Polarization dependent transmission through thin gold films bearing arrays of elliptical nanopores and assembled at transparent substrates is explored. Far field transmission spectra with incident light polarized along the short and long axis of the ellipses show asymmetric peaks. Near-field finite difference time domain simulated electric field profiles suggest these features are related to Fano resonances between the (± 1, 0) Surface Plasmon Polariton mode and the ( ± 1, 0) Rayleigh Anomaly. The unique spectral signature of these samples makes them attractive for visible and near infrared tags for anti-counterfeiting applications.
© 2012 OSA
Plasmonic nanostructures have recently attracted considerable interest due to their potential in photonic applications. Since Ebbesen et al  first observed Enhanced Optical Transmission (EOT) at specific wavelengths through sub-wavelength pores in metallic structures, much attention has been dedicated to these nanostructures. It has been shown that the observed transmission spectrum is dependent on the type and thickness of metal, the periodicity of the pattern, and the shape and orientation of the nanopore [2–4]. So far, numerous applications have been demonstrated based on EOT, including optical filtering [5–8] and biosensing [9–12]. A further benefit accruing from the use of nanopore arrays is that the shape anisotropy of elliptical or rectangular nanopores induces polarization sensitive transmission spectra in both individual nanopores and nanopore arrays, from the visible to the mid-infrared range [13–23]. Very recently, it was shown that this polarization could be used to tune the optical response in the visible spectral region when using arrays of crossed shaped nanoantennas, and that these devices had potential as encrypted tags for security application .
Recently, we developed a rapid facile approach for fabrication of elliptical nanopores in gold films using de-focused ion beam . In this work, we present the polarization dependence of light transmitted in both the visible and near infrared (NIR) through an array of elliptical nanopores in a thin gold film assembled at a glass substrate. A very strong spectral anisotropy was observed for spectra acquired using polarized excitation light along both the short and long axis of the ellipses showing distinct features, such as sharp peaks and spectral minima, in the 600-1200nm regions. 3D Finite Difference Time Domain (FDTD) simulations suggested that the observed peaks were related to Fano resonances between the (± 1, 0) Rayleigh Anomaly (RA) and the ( ± 1, 0) Surface Plasmon Polariton (SPP) mode on the Au/glass interface. Finally we demonstrated the potential use of such sample as visible and NIR tag for anti-counterfeiting applications.
2.1. Electron beam lithography fabrication
Patches of 60 µm × 60 µm periodic nanopore arrays were fabricated on thin films of chrome/gold (10nm/50nm) deposited on Si substrate using a focused ion beam (FIB). Elliptical nanopores with diameters of 150 nm and 350 nm were fabricated with a lattice period of 450 nm. The gold film was released from the underlying Si substrate by wet etching the chromium layer (Chrome Etchant Lodyne, Grower Chemicals Ltd.) for 10 minutes. The chip was then immersed into deionized water. The Si substrate sank to the bottom while the Au film floated on the water surface due to surface tension. Using this approach, the highly fragile gold films were easily freed from the silicon substrate in a contactless manner. Finally, the nanostructured film was flow assembled onto a glass microscope slide and allowed to dry for 24 h. After assembly, the film was observed to adhere strongly to the glass substrate via Van Der Waals interactions.
2.2. Optical characterization
To undertake spectral characterization, light from a xenon arc lamp was collimated and passed through a Glan-Thompson polarizer prior to illuminate the sample at normal incidence. Light transmitted through a nanopore array was collected using a 50x objective, spatially filtered using a pin hole and focused onto a fiber bundle. Transmission spectra were recorded using a monochromator coupled to a PMT (visible) or a germanium detector (NIR). All recorded spectra were normalized with respect to a glass substrate. Real color images were acquired using a commercial color CMOS chip with integration time of 1ms. NIR images were acquired by adding 750 nm long path filters in the collection path with an integration time of 60 ms.
2.3. FDTD simulations of nanopore arrays
3D-FDTD simulations were undertaken using JFDTD3D . The computational domain was chosen to be 450x450x1600 nm3 and the mesh was 5 nm in all directions. Periodic boundary conditions (450 nm) in the x and y directions were employed to simulate an infinite square array. Permittivity of gold was modeled using a Drude plus two-pole Lorentz model. Perfectly Matching Layers (PML) boundary conditions were imposed in the z direction in order to avoid reflections from the edges of the computational window. Transmission spectra were deconvoluted by Fourier transforming the simulated electric and magnetic fields on a surface above the gold followed by construction of the surface integral of the outward Poynting vector.
3. Results and Discussion
3.1. Experimental observations in the nanopore arrays
Figure 1(a) shows a scanning electron microscope image of an elliptical nanopore array fabricated using FIB in a thin gold film deposited on a Si substrate; inset, the surface of the film appears smooth and the edges of the nanopore sharp and well defined. Figure 1(b) shows an atomic force micrograph of a gold film bearing a nanopore array following its transfer by assembly onto a transparent substrate. Despite being only 50 nm in thickness, the film was sufficiently strong to survive the transfer. Two further benefits of the lift-off technique include: firstly the removal of the chromium adhesion layer that would otherwise dampen plasmonic effect on the Au-glass interface . Secondly, assembly of a film onto a new pristine substrate removes the possibility of substrate damage, arising during the FIB milling process, from interfering with optical transmission.
Figures 1(c)-1(d) show far-field transmission spectra with light polarized across and along the long axis of the ellipses, respectively. Avery good agreement between the simulated (red) and experimental (black) data, both in terms of peak position and overall trend was observed. The discrepancy in the relative intensity may be attributed to minor imperfections in the nanopore structure such as errors in pore dimension and sharpness of the pore edges or, to imperfections in the Drude-Lorentz model. All spectra exhibited a broad peak around λ = 490 nm, with a transmission of 20-35% (simulation and experiment, respectively). This peak can also be found in the spectrum of a plain gold film and corresponds to direct transmission (intraband transition) for gold and occurs irrespective of the incident polarization see Figs. 1(c)-1(d) . However, at wavelengths above 490 nm the recorded spectra differ significantly for the two polarization states.
When the incident light is polarized along the short axis of an ellipse, two peaks were observed (Fig. 1(c)), a broad peak in the near infrared at λ = 980 nm and a sharp, asymmetric peak at λ = 690 nm. On either side of this sharp peak, transmission reaches zero at λ = 735 nm and λ = 595 nm. These dips in the transmission spectrum can be correlated to the (1, 0)Glass and (1, 1)Glass Surface Plasmon Polariton (SPP) mode on the glass/metal interface, respectively. For normal incidence of light onto a square-symmetry periodic lattice, SPP modes occur at the wavelengths satisfying a two-dimensional grating coupling condition for surface plasmon [29, 30] (this equation is implicit as is dependent on λ):Table 1 shows the spectral positions of the predicted SPP modes for the glass/metal and metal/air interfaces with P = 450nm. As can be seen in this table, the predicted (1, 0)Glass and (1, 1)Glass occur at 721 nm and 573 nm, respectively, in close proximity to the 735nm and 595nm dips observed.
The peak at λ = 690 nm might also be associated with a Rayleigh anomaly. Rayleigh anomalies (RA) describe light waves diffracted to move in the plane of the surface and occur at wavelengths satisfying:Table 1, one can predict a (1,0)Glass RA at λ = 675 nm, close to the λ = 690 nm peak.
When the incident light was polarized along the long axis of an ellipse, a shoulder at λ = 570 nm in the low energy side of the 490 nm peak and a very sharp, asymmetric peak at λ = 735 nm with an associated minimum at 715 nm was observed see Fig. 1(d). This minimum value is in close spectral proximity to the aforementioned (1, 0)Glass SPP. The unusual asymmetric shape of the λ = 735 nm peak is indicative of a Fano resonance. Fano resonances are characteristic of systems where two transmission pathways interfere, a resonant and a non-resonant one, and have been observed in a variety of plasmonic nanostructures [31–35].
3.2. FDTD simulated near field profiles
To further understand the origins of the observed peaks, frequency-resolved near-field profiles were simulated at the wavelengths of interest. The top panel (Fig. 2(a) ) shows the plan view, on the glass side, of the electric field intensity at 960 nm when irradiated with light polarized along the short axis. The observed symmetric dipole-like distribution with maximum intensities in the direction of the incident field corresponds to the transverse Localized Surface Plasmon resonance (tLSP) of the nanopore. Figure 2(a) (bottom panel) shows a side elevation view of the electric field distribution, which is observed to be bound and concentrated on the top and bottom surfaces of the nanopore.
The top panel of Fig. 2(b) shows the plan view, on the glass side, of the electric field intensity distribution at 690 nm when irradiated with light polarized along the short axis. At this wavelength, the electric field is not bound to the rim of the nanohole anymore, but is instead concentrated in the area in between two adjacent holes and is thus supported by the film, confirming its SPP nature as suggested previously. While the excitation lies in the (1, −1) direction, it excites the (1, 0)Glass SPP mode. This is possible because the electric field vector of the incident light has a component in the direction of SPP propagation, i.e., , with kSP the wavevector of the SPP . As the periodicity is the same in the x and y direction, both the (1, 0)Glass and the (0, 1)Glass modes are excited, explaining why the distribution is symmetric with respect to the long axis of the ellipse. The side elevation view of the electric field distribution (Fig. 2(b)) clearly shows that the light is escaping into the substrate from the Au-glass interface, which is a signature of a Rayleigh anomaly . The interference between the (1, 0)Glass SPP and the (1, 0) Rayleigh anomaly then results in the Fano interference, characterized by the dip observed at 735mn in Fig. 1(c).
Figure 2(c) shows the electric field profile on the glass side at the minimum in transmission at 735nm. In the top panel, it can be seen that the field profile possesses a hybrid character, being both localized to the rim of the ellipse and propagating along the interface in the (1, −1) direction. This mode thus results from the combination of the tLSP of the nanohole and the (1, 0)Glass SPP at 690nm.
The side elevation shows the intensity on the air/metal interface is very weak, suggesting there is a destructive interference between the tLSP and the (1, 0)Glass SPP at this wavelength [21, 37]. Also note that on the glass/metal interface, the electric field is also starting to escape from the metal film, marking the onset of the RA.
Finally, Fig. 2(d) shows the electric field on the glass side at the second minimum at 595nm. Here again, the distribution is hybrid, being both bound to the edge of the hole, corresponding to the tLSP and also propagating in the (1,-1) direction, corresponding to the (1,-1)Glass. SPP . In a manner similar to the minimum at 735nm, the field profile suggest a destructive interference between the tLSP and the (1, −1)Glass SPP.
Figure 3 shows, of the electric field profiles, on the glass side, when irradiated with light polarized along the long axis. The top panel of Fig. 3(a) shows the electric field profile at λ = 735mn where a dipolar symmetric profile is observed with the maximum intensity in the (1,1) direction. This profile can be attributed to a combination of the longitudinal Localized Surface Plasmon resonance (lLSP) of the nanopore and the (1, 0)Glass SPP. As for the peak at 690nm in Fig. 3(b), the side elevation view of the electric field distribution of the bottom panel clearly shows that the light is escaping into the substrate from the Au-glass interface, suggesting a Rayleigh Anomaly.
The top panel (Fig. 3(b)) also shows the profile at the minimum at λ = 715mn, which is similar, however a lot less intense, to the one at λ = 735mn. As in Fig. 2(c), the side elevation shows the intensity on the air/metal interface is very weak, suggesting there is a destructive interference between the lLSP and the (1, 0)Glass SPP at this wavelength. Also note that on the glass/metal interface, the electric field is also starting to escape from the metal film, here again marking the onset of the RA.
3.3 Application as anti-counterfeiting tag
Polarized nanopore arrays have potential applicability for use in anti-counterfeiting applications, such as high end product labeling, drug packaging etc. Figure 4(a) demonstrates the concept of these devices where the lLSP and tLSP plasmon resonances have different frequencies in the visible and NIR regions of the spectrum. The CIE 1931 color coordinates of the nanopore arrays measured for different polarization angles exhibited different values ranging from (0.214, 0.306) to (0.317, 0.361). Real color transmission images obtained using incident white light polarized along and across the long axis have were recorded, see Fig. 4(b). The array appears white under polarization along the long axis (CIE coordinate 0.317, 0.361) and turns to sky blue (CIE coordinate 0.214, 0.306) as the polarization (or, conversely, the sample) is rotated by 90° such that light is polarized along the short axis Under the same conditions, the image turns from bright to dark in the NIR observed when a 700nm long path filter is placed in the optical path. This NIR spectral region is outside of the sensitivity of human eyes but may be resolved by a commercial CMOS camera. The inclusion of NIR spectral features adds a further level of security to an encrypted tag while the micron size of these arrays renders them difficult to see by the naked eye. Figures 4 (c) and 4(d) show the measured polarization dependence of the different plasmonic peaks. Spectra recorded by illuminating at angles other than along the short or the long axis could be related to a linear combination of these two spectra, suggesting the observed anisotropy was dictated by the shape of the nanopore, and not by the symmetry of the repetition pattern as previously reported [13, 23, 27]. The peaks at 570nm and 750nm are in phase and in quadrature with the 690nm peak. When the polarization of the incident light is parallel to the long axis, the lLSP intensity dominates and the tLSP is barely observable. In contrast, when the incident light is polarized parallel to the short axis the tLSP intensity dominates.
Simultaneously combining all these features (polarization dependent visible and invisible transmission – simultaneous white/bright under one polarization state and blue/dark by 90° rotation) in a security label would be quite unique and would be very challenging to illegally reproduce. Moreover, the transmission spectra could be further tuned by changing nanopore shapes, dimension or arrangement and metal used. Nanoimprinting or transfer printing could be used to make these samples in a scalable and cost-effective way.
In summary, arrays of elliptical nanopore in thin gold films have been fabricated by focused ion beam milling and assembled onto a glass substrate. These samples showed strong optical anisotropy, and their transmission spectra showed sharp asymmetric features. Optical response of the samples was simulated by FDTD and a good agreement was found between the simulated and experimental data. Analysis of the simulated near-field profile suggested the observed peaks were related to Fano resonances between the ( ± 1, 0) Surface Plasmon Polariton mode and the ( ± 1, 0) Rayleigh Anomaly. Polarized transmission images acquired using commercial CMOS color chip showed simultaneous eye visible and eye invisible anisotropies. The optical signature of these arrays is quite unique and would be difficult to reproduce using other techniques, making them attractive for security tag applications.
This work was supported by the EU-funded project Phast-ID (FP7-ICT-2009-5-258238) and Science Foundation Ireland under the Research Frontiers Programme (SFI/09/RFP/CAP2455).
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