1. Introduction
Periodically nanostructured noble metals are widely used to tailor plasmonic responses in current research[1-4]. The phenomenon of extraordinary optical transmission (EOT), which was first reported by Ebbesen et al.[1], has attracted extensive interest in recent years for their potential applications. The EOT was found through an optically opaque metal film perforated with regular arrays of submicrometer holes. The phenomenon was due to tunneling through the surface plasmons formed on each metal-dielectric interface. A lot of work has been done to study their dependence on lattice symmetry, metal film thickness, adjacent dielectric media, shape of the holes, and so on[5, 6]. These metallic films were fabricated on a flat substrate and the holes were made using focused ion-beam milling, or electron-beam lithography combined with reactive ion etching.
Metal film over colloidal crystals exhibits optical transmission properties of striking resemblance with perforated flat films, although the morphology is rather different[7-11]. Attempts have been made in both visible[8-10] and infrared[7] frequencies to exploit the optical response. Ordered arrays of gold half shells were constructed by controlled gold vapor deposition on the colloidal crystal substrate. Nanosphere lithography[12] was used as the sample production technique. Compared to conventional lithography, this method has several advantages, such as the relative ease of large casting, high quality and low cost.
In this report, we study the infrared transmission properties of gold films coated on ordered two-dimensional non-close-packed polystyrene colloidal crystal. Here we use the reactive ion etching method to fabricate the non-close-packed polystyrene colloidal crystal, which is used as a substrate. Then the gold films deposited on the substrate consist of gold half-shells on the PS spheres and gold film with a 2D array of holes on the glass substrate. We study the transmission dependence on the thickness of the gold film and the incident angle of the light. To support the experimental measurements we performed FDTD simulations, and the simulation results help us to uncover more details about the electric field distribution. This study is interesting not only for the understanding of the anomalous optical properties of periodically nanostructured noble metals but also for their potential applications for optical sensors, surface-enhanced Raman scattering, and solar cells[13-15].
2. Experiments
2.1 Sample preparation
We prepared the two-dimensional close-packed colloidal crystal monolayers by a vertical surface method[16]. PS particles were first assembled into a close-packed monolayer on a vortical water surface; the monolayer then was transferred onto the substrate by a withdrawer. A hexagonal two-dimensional close-packed colloidal crystal monolayer was organized on the glass substrate. The diameter of the PS spheres is 2.5
2.2 Characterization
The surface of the structured Au film was investigated by a Hitachi S-4800 scanning electron microscope (SEM). Optical spectra were obtained on a spectrophotometer (Hitachi UV-4100).
2.3 FDTD simulation
The optical transmission properties were simulated using the finite-difference time-domain method (FDTD, Lumerical). A unit cell of the structure was simulated under periodic boundary conditions in the
3. Results and discussion
3.1 Morphology and transmission properties
Figure 2(a) shows the SEM image of a hexagonal 2D non-close-packed colloidal crystal monolayer coated with gold film. After the RIE process, the diameter of the PS spheres reduced to 1.6
A typical transmission spectrum of the microstructured gold film (thickness is 20 nm) compared with that of the bare non-close-packed colloidal crystal monolayer is shown in Fig. 2(b). The overall transmission of the structured Au film is strongly decreased as compared to the transmission of the etched PS spheres. Furthermore, the optical spectrum of the gold film is significantly structured, and three transmission resonances are clearly seen from it. In particular, a strong extraordinary transmission peak at 2010 nm is observed, which is much stronger than the other two peaks appearing at 1430 nm and 1110 nm, respectively. For metallic films with a regular array of submicrometer holes, the transmission peaks appear at
3.2 Transmission dependence on the thickness of gold film
To further study the influence of the thickness of gold film on transmission spectra, three samples with thicknesses of 20, 25 and 30 nm gold films were fabricated. In Fig. 3 the transmission spectra of microstructured gold films with different thicknesses are compared. The main features in the three transmission spectra were kept. However, blue-shifts of the resonances were observed when the thicknesses of the gold films increased from 20 to 30 nm. The strongest peak shifted from 2010 to 1875 nm. For ultra-thin metal film, the dielectric constant changes with the thickness of the metal film. With the change of the dielectric constant of the gold films, the resonances display a blue-shift[18]. On the other hand, the transmission decreased slightly as the thickness of the gold film increased. It provided an easy way to adjust the transmission response in a long wavelength range by simply controlling the thickness of the gold film.
3.3 FDTD simulations
The simulated periodic structure is illustrated in Fig. 4(a). The dotted line indicates the cross-section in which the electric fields are mapped, as discussed below.
Figure 4(b) presents the simulated and measured transmission spectra of the microstructured gold films (20 nm gold film over 1.6
In order to gain more information on the nature of excitations presented in the metallic microstructures, we computed the configurations of the electric field intensity at two selected wavelengths: 2011 nm, at the transmission maximum, and 1623 nm, at the transmission minimum. Only the result for
3.4 Angle-resolved measurements using polarized light
To investigate further information about SPP resonances of the metallic microstructures, angle-resolved measurements are performed using polarized light from
When the periodically metallic microstructures are radiated, SPPs can be excited if their momentum
Ksp=K0√εmεiεm+εi=K0sinθ±Gpq |
(1) |
where
4. Conclusions
In conclusion, we have fabricated metallic microstructures on ordered two-dimensional non-close-packed polystyrene colloidal crystals. Extraordinary optical transmission can be observed though the gold film. The simulated results match the experimental ones quite well, and the electric field distribution is simulated at both transmission maximum and minimum wavelengths. The observed transmission dependence on the metal film thickness shows that the transmission response can be adjusted by simply controlling the thickness. To obtain more information about SPP resonances of the metallic microstructures, angle-resolved measurements are performed using s-and p-polarized light. The transmission spectra were very sensitive to the polarization of the light, which are determined by the incident angle