Circular dichroism from single plasmonic nanostructures ... · Circular dichroism (CD) studies on single nanostructures can yield novel insights into chiroptical physics that are

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Circulardichroismfromsingleplasmonicnanostructureswithextrinsicchirality

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Nanoscale

PAPER

Cite this: Nanoscale, 2014, 6, 14244

Received 3rd August 2014,Accepted 25th September 2014

DOI: 10.1039/c4nr04433a

www.rsc.org/nanoscale

Circular dichroism from single plasmonicnanostructures with extrinsic chirality†

Xuxing Lu,‡a Jian Wu,‡a,b Qiannan Zhu,a Junwei Zhao,a Qiangbin Wang,a Li Zhanb

and Weihai Ni*a

Circular dichroism (CD) studies on single nanostructures can yield novel insights into chiroptical physics

that are not available from traditional ensemble-based measurements, yet they are challenging because

of their weak signals. By introducing an oblique excitation beam, we demonstrate the observation and

spectroscopic analysis of a prominent plasmonic chiroptical response from a single v-shaped gold

nanorod dimer nanostructure. We show that circular differential scattering from the obliquely excited

gold nanorod dimer yields a characteristic bisignate peak-dip spectral shape at hybridized energies of the

dimer. This chiroptical response can be ascribed to extrinsic chirality which depends on the geometry

configurations of the chiral arrangement. Due to strong near-field coupling, the dipole orientations of the

hybridized resonance modes can be in favor of the incident circularly polarized light where a maximum

g-factor of ∼0.4 is observed. Promising applications of this chiroptical arrangement as a key componentcan be in electronics, photonics, or metamaterials.

1. Introduction

Chirality is a property possessed by a special category ofobjects ranging from biological or chemical substances1,2 toartificial metamaterials.3 These objects possess the featurethat they cannot be superimposed with their mirror images.The optical response of chiral objects differs distinctivelyunder the excitation of incident light with left- and right-handed circular polarization (LCP and RCP). Circular dichro-ism (CD) spectroscopy, an optical analytic method for chiralmaterials, provides detailed structural information by compar-ing the difference in spectral absorption under the excitationof light with two circular polarization states.4 Recently, three-dimensional (3D) chiral artificial nanostructures have drawnextensive attention because they can create negative refractionof light.3,5 Moreover, they can also provide “chiroptical hot-spots” for the detection of weak molecular signals by generat-ing superchiral electromagnetic fields.6,7 A variety of methodshave been developed towards the creation of 3D chiral nano-structures either on substrates6,8,9 or in solutions.10–15 Com-

pared to 3D chiral nanostructures, two-dimensional (2D)achiral nanostructures that can show the same effect undercertain conditions are, however, significantly less acknowl-edged. A 2D achiral plasmonic nanostructure, when excitedobliquely at a particular incident angle, can exhibit so-called“extrinsic chirality”, and the plasmonic CD (PCD) response ofsuch a nanostructure is nonzero. This is due to the symmetrybreaking by introducing the excitation light beam, and a geo-metrical chiral arrangement is formed by taking account ofboth the planar nanostructure and the oblique incident beam.It was first observed in liquid crystals.4 Zheludev et al.extended their study into artificial 2D metamaterials, whereextrinsic chirality leads to exceptionally large circular dichro-ism in the microwave region.16,17 Very recently, Kato et al. per-formed CD measurements on individual carbon nanotubesinduced by extrinsic chirality, where giant CD was observedusing a photoluminescense detection configuration.18

Traditional spectroscopy measurements are carried out onensemble samples where a great number of plasmonic nano-structures contribute to the overall signal. As a consequence,the spectral response is inhomogeneously broadened due tothe population average over these nanostructures, which couldbe very different from each other in geometry. Furthermore,plasmonic nanostructures that are synthesized, assembled, orstabilized in solutions take random orientations with respectto the direction of the incident light. The orientation averageover all the incident angles hinders in-depth studies. Plasmo-nic nanostructures usually possess large scattering cross sec-tions,19,20 which make them ideal candidates for detection

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr04433a‡Xuxing Lu and Jian Wu contributed equally.

aDivision of i-Lab, Key Laboratory of Nano-Bio Interface & Collaborative Innovation

Center of Suzhou Nano Science and Technology, Suzhou Institute of Nano-Tech &

Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China.

E-mail: [email protected] of Physics and Astronomy, Key Laboratory for Laser Plasmas (Ministry

of Education), State Key Lab of Advanced Optical Communication Systems and

Networks, Shanghai Jiao Tong University, Shanghai, 200240, China

14244 | Nanoscale, 2014, 6, 14244–14253 This journal is © The Royal Society of Chemistry 2014

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www.rsc.org/nanoscalehttp://crossmark.crossref.org/dialog/?doi=10.1039/c4nr04433a&domain=pdf&date_stamp=2014-10-31http://dx.doi.org/10.1039/c4nr04433ahttp://pubs.rsc.org/en/journals/journal/NRhttp://pubs.rsc.org/en/journals/journal/NR?issueid=NR006023

and broadband spectroscopic analysis at a single-particle level.Single-particle approaches21–23 based on dark field scatteringtechniques have proven themselves to be very successful andessential for in-depth studies with high sensitivity and selecti-vity by excluding the averaging effects. However, so far, obser-vation of pronounced CD signals from single plasmonicnanostructures are hardly achievable on systems with eitherintrinsic or extrinsic chirality due to limitations either insignal strength or experimental configurations. Nevertheless,as a powerful tool, single-particle CD spectroscopy is expectedto be greatly significant for understanding the PCD responsefrom individual subwavelength building blocks or, in otherwords, metamolecules of artificial metamaterials.16,24

Herein, by introducing an oblique excitation beam, wedemonstrate the observation and spectroscopic analysis of aplasmonic chiroptical response from a single v-shaped goldnanorod dimer nanostructure. We show that circular differen-tial scattering from the obliquely excited gold nanorod dimeryields a characteristic bisignate peak-dip spectral shape athybridized energies of the dimer. For plasmonic nano-structures, the scattering usually possesses a fixed ratio ofextinction coefficients,19 and therefore, the measured differen-tial scattering can be representative for the differential extinc-tion that defines the CD response. The observed chiropticalresponse can be ascribed to the extrinsically chiral systemcomprising the dimer structure and its excitation arrange-ment.16,17 The differential scattering intensity can be finelytuned by altering the in-plane orientation angle ϕ of the dimerstructure, the incident angle θ, and the structure angle βbetween the long axes of the two nanorods in the dimer,which indicates high flexibility of the experimental arrange-ment. Due to strong near-field coupling, dipole orientations ofthe hybridized resonance modes can be in favor of the inci-dent circularly polarized light where a maximum g-factor of∼0.4 is observed. Promising applications of this chiral arrange-ment as a key component can be in electronics, photonics, ormetamaterials.

2. Results

Fig. 1a illustrates the schematic design of our experiment. Twogold nanorods were self-assembled into a dimer in solutionand thereafter immobilized on a glass substrate with a specificangle between their long axes. Unlike other artificial three-dimensional (3D) chiral architectures, the gold nanorod dimeris actually a 2D achiral structure with a plane of mirror sym-metry in the middle of the long axes of the two nanorods.However, the symmetry is broken when the dimer is illumi-nated by an incident beam at an oblique angle. The twotogether form a geometry arrangement which cannot be super-imposed with its mirror image and thus the whole arrange-ment is chiral. This kind of arrangement forms the basis of so-called extrinsically chiral systems.

Gold nanorods were chosen to be the building blocks forthe 2D plasmonic dimer structures because their anisotropic

shape is ideal for the formation of a 2D dimer structure if theangle between their long axes is nonzero, while the dimerformed by two spherical gold nanoparticles can only yield aone-dimensional dimer structure. Moreover, the scatteringcross section of gold nanorods is much larger than sphericalgold nanoparticles of the same volume and their longitudinalplasmon resonance is in the visible range, which make thempreferable in our experiment.19 The gold nanorods were grownin aqueous solutions using a seed-mediated method.25,26 Theas-synthesized gold nanorods dispersed in 0.1 M cetyltrimethyl-ammonium bromide (CTAB) solution show an ensembletransverse plasmon resonance at around 520 nm and a longi-tudinal one at around 704 nm. The average diameter, length,and aspect ratio of the gold nanorods are 24 ± 2 nm, 69 ±5 nm, and 2.9 ± 0.3, respectively, which is confirmed by TEMmeasurements.

The gold nanorods were self-assembled in an end-to-endfashion through a hydrogen bonding-directed assemblymethod in aqueous solutions using cysteine (CYS) as linkermolecules27 (Fig. S1, ESI†). The assemblies of gold nanorods

Fig. 1 Observation and spectroscopic analysis of the chiropticalresponse from single gold nanorod dimer structures. (a) Schematicdesign of the experiment. The gold nanorod dimer structure is illumi-nated by circularly polarized light at an oblique incident angle of θ =60°. The circular polarization can be either LCP (in blue) or RCP (in red).(b) The SEM image of individual gold nanostructures (highlighted incircles) deposited on the glass substrate. (c) Zoomed-in image showingthe structure highlighted in the red circle in (b). Two individual goldnanorods with a structure angle of β = 80° between their long axes areclearly identified. The green arrow indicates the direction of incidentlight. (d) Corresponding dark-field scattering image of (b). (e) Dark-fieldscattering spectra of the gold nanorod dimer shown in (c) under exci-tation of the incident light with two circular polarization states, LCP (inblue) and RCP (in red). (f ) Differential scattering spectra obtained bysubtracting the scattering spectrum of RCP from LCP in (e).

Nanoscale Paper

This journal is © The Royal Society of Chemistry 2014 Nanoscale, 2014, 6, 14244–14253 | 14245

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http://dx.doi.org/10.1039/c4nr04433a

were deposited from the solution onto a clean cover glass slideand inspected using SEM and an optical microscope where across bar on the glass slide was used as an alignment mark.SEM images of the dimer structure were obtained before theoptical measurement under the dark-field microscope(Fig. 1b). The dimer structure, consisting of two individualgold nanorods with a specific angle between their long axes,are clearly observable in the zoomed-in SEM image (Fig. 1c).

Dark-field scattering of individual dimer structures wasmeasured using the experimental design shown in Fig. 1a,which facilitates the oblique excitation of single nano-structures by circularly polarized light (CPL). Experimentaldetails for this single particle chiroptical response measure-ment can be found in the methods section. Fig. 1d shows atrue-color dark-field scattering image obtained of the samearea as the SEM image (Fig. 1b). With the help of the pre-viously marked cross bar, the sample was coarsely alignedunder the optical microscope to the same area where the SEMimage was taken. Gold nanorod structures appear as diffrac-tion-limited red spots and form a pattern on the dark-fieldimage. By carefully comparing it with the SEM image, each redspot in the optical image can be identified with its structuraldetails. For example, the red spot highlighted in the center ofFig. 1d is confirmed as a dimer structure with the architectureand orientation shown in Fig. 1c. Under the excitation of theincident light with two polarization states, LCP and RCP, thescattered light from the dimer structure was collected by theobjective and thereafter analyzed spectroscopically with itsscattering spectra recorded (Fig. 1e). Both spectra show twopronounced scattering peaks at about 670 nm and 850 nm,corresponding to hybridized antibonding and bonding modes,respectively, which indicate that the plasmonic resonances ofthe two individual nanorods of the dimer are strongly coupled.Note that the scattering corresponding to the transverseplasmon mode at around 520 nm is hardly observable due toits small scattering cross section.28 The LCP and RCP scatter-ing spectra differ drastically in peak intensities. Subtractingthe scattering of RCP from LCP yields a peak followed by a dipin the spectrum representing the chiroptical response fromthe single dimer structure (Fig. 1f).

Because the chirality originates from the geometry arrange-ment formed by the 2D dimer structure and the incident light,the relative orientation between the two is one of the crucialvariables that determine the chiral property as well as the chir-optical response. During the dark-field scattering measure-ments, the sample was placed on a rotating stage so that thein-plane azimuthal angle ϕ of the dimer structure can befinely tuned with respect to the incident light. Starting froman arbitrary initial angle ϕ, the sample stage was rotated clock-wise at a step of 10° and the corresponding LCP and RCP scat-tering spectra from the same dimer structure were recorded(Fig. S2, ESI†). The dimer structure with the structure angle ofβ = 80° was investigated (Fig. 1c). The incident angle θ waskept unchanged at 60° during the measurements. The leftpanel of Fig. 2a shows the measured differential scatteringspectra at every 20° interval. ϕ = 0° is defined as when the

bisector of the angle of the dimer is perpendicular to the inci-dent direction, and the exact scenario shown in Fig. 1c isachieved. Starting from an initial angel of ϕ = −20°, the differ-ential scattering peak intensity increases with the increase inϕ and reaches a maximum at ϕ = 0°. A continuous increase ofϕ results in a decrease in the differential scattering intensity

Fig. 2 Chiroptical response at various values of the in-plane orientationangle ϕ of the dimer structure. (a) Comparison between measured (left)and calculated (right) CD spectra of the dimer with β = 80°. Differentialscattering spectra corresponding to different in-plane orientation anglesϕ, varying from −20° to 160° at a step of 20° while keeping θ = 60°unchanged, are shown from top to bottom. The increment of the majorticks is 2000 counts for ScatLCP-RCP and 1 for Qscat, LCP-RCP, respectively.(b) Dependence of peak intensity on ϕ. Measured peak intensities at670 nm (circles in red) and 850 nm (squares in black) as a function of ϕare compared with those from calculations (solid curves).

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from the maximum, diminishing to zero at ϕ = 90°. Furtherincreasing the angle inverts the differential scattering spectrato the opposite direction. Fig. 2b clearly shows this sinusoidal-like dependence of the differential scattering peak intensity onthe in-plane orientation angle ϕ.

In order to understand the chiroptical response from theplasmonic nanostructure with this extrinsic chirality, ananalytical method, coupled-dipole approximation (CDA),29–32

was employed. In the CDA calculation, the gold nanoroddimer on the glass substrate is modeled as two identicalprolate ellipsoids surrounded by a homogeneous medium withan averaged refractive index n = 1.32, where the average of thehomogeneity is taken over air (n = 1) and glass substrate (n =1.5) for simplicity.33 A schematic of the geometry used in thecalculation is illustrated in Fig. S3 of the ESI,† where β is theangle between the long axes of the two ellipsoids. β can bechanged by rotating both ellipsoids in the opposite directionaround their respective point which is defined at distance Dfrom their center on their axis. d is indicated as the distancebetween the rotation points of the two ellipsoids. Geometricparameters, a = 3.65b, D = 1.46b, and d = 2.2b, are properlychosen in the calculation so that a best fit to the measuredspectrum can be achieved, where a and b are the major andminor axis radii of the ellipsoid, respectively. The dimer isexcited by a plane wave with circular polarizations, LCP orRCP, at a given incident angle θ. The efficiency factor of scat-tering Qsca as a function of wavelength at various ϕ in therange from 0° to 180° was obtained by performing CDA calcu-lations (Fig. S2, ESI†). As shown in Fig. 2, the calculatedresults are found to be generally in good agreement with thosefrom the measurements. Some mismatches are believed to berelated to the errors in the single particle measurements andin the positioning of the linear polarizer.

The relative orientation of the two gold nanorods in thedimer plays an important role in determining the hybridizedplasmonic resonance properties.34 To resolve this plasmonicstructural dependence of the extrinsic chirality, we explored thechiroptical response from single gold nanorod dimers byvarying the structure angle β. β, the angle between the long axesof the two nanorods in the dimer, was identified and measuredusing SEM (Fig. S4, ESI†). During the measurement, the azi-muthal angle ϕ was kept constant by finely rotating the samplestage to an angle when the symmetry line of the dimer was per-pendicular with respect to the incident direction, and other geo-metry parameters were also kept constant. Due to the sizedifference in the individual gold nanorods forming the dimerswith various β, their scattering spectra are hardly comparable.In order to exclude this size effect, we carried out a normaliza-tion treatment where both LCP and RCP scattering spectra fromdimer i were divided by a factor A = Pi/max(Pi), where Pi is thesum of the highest peak intensity of LCP and RCP scatteringspectra, and max() returns the biggest of all the dimers investi-gated. The scattering spectra of single dimers with variousβ were thus obtained after carrying out this treatment (Fig. S5,ESI†). The measured differential scattering spectra from thedimers with various β are shown in the left panel of Fig. 3a.

CDA calculations were performed to investigate the depen-dence of the chiroptical response on the structure angle β.Based on the calculations, LCP and RCP scattering spectrafrom dimers with various β were obtained (Fig. S5, ESI†). Cal-culated differential scattering spectra at experimental β valuesare selected and compared with the measured spectra, whichis shown in Fig. 3a. Fig. 3b compares the chiroptical responsebetween the measurements and calculations by plotting thedifferential scattering peak intensity of both bonding modeand antibonding mode as a function of β. With the increase inthe structure angle β, the peak intensity first increases, reachesa maximum, and then decreases. At the same time, as shown

Fig. 3 Chiroptical response at various values of the structure angle β.(a) Comparison between measured (left) and calculated (right) differen-tial scattering spectra from the dimers with β = 0°, 28°, 65°, 80°, 103°,and 180°. Other geometry parameters are kept constant. θ is fixed at60°, and the azimuthal angle ϕ is kept constant by keeping the sym-metry line of the dimer perpendicular with respect to the incident direc-tion. The increment of the major ticks is 5000 counts for ScatLCP-RCPand 2 for Qscat, LCP-RCP. (b) Dependence of peak intensity on β. Measuredpeak intensities of bonding mode (circles in red) at around 850 nm andantibonding mode (squares in black) at around 670 nm as a function of βare compared with those from calculations (solid curves). (c) The samefor the dependence of peak wavelength on β.

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in Fig. 3c, the wavelengths of the bonding and antibondingmodes experience blue and red shifts, respectively. The resultsfrom the measurements are in good agreement with thosefrom the calculations, except for the dimers with β = 0° and180°. In these cases, the calculation results suggest that nochiroptical response can be observed because the two indivi-dual gold nanorods are aligned either parallel or in a straightline and they are in the same plane as the incident beam sothat the whole arrangement is not chiral. However, a peak or adip appears in the measured differential scattering spectrum.By carefully examining the dimer structures in the SEMimages, it is found that the dimer with β = 0° actually consistsof two gold nanorods of different lengths, and the other withβ = 180° has a small offset between the two gold nanorods(Fig. S4a and e, ESI†). It is not surprising to find these imper-fections in the resulting dimer structures because the dimersare formed by gold nanorods that have a size distribution, andthe dimers can also be disrupted by Brownian motion duringthe deposition in aqueous solution.

So far, the dependences of the chiroptical response on thein-plane orientation angle of the dimer structure and the struc-ture angle between long axes of the two nanorods in the dimerhave been investigated both experimentally and theoretically.The third factor, the incident angle θ, can also be crucial indetermining the chiral arrangement. However, due to thelimits of our experimental setup, θ cannot be changed at will.We instead show the calculated results for the dimer with thestructure angle β = 80° at various values of the incident angle θ(Fig. S6, ESI†). Finally, the dependences of the differential scat-tering spectral response on the three angles are summarized(Fig. S7, ESI†).

3. Discussion

Such an extrinsically chiral system consists of a simple andelegant plasmonic structure formed by a gold nanorod dimerand an oblique incident beam, yet exhibits a versatile andtunable chiroptical response. This response results fromintense interactions between the incident CPL and the plasmo-nic structure in its propagation path. In order to gain deeperinsights into the physics behind the chiroptical response, wefirst conceived a simple plasmonic hybridization picture forthe description of the electromagnetic behavior of the goldnanorod dimer under the excitation of CPL. As shown inFig. 4a, when the two gold nanorods are placed close to eachother, due to strong plasmonic coupling the degenerate plas-monic resonance of the individual gold nanorods is split intotwo hybridized resonance modes at lower and higher energies,corresponding to bonding and antibonding plasmonic modes,as well as anti-symmetric and symmetric collective chargedensity oscillations, respectively. Under the excitation of CPL,the anti-symmetric and symmetric oscillations are evidencedby charge density profiles obtained using the finite-differencetime-domain (FDTD) method (Fig. S9, ESI†). Considering theirsymmetric or anti-symmetric characters together with the

inherent chirality of the CPL, the two plasmonic modes inter-act with the oblique incident light with LCP and RCP indifferent manners, which can be used to explain the lineshape of the bisignated differential scattering spectrum. Todescribe this physical picture, we decompose the plasmonicstate into symmetrical and anti-symmetric components. Here,we neglect the contribution from the transverse modes forsimplicity. The resultant dipole moment can be viewed as acomposition of two individual dipoles oriented along theirown axes: p(ω) = [p1(ω)e1]⊗[p2(ω)e2], and its decompositioninto symmetric and anti-symmetric components can beexpressed in a simple form: p(ω) = p0cs(ω)es + p0ca(ω)ea. Here,p1(ω) and p2(ω) denote the two individual dipoles in the fre-quency domain, p0 is the dipole moment of the individualnanorod oscillating at its resonant frequency, and ca(ω) andcs(ω) are expansion coefficients of the anti-symmetry andsymmetry plasmon modes, which can be expressed as ca(ω) = and cs(ω) = . The base vectors e1 = [−cos(β/2)-sin(β/2),0]T and e2 = [cos(β/2)sin(β/2),0]

T are used to representtheir orientations, respectively. es = (−e1)⊗e2 and ea = e1⊗e2are two orthogonal base vectors corresponding to the sym-metry and anti-symmetry system. It’s easy to find that the twodipoles are related to the expansion coefficients in the form ofp1(ω)/p0 = ca(ω) − cs(ω) and p2(ω)/p0 = ca(ω) + cs(ω), which indi-cate that a phase parameter Δφ = arg[(ca + cs)/(ca − cs)] can beintroduced to describe the relative oscillation of the twodipoles.

Here we consider a typical arrangement with ϕ = 0°, β = 80°,and θ = 60°, which is just the scenario shown in Fig. 1a. Bydecomposing the resultant dipole into symmetric and anti-symmetric components, the phase parameter and the expan-sion coefficients are derived and plotted in Fig. 4b. One canclearly find that symmetric (solid line) and anti-symmetric(dashed line) components play a dominant role at higher (ħω+= 1.87 eV) and lower (ħω− = 1.47 eV) hybridized energies,respectively. The scattering coefficient (the right panel ofFig. 4b) at the two resonance frequencies can be determinedby estimating the expansion coefficients. For example, as|cs,L(ω+)| > |cs,R(ω+)| and |ca,L(ω−)| < |ca,R(ω−)|, it’s obvious thatthe scattering coefficient for LCP and RCP will obey: Qsca,L(ω+)> Qsca,R(ω+) and Qsca,L(ω−) < Qsca,R(ω−), thus the CD spectrahave the bisignated line shape with a peak at ω+ and a dip atω−. The middle panel of Fig. 4b shows the absolute value ofΔφ(ω) for LCP and RCP, which describes the relative phase ofthe two individual dipoles. As depicted in the plot, |Δφ(ω)| hasthe value of π at ω+ and 0 at ω−, which means that the twodipoles are oscillating in phase at lower hybridized energy(bonding mode) and in anti-phase at higher hybridized energy(anti-bonding mode). At these frequencies, the sum of the con-tribution from the two individual dipoles can thus bedescribed as a resultant dipole oscillating linearly in a fixeddirection, indicating that the dipole response (bright mode)dominates in the scattering process.

The microscopic mechanism of the incidence-inducedextrinsic chirality from the dimer system can be easily under-stood. As shown in Fig. 4d, in the view of the incident beam,

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the two nanorods are actually placed one after another in a“screwed” way, as the first and second rods form a helical con-figuration along the propagation direction of the incidentlight. Depending on the excitation wavelength, symmetric oranti-symmetric resonance modes with their specific configur-ations of dipole orientations can be excited. One configurationis just in favor of a certain inherent chirality of CPL, while theother is not. To be specific, the dipole orientations of the sym-metric mode match well with the spatial evolution of the elec-tric field vectors of LCP (Fig. 4d, left), while they are

unfavorable for RCP. On the contrary, for the anti-symmetricmode, a favorable match is found with RCP (Fig. 4d, right). Asa result, the LCP and RCP incident light “feel” distinctivelyand hence excite the hybridized plasmonic modes in differentmanners, which gives rise to the differences in far field scatter-ing properties. The LCP and RCP scattering cross sections ofthe dimer are therefore discriminated at these resonant wave-lengths. This physical picture can be used to explain bothexperimental and calculated results for the angular depen-dence of differential scattering intensities. For example, the

Fig. 4 Plasmonic hybridization and chiroptical properties of the gold nanorod dimer with extrinsic chirality. (a) Energy level diagram describing theinteraction of two degenerate longitudinal modes (ω0 = 1.70 eV) resulting in a blue-shifted (ω+ = 1.87 eV) symmetric mode (antibonding) and a red-shifted (ω− = 1.47 eV) anti-symmetric mode (bonding). p1 and p2 represent the dipoles of the two nanorods, and p is the resultant dipole of the two.(b) Left: calculated scattering coefficient as a function of energy for the individual nanorod (black) and the dimer excited by light with LCP (blue) andRCP (red). Middle: phase parameter as a function of energy describing the relative oscillations of the two rods. Right: symmetric (solid lines) andanti-symmetric (dashed lines) expansion coefficients as a function of energy for the LCP (blue) and RCP (red) excitation. (c) Reference frame for theextrinsically chiral arrangement. The green arrow indicates the incident CPL with θ = 60° in the yz-plane. The gold nanorod dimer lies in the xy-plane. Observation direction for far field scattering is along z-axis. (d) Interaction between the incident CPL and the dimer in the view of the propa-gating path. Left: dipole orientations of the symmetric mode match well with the spatial evolution of electric field vectors of LCP, while they areunfavorable for RCP. Right: for the anti-symmetric mode, a favorable match is found with RCP. (e) Oscillations of the electric field vector of thedimer in the xy-plane under the excitation of the incident light with LCP and RCP. The oscillation is found to be linear at resonance energies of anti-symmetric (left panel) and symmetric (right panel) modes, while it shows ellipsoid shapes at energies other than these two resonances (mid-rightand mid-left panels).

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differential scattering reaches a maximum when ϕ = 0°, β =90°, or θ = 45°. It is under these optimized conditions thatmaximum overlap is achieved between the spatially distributedelectric field vectors of the CPL and the dipoles of the dimerand gives rise to maximum differential scattering intensities.On the contrary, the differential scattering is zero when ϕ =90°, 180°, β = 0°, 180°, or θ = 0°, 90°, because at these anglesno such overlap can occur. In addition, the CD calculations ofa 1D gold nanorod under the same configuration indicate noobservable optical activity (Fig. S10, ESI†). The axial symmetryof the gold nanorod can be broken by the incident beam, andthe two form a 2D arrangement which has a mirror symmetryand therefore is not chiral. This suggests that a 2D plasmonicstructure is a prerequisite for obtaining extrinsic chirality.

In order to understand the scattering process and discrimi-nate the contribution of true CD from the measured differentialscattering, we further explored the far field scattering propertiesof the dimer system by deriving the Mueller matrix of scatteringbelonging to the extrinsically chiral arrangement. For simpli-city, here we set the observation direction of the far field scat-tering along the z-axis (Fig. 4c). We express the scattering fieldas the composition of left and right circularly polarized light:

rEsca ¼ ajeL þ bjeRwhere r is the distance from the origin of coordinates to the obser-vation position, the subscript j = L,R denotes the incident polariz-ation state, and the expansion coefficients can be expressed as:

aj ¼< eLjrEsca >; bj ¼< eRjrEsca >:The complex Mueller matrix Sij relates the electric field of

the incident CPL with the circular polarized far field scatteringof the dimer structure in a form of:

Esca;i ¼ SijEinc;j;where a simple relation can be obtained:

rSLj ¼ aj; rSRj ¼ bj:By solving aj and bj, one can obtain all the Mueller matrix

elements and plot them as a function of wavelength (Fig. S10,ESI†). The phase delay of LCP and RCP waves, arg(rSLL) andarg(rSRR), exhibits a similar dispersion trend in the visiblerange. This is because the CPL does not really propagatethrough any medium, and therefore circular birefringence isnot realistic for a scattering-based measurement. The scatter-ing of LCP and RCP waves, |rSLL|

2 and |rSRR|2, exhibits a big

difference at the two plasmonic resonance modes, which givesrise to a distinct true dichroism response Δ = |rSLL|2 − |rSRR|2

(Fig. S10d†). |rSRL|2 and |rSLR|

2 show very similar dispersionsto |rSLL|

2 and |rSRR|2. By taking account of both circular polar-

ization states of the scattering, one can obtain the measureddifferential scattering, |rSL|

2 − |rSR|2 = |rSLL|2 + |rSRL|2 −(|rSRR|

2 + |rSLR|2) (Fig. S11f, ESI†). Note that the measured

differential scattering has the same shape as dichroism Δ yethas doubled in intensity. This means that Δ contributes justhalf of the measured differential scattering. This half contri-bution can be readily understood. As shown in Fig. 4e, the

oscillation is found to be linear at resonance energies of theanti-symmetric (left panel, Fig. 4e) and symmetric (right panel,Fig. 4e) modes. The radiation of the linearly oscillating dipolegives rise to a linearly polarized electric field which can bedecomposed into LCP and RCP parts with the same half inten-sity, |rSLL|

2 = |rSRL|2 and |rSLR|

2 = |rSRR|2. Therefore, evalu-

ation of the true dichroism based on |rSLL|2 − |rSRR|2 gives

rise to the half of the measured differential scattering. At ener-gies other than these two resonances, the resultant dipoleoscillations exhibit ellipsoid shapes (mid-right and mid-leftpanels, Fig. 4e), which results from a mixed state of the tworesonance modes. Therefore, the measured differential scatter-ing can correctly reflect the true dichrosim by a factor of 2.

Fig. 5a depicts a calculated far field scattering profile fromthe dimer excited by incident light with LCP at the frequencyof ω−. One may instantly recognize this doughnut-like scatter-ing profile as typical from a point dipole, which confirms thatthe contribution of the two individual dipoles is equivalent toa resultant one at the hybridized frequencies. Those for RCP atthe frequency of ω− and LCP at both frequencies are alsoplotted in Fig. 5. The resultant dipole moment with its unitvector orientates in the direction of the symmetry axis of theprofile. In the left and right panel of Fig. 4e, we plot the oscil-lation directions of the resultant dipoles when the dimer isexcited by incident light with LCP and RCP at the frequenciesof ω+ and ω−. It’s clear that all these vectors lie in the xy-planebut are separated into two bands corresponding to the sym-metry and anti-symmetry oscillations of the dimer system.However, these vectors are found to deviate from their corres-ponding symmetric (x) or anti-symmetric (y) axes by smallangles. This deviation indicates that the symmetric or anti-symmetric system is broken due to the inherent chirality ofCPL. The deviation angle is determined by the relative values ofcs and ca. For example, as |ca,L(ω+)|/|cs,L(ω+)| < |ca,R(ω+)|/cs,R(ω+)|,

Fig. 5 Calculated far field scattering profiles from the dimer excited byincident light with LCP and RCP at the two resonance frequencies. Nor-malized far field profiles for (a) LCP at the frequency of 1.47 eV (bondingmode), (b) LCP at 1.87 eV (anti-bonding mode), (c) RCP at 1.47 eV(bonding mode) and (d) RCP at 1.87 eV (anti-bonding mode).

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RCP induces a larger deviation from the symmetric axis (x)compared to LCP (Fig. 4e). A similar conclusion can be drawnfor the low frequency (ω−) band with the anti-symmetry oscil-lation considered where we get a larger deviation for LCP atthat frequency. At hybridized frequencies, the shapes of the farfield scattering profiles are exactly the same because the twodipoles can be equivalent to a resultant one. However, the situ-ation is complicated at a different frequency. For example, asshown in Fig. S12 (ESI†), the far field scattering profile plottedat 1.7 eV loses this dipole nature, suggesting a mixture of thetwo hybridized states.

One of the key parameters that can assess the intensity ofthis interaction is the anisotropy factor g. The g-factor hasbeen studied and evaluated in many systems ranging fromnatural substances to artificial plasmonic nanostructures.Natural substances usually show g-factors on the order of10−7–10−5,1 while those of artificial plasmonic nanostructuresare usually higher depending on the asymmetry arrangement.For example, DNA-bridged gold nanorod assemblies exhibit ag-factor on the order of 10−3.13 A maximum g-factor of ∼0.14was found in 3D plasmonic oligomer arrays with 100 nm thick-ness.8 Once these individual particles in the oligomer toucheach other, the chiroptical response is enhanced due to theexcitation of a charge-transfer plasmon.9 The magnitude of theg-factor can be related to the helical movement of the displace-ment currents in the chiral structure, which is driven by theCPL. Therefore, structures in favor of promoting this displace-ment current will possess high g-factor values. In our system,the dimer is formed by two end-to-end assembled gold nano-rods separated by a gap less than 2 nm, where strong near fieldcoupling can occur (Fig. S8, ESI†). In addition, due to the ani-sotropic shape, the damping of the electron oscillation in thegold nanorod is much lower than in the spherical derivative.28

High g-factor values can thus be expected in our structure. By astraightforward calculation based on the results shown inFig. S11 (ESI†), the factor g = 2(|rSLL|

2 − |rSRR|2)/(|rSL|2 +|rSR|

2) should take half the value of the measured one, 2(|rSL|2

− |rSR|2)/(|rSL|2 + |rSR|2). Therefore, the g-factor of our extrinsi-cally chiral system reaches a maximum of ∼0.4 at about830 nm (Fig. S13, ESI†), which suggests strong interactionsbetween the CPL and the dimer.

4. Conclusions

In summary, we have demonstrated a new method for theobservation and spectroscopic analysis of the chiropticalresponse from single plasmonic nanostructures by applyingdark-field scattering techniques in CD measurements. Thedifferential of the scattering yields a peak followed by a dip, orthe reverse at hybridized energies, in the spectrum representingthe chiroptical response from the single dimer structure. Thisresponse has been ascribed to the extrinsically chiral systemcomprising the dimer structure and its excitation arrangement.The differential scattering intensity has been proven to bedependent on the in-plane orientation angle ϕ of the dimer

structure, the incident angle θ, and the structure angle βbetween the long axes of the two nanorods in the dimer, whichindicates high flexibility of the experimental arrangement. Dueto strong near field coupling, dipole orientations of the hybri-dized resonance modes can be in favor of the incident circularlypolarized light where a maximum g-factor of ∼0.4 is observed.Promising applications of this chiral arrangement as a key com-ponent can be in electronics, photonics, or metamaterials.

5. MethodsPreparation of gold nanorods

The gold nanorods were prepared using a silver ion-assistedseed mediated method.25,26 A seed solution for gold nanorodgrowth was prepared using a NaBH4 (Sigma Aldrich) reductionof 5 mL 0.25 mM HAuCl4 (Acros) in an aqueous 0.1 M CTAB(Sigma Aldrich) solution. 6 μL of the seed solution was addedto 10 mL of a growth solution containing 0.1 M CTAB, 0.5 mMHAuCl4, 0.8 mM ascorbic acid (Sigma Aldrich), 0.12 mM silvernitrate (Sigma Aldrich), and 18.6 mM HCL. The as-synthesizedgold nanorods dispersed in a 0.1 M CTAB solution show anensemble LSPR wavelength at 704 nm. The average diameter,length, and aspect ratio of the gold nanorods are 24 ± 2 nm,69 ± 5 nm, and 2.9 ± 0.3, respectively.

Assembly of gold nanorods

The gold nanorods were self-assembled in an end-to-endfashion through a hydrogen bonding-directed assemblymethod27 in aqueous solutions using CYS (Sigma Aldrich) aslinker molecules. For the assembly, 0.3 mL of the as-preparedgold nanorod solution was diluted to 1 mL. The pH of the goldnanorod solution was tuned to be acidic by adding 0.03 mL of1 M HCl. 0.03 mL of 30 mM CYS was then added to the solu-tion. The mixture was transferred into a cuvette which wasplaced in the thermal static cell holder (40 °C) of a spectropho-tometer (Agilent Cary 60). The assembly of gold nanorods wasmonitored and recorded by the spectrophotometer every 5 min.

Deposition of gold nanorod assemblies and their electronmicroscope characterization

When the extinction peak of the dimers at 870 nm reached themaximum in the spectrum, the assemblies of gold nanorodswere deposited from the solution onto a clean cover glass slide(Deckglaser Cover Glasses), rinsed a couple of times withethanol, and blown dry with nitrogen gas. After the deposition,a cross bar was marked on the glass slide for the alignment ofindividual dimer structures on the substrate under SEM andthe optical microscope. SEM images of the gold nanorodassemblies on the glass slide were taken using a FEI Quanta250 FEG microscope equipped with a back scattered electrondetector. TEM images were obtained using an FEI Tecnai 20Transmission microscope operating at 200 kV.

Single-particle differential scattering measurement

Dark-field scattering of individual dimer structures wasmeasured using the experimental design shown in Fig. 1a. By

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tuning the mirrors, broadband white light from a 100 Wquartz-halogen-tungsten lamp was steered obliquely to thecenter of the sample, which was placed on the stage of anOlympus BX53 optical microscope. The incident angle θ wasadjusted to 60° so that the direct light was kept out of theaperture of the objective and only the scattering light was col-lected. A polarizer, a broadband λ/4 wave plate (ThorlabsAQWP05M-600), and a lens with 3 cm focal length were placedin sequence in the light beam. The angle between the fast axisof the λ/4 wave plate and the polarizer was adjusted by ±45° sothat the linear polarization could be transformed into eitherLCP or RCP. A long working distance objective (50X, N.A. = 0.5,W.D. = 10.6 mm) was used to collect the scattered lightfrom the gold nanorod dimers, which also facilitates theoblique incident light reaching the sample. The optical micro-scope was integrated with an Acton SpectraPro 2750 mono-chromator and a Princeton Instruments Pylon 400BR digitalcharge-coupled device camera which was cooled to −120 °Cusing liquid nitrogen. The scattered light from individualdimer structures can therefore be imaged and analyzedspectroscopically.

CDA calculations

CDA was employed to investigate the chiroptical response ofthe extrinsically chiral system. A schematic of the geometryused in the calculation is illustrated (Fig. S3, the ESI†). Eachgold nanorod is approximated as a prolate ellipsoid with apolarization tensor α$j ¼ Tj�1α$0Tj ( j = 1,2), where α$0 is thediagonal matrix with one longitudinal element αL and twotransverse elements αT in the principal axes system of therods, and Ti is the rotation matrix which relates the ellipsoidframe with the lab frame. By using the Rayleigh–Gans approxi-mation, the polarization elements can be given as:35,36

αj ¼ V4πε� εm

εm þ ðε� εmÞLjwhere V is the volume of the nanoparticle, εm is the dielectricconstant of the surrounding medium, and ε is the dielectricfunction of gold.37

The depolarization factors Lj are defined as

LL ¼ 1� e2

e212e

ln1þ e1� e

� �� 1

� �

LT ¼ 1� LL2where the eccentricity e is defined as e ¼

ffiffiffiffiffiffiffiffiffiffiffiffi1� b2a2

q, and a and b

are the major and minor radii of the ellipsoid, respectively.35,36

The polarization of each ellipsoid can be expressed as:31

pj ¼ α$j � ðEinc;j þ G$j;i � piÞ

where Einc,i = E0e(s,p,L,R)exp(ik·ri − iωt ) is the electric field at ridue to the incident plane wave. The subscript labels s and pdenote two orthogonal polarized beams of light with s- andp-polarization, respectively. L and R denote the CPL with LCPand RCP, respectively. The unit vectors of the s polarization es

(perpendicular to the incident plane), p polarization ep (paral-lel to the incident plane), and the wave vector ek were chosento obey right-handed rule ek = es × ep. The LCP and RCP unitvector eL,R was defined as eL;R ¼

ffiffiffi2

pes + iep� �

=2 in the calcu-lation. G

$j;i�pi is the electric field due to the dipole at position

ri (i ≠ j ):31

G$j;i�pi ¼ expðikrjiÞrji3

ð1� ikrjiÞrji2

� 3rjiðrji � piÞ � rji2pi

�

� k2rji � ðrji � piÞ�

where rji = rj − ri, rji = |rj − ri|Therefore, the coupled equation for the dipole pj ( j = 1,2)

can be written as

X2i¼1

ðδj;i I$ � α$�G$ j;iÞ�pi ¼ α$�Einc;j

By solving these equations for the unknown polarizationspj, the extinction and absorption cross section can be calcu-lated as:

Cext ¼ 4πkEincj j2

X2j¼1

Im E*inc;j �pj� �

Cabs ¼ 4πkEincj j2

X2j¼1

Im pj �ðαj�1Þ*�p*j

�� 2

3k3pj�p*j

�

with the scattering cross section Csca = Cext − Cabs.The CD cross sections are given by

Cext;CD ¼ Cext;L � Cext;R

Cabs;CD ¼ Cabs;L � Cabs;R

Csca;CD ¼ Csca;L � Csca;RThe scattering far field is given by31

Esca ¼ k2 expðikrÞ

r

X2i¼1

expð�ikr̂�riÞðr̂r̂ � I$Þ�pi

As the differential scattering cross section is related withthe scattering far field in the form of

dCscadΩ

/ r2jEscaj2 ¼ k2X2i¼1

expð�ikr̂�riÞðr̂r̂ � I$Þ�pi

2

;

we express the far field as

r2 Escaj j2 ¼ k2X2i¼1

exp �ikr̂�rið Þ r̂r̂ � I$� ��pi

2

which is independent of r.

FDTD calculations

A software package, FDTD Solutions, from Lumerical Solu-tions, Inc., was employed to simulate the near field profiles of

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the gold nanorod dimer. A Drude–Lorentz model was used torepresent the dielectric function of bulk gold.38 The refractiveindex of the surrounding medium was taken to be n = 1.32.Light sources with LCP and RCP at an incident angle of 60°were used in the simulation. The gold nanorods were modeledas a cylinder with two hemispherical tips at both ends. Thewidth and length of the gold nanorods are 23 nm and 69 nmrespectively. The gap distance between the two nanorods is1 nm, the structure angle of the dimer is 80°, and the meshgrid used in the calculation was set at 0.5 nm.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (grant no. 21271181 and 21473240), Min-istry of Science and Technology of the People’s Republic ofChina (Inter-governmental S&T Cooperation Project, grant no.2013-83-6-10) and Thousand Youth Talent Program of China.

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Circular dichroism from single plasmonic nanostructures ... · Circular dichroism (CD) studies on single nanostructures can yield novel insights into chiroptical physics that are