2 floated on the water surface. The shape of the SRR film was maintained by the surface tension even though it was very thin (about 230 nm thickness). The consistently dark surface demonstrates that SRRs are densely packed. The separated SRR film can be transferred from the water to anywhere required. In order to demonstrate this, the SRR film was stacked on a fingernail, to which it adhered quite well because of its thinness and flexibility, as shown in Fig. 2(c).
For multi-layered SRR films, the surface of each SRR film should be as flat as possible. The surface topography before and after embedding was therefore measured by atomic force microscopy (AFM), the results of which are shown in Fig. 3. Before embedding, the height of the top of the SRRs with nanospheres is about 100 nm, as shown in Fig. 3(a). To examine the planarization of the SRR film surface via the PS film, some kind of concentration of PS in chloroform was spin-coated on SRRs. As a result, after embedding via the 1.5wt% one, the roughness of the SRR film surface was reduced within 5 nm, which appeared to be quite sufficient for the SRRs in the next layer, which was arranged in parallel under the SRRs, as shown in Fig. 3(b).
By stacking the SRR films with the same relative orientation, a 4-layered SRR film was fabricated. A part of the 4-layered SRR film was milled with a focused ion beam (FIB) in order to observe the cross-sectional surface. An SEM image of the cross-section of the 4-layered SRR film is shown in Fig. 4(a). SRRs in each layer are arranged in parallel, as shown in Fig. 4(b). This demonstrates the each SRR film surface was sufficiently flat. The SRR film thickness was about 230 nm. The distance between SRRs in each layer was about 160 nm.
3. Optical measurements
In order to investigate the optical properties of multi-layered SRR films, the transmission spectra were measured at normal incidence. The measurement range was from λ = 490 nm to λ = 1650 nm using a multichannel spectrometer. The transmission spectra were normalized by the transmission spectra of the Pyrex glass substrate. We first investigated the transmission characteristics of multi-layered (i.e., 1–4 layers) SRR films with the same orientation for all layers.
Figure 5 shows the transmission spectra of multi-layer SRR films with different numbers of layers. Two resonant dips appear in the transmission spectra for both Ex and Ey polarizations, each defined in the inset of Fig. 5. This polarization dependence of the spectra is consistent with the characteristics of isolated, single SRRs investigated in our previous studies [17, 18]. For the Ey polarization, the resonant dips at around λ = 770 nm and λ = 550 nm are the result of fundamental and second-order localized surface plasmon (LSP) modes, respectively. For the Ex polarization, the resonant dips at around λ = 1090 nm and λ = 660 nm are the result of fundamental and second-order LC resonance modes, respectively. In fundamental and second-order LSP modes and second-order LC resonance modes, the rate of increase of the depth of the dip was almost linear. This phenomenon follows the Lambert–Beer's law. On the other hand, the rate of increase of the depth of the resonance dip of fundamental LC resonance was not linear. In addition, the dip wavelength shifts, and the full width at half maximum increases. Such a spectrum distortion occurred when the layer number was three or more.
One of the potential causes of this distortion is a mutual electromagnetic interaction that occurs between the SRRs in different layers. Another cause is Fabry–Perot interference. It is reported that the resonance spectrum changes if the distance between metallic nanostructures is smaller than the structural diameter [15, 22]. In this sample, the SRR in the upper layer is not necessarily directly above the SRR of the lower layer because the distribution of the SRR in the plane is random. According to Fig. 4(b), the spacing of upper and lower SRR is at least 160 nm or more. The diameter of the SRR is approximately 130 nm, and the thickness is 70 nm or less. Therefore, the interaction might be weak. On the other hand, the Fabry–Perot interference will influence the spectrum by increasing the layer number. Therefore, we are predicting that the Fabry–Perot resonance contributes to the spectrum distortion that appears in Fig. 5(a). The distribution of SRR that we made was random in the film. Therefore, at present, because of the complex nature of the interaction between SRRs, an accurate discussion cannot be done. We are currently executing a simulation and some further experiments are planned, based on which we can clarify the optical properties in the near future.
We established a fabrication procedure for peelable polymer films that contain narrow gap crescent-shape SRRs. In addition, we succeeded in composing a 3D metamaterial by using our technique for depositing the films. The SRR monolayer films have macroscopic dimensions of over 10 × 10 mm2 (area), and a thickness of about 230 nm. Our low-cost and high-throughput method is advantageous because, once SRR films are fabricated, 3D metamaterials can be obtained by repetition of the stacking procedure.
In this investigation, we simply stacked SRR films on a flat substrate. However, we suspect that a more intriguing extension of these peelable films concerns their free transportability and high flexibility. These properties can be potentially applied to the transportation of SRR films to structured substrates on which SRRs cannot be fabricated by conventional techniques. We anticipate further development in the study of metamaterials through the use of our technique.
Ministry of Education, Culture, Sports, Science, and Technology of Japan (No. 22109007).
We would like to thank Prof. Shigenori Fujikawa at Kyushu University for his support in the preparation of nanospheres by electrostatic adsorption and selection of the materials. This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas from The Ministry of Education, Culture, Sports, Science, and Technology of Japan (No. 22109007).
References and links
1. V. G. Veselago, “The Electrodynamics of Substances with Simultaneously Negative Values of ε and μ,” Sov. Phys. Usp. 10(4), 509–514 (1968). [CrossRef]
3. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314(5801), 977–980 (2006). [CrossRef] [PubMed]
4. G. Dolling, C. Enkrich, M. Wegener, J. F. Zhou, C. M. Soukoulis, and S. Linden, “Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials,” Opt. Lett. 30(23), 3198–3200 (2005). [CrossRef] [PubMed]
5. V. M. Shalaev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K. Sarychev, V. P. Drachev, and A. V. Kildishev, “Negative index of refraction in optical metamaterials,” Opt. Lett. 30(24), 3356–3358 (2005). [CrossRef] [PubMed]
6. J. Valentine, S. Zhang, T. Zentgraf, E. Ulin-Avila, D. A. Genov, G. Bartal, and X. Zhang, “Three-dimensional optical metamaterial with a negative refractive index,” Nature 455(7211), 376–379 (2008). [CrossRef] [PubMed]
8. J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from Conductors and Enhanced Nonlinear Phenomena,” IEEE Trans. Microw. Theory Tech. 47(11), 2075–2084 (1999). [CrossRef]
10. F. Miyamaru, M. W. Takeda, and K. Taima, “Characterization of Terahertz Metamaterials Fabricated on Flexible Plastic Films: Toward Fabrication of Bulk Metamaterials in Terahertz Region,” Appl. Phys. Express 2(4), 042001 (2009). [CrossRef]
11. F. Miyamaru, S. Kuboda, K. Taima, K. Takano, M. Hangyo, and M. W. Takeda, “Three-dimensional bulk metamaterials operating in the terahertz range,” Appl. Phys. Lett. 96(8), 081105 (2010). [CrossRef]
13. C. Enkrich, M. Wegener, S. Linden, S. Burger, L. Zschiedrich, F. Schmidt, J. F. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic metamaterials at telecommunication and visible frequencies,” Phys. Rev. Lett. 95(20), 203901 (2005). [CrossRef] [PubMed]
14. W. T. Chen, C. J. Chen, P. C. Wu, S. Sun, L. Zhou, G. Y. Guo, C. T. Hsiao, K. Y. Yang, N. I. Zheludev, and D. P. Tsai, “Optical magnetic response in three-dimensional metamaterial of upright plasmonic meta-molecules,” Opt. Express 19(13), 12837–12842 (2011). [CrossRef] [PubMed]
15. N. Feth, M. König, M. Husnik, K. Stannigel, J. Niegemann, K. Busch, M. Wegener, and S. Linden, “Electromagnetic interaction of split-ring resonators: The role of separation and relative orientation,” Opt. Express 18(7), 6545–6554 (2010). [CrossRef] [PubMed]
16. S. Tanabete, Y. Nakagawa, T. Okamoto, M. Haraguchi, T. Isu, and G. Shinomiya;, “Fabrication and evaluation of photonic metamaterial crystal,” Appl. Phys., A Mater. Sci. Process. 112(3), 605–611 (2013). [CrossRef]
17. T. Okamoto, T. Fukuta, S. Sato, M. Haraguchi, and M. Fukui, “Visible near-infrared light scattering of single silver split-ring structure made by nanosphere lithography,” Opt. Express 19(8), 7068–7076 (2011). [CrossRef] [PubMed]
18. T. Okamoto, T. Otsuka, S. Sato, T. Fukuta, and M. Haraguchi, “Dependence of LC resonance wavelength on size of silver split-ring resonator fabricated by nanosphere lithography,” Opt. Express 20(21), 24059–24067 (2012). [CrossRef] [PubMed]
19. M. Retsch, M. Tamm, N. Bocchio, N. Horn, R. Förch, U. Jonas, and M. Kreiter, “Parallel preparation of densely packed arrays of 150-nm gold-nanocrescent resonators in three dimensions,” Small 5(18), 2105–2110 (2009). [CrossRef] [PubMed]
20. T. Serizawa, H. Takeshita, and M. Akashi, “Electrostatic Adsorption of Polystyrene Nanospheres onto the Surface of an Ultrathin Polymer Film Prepared by Using an Alternate Adsorption Technique,” Langmuir 14(15), 4088–4094 (1998). [CrossRef]
21. H. Watanabe, E. Muto, T. Ohzono, A. Nakao, and T. Kunitake, “Giant nanomembrane of covalently-hybridized epoxy resin and silica,” J. Mater. Chem. 19(16), 2425–2431 (2009). [CrossRef]
22. P. K. Jain, W. Huang, and M. A. El-Sayed, “On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation,” Nano Lett. 7(7), 2080–2088 (2007). [CrossRef]