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Letter pubs.acs.org/journal/apchd5

Leveraging Superchiral Light for Manipulation of Optical Chirality in the Near-Field of Plasmonic Metamaterials Lei Kang, Qiang Ren, and Douglas H. Werner* Department of Electrical Engineering and Center for Nanoscale Science, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Methods for generating enhanced chiroptical response are of great importance in biological and biochemical applications primarily due to the ubiquitous presence of chiral substances in the organic world. Exhibiting an unprecedentedly strong chiroptical response, metamaterials are considered as a good candidate for achieving the highly sought after enhanced optical chirality. Here we demonstrate that optical chirality in the near-field of planar chiral metamaterials can be controlled by the so-called superchiral light composed by two counter-propagating beams of circularly polarized light (CPL) of the opposite handedness. In contrast with the scenario of single CPL excitation, continuous manipulation of optical chirality including the handedness selective enhancement and switching effect is observed. Moreover, the volumetric examination reveals that the enhanced optical chirality is much more delocalized, indicating a further improved accessibility for plasmonic nanostructure based enantiomeric sensing. Finally, we demonstrate the potential of the proposed optical chirality manipulation for optical information processing in a proof-of-concept study involving a coherent imaging system. KEYWORDS: optical chirality, superchiral light, coherent, plasmonic, metamaterials

C

hiroptical response or, in particular, optical enantioselectivity enables a quantitative approach to differentiating chiral objects that exhibit identical scalar physical properties, based on their distinguishable optical interaction with circularly polarized light (CPL). Due to the fact that biochemical substances with enantiomers, i.e., pairs of stereoisomers that are mirror images of each other, are ubiquitous in the organic world, sensing and analyzing by means of chiroptical response is of extreme importance to the biological and biomedical sciences. For instance, by offering structural, thermodynamic, and kinetic information, circular dichroism (CD) spectroscopy, which exploits the difference in the absorption of CPL of the opposite handedness, has been widely employed in the quantitative analysis of the secondary structure or conformation of large biological molecules.1 Nevertheless, considering the extremely weak chirality present in most natural media, spectroscopic techniques capable of probing chiral macromolecules such as proteins and DNA at the nanogram level or lower still remain highly desirable. Beyond the applications in biosensing, chiral photochemistry,2,3 as an interdisciplinary method bridging asymmetric (or enantioselective) synthesis4 and photochemistry, has been demonstrated as an environmentally friendly technique for producing unequal stereoisomeric products, which are highly beneficial for, for example, drug effectiveness and safety. Furthermore, chiroptical response potentially offers additional degrees of freedom in optical information processing, which, however, may be based upon highly efficient manipulations of optical chirality in a reversible and dynamic manner. First introduced in a purely mathematical framework,5 the time-even pseudoscalar now referred to as optical chirality, © XXXX American Chemical Society

C=

ε0 E·∇ 2

×E+

1 B ·∇ 2μ0

× B (where ε0 and μ0 are the

permittivity and permeability of free space, respectively, while E and B denote the complex electric and magnetic fields, respectively), has been verified as a physical quantity describing the excitation rate (A) of an individual chiral molecule, following A ∝ αωUe − βC. The α and β in this expression correspond to the imaginary parts of the electric polarizability and the mixed electric−magnetic dipole polarizability of the molecule, respectively, while ω and Ue are the angular frequency and the local electric energy density of the surrounding field, respectively.6 As the microscopic mechanism for circular dichroism, the difference in the excitation rate A of a molecule illuminated by CPL (ΔA = A+ − A−, where the signs denote the handedness) is determined by the optical chirality that the molecule senses. Following a time-averaged (simplified) form of εω optical chirality,7,8 i.e., C = − 02 Im(E*·B), the value of C εω

± = ± 20c |E|2 , corresponding to CPL can be expressed as CCPL where the signs indicate the handedness. However, C±CPL, the maximum optical chirality that can be achieve by a plane wave,9 is rather moderate due to the intrinsic structure of CPL. On the other hand, the expression of the excitation rate also implies that, for certain analytes, solely increasing the intensity of CPL does not necessarily lead to more information in the CD signal, while high-level exposure may be problematic for biosubstance due to, for example, the photothermal effect. Compared with natural

Received: January 20, 2017 Published: March 20, 2017 A

DOI: 10.1021/acsphotonics.7b00057 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic of two counter-propagating beams of equal-intensity circularly polarized light of opposite handedness. In this scenario, electric field vectors of both waves rotate in the same clockwise direction and an optical standing wave along the z-axis is expected (not shown). The projections of the fields onto the xy-plane are also plotted on the right, in which the field vectors produce deconstructive interference associated with a phase difference between the two waves of Δφ = π. (b) Illuminating a chiral nanostructure with superchiral light of variable phase Δφ enables coherent manipulation of optical chirality arising from the interference of the chiral field in the near-field regime. (c) Field mappings in a plane z = 60 nm for the case depicting single CPL impinging on a gammadion structure unit cell at a wavelength of 2.01 μm. |E0| (|H0|) is the magnitude of the electric (magnetic) field of the incident wave.

polarized wave allow us to attribute the observed optical chirality manipulation to the coupling between the plasmon resonances during chiral excitations of the superchiral light. We also demonstrate that the proposed chiroptical manipulation holds for various planar plasmonic metamaterials. Furthermore, by presenting a proof-of-concept study, we illustrate that a combination of planar chiral metamaterials and chiral compound thin films can potentially enable controllable imaging systems.

media, plasmonic chiral metamaterials have been utilized to demonstrate an orders of magnitude enhancement in the interaction with CPL. In particular, remarkably enlarged optical chirality has been generated in the vicinity of three-dimensional (3D) metamolecules.10−23 Nonetheless, it has been pointed out that, in general, the huge optical chirality produced by 3D chiral metamaterials would be too localized to be effectively sensed by analytes.7 On the other hand, the significantly enhanced and easily accessible optical chirality of planar nanoarchitectures, such as spiral and gammadion nanostructures, offers the potential for applications in improved chiroptical analysis. In addition to the understanding of optical chirality, Tang and Cohen have proposed an alternative embodiment for a better enantioselectivity by introducing the so-called superchiral light that is constructed by two counter-propagating beams of CPL of opposite handedness.6,24 The interference leads to greatly enhanced chiral asymmetry around the nodes of the standing wave and enables the enhanced excitation rate of chiral molecules localized in these regions. However, the corresponding thickness of the functional layer is severely restricted (on the order of ∼10 nm),24 which is challenging in practical applications. Furthermore, we note that superchiral near-fields can be generated with different forms of excitation wave polarization.25,26 In this paper, we show that by introducing superchiral light the accessible optical chirality of planar plasmonic metamaterials can be significantly enhanced and, more importantly, can be purposely tailored in a coherent manner, including a nearcomplete suppression. Studies of the excited near-field of chiral plasmonic nanostructures impinged by a single circularly

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RESULTS AND DISCUSSION Compared with linearly (and randomly) polarized waves, circular polarization, where the electric field vector E rotates uniformly in a plane at the wave frequency, creates electromagnetic waves that produce a helix-shaped pattern as they propagate. This property offers a unique opportunity for circularly polarized waves to interact with the special class of media that possess molecularlevel chirality. However, the corresponding interaction strength is in general restrained by the dissymmetry of CPL. By exploiting the interference between two counter-propagating plane waves of CPL with opposite handedness, i.e., of superchiral light, Tang and Cohen investigated the significantly enhanced chiral asymmetry compared with that of a single circularly polarized wave.6,24 Figure 1a presents a schematic showing the two components of superchiral light, in which the electric field vectors of both waves rotate in the same clockwise direction and accordingly may result in an optical standing wave along the zaxis (not shown). Here, we define a phase difference Δφ between the two waves, which represents the relative phase difference between the electric field vectors of the two waves in the z = 0 B


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Figure 2. Field maps of the gammadion structure for different circularly polarized excitation schemes. Different components of the (a) electric and the (b) magnetic field in a unit cell are plotted in a plane at z = 60 nm for illumination by a single CPL wave (as illustrated in Figure 1b) at a fixed wavelength of 2.01 μm.

Figure 3. Evolution of the optical chirality in the vicinity of the gammadion structure as a function of Δφ. The 2D maps of the optical chirality in a unit cell are considered at z = 60 and −60 nm planes for two scenarios: superchiral light with polarization matching (first row in (b) and (d)) and mismatching (second row in (b) and (d)) the handedness of the gammadion. For comparison, the corresponding optical chirality resulting from excitation of a single CPL wave is also included in (a) and (c).

enhanced enantiomeric sensing.7,8 In sharp contrast to superchiral light, the proposed improvement in the excitation rate A of chiral molecules mainly arises from the enhanced optical chirality. It should be noted that, despite the pronounced farfield chiroptical responses that have been intensively studied in 3D metamaterials, enhanced optical chirality with better accessibility has been demonstrated in planar nanostructures. For instance, a 20-fold increase in optical chirality was reported within the 100-nm-thick region above a nanospiral structure,7 which distinguishes it from the extremely localized behavior found in superchiral light. A closer examination of the nanostructure-based approach reveals that, owing to locally

plane. Under this definition, the interference nodes that facilitate the enhanced asymmetry discussed in ref 6 were correlated with Δφ = nπ (where n is an odd integer), corresponding to the vectors depicted on the right of Figure 1a along with the projection of the waves’ trajectory. It can be seen that the electric field density at these nodes vanishes (when |ELCP| = |ERCP|) due to the interference, which provides the origin of the enantioselectivity enhancement, while, simultaneously, the energy density of magnetic field achieves its maximum, indicating that the optical chirality is independent of position along the z-axis.27 The handedness-selective chiral field observed in the vicinity of plasmonic chiral metamaterials suggests a distinct approach to C


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Figure 4. Enhancement of optical chirality in three-dimensional space in the vicinity of the gammadion structure as a function of Δφ. (a) and (c) ((b) and (d)) correspond to the scenario in which the gammadion is simultaneously excited by a +z-propagating LCP (RCP) wave and a −z-propagating RCP (LCP) wave. The ranges of z (e.g., z ∈ [10, 110] nm) indicate the regions over which the integration was performed in order to determine the optical chirality (see Figure 1b for the coordinates used in the calculation). The results represent the volumetric enhancement of optical chirality relative to that achieved by excitation with only a single circularly polarized wave.

present a series of detailed plots of the simulated field distribution in Figure 2, which clearly reveals the phase relationship between the different field components. As can be seen from Figure 2a and b, each pair of the field components corresponding to the two excitations exhibits features of similarity in both amplitude and phase, and the slight discrepancy is attributed to the nonzero magnetoelectric parameter associated with the gammadion structure.27 It is not difficult to deduce that, for the scenario where we have simultaneous illumination with a pair of CPL waves as illustrated in Figure 1b, interference between the plasmon resonance induced chiral fields can be expected. This provides the basis for manipulating optical chirality in a dynamic manner. Furthermore, we note that, for superchiral light with opposite polarization (RCP and LCP waves propagating along the z and −z direction, respectively), the interference of the induced chiral field proposed here may hold, as illustrated in the following discussion. To illustrate this control capability, in Figure 3 we depict the evolution of the optical chirality in the vicinity of the gammadion structure as a function of Δφ, which was obtained by employing a superchiral light excitation in the simulation domain (see Methods for details). We note that, to indicate the enhancement and the handedness of the local chiral field, the optical chirality results shown in Figure 3 have been normalized by |CCPL|, i.e., the optical chirality of the CPL without the nanostructure present.7 As shown in each row of Figure 3, compared with the single CPL illumination scenario (Figure 3a and c, which were obtained by respectively substituting the field data shown in Figure 2 and Figure S1 into the equation of optical chirality), the optical chirality of the structure dramatically varies with Δφ, the phase difference between the two components of the superchiral light. In particular, it is evident that when Δφ is around zero, optical

enhanced electric (especially the component perpendicular to the surface of the structure, i.e., Ez) and magnetic fields in the near-field regime, a plasmon resonance facilitates the enhanced optical chirality. Although magnetism is generally negligible in optics, a plasmon-enabled enhanced magnetic field may play a significant role in nanostructure-based enantiomeric sensing, recalling that C ∝ Im(E*·B). Indeed, a phase shift between E and B is the basis of a nonzero optical chirality. In fact, as we discuss below in detail, beyond the enhancement, the reciprocity28 and the relatively weak chirality of planar chiral nanostructures would imply that there is a coherent manipulation of optical chirality. We note that achiral structures, which inherently do not yield CD signals, have also been reported to assist optical chirality enhancement in localized regions due to the near-filed magnetoelectric coupling29,30 and excitation of surface plasmon polaritons,31 but are beyond the scope of this work. Without loss of generality, we first consider a planar gammadion structure28 illuminated with LCP and RCP waves propagating along the z and −z direction, respectively, as depicted in Figure 1b. Being normal to the z-axis, the 20-nmthick nanostructure is located at the origin of the coordinate system. In the depicted scenario, the polarization of both CPL waves matches the handedness of the gammadion, which brings about a moderate enhancement in C.7 We note that, for CPL of the opposite handedness, the following discussion regarding the near-field excitation would still be valid but with a different absolute value. The simulated field distribution in a plane 50 nm above the surface of the gammadion (z = 60 nm) is shown in Figure 1c (see Supporting Information for field maps in the z = −60 nm plane). Prominent enhancements in both the electric and magnetic fields are observed for both excitations. To gain a better understanding of the excited chiral field behavior, we D


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Figure 5. Universality study of the proposed coherent manipulation of optical chirality in planar chiral nanostructures. Three-dimensional rendering of the optical chirality enhancement as a function of Δφ for a (a) gammadion (w1 = g1 = 80 nm; λres = 2.01 μm), (b) spiral (w2 = 80 nm, g2 = 53 nm; λres = 1.52 μm), (c) twisted split-ring resonator (SRR) (w3 = 50 nm, g3 = 100 nm; λres = 1.78 μm), and (d) dual-asymmetric SRR (w4 = 100 nm, g4 = 125 nm, g5 = 50 nm where the arc angle difference is 40°; λres = 1.84 μm).

single circularly polarized wave excitation, i.e., En3D SCL = ∫ CSCL dV/∫ CSingle dV. The corresponding direct comparisons of the optical chirality are provided in the Supporting Information. It can be seen that, when Δφ varies around 0 (or 2π), the C enhancement is spatially selective and closely related to the handedness matching condition. Poulikakos and coauthors have shown that this behavior is related to the fact that the optical chirality density is not a conserved quantity in nonvacuum systems.25 In the case of handedness matching (Figure 4a and c), the pronounced enhancement only accumulates in the region z > 0. In contrast, for superchiral light of the opposite handedness (Figure 4b and d), significant enhancement is observed for negative values, indicating that the sign of C is flipped when z < 0, especially within the region rather close to the structure surface (e.g., z ∈ [−110, −10] nm). Results in Figure 4c indicate the further improved accessibility of the optical chirality, which is attributed to the constructive interference of the fields in the vicinity of the chiral nanoarchitectures.7,32−34 On the other hand, consistent with the results shown in Figure 3, the “switching-off” effect observed in Figure 4 as Δφ approaches π is obviously independent of the handedness matching condition and region of examination. It is worth pointing out that, besides the phase, field amplitude modulation of the superchiral light provides another degree of freedom in this control, especially when the reciprocity of the chiral nanostructures is considered. In order to provide a complete picture, we have performed a comprehensive study of the superchiral light facilitated manipulation of the optical chirality, the results of which are reported in the Supporting Information. We emphasize that this superchiral light enabled drastic optical chirality manipulation is in fact a universal phenomenon in planar chiral metamaterials. To elucidate this further, we calculate the 3D chiral field of three more typical planar nanostructures and

chirality experiences a pronounced enhancement relative to the case of single CPL excitation. More importantly, optical chirality vanishes or, namely, is switched off when Δφ approaches π. For other values of Δφ, the optical chirality varies continuously. This observed behavior of optical chirality suggests that the potential excitation rate A of chiral molecules in the vicinity of the nanostructure can be modulated dramatically as a function of the phase of the superchiral light. In addition, it is clear that the variation of this optical chirality exhibits a phase periodicity of 2π, which unambiguously reveals the coherent nature of the proposed manipulation scheme. It can also be seen that, including local sign flipping, the optical chirality at certain spatial positions varies asymmetrically relative to Δφ = 0, revealing the highly complex nature of the chiral field interference in the nearfield regime. Furthermore, Figure 3 reveals that the proposed manipulation of optical chirality can be applied to superchiral light with polarization that matches and mismatches the handedness of the planar nanostructure and that holds in different regions of interest (e.g., the planes at z = 60 and −60 nm). We note that, in order to achieve reasonable comparisons, the total input power (P) used in the simulations was set to a SCL constant value, i.e., PSCL = PSingle where PSCL = PSCL LCP + PRCP and SCL SCL PLCP = PRCP. As discussed previously, planar chiral nanostructures offer the potential for creating a new class of enhanced enantiomeric sensing devices due to their accessible optical chirality. In light of this, it is essential to examine the optical chirality created by superchiral light in the three-dimensional space around the nanostructure. Figure 4 illustrates the corresponding spatially dependent enhancement of optical chirality produced by the gammadion structure as a function of Δφ. It is obtained by evaluating the ratio between the 3D integral of the optical chirality produced by the superchiral light excitation and that of a E


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Figure 6. Chiroptical imaging system based on the manipulation of the optical chirality corresponding to planar chiral metamaterials. (a) Schematic of the chiroptical imaging platform for the proof-of-concept demonstration. A phase-modulated superchiral light wave, which is obtained as the combination of a uniform LCP wave and an RCP wave with predetermined phase profile, is utilized to achieve a desired optical chirality distribution. For simplicity, the RCP wave is set to carry a periodic phase modulation for every fifth unit cell in the x direction. From the left to right unit cells, two RCP wave phase patterns, i.e., [0.50, 0.25, 0, 0.25, 0.50]π and [1, 0.75, 0.50, 0.25, 0]π, are considered, and the corresponding enhanced optical chirality is shown in (c) and (d), respectively. For comparison, results of a single uniform LCP wave excitation (k ∥ z) are also included in (b).

superchiral light. This offers the potential for developing enhanced chiroptical phenomenon based displays. We emphasize that the feature size of the proposed imaging system may not be dominated by the geometric dimensions of the individual metamolecules, but rather by those of the secondary components, such as the width of the arms of the gammadions and the slits of the asymmetric split-ring resonators (SRRs), which accordingly may be utilized to achieve images with subwavlength information. Furthermore, considering the nature of plasmon resonances of planar nanostructures, we envision that the proposed imaging system would be less wavelength-sensitive, while less optical chirality contrast between meta-atoms that experience a different relative phase is expected when the system is operated at wavelengths away from resonance peaks. In summary, we have demonstrated superchiral light based coherent manipulation of optical chirality in the near-field of planar chiral metamaterials. Compared with that induced by a single circularly polarized excitation, pronounced enhanced as well as highly suppressed optical chirality can be achieved by illuminating a chiral nanostructure with counter-propagating beams of CPL with the opposite handedness, i.e., superchiral light with a properly chosen phase. The enhanced optical chirality is much more delocalized, indicating a further improved accessibility for plasmonic nanostructure based enantiomeric sensing. Furthermore, by performing a proof-of-concept study, we demonstrate that the proposed optical chirality manipulation can potentially enable chiroptical platforms such as coherently controllable imaging systems.

illustrate the corresponding enhancement of optical chirality (relative to that of CPL) in Figure 5. For comparison, those obtained from single circularly polarized excitation are also included in the left column of Figure 5. The manipulation as a function of the superchiral light phase Δφ is explicit, although the magnitude of enhancement varies between structures, originating from their intrinsic chiroptical response. In fact, beyond the absolute value enhancement, the observed control of C illustrated in Figure 3, along with the unit-cell nature of metamaterials, potentially provides a means for constructing chiroptical platforms that offer more degrees of freedom for optical information processing. Here we demonstrate this concept by presenting a proof-of-concept study on a controllable imaging system that consists of an array of gammadion structures covered by a chiral thin film, for instance, that exhibits excitation rate A dependent fluorescence emission.24 Without loss of generality, as illustrated in Figure 6a, we consider superchiral light as the excitation with a specific wavefront profile: the LCP wave component that propagates along the z direction has a uniform phase, while the counter-propagating RCP wave component possesses a periodic phase modulation in the x direction.35 This corresponds to superchiral light with the simple 1D periodic (along the x-axis) phase distribution of Δφ. By employing periodic boundary conditions, we calculate the chiral field and obtain the optical chirality enhancement maps over a period of five metamolecules driven by the phase-modulated superchiral light, as illustrated in Figure 6c and d. For the sake of comparison, the results for a single LCP wave excitation (k ∥ z) with a uniform wavefront are also included in Figure 6b. Consequently, when the metamaterials are covered by, for example, a 10-nm-thick film consisting of enantioselective fluorophores,24 the induced fluorescence intensity can then be encoded according to the local phase distribution of the

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METHODS Simulation. Full-wave electromagnetic simulations were performed using the COMSOL multiphysics software package. To calculate the optical chirality, the complex field amplitudes F


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(9) Canaguier-Durand, A.; Genet, C. Chiral Near Nields Generated From Plasmonic Optical Lattices. Phys. Rev. A: At., Mol., Opt. Phys. 2014, 90, 023842. (10) Plum, E.; Fedotov, V. A.; Schwanecke, A. S.; Zheludev, N. I.; Chen, Y. Giant Optical Gyrotropy Due to Electromagnetic Coupling. Appl. Phys. Lett. 2007, 90, 223113. (11) Decker, M.; Ruther, M.; Kriegler, C. E.; Zhou, J.; Soukoulis, C. M.; Linden, S.; Wegener, M. Strong Optical Activity from Twisted-cross Photonic Metamaterials. Opt. Lett. 2009, 34, 2501−2503. (12) Liu, N.; Liu, H.; Zhu, S. N.; Giessen, H. Stereometamaterials. Nat. Photonics 2009, 3, 157−162. (13) Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; Freymann, G.; Linden, S.; Wegener, M. Gold Helix Photonic Metamaterial as Broadband Circular Polarizer. Science 2009, 325, 1513−1515. (14) Decker, M.; Zhao, R.; Soukoulis, C. M.; Linden, S.; Wegener, M. Twisted Split-ring-resonator Photonic Metamaterial with Huge Optical Activity. Opt. Lett. 2010, 35, 1593−1595. (15) Zhao, Y.; Belkin, M. A.; Alu, A. Twisted Optical Metamaterials for Planarized Ultrathin Broadband Circular Polarizers. Nat. Commun. 2012, 3, 870. (16) Abdulrahman, N. A.; Fan, Z.; Tonooka, T.; Kelly, S. M.; Gadegaard, N.; Hendry, E.; Govorov, A. O.; Kadodwala, M. Induced Chirality Through Electromagnetic Coupling Between Chiral Molecular Layers and Plasmonic Nanostructures. Nano Lett. 2012, 12, 977−983. (17) Hentschel, M.; Schaferling, M.; Metzger, B.; Giessen, H. Plasmonic Diastereomers: Adding up Chiral Centers. Nano Lett. 2013, 13, 600−606. (18) Frank, B.; Yin, X. H.; Schaferling, M.; Zhao, J.; Hein, S. M.; Braun, P. V.; Giessen, H. Large-Area 3D Chiral Plasmonic Structures. ACS Nano 2013, 7, 6321−6329. (19) Yin, X.; Schäferling, M.; Metzger, B.; Giessen, H. Interpreting Chiral Nanophotonic Spectra: The Plasmonic Born-Kuhn Model. Nano Lett. 2013, 13, 6238−6243. (20) Cui, Y. H.; Kang, L.; Lan, S. F.; Rodrigues, S.; Cai, W. S. Giant Chiral Optical Response from a Twisted-Arc Metamaterial. Nano Lett. 2014, 14, 1021−1025. (21) Kang, L.; Lan, S. F.; Cui, Y. H.; Rodrigues, S.; Liu, Y. M.; Werner, D.; Cai, W. S. An Active Metamaterial Platform for Chiral Responsive Optoelectronics. Adv. Mater. 2015, 27, 4377−4383. (22) Esposito, M.; Tasco, V.; Cuscunà, M.; Todisco, F.; Benedetti, A.; Tarantini, I.; Giorgi, M. D.; Sanvitto, D.; Passaseo, A. Nanoscale 3D Chiral Plasmonic Helices with Circular Dichroism at Visible Frequencies. ACS Photonics 2015, 2, 105−114. (23) Zhao, Y.; Askarpour, A. N.; Sun, L.; Shi, J.; Li, X.; Alù, A. Chirality Detection of Enantiomers Using Twisted Optical Metamaterials. Nat. Commun. 2017, 8, 14180. (24) Tang, Y.; Cohen, A. E. Enhanced Enantioselectivity in Excitation of Chiral Molecules by Superchiral Light. Science 2011, 332, 333−336. (25) Poulikakos, L. V.; Gutsche, P.; McPeak, K. M.; Burger, S.; Niegemann, J.; Hafner, C.; Norris, D. J. Optical Chirality Flux as a Useful Far-Field Probe of Chiral Near Fields. ACS Photonics 2016, 3, 1619− 1625. (26) Kramer, C.; Schäferling, M.; Weiss, T.; Giessen, H.; Brixner, T. Analytic Optimization of Near-Field Optical Chirality Enhancement. ACS Photonics 2017, 4, 396−406. (27) Bliokh, K. Y.; Nori, F. Characterizing Optical Chirality. Phys. Rev. A: At., Mol., Opt. Phys. 2011, 83, 021803. (28) Wang, B.; Zhou, J.; Koschny, T.; Kafesaki, M.; Soukoulis, C. M. Chiral Metamaterials: Simulations and Experiments. J. Opt. A: Pure Appl. Opt. 2009, 11, 114003. (29) Alizadeh, M. H.; Reinhard, B. M. Plasmonically Enhanced Chiral Optical Fields and Forces in Achiral Split Ring Resonators. ACS Photonics 2015, 2, 361−368. (30) Nesterov, M. L.; Yin, X.; Schäferling, M.; Giessen, H.; Weiss, T. The Role of Plasmon-Generated Near Fields for Enhanced Circular Dichroism Spectroscopy. ACS Photonics 2016, 3, 578−583.

were obtained from unit-cell-based simulations using periodic boundary conditions. A simple Drude model was employed for the material property of gold, ε(ω) = 9 − ωp2/[ω(ω + iωc)], where the plasma frequency is ωp = 1.37 × 1016 Hz and the scattering frequency is ωc = 1.2 × 1014 Hz. For simplicity, all simulated nanostructures were embedded in air. To investigate the manipulation behavior, the nanostructures were independently excited by two counter-propagating circularly polarized waves of the opposite handedness, and then, the interferenceenabled chiral fields were obtained as a superposition of those resulting from the two individual excitation channels. Rendering. The software package POV-Ray was utilized to better visualize the 3D distribution of the optical chirality in the vicinity of the metamolecules. Three-dimensional optical chirality data obtained from simulations were imported and rendered as transparent media, along with the schematic of the nanostructures. A detailed discussion of this visualization method can be found in ref 7.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00057. Additional simulation results, including near-field excitation by a single circularly polarized light wave; absolute optical chirality in 3D regions; manipulation of the optical chirality enhancement; the corresponding manipulation of absolute optical chirality (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (D. H. Werner): [email protected]. ORCID

Lei Kang: 0000-0001-7718-7756 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Penn State Materials Research Science and Engineering Center (MRSEC, NSF DMR1420620).

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REFERENCES

(1) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism: Principles and Applications; Wiley-VCH: New York, 2000. (2) Inoue, Y. Asymmetric Photochemical Reactions in Solution. Chem. Rev. 1992, 92, 741−770. (3) Ramamurthy, V.; Schanze, K. S. Organic Molecular Photochemistry; Marcel Dekker: New York, 1999. (4) IUPAC, Compendium of Chemical Terminology, 2nd ed. (the ″Gold Book″); 1997. (5) Lipkin, D. M. Existence of a New Conservation Law in Electromagnetic Theory. J. Math. Phys. 1964, 5, 696−700. (6) Tang, Y.; Cohen, A. E. Optical Chirality and Its Interaction with Matter. Phys. Rev. Lett. 2010, 104, 163901. (7) Schäferling, M.; Dregely, D.; Hentschel, M.; Giessen, H. Tailoring Enhanced Optical Chirality: Design Principles for Chiral Plasmonic Nanostructures. Phys. Rev. X 2012, 2, 031010. (8) Schäferling, M.; Yin, X.; Engheta, N.; Giessen, H. Helical Plasmonic Nanostructures as Prototypical Chiral Near-Field Sources. ACS Photonics 2014, 1, 530−537. G


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(31) Alizadeh, M. H.; Reinhard, B. M. Enhanced Optical Chirality through Locally Excited Surface Plasmon Polaritons. ACS Photonics 2015, 2, 942−949. (32) Hendry, E.; Carpy, T.; Johnston, J.; Popland, M.; Mikhaylovskiy, R. V.; Lapthorn, A. J.; Kelly, S. M.; Barron, L. D.; Gadegaard, N.; Kadodwala, M. Ultrasensitive Detection and Characterization of Biomolecules Using Superchiral Fields. Nat. Nanotechnol. 2010, 5, 783−787. (33) Meinzer, N.; Hendry, E.; Barnes, W. L. Probing the Chiral Nature of Electromagnetic Fields Surrounding Plasmonic Nanostructures. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 041407. (34) Ogier, R.; Fang, Y.; Svedendahl, M.; Johansson, P.; Käll, M. Macroscopic Layers of Chiral Plasmonic Nanoparticle Oligomers from Colloidal Lithography. ACS Photonics 2014, 1, 1074−1081. (35) Kao, T. S.; Jenkins, S. D.; Ruostekoski, J.; Zheludev, N. I. Coherent Control of Nanoscale Light Localization in Metamaterial: Creating and Positioning Isolated Subwavelength Energy Hot Spots. Phys. Rev. Lett. 2011, 106, 085501.

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