Large-area plasmonic photonic structures containing a proportion of quasicrystals can be fabricated by a solution-processable method. A photoresist film is exposed to a multi-beam interference pattern to form a quasicrystal template. A gold nanoparticle colloid is then spin-coated onto the template. An inverse pattern can be obtained after annealing to afford greater control over the sample morphologies and spectroscopic characteristics. Coupling between the waveguide modes and particle plasmons strengthens with increasing annealing temperature. After mode degeneration is removed, a multi-mode coupling process is observed. These results are helpful in understanding the mechanisms and design strategies of complex plasmonic nanostructures.
© 2013 Optical Society of America
Complex plasmonic structures such as photonic quasicrystals have been investigated extensively because of their unique physical properties, which include plasmonic bandgaps , local field enhancement and localization [2–4], enhanced optical transmission  and superfocusing . The structures from these studies have been applied to sensors , lasers  and surface-enhanced Raman spectroscopy . To date, several techniques have been developed for metallic photonic quasicrystal fabrication, including electron beam lithography [2,10], holography , and direct imprinting . Recently, a solution-processable method was introduced for fabrication of periodic plasmonic nanostructures [13,14] and aperiodic nanostructures [15,16]. In this paper, we demonstrate a simple fabrication technique to produce large area and low cost plasmonic nanostructures containing a proportion of quasicrystals that is based on a solution-processable method. The resulting structures are quite different to the nanoparticle-embedded quasicrystals that were made by holography , and can be termed complex plasmonic quasicrystals . The coupling between the waveguide modes and the particle plasmons emerges after high temperature annealing or removal of the mode degeneration. In addition, the morphology and spectra of the plasmonic nanostructure can be modified by changing the annealing temperature.
2. Fabrication of complex quasicrystal templates
A customized hexagonal prism is used to create the interference pattern . The optical layout for fabrication is depicted in Fig. 1(c). The top surface of the hexagonal prism is shielded with a black paper, which guarantees that the six interference beams overlap on the surface of the recording material. A He-Cd laser (Kimmon Koha Co., Ltd, Japan) operating at 325 nm is used as the ultraviolet light source. The pattern consists of periodically distributed 12-fold quasicrystals, and is denoted by the white lines shown in Fig. 1(a). Triangles and rectangles are the basic elements of the pattern. A positive photoresist (Rohm and Haas Electronic Materials Ltd.; Model S1805) is used as the recording medium to write the dark fringe pattern indicated in Fig. 1(a). An inverse pattern can also be recorded using a negative photoresist (Allresist GmbH; Model AR-N 4340), as shown in Fig. 1(b). A 180-nm-thick indium tin oxide (ITO) coated glass (20 × 20 × 1 mm) is used as the substrate. Atomic force microscopy (AFM) images of the quasicrystal templates show the large-area homogeneity that was produced by interference lithography. The height of the pattern in the negative photoresist is approximately 90 nm, which is about twice the size of the pattern in the positive photoresist. This indicates that it is easier to fabricate a nanostructure with a higher height-width ratio when using the negative photoresist.
The complex quasicrystal on the ITO glass substrate can be considered to be a waveguide grating structure, where a narrowband waveguide resonance mode can be excited. During the measurements, a tungsten halogen lamp (HL-2000) is used as a nonpolarized white light source. Figure 2 shows the extinction spectra of the photoresist template in Fig. 1(a) for two orthogonal polarization directions. It is defined that incident light is s polarized (p polarized) when its electric field is perpendicular (parallel) to the incidence plane. The small differences between the two spectra are because of the slightly different effective refractive index values of the nanostructure for the s and p polarizations. There are four main peaks (which are identified by symbols) in the extinction spectra, which are four waveguide modes corresponding to the ring of diffraction peaks in the Fourier transform of the structure in Fig. 1(a) . The envelope shapes of the extinction spectra are mainly caused by the absorption of the photoresist film, which is denoted by the blue line in Fig. 2.
3. Fabrication of complex plasmonic quasicrystals using annealing
A colloid of gold nanoparticles in chloroform with a concentration of 100 mg/ml is spin-coated (at 2000 rpm) onto the photoresist template in Fig. 1(a). The sample is then heated to different temperatures for 20 min in a muffle furnace, and is subsequently cooled to room temperature. The results shown in Fig. 3 demonstrate that the morphology of the plasmonic nanostructure can be controlled by varying the annealing temperature. The height of the pattern after annealing at 450°C is much higher than that of the other patterns. This can be attributed to the fact that the photoresist will be removed completely at temperatures above 450°C . The gold nanoislands also become more polished and clean, and thus can be classed as a plasmonic nanogalaxy . Obviously, the numbers, height and sizes of the gold nanoparticles vary considerably for different annealing temperatures, and will thus exhibit different optical properties. This aspect will be discussed in detail later in the paper.
Figure 4 shows the extinction spectra of the complex plasmonic structures that were shown in Fig. 3. The broad envelope is primarily ascribed to the particle plasmon resonance of the gold nanostructure (the dash-dotted line in Fig. 5(a)). The optical properties are significantly improved by refurbishing the gold nanoparticles and removing the photoresist template either partially or completely at a higher temperature. Different spectroscopic characteristics are obtained for different annealing temperatures, as shown in Fig. 4. For the plasmonic nanostructure that was annealed at 450°C, strong coupling between the waveguide modes and the particle plasmons can be observed for both the s and p polarizations, and which can be identified via the narrowband reduction in the extinction spectra (indicated by the red arrows in Fig. 4). This means that two enhanced transmission peaks can be observed around 750 nm. Note that the two waveguide modes around 550 nm become much weaker when compared with the results shown in Fig. 2. Thus, a higher annealing temperature has a greater influence on the smaller period nanostructures.
The tuning properties of the waveguide modes in the complex plasmonic quasicrystals are demonstrated in Fig. 5. The incidence angle changes from 0° to 30°, and is defined as the angle θ shown in the inset of Fig. 5(a). The behavior of the waveguide modes in the complex plasmonic quasicrystals is more complicated when compared with that of their periodic counterpart . More waveguide modes emerge following the removal of the mode degeneracy for large incidence angles (indicated by the red arrows in Fig. 5(b)).
To further eliminate the effects of the mode degeneracy of the complex plasmonic quasicrystal, the extinction spectra are measured by rotating the sample around the axis of the incident beam, i.e., by changing angle α while maintaining a fixed angle θ, as shown in Fig. 5(a). For example, for the s polarization, more than six waveguide resonance modes appear. The evolution of the extinction spectra is illustrated in Fig. 6, where the angle α changes from 0° to 60° while maintaining angle θ at 25°. In other words, the sample is rotated and the rotation axis is perpendicular to the sample surface, which guarantees that the relationship between the electric field vector of the incident light and the incidence plane is fixed. The pattern on the sample will repeat every 60° with respect to the k-vector of the incident light as shown in the inset of Fig. 6. Theoretically, the two extinction spectra, which are identified by symbols and in Fig. 6, should be identical. From Fig. 6, the differences between the spectra imply that the sample is no longer strictly symmetric due to the missing of some nanoparticles during the high temperature annealing process.
Figure 7 shows two typical diffraction patterns from the fabricated structure at different incident wavelengths. The quasicrystal structures show no diffraction rings, which indicates that the fabricated structure is a complex photonic structure with a portion of 12-fold quasicrystals, i.e., complex quasicrystals. In addition, an interesting fine structure in the diffraction pattern can be observed at larger incident wavelengths. All the diffraction spots in Fig. 7(a) will split into several spots, as shown in Fig. 7(b). The reason for this behavior is that the complex quasicrystal is in a microcrystalline state. For the microcrystalline case, each diffraction spot consists of several spots , producing a one-to-many laser fanout phenomenon. The spots in Fig. 7(a) are too close to be resolved due to the fact that blue light is less diffracted than red light.
We fabricated complex plasmonic nanostructures containing a proportion of 12-fold quasicrystals using colloidal gold nanoparticles. The spectroscopic characteristics and the morphologies of the fabricated structures can be conveniently controlled by varying the annealing temperature. Multiple waveguide modes can be observed and measured easily by removing the mode degeneration of the complex quasicrystal structure. The results presented here may be helpful in the research and design of complex plasmonic nanostructures.
The authors would like to acknowledge the financial support of the 973 Program (grant no. 2013CB922404), the National Natural Science Foundation of China (grant nos. 11104007 and 11274031), the Beijing Natural Science Foundation (grant nos. 1132004 and 4133082), the Beijing Educational Commission (grant no. KM201210005034), and the Beijing Nova Program (grant no. 2012009) for this work.
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