We show that an elliptical nanohole array (ENA) penetrating through a metal/dielectric/metal (MDM) film produces multi-band circular dichroism (CD) in the near infrared (N-IR) region when the incident light is off-normal incidence. This extrinsic CD is a result of the elliptical symmetry of the nanohole, which gives rise to different polarization modes along its short and long axes. These two polarization modes introduce a net polar vector that forms a chiral triad with the off-normal incidence and the vector normal to the plane of the ENA. The proposed structure possesses four resonance peaks arising from the excitation of both internal- and external- surface plasmon polariton (SPP) modes with different diffraction orders to those wavelengths at which they couple to the incoming light. The formation of these resonance peaks is responsible for the multi-band CD.
© 2014 Optical Society of America
Metamaterials (MMs) are manmade effective media that possess properties and capabilities that do not occur in nature, such as negative refractive index , perfect imaging , and invisible cloaking . Besides these interesting applications, recent progress in MMs has opened new routes to achieve circular birefringence and circular dichroism(CD), which are jointly referred to as optical activity (the ability to rotate the polarization state of light) [4–8]. For example, MMs with helix resonators , twisted U shape split ring resonators , or gammadion resonators  are typical chiral MMs with circular polarized eigenmodes. Alternatively, optical activity is also observed in non-chiral MMs under an oblique incidence which are termed as “extrinsic chiral” structures [11,12], such as periodically repeating metallic spheres , asymmetrical split rings  or nanorings . In terms of practical applications, MMs consisting of achiral resonance elements have been shown to be as efficient as those with chiral particles in diffraction experiments while having simpler patterns that are easier to fabricate .
However, these MM based polarization rotators typically operate over narrow frequency band, owing to the narrow resonance of the plasmonic eigenmodes . As an attempt to solve this problem, people have broadened the bandwidth of optical activity by using gold helix MMs [18,19] as well as stacking multiple polarizers or introducing a gradient in the helical pitch [20,21]. Even so, modest bandwidth increases are usually obtained at the expense of thicker devices, which are difficult to fabricate and integrate into today’s nanophotonic systems. One way to overcome this difficulty is to use planar MM structures comprising of strongly scattering anisotropic particles that are able to excite multiband optical activity. For example, Ma et al. have proposed a multi-band circular polarizer using multilayered planar spiral MMs . Shi et al. have demonstrated a stereometamaterial with twisted asymmetric split-ring resonators to create three cross-polarization transmission peaks . Zarifi et al. have presented a dual band optical activity using semi-planar chiral MMs . Xie et al. have proposed a multi-band circular polarizer using a double-layered Archimedean spirals array in which the bottom spirals twist 90° compared to the upper spirals .These MM structures work in the GHz region because the fabrication of planar MMs with multiband optical activity in the optical region is hindered by the minimum realizable features, which is imposed by the diffraction limit. Thus, an effective method for obtaining the MM with multiband optical activity in the optical spectral region is desirable and necessary for practical applications.
Following pioneering work in 2003 , chiral and non-chiral planar(2D) MMs [26–28] were introduced as a distinct class of structures to achieve optical activity, which can be understood in terms of plasmon modes leading to local electric-dipole moments. Lately both chiral and non-chiral MMs based on coupling in a two layer (metal/dielectric/metal, MDM) structure have been proposed to provide stronger polarization effects than for single metal film structures [29,30]. In such multilayer (3D) MMs, the local magnetic resonance caused by the antisymmetric oscillation modes of the two coupled layers can enhance circular polarization difference effects relative to an identical lateral structure with a single metal layer where only electric dipole resonances occur. Therefore, multilayer MMs have recently emerged as one of the most prominent topics of chirality research. In our earlier work , we numerically showed an enhanced CD is obtained using an elliptical nanohole array (ENA) embedded in the MDM multilayer. However, the CD was only possible across a narrow single band in the visible region. More recently, it has been shown that MMs consisting of a stack of MDM films perforated by an array of subwavelength round holes may exhibit several resonance transmission peaks at different frequencies in the optical region [31,32]. These multiple resonance peaks are associated with the excitations of either internal or external surface plasmon polaritons (SPPs) on the double-layer hole array structure . Inspired by this interesting phenomenon, the present study demonstrates the use of fishnet MMs for realizing the multiband extrinsic CD in the near infrared (N-IR) region by tilting the structures off the symmetric axes relative to the light incidence plane.
Our structure consists of an ENA embedding through Au/Dielectric/Au plates. An elliptical nanohole is a 2D anisotropic system that possesses two principle polarization directions along the two orthogonal principle diameters of the hole. Four transmittance resonance peaks linked to the multiband CD can be obtained when illuminated by an off-normal circular polarized wave. These peaks come from the excitation of both internal- and external- SPP modes with different diffraction orders at those wavelength. These modes couple to the incoming light via grating coupling on a multilayer hole array structure. Under an oblique incidence, the transmittance of the structure depends on the state of the polarization (left or right circular polarization) if only the two principle axes of the elliptical nanoholes are not contained in the plane of incidence . In such a configuration, the experimental arrangement is chiral. This fact is commonly known as pseudo or extrinsic chirality . The experimental realization of the small elliptical pores embedding through the multilayer structure sounds challenging but leveraging off the recent developments in the fabrication of arrays of ultrasmall round pores penetrating suspended MDM structure  and nanocale photonics crystals in the N-IR region  may be able to address this problem. Nevertheless, the simple pattern of the elliptical holes will reduce the difficulty of fabrication compared with the chiral MMs consisting of helix or gammadion elements and nonchiral MMs consisting of arc or sphere resonators. This proposed structure will be useful for detection of a broad range of stereochemical and biological agents as well as have potential applications in the field of multiband circular polarizer and infrared vibrational optical activity .
2. Fishnet metamaterials and simulation method
The fishnet MMs consist of a two gold layers (60nm thick Au) spaced by a 200 nm thick dielectric interlayer with an inter-penetrating ENA shown in Fig. 1(a), the unit cell is shown in Fig. 1(b) and 1(c) shows the chiral triad in the ENA. Here, in order to achieve the CD, the incident wavevector k, the vector normal to the surface n, and one of the two primitive diameter vectors(a or b) must form a chiral entity as shown in Fig. 1(c) in red. The lattice constant of the elliptical hole is L = 400nm, the diameters of the elliptical holes are d1 = 340nm and d2 = 170nm, θ is the angle between the incident wavevector k and the vector n, which is normal to the plane of the ENA (x–y plane). φ is the rotation angle between the parallel (to the x–y plane) component of the wave vector and the x axis. The z-axis is normal to the structure surface and the x-y plane is parallel to the structure surface. The structure is considered to be suspended in a vacuum. The structure is periodically extended along the x and y axes. The Au bottom layer interacts with the upper Au layer to excite strong magnetic resonances. Au is selected as the metal due to its stability and low ohmic loss. The structures are simulated using a commercial software ‘3D EM Explorer Studio’, which uses the finite difference time domain(FDTD) method. The dielectric properties of Au as given by Johnson & Christy are used . The scalar value of the refractive index of the dielectric interlayer is 3.42. A plane wave is incident onto the structure at an oblique angle θ, as described in Fig. 1(a). Periodic boundary conditions were used in the x and y directions. The FDTD mesh size was 2 nm to provide an accurate calculation on the plasmonic effect. It is known that CD can primarily pass light of circular polarization of one handedness, while suppress the transmittance of light of the other handedness . Most materials do not discriminate between right- and left circular polarizations in transmittance, so that where R and L stand for right- and left circular polarizations, is zero for all frequencies; a nonzero indicates the existence of CD and enantiomeric asymmetries . Therefore, here the CD is defined as the result of the difference in the transmittance of right- and left circularly polarized light: [40,41]. In 3D EM Explorer Studio, we can specify the phases for the Ep and Es components, where Ep is p polarization of the incident E-field and Es is s polarization of the incident E - field. The phase difference between Ep and Es is 90° for right circular polarization (RCP) and −90° for left circular polarization (LCP). The Ep and Es amplitudes are the same for both LCP and RCP.
3. Simulation results and discussion
In Fig. 2(a), we calculate the transmittance spectrums of LCP and RCP with θ = φ = 45° where four transmittance peaks are observed at the resonant wavelengths of 1090nm, 1170nm, 1430nm and 1620nm. These transmittance peaks are caused by the electric and magnetic resonances in the MDM-ENA, which can in turn contribute to the multiband CD shown in Fig. 2(b). Here, CD occurs when the direction of the light propagation is a ‘screw direction’ of the unit cell. In such a configuration, the unit cell doesn't have an inversion center; an oblique incidence provides no reflection symmetry in the plane perpendicular to the propagation direction; both oblique incidence and anisotropic elliptical hole occur with no inversion or mirror rotation axis along the propagation direction; the polarization vector (a or b) is not parallel to the incident plane hence leading to no reflection symmetry for any plane containing the propagation direction. Therefore, this experimental geometry is chiral and supports optical activity .
To further understand the mechanism of the multiband resonance in the MDM-ENA under off-normal incident light, it is useful to study the dispersion relation of the SPP modes within the multilayer structure. Both the internal and external SPP modes in the multilayer MMs are similar to those of the same structure without resonant elements, i.e. MDM films [31,32], where the internal-SPP mode resonates in the inner surfaces of the metal layers(metal-dielectric interface) and the external SPP mode resonates in the outer surfaces of the metal layers(metal-air interface). Therefore, the SPP dispersion relation of the MDM-ENA can be approximated by that of the MDM structure without pores. In Fig. 3, we have calculated the SPP modes dispersion relation of the Au-Dielectric-Au sheets with the top Au film thickness of 60 nm, middle dielectric film thickness of 200 nm and bottom Au film thickness of 60 nm by means of a standard procedure in a commercial software Lumerical FDTD Solutions. The CD spectra of the MDM-ENA is depicted together with the dispersion relation of the Au-Dielectric-Au films. Here, four main peaks appear in the simulated frequency range, which corresponds to the first- and second-order transmission resonances according to Eq. (1) when (i, j) = (1,0) and (1,1), respectively.42]. The peak positions corresponding to selected values of (i, j). Particularly, there are two distinct separate peaks corresponding to the odd and even symmetric (1,0) external-SPP modes, and another two peaks are associated with the (1,0) and (1,1) internal-SPP modes. Therefore this structure has four different SPP modes although some of them are on the same diffraction orders. Figure 3(a) shows that the internal (1,0) and (1,1) diffraction order modes of the Au-Dielectric-Au trilayer are excited around the resonant wavelengths of 1836 and 1095nm. Noted that two distinct peaks, which correspond to the odd (1,0)+ and even (1,0)- symmetric external-SPP modes, appear in the CD spectrum in Fig. 3(b), where the subscripts + and − stand for odd and even symmetry external-SPP modes, however the dispersion relation of the Au-Dielectric-Au trilayers in Fig. 3(a) predicts a single peak at the wavelength of 1345 nm for both external modes. This is because the external SPPs on opposite interfaces experience increased coupling through the holes, which the Au-Dielectric-Au trilayer model does not take into account. As can be seen, the four SPP modes for the simple MDM structure do not perfectly match the four CD peaks at the resonance wavelengths of 1620 nm, 1170nm, 1090nm and 1430nm for the MDM-ENA. This difference is because the dispersion relation of the SPP modes, which was used as matching condition, does not include the resonant elliptical apertures, which cause scattering losses and resonance shifts . At the internal-SPP modes, antiparallel currents are excited at opposite internal metallic interfaces, closed by an electric displacement current. The formation of a virtual current loop between the metallic layers leads to the magnetic dipolar moments . However, the external-SPP resonances do not show a magnetic response since the displacement current does not form a virtual current loop with the electric current. Therefore, the origin of the external-SPP mode in the MDM multilayer structure can be traced to the electric resonance.
Figure 4 shows the spectra of the CD for the MDM-ENA for different thicknesses of dielectric interlayer Td = 150nm, 200nm and 250nm. It can be seen that the value of CD decreases as the dielectric layer becomes thicker, which is a result of increased losses. The spectra of the CD red shifts with increasing thickness of the dielectric interlayer. Specifically, it shows that the spectra of the CD for both Td = 150nm and 250nm fluctuate more than Td = 200 nm. It is because that the frequency of the magnetic resonant dipole in the multilayer structure shifts if the thickness of the dielectric layer changes, thus the magnetic resonant dipole needs to be relocated by means of redesigning the geometry of ENA to obtain the impedance match to a vacuum .
In order to characterize the dependence of CD on the angles of rotation φ, we simulate CD at different values of φ for a fixed incident angle θ = 45°, as shown in Fig. 5(a). Circular difference effects are not present for the structure orientation φ = 0° since the anisotropic axis of the structure is in the incident plane hence leading to a mirror plane of the experimental geometry . However, the multiband CD can be observed for the nonzero values of φ from 15° to 60°. In the case of φ = θ = 45°, CD at 1430 nm can achieve the highest magnitude of 0.108. Importantly, the CD changes the sign between the positive and negative rotation angles of φ, whereas the magnitude of the CD remains unchanged. Therefore, this experimental configuration exhibits different enantiomers provided by changing the rotation angle φ. In Fig. 5(b), we simulate the transmittance CD spectra at various incident angles θ by fixing φ = 45°. As can be seen, no CD is obtained for θ = 0° and the CD response increases with θ. Particularly, the CD can be significantly enhanced for θ > 30°. Although this structure can obtain the maximum value of 0.16 of CD at 1390nm for θ = 60°, it exhibits negative CD in the wavelength region from 1140nm to 1230nm. Therefore, in this work we are focus on the CD at φ = θ = 45°.
In Fig. 6(a), we present two CD spectra with normal(θ = φ = 0°) and oblique(θ = φ = 45°) incidences. For the case of θ = φ = 0°, the structure does not show any CD. Figure 6(b)-6(e) show FDTD simulation snapshots of the field distributions of total electric field intensity at 1430nm (external order(1,0)-) on the gold-air interface with LCP and RCP light both at normal incidence (θ = φ = 0°), where no CD is obtained, and for θ = φ = 45°, where a substantial CD response is observed. For θ = φ = 0°, it shows that the field distributions for the two circular polarization are exact mirror images of each other. The difference of these field patterns cancels hence leading to a zero CD. For θ = φ = 45°, it shows that the field patterns are significantly different between the LCP and RCP light. Moreover, the field pattern appears asymmetric over the nanohole array under the oblique incidence since the time of the pulse propagating through different regions of the structure is unequal. The asymmetric field patterns of the two oblique circular polarized electric fields provide asymmetric transmittance shown in Fig. 2(a). The phase of the resonant modes excited by these two circular polarizations give rise to strong interferences of the modes at the apertures, resulting in different transmittances of the LCP and RCP light. Figure 6(f)-6(i) shows the snapshots of the E field distributions at 1620nm (inner order(1,0)) on the gold-dielectric interface for both normal incidence (θ = φ = 0°) and oblique incidence(θ = φ = 45°). For θ = φ = 0°, the E field distributions for the LCP and RCP light exhibit a mirror symmetry, leading to a zero CD. For θ = φ = 45°, the asymmetric field patterns is observed, resulting in a non-zero CD.
In conclusion, a multiband CD around the surface plasmon frequencies in the N-IR region is observed in planar non-chiral MDM-ENA. At off-normal incidence both internal- and external SPP modes with different diffraction orders are excited that cause a different transmittance of the two circular polarization states through the MDM-ENA around the multiple frequency bands. This transmittance difference is due to the extrinsic chirality induced by the mutual orientation of the MDM-ENA and the incident light. This proposed structure can be used in sensing a broad range of stereochemical and biological agents as well as to explore potential applications in the field of multiband circular polarizer and infrared vibrational optical activity.
We acknowledge the financial support from National Natural Science Foundation of China (Grant No. 61172059,51302026), Ph.D Programs Foundation of Ministry of Education of China (Grant No. 20110041120015), Postdoctoral Gathering Project of Liaoning Province (Grant No. 2011921008) and The Fundamental Research for the Central University (Grant No. DUT14YQ109).
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