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Plasmonic Chiral Nanostructures: Chiroptical Effects and Applications Yang Luo, Cheng Chi, Meiling Jiang, Ruipeng Li, Shuai Zu, Yu Li, and Zheyu Fang* (circular birefringence) and absorption losses (circular dichroism) with the circu larly polarized light (CPL) illumination.[10] CB arises from the difference in the real part of refractive index, leading to a dif ferent velocity for LCP and RCP compo nents, and thus results in the polarization rotation of the linearly polarized incident light. CD corresponds to the difference in the imaginary part of refractive index, resulting in a distinct absorption loss for LCP and RCP excitations. Besides the con ventional CD and CB, asymmetric trans mission (circular conversion dichroism) is another fundamental chiroptical pheno menon, which exists in the non-diffracting array, referring to different LCP-to-RCP and RCP-to-LCP conversion efficiencies.[11] All of these chiroptical phenomena have been successfully applied in the spectros copy for identifying special arrangements of chiral matters in biology, chemistry and physics as efficient diagnostic tools.[12–16] However, the chirop tical response in natural chiral materials is relatively weak due to the small electromagnetic (EM) interaction volume,[17] hence limits its further applications. Recent progress in plasmonics paves the way for the enhancement of chiroptical response.[18–20] Surface plasmons (SPs), as the collective electrons oscillation at the dielectric and metal interface,[21–23] present the capacity of light confine ment and field enhancement, which significantly improve the strength of light-matter interactions.[24–28] With the up-to-date nanofabrication technology, the study field of chirality has been extended from traditional chiral molecules to 3D metallic nanostructures.[29–32] Chiroptical responses of metallic meta molecules have been widely investigated,[33–35] and applied in various fields, such as biosensing,[36] chiral catalysis,[37] polari zation tuning,[38] and chiral photo detection.[39] The 3D metallic structure exhibited giant optical activity response because of the strong interaction between electric and magnetic resonant modes.[40,41] Different from 3D chiral ensembles, planar chiral structures show none chiral effect, as they can always coincide with their mirror images. However, 2D chirality was successfully found in the quasi-two-dimensional (quasi-2D) chiral structure.[42] Moreover, recent reports show that even achiral nanomaterials have the ability to generate strong CD under an oblique CPL illumination.[43] This kind of extrinsic chirality arises from sym metry breaking of the incident light and the quasi-2D material, which is quite different from the intrinsic chirality of 3D chiral

The plasmonic chiroptical effect has been used to manipulate chiral states of light, where the strong field enhancement and light localization in metallic nanostructures can amplify the chiroptical response. Moreover, in metamaterials, the chiroptical effect leads to circular dichroism (CD), circular birefringence (CB), and asymmetric transmission. Potential applications enabled by chiral plasmonics have been realized in various areas of nanoscience and nanotechnology. In this review, both basic theories and state-of-the-art studies on plasmonic chiroptical effects are summarized. Molecular chiroptical effects are drastically enhanced by metallic nanostructures that can generate a “superchiral” field, which arises from the strong electromagnetic interactions. Both intrinsic and extrinsic plasmonic chiral metamaterials formed by the periodic arrangement of metallic nanostructured units show high levels of CB, CD, and asymmetric transmission. Consequent applications including photo detection, molecular sensing, and chirality tuning are discussed, and a perspective of emerging concepts such as Pancharatnam− Berry (PB) phase in this booming research field is presented.

1. Introduction Chirality refers to a certain handedness in geometry with a mirror image that cannot coincide with itself.[1,2] DNA double helix, sugar, quartz, cholesteric liquid crystals and biomolecules are chiral structures. Mirror images with opposite chirality are enantiomers, and can be divided into left- and right-hande dness, respectively. Three-dimensional (3D) chiral materials usually show two important chiroptical responses: circular bire fringence (CB), which means the capacity to induce the rotation of the polarization plane,[3] and the circular dichroism (CD), which implies different absorption levels for left-handed (LCP) and right-handed circularly polarized (RCP) light.[4,5] Meas uring chiroptical response is a critical method to identify enan tiomers, and is important in life science, analytical chemistry, biochemistry and medical science.[6–9] The difference of both real and imaginary parts of the refrac tive index of the chiral material results in distinct phase delay

Y. Luo, C. Chi, M. Jiang, R. Li, S. Zu, Y. Li, Prof. Z. Fang State Key Lab for Mesoscopic Physics Academy for Advanced Interdisciplinary Studies Collaborative Innovation Center of Quantum Matter Peking University Beijing 100871, China E-mail: [email protected]

DOI: 10.1002/adom.201700040

Adv. Optical Mater. 2017, 5, 1700040

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structures. Understanding the origin of these chiroptical effects allows easier design of nanostructures and control of chiral interactions. Therefore, a systematic summary of this emerging field is highly desired. With the introduction of the basic theory and recent advances of the plasmonic chirality, we present a system atic and proactive thinking of the field. We believe that the deep understanding of plasmonic chiroptical effects can promote fur ther interdisciplinary studies and stimulate future applications. In this review, traditional chiral molecules and metallic met amolecules are first introduced. Then, the chiroptical response of quasi-2D and 3D periodic arrays, and potential applications for optical force, detecting and sensing, as well as chirality tuning are discussed. Finally, we summarize this review and give a perspective of the Pancharatnam–Barry phase for the chiral investigation.

2. Chiral Molecules, Chiral Metamolecules and Chiral Interactions

A± =

2.1. Chiral Molecules In the natural world, there are various chiral molecules caused by diverse symmetry breaking, for example, central chi rality (Figure 1a), helical chirality (Figure 1b), axial chirality (Figure 1c) and planar chirality (Figure 1d), inspiring the inves tigation of chiral properties. Based on the study of chirality, abundant applications have been realized in many fields of nanoscience, such as material synthesis and catalysis,[44] optical communications,[45] light harvesting,[46] and light emitting devices.[47] When the chiral structure interacts with monochromatic light, based on the Maxwell’s equations and the general consti tutive relations,[10] an electric dipole p and a magnetic dipole m can be produced p = α E − iGB m = χ B + iGE

Zheyu Fang is a professor at the School of Physics, Peking University, China. He received his Ph.D. in physics from Peking University with Prof. Xing Zhu. Then he worked with Prof. Naomi J. Halas and Prof. Peter J. Nordlander as Postdoc at Rice University. His current research interests are plasmonics, near-field optics, and optoelectronic nanomaterials and devices.

(1)

where α represents the electric polarizability, χ represents the magnetic polarizability and G is the isotropic mixed electricmagnetic dipole polarizability. As for the electric quadrupole, although its contribution is in the same order as that of mag netic dipole, its contribution to the differential absorption aver ages to zero in isotropic materials. Based on Equation (1), the rate of excitation of molecules A is[48]

ω 2 α ′′ E + χ ′′ B 2

2

± G′′ω Im ( E * ⋅ B )

(2)

where A+ and A− are the absorption rates of RCP and LCP, respectively. The above derivation implies that chiral molecules have dif ferent absorption rate to the left- and right-handed polarized light. This optical response difference is named as circular dichroism (CD). CD spectroscopy has become an important method to characterize chirality of molecules, and can be defined as

CD =

A+ − A− A+ + A−

(3)

Besides CD, circular birefringence (CB) can also be used to study chiroptical effects. When light passing through a chiral structure, the difference in velocity between LCP and RCP can result in the polarization plane rotation of the incident light, which can be described by the index of refraction as, ∆n = n + − n −

(4)

where n+ and n− are the real part of refraction index for RCP and LCP, respectively.

Figure 1. Different kinds of natural chiral molecules. a) Central chiral molecule. b) Helical/spiral chiral molecule. c) Axial chiral molecule. d) Planar chiral molecule.


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CD and CB, as two of chiroptical phenomena, which depend on the imaginary and real part of the material refraction index, are connected with each other via Kramers–Kronig relationship.

2.2. Plasmonic Chiral Metamolecules When metallic nanostructures, especially noble metal, illu minated by light with proper energy and momentum, SPs can be excited. Surface plasmon polaritons (SPPs) and local ized surface plasmons (LSPs) have been used to enhance the electric field, and excite higher electric and magnetic modes, leading to a series of fantastic optical phenomena and applications,[49–52] such as the surface-enhanced Raman spectroscopy,[53] extraordinary optical transmission,[54] tipenhanced Raman spectroscopy,[55] and enhanced fluorescence spectroscopy.[56] With the development of nanotechnology, a deeper insight of SPs was further revealed, stimulating lots of active research fields, such as the quantum plasmonics,[57] graphene plasmonics,[58] Fano resonance,[59] and nonlinear nano-optics.[60] Combined with plasmonic metal materials, multifarious chiral metamolecules have been designed.[30,61,62] Com paring to the natural chiral molecule, artificial metamolecules acted as versatile platforms to manipulate electric field at the nanoscale with negative index.[63] Because SPs are sensitive to the surrounding medium, and the material permittivity and morphology, plasmonic chiral metamolecules can display strong chiroptical response and high sensitivity to the CPL incidence. However, there are still some obstacles for the future applica tion. The fabrication of complex metastructures is limited by the condition of current technology, especially for the precise fabrication and manipulation molecules at the nanoscale. And the enhancement mechanism of plasmonic chiral structures is still unclear. Moreover, how to achieve chiroptical response in the visible range needs more efforts. In this section, we introduce different chiral metamolecule structures that were designed to solve the above problems. Top-down approaches such as electron-beam lithography (EBL),[64,65] focused ion beam(FIB),[66] glancing angle deposi tion (GLAD),[67] and nanoimprint methods[68] are used for the fabrication of plasmonic structures, which provide the access to plenty of far-reaching progress in the investigation of meta molecules. Figure 2a shows the SHG-CD signal of gold G-shape chiral nanostructure fabricated via the EBL method.[42] In this structure, the SHG signal showed a ratchet wheel pattern. By comparing the SHG-CD signal of different G-shape arrange ments, it could be inferred that the arrangement of the struc ture was important for the emission signal, and the suprastruc tural plasmon modes were displayed by ratchet wheel patterns which depended on the handedness of the structure. This work has profound significance on the relationship between CD and chiral structure, opening a route for the chirality study. Besides the G-shape nanostructure, the planar gold hep tamer nanostructure was reported to show strong chiroptical response at the Fano resonance (Figure 2b),[69] which origi nated from the interference of magnetic quadrupole and electric toroidal modes, and the Fano interference enhanced


the field interaction between the metallic nanoparticles, resulting in the strong chiroptical response. By carefully manipulating the separation distance and the interparticle angle of heptamers, a nearly 30% chiral response at Fano resonance was achieved, which is much higher than the pre vious works. Additionally, a two layered L-shape stereo structure as shown in Figure 2c, was fabricated by the top-down approach. The method allowed precise control of the relative position between L-shape gold molecules in different directions, leading to various chiroptical phenomena.[70] As the horizontal displace ment S between two L-shape gold nanostructures increasing, the CD signal decreased to nearly vanishing and then increased with opposite sign value. The origin of this phenomenon was that the horizontal displacement S between the two L-shapes resulted in an inversion of the whole plasmonic mode of the L-shape ensemble, leading to the reversion in the sign of the CD spectrum. By manipulating the relative displacement of two L-shapes, the control of CD spectrum can be realized by tuning hybridized plasmon modes. Molecular self-assembly approach, as a relatively cheaper, faster and more productive bottom-up method to assemble metamolecules, is especially used for the fabrication of stereo structures.[71] It provides an effective way to easily control and tailor the arrangement of nanoparticles, and to form chiral metamolecules with complicated geometry. The strategy for constructing nanomolecules based on the DNA structure was first proposed in 1996, which provided a method to allow syn thesizing much more complex structures for future studies.[72] Based on this idea, many nanostructures with complex geom etries have been fabricated. As shown in Figure 2d,[73] by taking the advantage of the handedness of DNA structures, gold nanoparticles were assembled into helical arrangement at the DNA scale and displayed enhanced CD signals. When plasmonic waves were propagating along the gold nanohel ical structure, the absorption of CPL, with the handedness in accordance with the structure, was largely enhanced. Addi tionally, through depositing silver nanocoatings on gold par ticles, the blue-shift of the CD peak was observed, because of the plasmon resonance wavelength for silver is shorter than the one for gold. This implied the potential ability to control the chiroptical response by the molecular self-assembly based on DNA structure. Other molecules were also found that can act as predominant roles in assembling chiral structures. In Figure 2e, sodium carbonate (SCA), sodium citrate (SCI) and DNA molecules that worked as connectors to link the gold nanorods, successfully assembled side-by-side (SBS) gold nanorod pairs.[74] Here, DNA molecules were left-handedness, SCA and SCI linkers were racemic and achiral, respectively. SBS oriented pairs and “ladders” displayed chiral signals by breaking the centrosymmetry of parallel structures through producing dihedral angles between nanorods. By choosing different triggers, different enantiomeric conformations can be achieved: DNA linkers gave rise to the levorotatory enan tiomer, and SCA/SCI triggers resulted in the dextrorotatory enantiomer. Planar V-shaped gold dimer also can be obtained by the molecular self-assembly method, as illustrated in Figure 2f. By inducing an extra oblique CPL excitation, the planar gold

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Figure 2. a) The SHG signal acts as a function of angle, and oriented circles indicate LCP and RCP light. The inset is the SEM image of planar gold G-shape nanostructure, and the scale bar is 2.4 μm. Reproduced with permission.[42] Copyright 2009, American Chemical Society. b) Scattering intensity spectrum of the gold nanoheptamer under the illumination of normal incident LCP and RCP light. The inset is a SEM image of the structure, and the scale bar is 200 nm. Reproduced with permission.[69] Copyright 2016, Royal Society of Chemistry. c) Illustration of horizontal displacement S between two L-shapes and the corresponding SEM image. Reproduced with permission.[70] Copyright 2015, American Chemical Society. d) Illustration of DNA tailoring the arrangement of nanoparticles to form self-assembled gold nanohelices. Reproduced with permission.[73] Copyright 2012, Nature Publishing Group. e) Schematic view of chiral gold NRs dimers made with SCA, SCI and DNA linkers. Reproduced with permission.[74] Copyright 2013, Nature Publishing Group. f) Schematic of extrinsic chirality of the planar V-shaped dimer caused symmetry breaking by oblique CPL incidence. Reproduced with permission.[75] Copyright 2014, Royal Society of Chemistry.

nanorod dimer yielded chiroptical response.[75] The selfassembly V-shape gold nanorod dimer was an achiral structure with planar mirror symmetry along the direction of the angle bisector of V-shape. However, the nanorod cannot geometrically coincide with its mirror image because the mirror symmetry is broken by the external oblique excitation. This kind of chirop tical response can be considered as the extrinsic chirality, or pseudochirality.[76]


2.3. Chiral Interactions Understanding of the chiral interaction is important, which helps us get access to the insight of physics, and on the other hand, principles can be used to guide potential applications. For example, a recent study proposed a quantum network on the basis of chiral-light-matter interaction.[82] The chiral interac tion originated from two quantum dots, which can build the

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quantum network, allowed high quality two-qubit parity inter modal measurements. In order to quantify the enhancement of chiral response, optical chirality (OC), which is represent by C, is introduced as:[83]  1 1 C = ε 0 E ⋅ ∇ × E + B ⋅ ∇ × B  2 µ0 

(5)

where E and B represent the electric field and magnetic field, respectively. In this section, we mainly discuss the interactions between 1) light and plasmonic chiral nanostructures, 2) plasmonic chiral nanostructures and achiral molecules, and 3) plasmonic achiral nanostructures and chiral molecules. The structural and electromagnetic origin of chiroptical properties of plasmonic nanostructures has been theoretically studied. Comparing the optical chirality difference among various plasmonic nanostructures, from 2D to 3D, the principle of tailoring enhanced optical chirality has been demonstrated (Figure 3a).[77] For planar two-armed spiral that showed non stereochirality, its strong near-field optical chirality has been observed with C ≈ 50, which was higher than that in planar gammadion. Besides, the difference of spatial distribution of C between LCP and RCP illumination was easily to be observed, which was different from that of 3D chiral structures. For the 3D twisted split SRR, strong far-field chiroptical response could be obtained, and the spatial distribution of C for LCP and RCP excitation were the same. This work has provided a route for the design of plasmonic nanostructures with high optical chirality. The study of optical chirality C gave a way to describe chiral structures, which can further the study of light-meter interac tions and stimulate future applications, for example, enhanced localized chiral hot spot in nanospiral can be used to increase the adsorption rate of biomolecules. Hybridizations of chiral plasmonic nanostructure and inor ganic achiral structure have generated interesting chiroptical response. As illustrated in Figure 3b, the coupling between charge carriers of achiral ZnO nanopillars and gold chiral shell resulted in chiral signal of achiral ZnO nanopillars.[78] The handedness of chiral gold nanocluster can be “transferred” to its adsorbate achiral molecule, and Figure 3c is the schematic of Au38(2-PET)24 nanocluster.[79] In this ensemble, the interaction between achiral molecules and chiral gold nanostructure broke the mirror symmetry, resulting in the chirality of molecules. The interaction between achiral plasmonic nanostructures and chiral molecules also has attracted tremendous interests. Many works have been reported to explore inherent proper ties and yielded a series of interesting phenomena. As the structure shown in Figure 3d, when the molecule was placed between a plasmonic dimer, and the dimer axis was parallel to the molecular dipole, CD signal originated from chiral mole cule can be enhanced in the visible spectral range.[80] The hot spot generated from metallic dimer amplified the Coulomb interaction between the dimer and chiral molecule. Moreover, the interaction resulted in strong dissipation of the dimer struc ture at plasmon wavelength, which was the origin of giant CD signal. Apart from the near-field enhancement mechanism, the far-field coupling can also contribute to CD enhancement.


Near-field mechanism that refers to the interaction in a rela tively short scale (in general, the short scale interaction refer ring to the interaction distance r < λ /2π), is a localized effect with rapid decay velocity, while the far-field interaction, which exists in the r ≥ λ/2π range, is a long range effect with rela tively low damping. The AFM image of the hybrid nanostruc ture composed of the achiral gold cross array and chiral Flavin mononucleotide (FMN) film was shown in Figure 3e.[81] The CD signal was obtained as the film thickness increase to several hundred nanometers, which was beyond the scope of nearfield mechanism. The reason for the phenomenon was that the radiative coupling between the achiral gold cross and chiral molecule induced giant plasmonic chirality in the visible range, which was much more efficient than that from near-field mech anism, and could reach the factor of 103.

3. Periodic Chiral Nanostructures Periodic nanostructures, due to the repeated arrangement of unit cell along x- and y-direction, display a series of unique optical properties.[84–86] For example, for photonic crystal, its repetitive arrangement of refractive index makes it easy and effective to manipulate optical properties.[87] When light passing through periodic subwavelength hole arrays, extraor dinary optical transmission arises.[88] The interaction between periodic chiral metallic nanostructures and CPL, gives rise to interesting and unique chiral phenomena.[89–91]

3.1. Jones Matrix The optical response of periodic chiral structures can be described by the frequency-dependent Jones matrix, which is a 2 × 2 matrix with four complex elements. In the linear metamaterial, the transmission field Et can be described as  txx Et =   tyx

txy   I x  tyy   I y 

(6)

where t is the transmission matrix, and x and y representing different polarization directions. When the incident light is cir cular polarized, Equation (6) can be written as  t E t =  ++  t−+

t+−  0 E t−− 

(7)

In this equation, + and – are RCP and LCP, respectively, and E0 is the incident field. Based on this, CD can be described as 2

2

2

∆tsymmetry = t++ − t−− = txx − tyy

2

(8)

Besides CD, the asymmetric transmission is described as the different LCP-to-RCP and RCP-to-LCP conversion efficiencies, which are reversed for opposite propagation directions of the incident light. Through a series of coordinate transform, asym metric transmission can be expressed as

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Figure 3. a) Optical chirality enhancement of double spiral planar structure (left) and twisted split ring resonator (SRR) (right). Reproduced with permission.[77] Copyright 2012, American Physical Society. b) Schematic view of left- and right-handed structures composed of chiral gold shell covering on achiral ZnO nanopillar. Reproduced with permission.[78] Copyright 2013, American Chemical Society. c) Illustration of Au38(2-PET)24 nanocluster. Reproduced with permission.[79] Copyright 2015, Nature Publishing Group. d) Schematic of the coupling effect between chiral molecule and plasmonic dimer. Reproduced with permission.[80] Copyright 2013, American Physical Society. e) AFM image of the gold cross array with a FMN film. Reproduced with permission.[81] Copyright 2012, American Chemical Society.

2 2 2 2 ∆t asymmetry = t+− − t−+ = txy − tyx = −∆t asymmetry

(9)

Here, arrows indicate the propagation direction of forward and backward. It can be inferred from Equation (9) that the asym metric transmission results from the offdiagonal elements t+ and t- +. 3.2. Quasi-2D Periodic Chiral Nanostructures Metallic chiral nanostructures exhibit exceptional proper ties when separated structures are configured into periodic arrays. For each unit, the in-plane rotating manipulations are inequivalent to their mirror reflections. For periodic arrays, the near-field condition is influenced by the adjacent structure


unit, resulting in the generation of unusual modes, and several diffraction poles can be observed if the structure period is longer than the illuminating beam wavelength. The polariza tion dependent spectroscopy has been observed in a metallic gammadion structure array on a dielectric substrate. Under the illumination with different polarizing states, the gammadion array displayed the ability of elliptization to diffracted beams, as shown in Figure 4a.[92] Quantities of chiral quasi-2D metas tructures were presented, which revealed attractive physics and inspired more practical applications.[95–97] However, some geo metrically chiral structures presented little CD signal in both near-field and far-field under the illumination of LCP and RCP (upper part of Figure 4b).[93] Recently, experimental observation of chiral near-field dis tributions has been achieved by a scattering-type scanning near-field optical microscopy method (SNOM) under the

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Figure 4. a) Azimuth rotation of right- and left-handed chiral gammadion structures, inset is SEM image of the structure. Reproduced with permission.[92] Copyright 2004, Elsevier. b) SEM image, near-field electric distribution and the corresponding transmission spectra of periodic rosette structure (upper) and Fano resonant antennas (lower). Reproduced with permission.[93] Copyright 2016, American Chemical Society. c) Schematic view of optical conversion of CPL by quasi-2D chiral structure (left). Asymmetric transmission spectra of four types of quasi-2D chiral metamaterials under the forward/backward illumination (right). Reproduced with permission.[94] Copyright 2008, American Chemical Society. d) Transmission spectra of periodic SRRs at different incidence angles, the insets show the rotating axis of samples. Reproduced with permission.[43] Copyright 2012, American Physical Society.

illumination of CPL. The distribution of the local CD signals coming from chiral nanostructures explained the relation ship between chiroptical signal and nanoscale chirality.[91] A recent work showed that circularly polarized selective nearfield distribution explained the far-field chiral optical reac tions, in which several propagating waves in different direc tions were observed, and can analogize to nanoantennas (upper part of Figure 4b).[93] In this single-layered structure which was in rosette geometry, it was believed that a signif icant magnetic mode was lost and thus caused weak chiral response. Besides, it can be explained by the short arm of rosette structure. Because the arm was much shorter than the wavelength, no propagating wave could be supported by the structure, while only the standing wave existed on the arm of the structure, which was similar to single-layered 1-turn Archimedean spiral structure. In those antenna arms, only linear polarized spiral radiation was induced, thus the near-field electrical distribution was almost the same under the LCP and RCP illumination. In another single layer asymmetric structure, the near-field distributions were significantly different for LCP and RCP


illumination. As shown in the lower part of Figure 4b, this metasurface was comprised of several nanoantennas, which supported coupled dipoles and monopoles.[93] Under LCP illumination, both monopole and dipole modes were excited in the antenna, while the field was negligible in monopole antenna and the horizontal wire when they were under the RCP illumination. The strong chiral near-field reaction came from the Fano resonance coupling between the electrical dipole and monopole, thus the additional absorption was induced in the antennas structure only under LCP illumina tion and finally caused strong CD signal in the far-field. The method of real space mapping technique enabled the experi mental observation of chiral near-field distributions in the asymmetric nanoantenna, and provided an alternative way to analyze and design quasi-2D plasmonic chiral structures. Considering the space resolved observation ability, hot spots in the plasmonic chiral structures, where the near-field chi rality was stronger, can be observed and located. This was inspiring because biomolecules can be detected in the chiral optical field, while the sensitivity benefited from the chiral field density. By constricting biomolecules in those hot spots,

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it was promising to realize advanced biosensing solutions of enhanced sensitivity. Quasi-2D nanostructured plasmonic metamaterials also showed asymmetric transmission properties at opposite inci dent directions. A research of this distinctive property was reported that according to the Lorentz Reciprocity Lemma, mag netization was necessary for asymmetric propagation in most optic media. However, it was possible that the transmission of two sides of the medium was different if it could partially convert the polarization state of the light.[98] As shown in the left part of Figure 4c, after transmitted through the plasmonic structure, a portion of the incident RCP beam was converted to LCP light, while for opposite incident directions the ratio was different.[94] In the right part of Figure 4c, the existence of asymmetric transmission was proved by the illuminating circu larly polarized beam from opposite incident direction. Besides of the intrinsic chirality, extrinsic chirality was found in quasi-2D chiral structures. A periodic structure has been reported, of which the comprising unit showed both 2D and 3D symmetry, while the scattering signal showed optical chi rality when the incident beam broke the symmetry.[76] Generally speaking, optical chirality came from the near-field electrical distribution supported by asymmetric plasmonic structures. For quasi-2D structures with C3v and higher symmetry, like gammadion structures, chirality results from the symmetry breaking along the z-axis of the substrate. For structures with low rotational symmetry, chirality actually originated from modes. However, some symmetric structures exhibit similar optical chirality when the excitation light is at an oblique inci dent angle. It has been proved that every quasi-2D nanostruc ture had the intrinsic nature of extrinsic chirality, by inves tigating polarizing tensors and scattering cross sections of SRRs under chiral illumination. When electric polarizability and magnetic polarizability were nonzero in the ring structure under an oblique excitation, maximum transparency could be reached, while the minimum emerged under the vertical inci dence. Non optical activity was observed at a normal incident angle. Figure 4d shows the transmission spectra of SRRs under LCP and RCP illumination at different incident angles, which can be modeled as LC resonators.[43] The magnetic mode was excited at the wavelength of 1600 nm which corresponded to the minimum in transmission spectrum under normal illumi nation. The LC resonance only depended on Ex and Hz compo nent, and was independent with Ey. Additional Hz component was provided by the illuminating beam at increasing incident angle, which leaded to decreasing transmission. Under RCP illumination, the transmission increased continuously and became nearly transparent at −50 degree, which was similar to the situation with opposite handed illumination at 50 degree incident angle. Additionally, when the incident axis and struc tural axis of split rings were in the same plane, this system was achiral, because it was without the symmetry breaking caused by the incident beam.

4. Applications

3.3. 3D Periodic Chiral Nanostructures In this part, we discuss periodic chiral stereostructures and their chiroptical responses. 3D metastructure as right-handed


helices with two pitches shown in the inset of Figure 5a, was introduced to generate broadband CD signals.[99] The normal incidence transmission spectrum showed that the transmit tance of LCP light (depicted in red) was much larger than that of the RCP light (depicted in blue) in a broad spectrum from 3 to 6.5 µm, which is promising for the broadband cir cular polarizer device. Besides, the “host–guest” hybrid system which introduced CD signal was shown in Figure 5b.[100] The introduced plasmonic superstructure was composed of artificial chiral and achiral molecules. The CD spectra of this plasmonic superstructure where the gold bar was shifted around the chiral component was measured by a Fourier-transform infrared spectrometer. The observed results showed that the unusual CD signal could be obtained at the resonance wavelength of the achiral part in the “host-guest” plasmonic metastructure, which matched the simulated results properly. At the resonance wavelength of the achiral component, the CD feature originated from near-field interactions was impressible to the structure inner position, which was meaningful in the field of ultrasensi tive chiral sensing. The CB within the chiral metamaterial was also revealed. Chiral effects of the twisted-arcs array with a thickness of around λ/6 has been measured. The SEM image in the inset of Figure 5c showed the structure of the chiral metamaterial. And the result showed the average index contrast of −0.42 in the refractive index within the central region for circular polariza tions of opposite handedness. This design offered a method to produce significant CD in the near-infrared wavelength range, opening the gate of applying chiral metamaterial in integrated photonics. Besides CD and CB, asymmetric transmission has also been discovered in stereo nanoarrays. The structure was shown in the inset image of Figure 5d.[102] The measurement results showed that when the incident light illuminated the metamaterial in opposite directions, elements of Jones matrix acted differently: the module of txy and tyx interchanged while txx and tyy remained the same. The spectra matched the sim ulation results properly. The experimental observation and theoretical analysis of asymmetric transmission in these low symmetry metamaterials, enriched the variety of transmis sion functionalities and provided ideas for designing photonic metamaterials.[98] As shown in the inset image of Figure 5e, a two-layered U-shaped SRR array was investigated. The meas ured transmission spectra (Figure 5e) revealed the relative peak locations and the transmission conversion difference of the x-polarized circularly incident light propagating from for ward (upper plane) and backward (lower plane) directions.[103] This asymmetric transmission along the normally incident direction polarized waves direction in this reciprocal struc ture, made the isotropic and linear materials potential can didates for future optical chirality tuning device at terahertz range.

As the basic theory and particular properties have been reviewed above, potential applications of plasmonic chiral nanostructures attract growing interests in lots of fields, such

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Figure 5. a) Transmission spectrum of right-handed helix with two pitches, which displays a broad CD signal. Reproduced with permission.[99] Copyright 2009, American Association for the Advancement of Science. b) CD spectrum of the hybrid plasmon structure. The inset is the schematic of “host-guest” structure. Reproduced with permission.[100] Copyright 2015, Royal Society of Chemistry. c) CB signal of dual-layered twisted–arcs array. The inset is the SEM image of the twisted-arcs unit. Reproduced with permission.[101] Copyright 2014, American Chemical Society. d) The squared modulus of the four Jones matrix elements of the two-layered superstructure. Reproduced with permission.[102] Copyright 2010, American Physical Society. e) Transmission spectra of forward (upper) and backward (lower) exiting light. The inset is the schematic of two-layered U-shaped SRRs. Reproduced with permission.[103] Copyright 2012, American Physical Society.

as chiral optical force, detecting, biosensing, switching and con trolling photoluminescence (PL).

4.1. Optical Force Light possesses momentum and energy, which can give driving force to particles by the interaction of light and plasmonic chiral nanostructure. Two types of optical forces are reviewed


here. One comes from linearly polarized light, and the other derives from CPL. Plasmonic nanomotors driven by linearly polarized light have been demonstrated.[104] Figure 6a showed the image of nanomotor. The golden gammadion was sandwiched between two layers of silica disks. When light illuminated on the plas monic gold nanostructure, optical force was generated and could drive the silica disk to rotate, even when the volume of the disk was 4000 times larger than the motor. As shown in

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4.2. Detecting and Sensing Artificial plasmonic chiral structures play an important role in detecting chirality, and in this part, we mainly discuss ultracompact CPL detector and chiral molecule sensing. Precise CPL detectors have been experi mentally manufactured. However, for con ventional CPL detectors, multiple optical elements impeded the development of inte grated CPL detection. A latest work presented an ultracompact CPL detector, which was composed by chiral plasmonic meta-molecule array, metal back plane and dielectric spacer (Figure 7a).[115] The distinguishable CPL signal was based on the control of hot elec tron injection and the manipulated CD of chiral plasmonic nanostructures. When CPL illuminated on the silicon wafer, the chiral metamaterial had the ability of selectively absorbing particular chiral photons and reflecting the other chiral photons. Then, the hot electrons with higher energy arose from the chiral metamaterial, overcame the Figure 6. Schematic of structures for optical force related to chirality. a) Illustration of plas- Schottky barrier and jumped to the Schottky monic nanomotors made up of manufactured gold gammadion nanostructure and two silica interface. Hence, the photocurrent can be disks. b) The rotation speed of 25-in-1 motor sample under the illumination of linearly polarized detected.[39] The reported CD spectrum light. Reproduced with permission.[104] Copyright 2010, Nature Publishing Group. c) Nano revealed that the CD signal of right-handed particle suffers optical lateral force under CPL incidence. Reproduced with permission.[114] metamaterials reached the peak of nearly Copyright 2015, Nature Publishing Group. 90% at the resonant wavelength under RCP illumination, and it performed similarly with left-handed metamaterials under LCP illumination. This ultra Figure 6b, dots were the measured data and the dashed line compact detector contributed to easily distinguishing LCP from was depicted corresponding to experimental data. The speed RCP through the camera images and photocurrent. Moreover, and direction of the torque can be tuned by adjusting the wave the change of a source-drain bias added to the Schottky diodelength of incident light. The green part with positive sign rep based photodetectors brought different photocurrent. On one resented the anticlockwise rotation, and the purple part repre hand, a negative one resulted in increasing photoresponsivity. sented the clockwise rotation. Besides, the torque of different On the other hand, a positive one led to high polarization dis motors can be added to rotate the same disk, which has been crimination ratio and selectivity. All these manipulations ben proved to be faster. Plasmonic nanomotors provided candidates efited the ultracompact CPL detectors to meet growing needs to for biological DNA reconfiguration and nanoscopic solar light practical applications, such as fiber optical sensors, communi reserve.[105–107] cation techniques and image devices.[115] In the last decades, optical tweezers driven by optical forces have been widely applied in moving, capturing and In most cases, chiral biomolecules detection has been rotating nanoparticles.[108–112] It was theoretically verified achieved by measuring the EM fields via manipulating plas monic structures.[116,117] It has increased the sensitivity of effec that CPL exerted on gold nanoparticles could modulate the spinning of nanoparticles,[113] which paved the way for nano tive refractive index measuring. Ultrasensitive detection of bio molecules with superchiral EM fields has been achieved.[118] stirring. In addition, lateral forces generated from the illumi [114] nation on particles explained by spin-orbit coupling theory The inset of Figure 7b shows the left-handed and right-handed gold gammadions. The three modes I, II and III represented have been reported. The lateral force directed along the light the peaks where the local index of refraction in the ambient propagation direction and at the same time, along other medium made a difference. The CD spectra reflected that directions. As shown in Figure 6c, particles obtained contrary superchiral EM distribution of planar chiral metamaterials scattered EM momentum and recoil force under incident (PCMs) were in mirror symmetry for left-handed and rightlight with different handedness, and the background wave handed gammadions. Besides, the marked three extremes described the distribution of surface plasmon polarization. By showed the maximum sensitivity to the refractive index tuning the polarization of the incident light, particles move changes of ambient medium, and the optical properties were towards different directions. The scale of lateral force was as affected by β-sheet of biomolecules. The phenomena provided large as other optical forces. Consequently, this demonstra tion offered possibilities to achieve more accurate control of advanced idea for biosensing[19] and biospectroscopy. Addition particles. ally, a recently study has provided an advanced method to detect


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increased to ≈55 zeptomoles, which offered a platform applied in biosensing and pharma ceutical synthesis.

4.3. Tuning Chirality Besides stereotypical chiral optical properties, control chirality are more suitable for the growing development of advanced devices. Tuning chirality reviewed here includes chiral switching, controlling photoluminescence (PL) and manipulating optical properties. Switching the chirality has attracted increasing attention for scientists due to the wide application in biology and pharma cology. Generally speaking, there are plen tiful methods to switch the handedness of materials by external stimuli, such as photo excitation,[122] electric field stimulus and tem perature control.[123] In most cases, chirality switching require the reconfiguration of the materials. A breakthrough has been made Figure 7. Schematic of structures for chirality detecting and sensing. a) Schematic of the ultra- that an active control was achieved without compact CPL detector. Reproduced with permission.[115] Copyright 2015, Nature Publishing rebuilding the structures. It was demonstrated Group. b) CD signals of opposite handed PCMs while dipped in distilled water. Reproduced that a kind of fabricated composite made up of with permission.[118] Copyright 2010, Nature Publishing Group. c) Schematic of detecting subwavelength metamaterials exhibited prop biomacromolecular secondary structure. Reproduced with permission.[119] Copyright 2016, erties of photoinduced handedness switching American Chemical Society. d) Schematic of detecting chiral enantiomers by the twisted metain terahertz wavelength.[124] The structure was material. Reproduced with permission.[121] Copyright 2017, Nature Publishing Group. fabricated to ensure the unit cell of periodic metamaterial was less than wavelength of terahertz waves. The reported results in Figure 8a showed that chiral biomacromolecular secondary structure.[119] A chiral both CD and ORD signals were reversed with photoexcitation solid-inverse structure was fabricated as shown in Figure 7c. from 0.9 to 1.14 THz. In addition, according to measurements One layer was in the shape of gammadion, and the other layer in ORD, the sign of ORD was reversed along with switching of was in the complementary geometry to the first layer. When chirality when the frequency was in 0.9 to 1.05 THz and 1.05 to chiral biomacromolecules entered the detecting structure, the 1.19 THz range, which was convenient to judge photoexcita EM field was changed, which responded to reflectivity spectra tion. Other evidence was the direction and major axis changes and chiroptical properties. According to the reflection spectra in polarization states. The demonstration provided an approach and optical rotatory dispersion (ORD), the front incidence was to switch the chirality of light. prior to the back incidence. Reflection spectra showed α-helical, Structures with particular handedness have the ability to β-sheet, and disordered motifs of the proteins under CPL illu control fluorescent emission selectively through the interac mination with different handedness. In the spectra, blue-shifts tion of metamaterials and incident light.[125] The active control can be observed because of the interaction of proteins and plas monic chiral structures. These results offered possibilities to of light-induced monolayer MoS2 fluoresce has been achieved design metamaterials and probe biomolecules chirality.[120] (Figure 8b).[126] The spin-orbit coupling[127,128] played a domi nant role in this process. The PL intensity was measured, In addition, enantiomers detection contributes a lot to drug which presented an obvious enhancement when the chirality of and medical applications. Owing to the weak CD signals, espe spiral ring was in accordance with the handedness of illumi cially in small scale, conventional detectors suffered limitations. nation light. On the contrary, when the spiral ring has the dif A progressive method to enhance CD effect for enantiomers ferent chirality with incident light, the enhancement cannot be detection has been provided, as shown in Figure 7d.[121] Twisted observed. A large increase in electric field intensity occurred in plasmonic metamaterial made up of a dielectric layer sand ring structure’s center. Except for the incident light polarization wiched with two metasurfaces has been fabricated. The metas state, the number of spiral rings also influenced the electric urfaces with different layers were placed with particular angles, field distribution. Under the illumination of LCP, rings with and the best angle was selected according to the measured CD even number turns arose from focusing effect while rings with spectra. This near-field effect for chirality enhancement took odd number turns cannot. The experiment provided potential the advantage of interactions between chiral metamaterial applications for spin-dependent light-emitting devices. inclusions and enantiomers. An obvious enhancement in CD It has been reported that chiral nanoantennas array could signals can be observed at the plasmonic resonance frequency. redirect the polarization state of light,[129] shown as Figure 8c. In this way, the sensitivity of detecting chiral molecules was


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Figure 8. Schematic of structures for chiral tuning chirality. a) CD spectrum before photoinduction (black curve) and after photoinduction (red curve). The inset is SEM image of the fabricated metamaterial. Reproduced with permission.[124] Copyright 2012, Nature Publishing Group. b) PL of MoS2 monolayer excited by CPL. The inset describes that spiral ring structures perform differently under CPL excitation. Reproduced with permission.[126] Copyright 2016, American Chemical Society. c) Schematic of plasmonic nanoantenna array which changes the direction of diffractive light. Reproduced with permission.[129] Copyright 2016, American Chemical Society. d) Measured ORD and reflectivity spectra of 100 nm and 30 nm TPSs with different handed “shuriken” structures. Reproduced with permission.[130] Copyright 2015, Wiley.

Nanoantennas were considered to be planar chiral structures.[98] For comparison, three arrays of nanoantennas with struc tures of achiral, D+ and D- were fabricated. The measured PL enhancement indicated distinguishable features between chiral structures under the excitation of LCP and RCP. Achiral struc ture responded similarly to CPL with different handedness. D+ antennas performed large PL enhancement for RCP and vice versa. It showed that the handedness of light can be changed by chiral structures. It has been presented that optical properties of a kind of plas monic arrays can been tuned by different thick films, shown as the insets in Figure 8d.[130] The fabricated template plasmonic substrates (TPSs) can be seen as a solid and inverse structure. The coupling of electric and magnetic fields enabled solid structure overlap with inverse one. Thus, the enhancement in hybridization of electric and magnetic modes coupling strength ened along with increasing film thickness. Moreover, the meas ured ORD and reflectivity spectra indicated the optical proper ties of different handed “shuriken” structures were changed relying on the film thickness. This demonstration offered a gate for artificially manipulating chiroptical properties in producing process, which was easier to applied in commercial fields.

5. Pancharatnam–Barry Phase Apart from the applications mentioned above, there are many phenomena waiting to be uncovered. Introducing chirality into


holography attracts our attentions and deserves research for its positive impact technology. In this field, the promotion in the efficiency of holography is a significant problem. The practical applications in holography are discussed here, and the related theories are mentioned in Figure 10. A designed structure that could highly improve the efficiency of holography was in discussion. Figure 9a shows the periodic hologram’s phase distribution and the operating principle.[131] The reflective com puter-generated hologram (CGH) was under a circularly polar ized incident beam. In this way, the holographic image can be produced by the reflected beam. The unit cell of the structure was composed of the MgF2 and gold film, with gold nanorods on the top. This design of geometric metasurface hologram can help the diffraction efficiency reach to 80% at 825nm, taking account of metal’s ohmic loss from visible to near-infrared wavelength. Compared to the conventional methods with the efficiency below 50%,[136] this design was a great progress in improving light utilization. 3D technique is of popular utilization for its convenience. A system to realize this kind of 3D optical holography has come into being. The hologram structure and reconstruction proce dure are represented in Figure 9b. Nanorods played as pixels of diffractive elements, and they can generate noninterval local phase profile under the incidence of CPL. When the metas urface which consisted of those nanorods was illuminated by CPL, the desired continuous local phase profile can be gener ated, which resulting in the opposite handedness transmis sion.[132] In order to avoid aliasing effects and offer enough

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Figure 9. Different kinds of holography. a) Illustration of the reflective computer-generated hologram (CGH) under the incident of CPL. Reproduced with permission.[131] Copyright 2015, Nature Publishing Group. b) Hologram structure and reconstruction procedure of 3D optical holography. Reproduced with permission.[132] Copyright 2013, Nature Publishing Group. c) Spin- and wavelength-dependent holography. Reproduced with permission.[133] Copyright 2016, Nature Publishing Group. d) Schematics of the helicity multiplexed metasurface hologram. Reproduced with permission.[134] Copyright 2015, Nature Publishing Group. e) Achromatic and highly dispersive metaholograms. Left: The achromatic metahologram generates the same image colors in the far field. Right: The highly dispersive metahologram independently projects distinct images. Reproduced with permission.[135] Copyright 2016, American Chemical Society.

computer-generated holography information, the number of pixels was controlled to realize the 3D object reconfiguration, in which the 3D holographic image arose in the Fresnel scale. Besides, nonlinear optics has attracted great attention for its special optical characteristics. And it can be applied into holography to realize selecting signals. As the spin- and wave length-dependent holographic images shown in Figure 9c, reconstructions of two letters “L” and “R” were in different cir cular polarization states.[133] As the incident wave remained the same, the phases of LCP and RCP were −φ and −3 φ, respec tively. These two geometric phases were generated in the for ward direction, where φ was called the Berry Phase. Thus the differences of the phase shifts on nonlinear signals for different circular polarization states were obvious. Their applications in holography were plentiful for different images which can be obtained here through controlling the transmitted light fre quency and spin. Similarly, introducing helicity multiplex into metasurface hologram can also realize this control. Figure 9c shows the schematics of the helicity multiplexed metasurface hologram.[134] The irradiation of LCP light that brought the reconstruction of different images on the opposite sides was in accordance with the incident light. And the locations of images “flower” and “bee” reversed when the illumination changed between LCP and RCP light. Each silver nanorod together with the background layer performed as a reflective-type halfwave plate. The reflected light had two circular polarization states and one of them had the additional phase delay called PB phase. So changes in the helicity of the CPL attributed to


reversing the phase profile. This work offered a method to realize multiplexed functionalities. The achromatic and highly dispersive holograms, contribu ting to improving the quality of the hologram images, have been in need for research. Figure 9e shows a related experi mental setup.[135] The achromatic metahologram generated the same image (a “flower”) for all red (633 nm), green (532 nm), and blue (473 nm) colors in the far field. The highly dispersive metahologram independently projected distinct red, green, and blue images: a “flower”, a “peduncle” and a “pot”, corre spondingly. And the metasurface fabricated by dielectric has been of great application in hologram. The light phases after the metasurface for different wavelengths were manipulated independently. Using the Gerchberg–Saxton (GS) algorithm, the required phase distributions were retrieved and then con verted into the in-plane orientations of the corresponding nanoblocks.[137] Related theories applied in holography have been studied, and they have potential application in many fields. The defi nition of PB phase have brought emerging concept such as PB phase metamaterial. As shown in Figure 10a,[138] the n-th nonlinear dipole moment can be expressed as Pnωθ,L = αθ(ELσ)n, and through transferring the reference, we have Pnωθ,σ = Pnωθ,L,σ e−σθi ∝ e(n−1)iσθ and Pnωθ,-σ = Pnωθ,L,-σ eiσθ ∝ e(n+1)iσθ. The nonlinear polarizabilities of this nanorod structure were αnωθ,σ,σ ∝ e(n−1)iσθ and αnωθ,−σ,σ ∝ e(n+1)iσθ. In this way the geometric phases (n−1)σθ and (n+1)σθ which have different circular polarization compared to the incident

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Figure 10. Pancharatnam−Berry (PB) phase and generalized laws of refraction and reflection. a) Principle of geometric-phase-controlled metamaterials. Left: The Rotation of the nanostructure in the laboratory frame introduces a geometric phase. Right: circular polarizations are generated. b) THG signals of metasurface. Reproduced with permission.[138] Copyright 2015, Nature Publishing Group. c) Illustration of the generalized Snell’s law of refraction. d) Experimental setup for y-polarized excitation. Reproduced with permission.[139] Copyright 2011, American Association for the Advancement of Science. e) Illustration of a refract dipole array. Reproduced with permission.[140] Copyright 2012, American Chemical Society. f) Schematics of the generation of SPP wakes by aperture antenna arrays. Reproduced with permission.[141] Copyright 2015, Nature Publishing Group.

wave, can be introduced. The image above showed its con crete effect on the linear wave that (1−1)σθ = 0 and (1+1)σθ = 2σθ these two geometric phases were formed along the forward direction. This generation of circularly polarized phase can also exist in nonlinear optics. Figure 10b shows third harmonic generation (THG) signals of metasurface. The phase-controlled diffraction of the RCP light was in fundamental range. RCP light and LCP light with circularly polarized THG signals were diffracted to the first and second order separately on the metasurface with nanorod structure with C2 symmetry. It was observed that the THG signals with different polarization to the fundamental beam were general ized on this C2 metasurface, and the THG signals with the different circular polarization were produced on that with the C4 symmetry sample. It was in accordance with the selection rule that the nanostructure with different symmetries can tune the signals of THG. The generalized laws of refraction and reflection, originated from the generalized Snell’s law, have been in consideration for the related anomalous phenomena. The derivation of the gen eralized Snell’s law of refraction was shown in Figure 10c,[139] where θt and θ meant the angles of the refraction light and the incident light, respectively, and dx was the distance between crossing points. The generalized Snell’s law took the abrupt phase shift into consideration and adjusts the Snell’s law to metamate rials. According to the Snell’s law, the phase difference between two different paths was zero. In the media with refractive indices ni and nt, the differences between them were sin(θi) dx and sin(θt) dx, respectively. Taking the abrupt phase shift at Adv. Optical Mater. 2017, 5, 1700040

the interface into consideration, the Snell law can be expressed as k0nisin(θi)dx + (φ + dφ) = k0ntsin(θt)dx + φ. Here k0 = 2π/λ0, and λ0 referred to the vacuum wavelength. So the equa tion can be simplified as ntsin(θt) − nisin(θi) = λ0dφ/2πdx, which was the generalized Snell’s law of refraction. The nonzero phase gradient led to the phenomenon that the incident lights with angles ±θi can bring different values of angles of refrac tion. Then the phenomena of anomalous refraction and reflec tion are in discussion. Figure 10d shows the experimental setup of y-polarized excitation on the antennas. From the dis cussion of the generalized Snell’s law, the possible angles for total internal refraction were θ c = arcsin( ± nt /ni − λ0 dφ /2π ni dx ) . The nonlinear relation between the θr and θi was revealed here, and then θc’ = arcsin(1 − λ0 dφ/2πnidx) could be obtained. The nonlinear influence of φ on the angles of refraction and reflec tion was referred as the anomalous refraction and reflection.[142] Although experimental results showed that the regularly reflected and refracted beams still exist, almost all the energy of incident light was transferred into the anomalous reflection and refraction.[139] Figure 10d shows the setup for the y-polarized excitation experiment. The gradient of the phase shift was nega tive. Anomalous reflection and refraction lights were accord with the incident light compared to the normal of the surface. Generalized laws of refraction and reflection have brought multifarious applications. For instance, it can be utilized in controlling propagation of light. As shown in Figure 10e, an abrupt phase change, resulting from the plasmonic meta surface, controlled the wave-front for CPL from visible to near-infrared wavelength range.[140] When the antenna was

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illuminated by the CPL, the scattered wave had a phase change related to dipole orientation. This application of PB phase in controlling propagation of light opened a gate for light manipu lation. Besides, these laws can be also applied in exciting SPPs. Figure 10f shows the generation of SPP wakes through aper ture antennas, which can introduce a phase shift with the same phase gradient.[141] The aperture antennas were separated in a distance shorter than λSSP to bring phase shifts, thus can con trol the illumination angle of SPP wakes. This condition was written as k0sin θΔx + Δϕ = ksppsinγΔx, and sin γ = sinθ/neff + dϕ/ ksppdx. Routing the SPP wakes became possible, meaning that PB phase can be conveyed by interacting with circularly polar ized light. Now the method of using such geometric phase shift is popular and efficient in this field. For future studies, PB phase is of great potential. One of the feasible developments is combining the principle of PB phase with hot electron injection, which helps promote and control coupled characteristics of 2D materials.[143,144] Besides, intro ducing the concept of PB phase can deepen the understanding of quantum Hall Effect and anomalous Hall Effect in graphene.[145] Moreover, the related electronic properties such as electron dynamics influenced by PB phase[146] is waiting to be presented.

6. Conclusions and Perspectives Plasmonic chirality is a growing discipline, and there are many unrevealed surprises and challenges, such as nonlinear chi roptical effects, strong coupling, ultrafast detecting, and chiral quantum optics. Thus, the study of plasmonic chirality is crit ical for the future science and technology development. Advanced fabrication methods including top-down and bottom-up approaches enable the assembly of various metallic chiral nanostructures, like nanoheptamer, nanohelix, twisted arcs array, and periodic two-layered SRR nanostructure. However, in order to implement further study and achieve practical applications, a higher accuracy and more productive method is in need. Latest research showed that controlling the symmetry matching of carbon nanotubes arrays and catalyzer can assemble the horizontal single-walled carbon nanotubes arrays with controlled chirality.[147] This idea can be used for reference for the future fabrication of plasmonic chiral nano structures. Based on artificial nanostructures, chiroptical effects such as CB, CD, and asymmetric transmission are excited and manipulated to stimulate potential applications, like chiral nanomotor, optical tweezer, chiral light detector, and biomo lecular sensor. Moreover, through tailoring the geometry of PB phase metamaterials, handedness can be introduced to holog raphy, which gives rise to the highly dispersive achromatic holo grams, advanced 3D optical holography technique, and non linear holography. Nevertheless, obstacles still exist for practical uses. For instance, the sensitivity is not high enough, leading to failure in the attogram probing. One feasible approach is to improve the detectors with lower noise and higher pixels, and another is to combine the sensing technique with other methods such as the integrating detector with nanofluidics.[148] As for the efficiency problem, a recent report pointed out that chiral graphene quantum dots were promising for drug delivery to achieve more efficient and selective phototherapies.[149]


In the future study, advanced characterization methods are needed for deeper nature. The near-field imaging of chiral optical density of state (ODOS) can act as a promising tool. Latest research showed that the near-field imaging of local ODOS revealed the chirality and spin-dependent local modes of the spiral plasmonic cavity.[150] Besides, the imaging by dichroicsensitive cathodoluminescence (CL) nanoscopy provided the explanation for the origin of dichroic hot-electron transfer at the interface between chiral gold SRRs and wide gap semicon ductor.[151] Moreover, the time-resolved detection of plasmonic chiral structures can provide the dynamics of electrons, which can be achieved by the ultrafast pump-probe technique.[152,153] The further study and deeper understanding of chiral interactions in plasmonic nanostructures can lead to significant studies and unprecedented applications in analytical chemistry, biochemistry, life science, and quantum communications.

Acknowledgement Y.L., C.C., and M.J. contributed equally to this work. This work is supported by the National Key Research and Development Program of China (grant no. 2017YFA0205700), National Basic Research Program of China (grant no. 2015CB932403, 2017YFA0206000), National Science Foundation of China (grant nos. 61422501, 11674012, 11374023, 61176120, 61378059 and 61521004), Beijing Natural Science Foundation (grant no. L140007), and Foundation for the Author of National Excellent Doctoral Dissertation of PR China (grant no. 201420), National Program for Support of Top-notch Young Professionals.

Conflict of Interest The authors declare no conflict of interest.

Keywords asymmetric transmission, circular birefringence, circular dichroism, plasmonic chiral nanostructures Received: January 15, 2017 Revised: March 30, 2017 Published online: June 9, 2017

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