Giant Chiral Optical Response from a Twisted-Arc Metamaterial

Jan 14, 2014 - In this Letter, we report giant chiral optical responses within a near-infrared band from a bilayered twisted-arc photonic metamaterial...
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Giant Chiral Optical Response from a Twisted-Arc Metamaterial Yonghao Cui,† Lei Kang,† Shoufeng Lan,‡ Sean Rodrigues,‡ and Wenshan Cai*,†,‡ †

School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332, United States ‡ School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Drive NW, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: We demonstrate enormously strong chiral effects from a photonic metamaterial consisting of an array of dual-layer twisted-arcs with a total thickness of ∼λ/6. Experimental results reveal a circular dichroism of ∼0.35 in the absolute value and a maximum polarization rotation of ∼305°/λ in a near-infrared wavelength region. A transmission of greater than 50% is achieved at the frequency where the polarization rotation peaks. Retrieved parameters from measured quantities further indicate an actual optical activity of 76° per λ and a difference of 0.42 in the indices of refraction for the two circularly polarized waves of opposite handedness. KEYWORDS: Metamaterials, nanophotonics, chirality, circular dichroism, optical activity

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circular dichroism and polarization rotatory power. Singlelayered chiral patterns were first demonstrated, partially due to the relative ease of fabrication, but the chiral responses in such planar metallic structures are usually weak and typically require oblique incidence.8,9 Circularly polarized waves are characterized by the twisting of their electromagnetic field vectors as they travel; therefore a pronounced chiral response naturally requires the structural variation along the propagation direction. The most widely practiced approach for the exploration of “the third dimension” in chiral metamaterials is aligned lithography, with angular offsets purposely assigned between adjacent layers. Experimental demonstrations following this strategy include dual-layer twisted rosettes,10 crosses,11 split-rings,12−14 gratings,15 L-shapes,16 coupled nanoparticles,17,18 and arrays of twisted nanorods of up to four stacks.19 Controlled deposition is another commonly used method to achieve geometrical variation in metallic chiral particles along the light propagation direction.20,21 Furthermore, the direct laser writing technique has been utilized to create delicate, three-dimensional architectures that can serve as circular polarization components such as broadband polarizers and beamsplitters in the near-to-mid infrared range.22−24 In this Letter, we report giant chiral optical responses within a near-infrared band from a bilayered twisted-arc photonic metamaterial. An earlier demonstration at microwave frequencies has revealed the twisted-arc design as a promising candidate for pronounced chiral effects.25 In this work, we designed and fabricated arrays of dual-layer twisted-arcs as a

olarization not only represents an intrinsic feature of optical waves but also offers an extra degree of freedom to manipulate light for various applications. In particular, circularly polarized light has its instant electric field vector directed along a helical trajectory and therefore possesses an inherently chiral nature. As a result, polarized light of opposite handedness, namely, left- and right-handed circularly polarized (LCP and RCP) waves, interact differently with structures that cannot be superimposed upon their own mirror image, a feature known as chirality. This feature is ubiquitous in the organic world, where it is found in sugar molecules, amino acids, and, on a larger scale, proteins, nucleic acids, and viruses. Consequently, the analysis of a chiral response is a powerful tool and is widely used in the structural characterization and spectroscopy of chemical and biomolecular substances. Important optical phenomena stemming from chirality include circular dichroism, which measures the differential extinction of the two circularly polarized waves of opposite handedness, and optical activity, which refers to the rotation of the plane of linearly polarized light when passing through an optical medium. These effects in natural materials, however, are generally very weak and are detectable only when there is a macroscopic path length of light in the material. Many years have passed since people started to overcome the constraint of naturally occurring media and began using artificially structured materials to obtain enormously strong chiral responses.1−3 Today the explosive development of metamaterials offers revolutionarily new insight into this endeavor. Metamaterials consist of engineered subwavelength building blocks and can possess tailored electromagnetic properties that are not available in nature.4−7 Recently, renewed interest has focused on chiral metamaterials with contrived © XXXX American Chemical Society

Received: December 10, 2013 Revised: January 9, 2014

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Figure 1. Structural geometry and SEM images of the chiral metamaterial. (a) Schematic of a unit cell of the twisted-arc metamaterial, wherein the silver arcs in each pair have an angular offset of α + β with respect to the stacking axis passing through the center of the arcs. The azimuthal angle φ is to be used in the characterization of polarization rotation. Geometrical parameters: α = 100°, β = 10°, r = 140 nm, w = 70 nm, t = 50 nm, and d = 180 nm. The unit cell is arranged in a two-dimensional square lattice with a lattice constant of 630 nm. (b) Oblique view of the sample under an electron microscope, with the magenta and green colors indicating the top and bottom arcs, respectively. Insets: Enlarged SEM images of a single meta-atom at the normal and oblique incidence, respectively.

Figure 2. Simulated optical properties of the chiral metamaterial. (a) Transmittance spectra of RCP and LCP waves. The insets indicate the polarization states of circularly polarized light with respect to a unit cell. (b) Polarization rotation angle (θ) of linearly polarized light and the resultant ellipticity (η) of the transmitted wave. (c) The difference in the refractive indices for circular polarizations of opposite handedness, Δn = nRCP − nLCP. (d−g) Induced electric current, with vector size and color indicating the current density, when the structure is illuminated by circularly polarized light under selected conditions.

(nRCP and nLCP) for the two circularly polarized waves of opposite handedness. The metamaterial in this work consists of an array of paired twisted arcs, where the two silver arcs in each building block are situated in two distinct layers and are angularly shifted along the cylindrical axis. All of the geometrical parameters are optimized from full-wave numerical simulations and the fine features of the dual-layered structure are achieved by aligned electron-beam lithography (see the Supporting Information for details). The thickness of the entire stack is 230 nm,

chiral metamaterial at optical frequencies. The transmission measurement with circularly polarized waves at normal incidence unambiguously indicates a circular dichroism of ∼0.35 at a wavelength of ∼1.4 μm when light passes through a metamaterial layer of ∼λ/6 in thickness. Moreover, a linear polarization rotation of ∼305° per wavelength along with a linear transmittance of over 50% was observed at ∼1.35 μm. Further analysis of retrieved parameters from measured quantities reveals an actual optical activity of approximately 76°/λ and a difference of 0.42 between the indices of refraction B

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The chiral parameters θ, η, and Δn = nRCP − nLCP retrieved from numerical simulations are shown in Figure 2b and c. To quantify the circular dichroism in the fabricated chiral metamaterial, we collected and analyzed the light transmission of the sample illuminated with circularly polarized waves. The transmission spectra, normalized by that of an unpatterned substrate, are illustrated in Figure 3a. The two curves are well-

approximately one-sixth of the applied wavelength of light. Figure 1 shows the schematic and the scanning electron microscope (SEM) images of the twisted-arc photonic metamaterial, in which false colors are used to illustrate the two-layered metallic arcs with well-defined angular offsets. We first performed full-wave electromagnetic simulations to elucidate the chiral optical characteristics in the proposed metamaterial. In the numerical modeling, the refractive indices of the substrate and the spin-on dielectric are set to 1.50 and 1.41, respectively, and tabulated data with adjusted loss factors accounting for fabrication imperfections are used for the dielectric function of silver.26,27 Within a prescribed wavelength range, the transmission of circularly polarized waves is critically sensitive to the spin angular momentum of light, as shown in Figure 2a. The transmission minima experienced by each circular polarization are separated by over 100 nm. The circular state of polarization is largely preserved in the transmitted wave, with a transmission conversion of less than 0.07 between the two circular polarizations (see Supporting Information). To elucidate the nature of the resonance behaviors in the transmission spectra, in Figure 2d−g we plot the distribution of the electric current density under selected illumination conditions. For both resonance modes of LCP at 1.29 μm and RCP at 1.47 μm, we observe a strongly induced current flowing in the arc pair in a rotating manner, which indicates a powerful twisting effect of the light field when the circularly polarized wave of proper handedness passes through the metamaterial. For the opposite polarization (e.g., LCP at 1.47 μm), the induced current becomes much weaker. This indicates a limited interaction between the light and the structure and, hence, a much higher transmittance. The twisted-arc meta-atom can be viewed as a coupled resonator system, where the strong chiral responses arise from the coupling between the two arcs.28,29 This effect is better visualized in the current mapping when the system is on-resonance. The induced currents in the upper and lower arcs flow against each other when the structure is under LCP illumination (Figure 2d) and flow in the same direction in the case of RCP incidence (Figure 2g). These features in the current distribution indicate that the antisymmetric and symmetric modes are excited in the twisted-arc structure at the LCP and RCP resonances, respectively. As the antisymmetric mode represents a higher energy in the coupled system than its symmetric counterpart, the LCP resonance occurs at a shorter wavelength than that of RCP, which agrees well with the numerical and experimental observations. Other than the transmission contrast, another characteristic of a chiral response is the rotation of the polarization angle when a linearly polarized light beam travels through the medium, an effect conventionally known as optical activity. The rotation angle θ for linear polarization is related to the transmission coefficients following θ = [arg(tR) − arg(tL)]/2, where tR and tL denote the complex transmission coefficients of RCP and LCP waves, and arg represents the phase angle. The optical activity originates from the difference between the indices of refraction nRCP and nLCP by θ = πt(nRCP − nLCP)/λ, where λ is the wavelength in free space and t is the effective thickness of the medium. In general, the linear state of polarization is not preserved in a chiral material, and the ellipticity of the transmitted wave is connected to the power transmittance TR = |tR|2 and TL = |tL|2 by η = atan[(TR − TL)/ (TR + TL)]/2. This allows us to deduce the chiral characteristics from measured data based on circularly polarized incidence.

Figure 3. Circular dichroism in the chiral metamaterial. (a) Measured transmission spectra of LCP (blue) and RCP (red) light waves at normal incidence. (b) Ellipticity η of transmitted light obtained from experiments (purple) and simulations (orange).

separated, with the transmission dip located at 1.31 μm (1.40 μm) for the LCP (RCP) illumination. At the RCP resonance wavelength of 1.40 μm, the transmittance values are 0.35 and 0.70 for RCP and LCP, respectively, which correspond to a circular dichroism of 35% in the absolute value, or 3 dB in decibels. The measured spectra are in fairly good agreement with the simulated results in Figure 2a, with discrepancies possibly arising from fabrication imperfections and geometrical variations. The wavelength dependence of ellipticity deduced from the measured transmission data are shown in Figure 3b, which fits reasonably well with the numerical results in Figure 2b. A pure linear polarization rotation is expected at a wavelength of 1.35 μm, where the transmittance TR and TL are perfectly balanced (TR = TL) and the transmitted light would exhibit a vanishing ellipticity (η ≈ 0). The polarization rotation of light passing through the chiral metamaterial was probed when the sample was illuminated with linearly polarized light at normal incidence. The polarization angle of the incident wave was set at φ (see Figure 1a for definition of coordinates), and the output light was analyzed using a linear polarizer cascaded with a polarization- and wavelength-calibrated spectroscopy system. Since linear polarization is not always preserved, the longer axis of the polarization ellipse is used to identify the polarization angle in general. Figure 4a depicts the polarization rotation angle θ as a function of the wavelength of linearly polarized light with φ = 90°. The experimental data nicely follow the trend predicted by numerical simulations, which is included in the same figure. A significant polarization rotation of −50° is observed at the wavelength around 1.35 μm, which translates to a polarization C

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Figure 4. Polarization rotation in the chiral metamaterial. (a) Dependence of the polarization rotation angle (θ) on the wavelength of linearly polarized light. The incident wave is polarized at φ = 90°, which corresponds to the y-axis in Figure 1a. The measured data (red circles) are plotted along with the simulated result (solid green curve). (b−d) Polar diagrams for the polarization states of the transmitted light at a series of wavelengths, including (b) 1.10 μm, (c) 1.35 μm, and (d) 1.60 μm, respectively.

rotation power of as large as 305° per λ. To the best of our knowledge, this is the largest value of optical rotation power observed in bilayered structures in the near-infrared region. Despite the strong chiral resonance, a transmission of over 50% is achieved at the wavelength where the polarization rotation angle reaches its maximum magnitude. Away from the prescribed operational band, the optical activity tails off, as indicated in both Figure 4a and the polar polarization diagrams in Figure 4b−d. For example, at wavelengths of 1.1 and 1.6 μm, the metamaterial exhibits a near-zero optical rotation and the linear polarization is perfectly preserved, as indicated by the ∼sin2ϕ polar function illustrated in Figure 4b and d. We note that the polarization rotation of linear polarized light (Figure 4a) does not unambiguously determine the optical activity in the sample. This is because the unit cell of our chiral metamaterial does not possess a perfect in-plane rotational symmetry, and the linear birefringence induces a polarizationangle dependent optical rotation power. Such an effect has been observed in a number of chiral structures with different levels of rotational symmetry10,12 and has been mitigated to a certain extent by a complicated arrangement of meta-atoms with substantially improved in-plane isotropy.13,14 To specify the linear birefringence and obtain the actual optical activity in the proposed metamaterial, we performed polarization analysis for linearly polarized incidence at a series of azimuthal angles φ (see the Supporting Information for details). The dependence of the polarization rotation angle (θ) on the orientation of the incident linear polarization at the wavelength of 1.35 μm is depicted in Figure 5a, which reveals the impact of the linear anisotropy on the polarization rotatory power. The actual optical activity is quantified as the average value of θ ≈ −13° within the deeply subwavelength metamaterial layer, which translates to approximately 76° per free space wavelength. The index contrast Δn = nRCP − nLCP at 1.35 μm deduced from the measured polarization rotation for different incident polarization angles φ is illustrated in Figure 5b, where an average

Figure 5. Optical activity and circular birefringence of the chiral metamaterial. (a) Polarization rotation angle (θ) as a function of the azimuthal angle (φ) of the linearly polarized incident light for the wavelength of 1.35 μm. The dashed lines indicate the maximum and average value of θ observed in the experiments, respectively. The quantity of θave reveals the true level of the optical activity in the metamaterial. (b−c) The difference in refractive indices, Δn = nRCP − nLCP, within (b, λ = 1.35 μm) and away from (c, λ = 1.10 or 1.60 μm) the chiral resonance as a function of φ. The dashed line in b indicates the average index contrast of −0.42 in the refractive indices for circular polarizations of opposite handedness. The empty circles represent the measured results, and the solid curves are to guide the eye.

index difference (Δn)ave of −0.42 is obtained within the central region of the chiral response band. In contrast, away from the chiral resonance at, for example, 1.10 and 1.60 μm, the polarization rotation and the circular birefringence are negligibly small, as indicated in Figure 5c. We have investigated the giant chiral response from a photonic metamaterial consisting of an array of dual-layered twisted-arcs. The neat design and the relatively straightforward fabrication procedure make it possible and practical for us to produce enormously strong circular dichroism and optical activity in the near-infrared frequency range. Further developments of this study may include the downscaling of the structure for the visible spectrum of light and the improvement in the rotational symmetry by suppressing the linear birefringence. Given the pronounced chiral effects and the deeply subwavelength thickness of the structure, we envision applications of the chiral metamaterial for ultracompact polarization components in integrated photonics, plasmonicenhanced sensing of biochemical substances with enantiomers, and circular dichroism spectroscopic analysis at the chip-scale. D

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ASSOCIATED CONTENT

S Supporting Information *

Detailed fabrication process, optical characterization methods, impact of linear birefringence on polarization rotation, and geometrical scaling effect on the strength of chirality. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1-404-894-8911. Fax: 1-404894-0560. Address: 777 Atlantic Drive NW, Atlanta, GA 303320250, USA. Author Contributions

Y.C. and L.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the start-up fund provided to W.C. from the Georgia Institute of Technology. S.R. acknowledges the support of the National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DGE-1148903.

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