Polarization-Sensitive Coupling and Transmission Dip Shift in

Article pubs.acs.org/JPCC

Polarization-Sensitive Coupling and Transmission Dip Shift in Asymmetric Metamaterials Yapeng Cao,†,‡ Yiyang Xie,† Zhaoxin Geng,*,†,§ Jian Liu,‡ Qiang Kan,† and Hongda Chen† †

State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China ‡ State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China § School of Information Engineering, Minzu University of China, Beijing 100081, China S Supporting Information *

ABSTRACT: Polarization-sensitive asymmetric metamaterial structures, which consist of both conductive and capacitive interactions between tilted gold nanorods, are investigated both theoretically and experimentally. The near-infrared transmission spectra of the fabricated metamaterial display two transmission dips related to the dipole modes of the nanorods. These dipole modes couple to each other, and the coupled mode is polarization-sensitive and converts between the bonding mode and the antibonding mode. This conversion leads to the polarization-sensitive and continuous resonance wavelength shifts of the dipole modes. This phenomenon is different from those observed in traditional Dolmens or tilted split ring resonators whose resonant modes do not show obvious wavelength shift. In addition, geometry tuning of the structure is also carried out, which greatly affects the polarization-sensitive resonance wavelength shifts shown above. Meanwhile, experimental results demonstrate that the metamaterial structure is able to rotate the polarization of light.

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INTRODUCTION

wavelength shift of a particular mode in Dolmen structures and nanorod dimers.24−28 Many of the reported anisotropic nanostructures are polarization selective. For the single plasmonic gold nanorod, which is the basic element composing a Dolmen structure or a nanorod dimer, its longitudinal dipole mode and transverse mode are excited under orthogonal polarization directions and are spectrally distinct. The intensity of the longitudinal mode is in direct proportion to cos2(θ), where θ is the angle between the polarization direction of the incident light and the major axis of the nanorod. As a result, gold nanorods show polarization-sensitive color and are applied as orientation sensors.25,29,30 Meanwhile, the spectral wavelength of the longitudinal mode in the gold nanorod is polarizationindependent25,26,29 for normally incident light (nanorods similar to ellipses are not considered here for they may have multiple optic axes). For structures containing more than one gold nanorod, the different sizes and orientations of the nanorods lead to different dipole resonances when the polarization angle of incident light rotates, as demonstrated in SRRs, nanorod dimers, trimers, and Dolmen structures. Nevertheless, for each of the spectrally distinct resonant modes, in most cases, the polarization state only changes the

Recently, metamaterials have attracted much attention and are gradually applied in various areas such as subwavelength imaging,1,2 electromagnetic cloaking,3−5 biosensing,6,7 and polarization controlling.8−11 Optical properties of diverse kinds of structures, such as split ring resonators (SRR), fishnets, bowties, and nanorods,12,13 have been thoroughly investigated. Among these structures, asymmetric nanostructures like the Dolmen structure and the nanorod dimer with plasmonic resonance properties have received special interest as they may support the so-called antibonding mode, which is usually accompanied with Fano resonances. Many researches have studied the factors which influence the antibonding in the Dolmen structure and the nanorod dimer. It is illustrated that the resonance intensity and resonance wavelengths of the coupled modes in these nanostructures are greatly dependent on the geometry parameters. The resonance wavelengths of the bonding and antibonding modes shift with parameters such as structure sizes and shapes, the aspect ratio of the nanorods, the nanogap size, the cross angle between the nanorods, and the relative positions of the nanorods.14−24 Apart from geometry tuning, the resonance property can also be controlled by rotation of the polarization direction of the incident light. However, the relevant studies focus on the polarizationdependent variation of the scattering intensities and excitation of different eigen modes and do not report on resonance © XXXX American Chemical Society

Received: December 10, 2014 Revised: March 1, 2015

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DOI: 10.1021/jp512296t J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

structure. Then, 30 nm thick gold film was deposited by electron beam evaporation. Finally, a lift-off process was performed to obtain the designed metamaterial. The transmission spectra were measured using the Olympus BX-RLA2 system. The spectra data were recorded by a nearinfrared spectrometer (Ocean NIRQuest). A broadband lamp was used as the source and a Glan-Taylor prism was used to achieve a linear polarized incident light. The Lumerical FDTD Solutions software was used to simulate the optical properties of the metamaterial. On the basis of the periodic structures designed, the transmission spectra of the metamaterials were extracted by monitoring the optical power flowing in and out of the volume with a plane wave light source. Periodic boundaries in x and y directions and the perfectly matched layer (PML) boundary in the z direction were applied in the simulation. The mesh sizes are 5 nm in x, y, and z directions.

coupling strength between light and does not affect the eigen frequencies of these modes. However, in a few exceptional cases, the varying polarization state changes the coupling between different resonant modes and makes the resonant modes different from the eigen ones. Thus, the resonance wavelengths may shift gradually with the polarization angle.31,32 Here, an asymmetric metamaterial, which is polarizationsensitive in the infrared area, is designed based on the finite difference time domain (FDTD) calculations and fabricated by electronic beam lithography (EBL) on quartz slide. The polarization-sensitive resonant wavelength shift and controlling of the antibonding mode of this metamaterial are explored in detail by changing the tilted angle of the gold nanorod. Then the influence of geometry parameters of the nanostructure on the polarization-dependent resonance shift is studied systematically, which gives one a better understanding of the experimental results.

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MATERIALS AND METHODS The metamaterial designed and fabricated in our experiment is composed of periodic structures (Figure 1). It is a combination

RESULTS AND DISCUSSION The normal incidence light is linearly polarized light traveling along the positive z axis. The transmission spectra of the metamaterial sample were measured when the incident light was polarized in different directions. When the polarization angle γ is 0°, the incident electric field is along the x axis in Figure 1a. The measured transmission spectra (smoothed) are shown in Figure 2, parts a and b. There exist two remarkable transmission dips: dip A at about 1032 nm and dip B at about 1234 nm. These two dips result from different resonant dipole modes in the nanorods. Dip A is mainly attributed to the resonant dipole mode in nanorod A2 and modified by the dipole mode in A1, as demonstrated by the simulated electric field profiles presented in Figure 2f. At dip A, when the light is vertically polarized, the dipole mode in A2 is strong, however, the dipole mode in A3 is weak. Though both the dipole modes in A1 and A2 are excited, the nanorod A2 plays an dominate role, since dip A has the minimum when γ is 90° (i.e., the light is polarized along the A2 nanorod). By contrast, dip B has the minimum when the polarization direction is parallel to A3. As the nanorod A3 has a larger aspect ratio than that of the nanorod A2, the resonance wavelength of the dipole mode of A3 is larger and corresponds to dip B. As demonstrated in Figure 2f, the electric field in A3 is greatly enhanced at dip B. Additionally, when the polarization angle γ varies from 90° to 150°, the resonance wavelength of dipole mode in A2 (dip A) shows a red shift. To see the dip shift more clearly, some of the transmission lines are normalized and depicted in Figure 2c. The eigen wavelength of the A2 dipole mode is 1032 nm, when the light is polarized along nanorod A2. Then dip A (λA) shifts from 1032 to 1089 nm when γ changes from 90° to 153°. Meanwhile, dip A shows little shift when γ changes from 0° to 90°. As for the resonance in nanorod A3, the wavelength of dip B (λB) shifts 117 nm, from 1149 to 1268 nm as γ changes generally from 155° to 280° (the angle 280° is the same as 100°), as shown in Figure 2d. When γ is between 120° and 150°, dip B disappears as the resonant mode in A3 is not excited. Figure 2e presents the simulated transmission spectra at different polarization angles. The positions of dip A are plotted as the white dashed line, which agrees well with the experimental results and shows a shift of dip A. This polarization-sensitive shift of transmission dips is peculiar as the resonance wavelengths of gold nanorods and SRRs are traditionally independent of polarization. The

Figure 1. Design and geometry of the metamaterial. (a) Structure model of the metamaterial. (b) Geometry sizes of the structure unit: periods in y and x directions are a1 = 465 nm, a2 = 585 nm, L1 = L3 = 280 nm, L2 = 220 nm, w1 = w3 = 80 nm, w2 = 90 nm, and the gold thickness t = 30 nm. (c) SEM image of the fabricated metamaterial.

of the tilted SRR structure and the tilted Dolmen structure, and is a little similar to structures presented in refs 6 and 24. The periods of the structure are a1 in y axis and a2 in x axis as illustrated in Figure 1b. The structure consists of three gold nanorods, named as A1, A2 and A3. The nanorod A2 lies along the y axis. The parallel nanorods A1 and A3 are tilted to introduce symmetry breaking. The angle between A1 and A2 is α. There is a narrow gap between nanorods A2 and A3, whose minimum width is about 30−40 nm. The scanning electron microscopy (SEM) image of the fabricated metamaterial is shown in Figure 1c (α = 52°). Structures with different α as 30, 45, 75, and 90° are also fabricated. The fabrication processes are briefly described as follows. First, a layer of PMMA EL4 resist was spin-coated on a quartz substrate and a 5 nm thick gold was sputtered on the resist in sequence. Second, the EBL process was used to define the B


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Figure 2. Tested and simulated transmission spectra. (a, b) Measured transmission spectra of the metamaterial sample at different polarization directions. The polarization angle is shown as γ. (c, d) Normalized transmission demonstrating the shift of spectra of dip A (c) and dip B (d). (e) Simulated transmission spectra of the metamaterial at different polarization directions. The white dashed line illustrates the shift of the transmission dip A. (f) The electric field profiles at the simulated wavelengths of dip A and dip B. The red arrows show the polarization directions.

Figure 3. Simulated transmission spectra of nanorods A2, A3 and the tilted SRR dependent on the polarization angle of the normal incidence light. (a, b) Transverse dipole modes and the longitudinal dipole modes of the nanorod A2 (a) and the nanorod A3 (b), respectively. (c) Spectral positions of the A2 longitudinal mode and the A3 longitudinal mode of the titled SRR, as marked by the vertical black lines.

simulated transmission spectra of individual nanorods A2 and A3 are demonstrated in Figure 3. The results show that the spectral position of both the transverse dipole mode (at 620 nm) and the longitudinal dipole mode (at 1034 nm) in nanorod A2 are polarization-independent (Figure 3a). This is

the same for nanorod A3 (Figure 3b). These eigen modes in nanorods A2 and A3 only show intensity variation when the polarization is rotated. A tilted SRR was also studied by FDTD simulation (Figure 3c). This asymmetric SRR has the same geometry size as that investigated in our experiment, except C


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which leads to polarization-sensitive variation in the coupled mode. When γ is 90°, A3 supports a quadrupole mode (Figure 4a). When γ changes to 120°, the mode in A3 turns into a mixture of the quadrupole mode and the transverse mode (Figure 4b). The hybridized mode in A3 is dominated by the transverse dipole mode at γ = 140° (Figure 4c) and then by the longitudinal dipole mode at γ = 150° (Figure 4d). The mode variation in A3 which depends on the polarization angle leads to the different coupling ways between nanorods A2 and A3 (i.e., bonding mode and antibonding mode) and finally give rise to the polarization-sensitive shift of dip A. This is a bit similar to that illustrated in ref 24, in which resonances corresponding to different modes are investigated. Meanwhile, the nanorod A1 enhances this effect as it is always in phase with A2. As seen from Figure 4, the coupling mode between A1 and A3 changes between the dipole mode (when γ = 90°) and the quadrupole mode (when γ = 150°) as γ varies. While γ is between 0° and 90°, the coupling between A2 and A3 is actually weak, as the bonding mode has localized field in the gap, which is relatively wide (∼40 nm). Therefore, A2 is less affected by A3 and acts more like an individual nanorod, so the shift of dip A is small. The mechanism of the shift of dip B (the A3 dipole mode) is similar to that of dip A. When γ varies from 155° (−25°) to 200° (30°), the conversion between the bonding mode and antibonding mode together with the mode variation in A2 mainly contributes to the dip shift, as shown in Figure 4, parts e and f. The smaller red shift of dip B corresponding to 80° − α < γ < 100° is mainly attributed to the mode variation in nanorod A2 where the longitudinal mode enhances and the transverse mode weakens. To confirm the analysis above, structures with different cross angles α (30, 45, 52, 75, and 90°) were fabricated, as shown in Figure 5a. The shifts of dip A of these structures depending on the polarization angle were compared in Figure 5b. We define the polarization angle where the resonance wavelength starts to increase as β. We suppose that when α increases, as the antibonding mode occurs near the range 180° − α < γ < 180°, the angle β will follow the angle (180° − α) and decrease. As shown in Figure 5b, the value of β shows a decrease from about 150 to 90°, when the angle α increases from 30 to 90°, indicating that the dip shift is associated with the rise of antibonding mode. The black curve in Figure 5b at the range 0° < γ < 110° is relatively flat, as the antibonding is not excited. When γ lies near 180°, dip A disappears and there is a discontinuity here. Dip B shows similar results, as plotted in Figure 5c. The black line shows increasing at the range −25° (155°) ∼ 20° and 80° ∼ 110°. Between the angles 110 and 155° is a discontinuity of dip B as the dipole mode in A3 is weak. A flat band lies between 20 and 70°, as the antibonding mode vanishes. When the angle α increase, the curves show the similar variation trend with that in Figure 5b. In addition, metamaterials with different structures were investigated with simulations to see how the structure asymmetry affects the dip shift. Structures such as the split ring resonator (SRR), tilted SRR, Dolmen structure, and dimers were thoroughly studied by FDTD method and the wavelength shift of dip A versus the polarization angle were compared in Figure 5d. The colorized curves corresponded to the structures in the same color shown in the bottom right corner. The results show that asymmetric structures (such as our structure presented above and tiled Dolmen, see the black and green lines) exhibit large shift of dip A (∼90 nm). Meanwhile, symmetric structures such as SRR (in red),

that nanorods A2 and A3 contact with each other thus there is no capacitive coupling between A2 and A3. The dipole resonant modes in A2 and A3 are excited most when γ is 90° and 38° respectively. The spectra show that both the A2 dipole mode and the A3 dipole mode remain unchanged in resonance wavelength position. What the polarization changes is the relative intensities of different modes. In Figure 3c, the minimum of the transmission lines change from the A3 mode to A2 mode when γ changes from 60 to 90° and seems to present a transmission dip shift. However, this is only resulted by the relative transmission intensities of these two modes, which are spectrally discrete. The variation of the transmission minimum is not continuous too, so it could not be denoted as a resonance wavelength shift. Compared with those in Figure 3c, the spectra in Figure 2b are quite different, where both the A2 mode and the A3 mode show significant shift in the spectral position. This may be attributed to the polarization-sensitive capacitive coupling between the nanorods. To better explain the resonance wavelength shift, we calculate the surface charge distribution of the structure by FDTD methods, as seen in Figure 4. It is found that the dip

Figure 4. Charge densities of the structure with α being 52° at different polarization angles for dip A (a−d) and dip B (e, f). Positive and negative charges are painted as red and blue, respectively. Black arrows: the electric field. Red arrows: the dipole modes. Blue arrows: the bonding mode (Pb) and the antibonding mode (Pa).

shift is simultaneous with the excitation of the antibonding mode in nanorods A2 and A3. The dipole modes in A2 and A3 couples through a nanogap which is about 40 nm wide. As the cross angle α is 52°, when the polarization angle γ is less than 128°, the two longitudinal dipole modes in the nanorods A2 and A3 are in phase and the structure supports a bonding mode (Figure 4a). When γ lies between 128° and 180°, these two dipole modes turn out of phase and the mode in the nanorods changes to antibonding mode (Figure 4d). Other modes like the transverse dipole modes in the nanorods are also excited to a certain extent and exert influences on the charge distributions, D


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Figure 5. Polarization-sensitive shift of resonance wavelength of different structures. (a) Structures with different cross angles α. (b) Relationship between the resonance wavelength of dip A and the polarization angle of light for structures in part a. The legends represent the angle α. The vertical dashed lines show the values of β for different structures. (c) Relationship between the resonance wavelength of dip B and the polarization angle for structures with different α in part a. (d) Shift of the resonance wavelength of dip A for different structures, according to simulation results.

Figure 6. Relationships between the structure geometry parameters and the polarization-sensitive shift in resonance wavelength for dipole modes in nanorods A2 and A3, respectively. (a) Tuning of the width of nanorod A2 (w2). The width of A3 (w3) remains unchanged as 80 nm and the gap is about 30 nm wide. (b) Tuning of the w3. The width of the nanorod A1 is always the same as that of A3. The parameter w2 is 80 nm and the gap is about 20 nm. (c) Tuning of the gap width. The parameter w2 is 80 nm and w3 is 70 nm. The wavelength shift of the A3 mode here is calculated when γ changes from −20 (160°) to +90°.

dip B when γ changes from −20 (160°) to +40° (220°). Figure 6a demonstrates that both the A2 mode and the A3 mode show a downward trend when the width of nanorod A2 increases. In Figure 6b, when the width of nanorod A3 grows larger, the wavelength shift of the A3 mode increases rapidly while the wavelength shift of the A2 mode shows a nonmonotonic variation and has the maximum value when the width of A3 is about 70 nm. In Figure 6c, we demonstrate that when the gap between nanorods A2 and A3 becomes wider, the A2 mode shows a decrease in wavelength shift but the A3 mode presents the opposite change. These simulation results agree with our experiment results. With certain tuning of the widths of the nanorods and the gap width, for instance, when A3 is slender, the wavelength shift may disappear, as demonstrated in Figure S2 in the Supporting Information. Nanostructures are usually sensitive to the substrates they stand on.33,34 For our metamaterials, substrates with larger refractive indexes lead to red shift of resonance positions. Therefore, the appropriate substrate is necessary and we select

Dolmen (in pink) and the asymmetric tilted SRR (in blue) only show negligible shift of dip A (less than 10 nm), which is likely to be the result of the spectral overlap with dip B, as mentioned in S1 of the Supporting Information. For SRR and Dolmen structures, nanorods A1 and A3 have opposite influences on the A2 dipole mode and the structure does not show a conversion between bonding mode and antibonding mode. The tilted dimer (in brown) possesses less asymmetry compared with a tilted Dolmen, thus its shift of dip A was much smaller than that of the latter. As analyzed above, the polarization-sensitive dip shift is enhanced by the nanorod A1, which gives larger asymmetry degree to the metamaterial. We also show that the wavelength shifts of the resonant modes in the nanorods of the structure can be tuned by changing the geometry parameters. In some cases, these shifts may disappear. We consider the shift of the transmission dip A when the polarization angle γ changes from 90° to 150° as the resonance wavelength shift of the A2 mode. As for the A3 mode, the wavelength shift is the variation of the transmission E


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Figure 7. Polarization rotation of light of our metamaterial. (a) Rotation of the light after traveling through the metamaterials (MMs). (b) Normalized transmission intensity of the substrate and the sample at different wavelengths at the polarization angle of 0°. (c) Relationship between the polarization rotation angle θ and the width of the nanorods A1 and A3.

shifts 119 nm, which is different from what happens in metamaterial structures such as SRRs, tilted SRRs, and Dolmens. Surface charge density calculations show that the coupled mode in the metamaterial converts between the bonding mode and the antibonding mode while the polarization angle changes. The results demonstrate the polarizationsensitive mode conversion and mode hybridization is the main reason for the shift of the resonance wavelength. The geometry tuning of the resonance wavelength shifts are also investigated and we show that the shift does not always emerge in similar asymmetric metamaterials. This phenomenon is useful in detecting the polarization state of light and modulating the transmission spectra. Moreover, this metamaterial is capable to rotate the polarization direction of the incident light by about 21.5°.

the isotropic and almost nondispersive quartz as the substrate in the experiments. To summarize, the polarization-sensitive shifts of the transmission dips is mainly attributed to the polarizationdependent capacitive coupling between different nanorods. The excitation of the antibonding mode and the hybridized modes of the longitudinal and transverse dipole modes in the nanorods change the coupled mode. The shifts can be tailored by tuning the geometry parameters of the nanorods in the metamaterial. Polarization Rotation. Another optical property of the designed metamaterial is that the metamaterial could rotate the polarization direction of the transmission light at certain wavelengths. In the experiment, a linearly polarized light beam propagates normally to the metamaterial sample, as depicted in Figure 7a. Then a polarizer was placed after the sample and rotated to detect the polarization of the transmission light. The relationship between the normalized transmission intensities and the angle of the polarizer is shown in Figure 7b. Notice that the intensities are normalized to the max transmission intensity, so a minimum of zero indicates that the transmission light is linearly polarized instead of elliptically polarized. The minima with small negative intensity values are caused by the noise error. The shift of the intensity peak is exactly the polarization angle rotated by the metamaterial. As depicted in Figure 7b, the polarization is rotated by about 21.5° at 1219.5 nm when the incident light is polarized at 0°. At other wavelengths, the polarization rotation angle is smaller, and in some cases the transmission light is elliptically polarized. The FDTD calculations demonstrated that the polarization rotation angle of light could be tuned by changing the size of the structure, as depicted in Figure 7c. The angle θ increased with W3 (the width of the nanorods A1 and A3). Besides, the gap between nanorods A2 and A3, the variation of length of nanorods, the thickness of gold and the cross angle α affected the angle θ. Therefore, by elaborate designing, the desired polarization rotation angle would be achieved.

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ASSOCIATED CONTENT

S Supporting Information *

Additional discussions, experimental results, and spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Z.G.) E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by fund from The Science and Technology Plan of Beijing (Z141100004214001), The Major State Basic Research Development Program of China (973 Program: 2009CB320303 and 2010CB934104), The National Natural Science Foundation of China (60978067) and The Science and Technology Research Funding of State Cultural Relics Bureau (20110135).

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CONCLUSION We have designed and fabricated an asymmetrical DolmenSRR-like metamaterial whose optical resonance properties are sensitive to the polarization direction of the incident light. Two parallel gold nanorods in the dimer are tilted by 38° and couple to the third gold nanorod conductively and capacitively, respectively. Through measuring the transmission spectra of the metamaterial and calculating the electric field profiles and the charge densities, the results illustrated that the spectral positions of the resonant modes shift with the polarization angle of the incident light. In particular, one of the dipole modes shifts 57 nm in the transmission spectra and the other

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Polarization-Sensitive Coupling and Transmission Dip Shift in

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