Microscopic Origin of Surface-Enhanced Circular Dichroism - ACS

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Microscopic Origin of Surface-enhanced Circular Dichroism Seojoo Lee, SeokJae Yoo, and Q-Han Park ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00479 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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Microscopic Origin of Surface-enhanced Circular Dichroism Seojoo Lee‡, SeokJae Yoo‡, and Q-Han Park*

Department of Physics, Korea University, Seoul, Korea. KEYWORDS: Circular dichroism, chiral molecule, chirality, biosensing, plasmonics, metamaterial

ABSTRACT: Circular dichroism (CD), the difference in absorption of two opposite circularly polarized lights by chiral molecules, can be significantly enhanced when molecules are adsorbed on the surface of nanostructures. We present a theory based on the Poynting’s theorem adapted for chiral media to analyze the surface-enhanced CD of a chiral molecule/nanostructure coupled system. Our theory clarifies the microscopic origin of surface-enhanced CD signals by showing that the enhanced CD has two forms, inherent and induced. The inherent CD is the direct molecular CD which becomes enhanced due to the strongly localized optical helicity density near the nanostructure. The induced CD, previously ignored, derives from asymmetric excitation and absorption of electromagnetic fields inside the nanostructures surrounded by chiral molecules upon the injection of two oppositely circularly polarized lights. Moreover, it is

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demonstrated that the induced CD can contribute significantly to the CD signals measured by surface-enhanced chiroptical spectroscopy.

Spectroscopic techniques utilizing the surface-enhanced optical signals of molecules near rough metal surfaces or nanostructures have proliferated rapidly in the past decade. Recently, inspired by some pioneering works1–3, surface-enhanced spectroscopic techniques have been applied to circular dichroism (CD) spectroscopy for chiral molecules4–12. CD spectroscopy measures the difference in absorption of two oppositely circularly polarized light sources by chiral molecules to obtain their stereochemical information, and molecular CD signals can be enhanced on nanostructures and metamaterials1–3. Despite the recent surge of interest in surfaceenhanced CD spectroscopy, a common consensus on the cause of surface-enhanced CD has not arisen because various mechanisms could contribute to the enhancement of CD, including the molecule-plasmon Coulomb interaction3,13,14, the strong local optical helicity of near fields1,7,11,15,16, the radiative coupling between molecules and nanostructures8,17, or the circular differential scattering of light by nanostructures embedded in chiral media6,18. The moleculeplasmon Coulomb interaction model correctly reproduces the total CD but only within the electrostatic approximation, and thus the theory cannot explain CD originating from chiral molecules attached to resonant nanostructures, in particular, recently reported CD enhancement by locally enhanced magnetic fields5,19. The contribution of locally enhanced electric and magnetic fields to CD has been treated rigorously by the optical helicity theory, but it failed to predict the induced CD originated from the lossy nanostructures. Here, we propose an analytic study based on Poynting’s theorem adapted for chiral media to identify the microscopic origin of surface-enhanced CD in chiral molecule/nanostructure coupled


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systems. Microscopically analyzing near-field energy dissipation inside the coupled system showed that CD signals macroscopically measured by transmission of the coupled systems have two distinct origins – inherent and induced. We find that the inherent CD of chiral molecules can be enhanced by local fields, and that the optical response of the nanostructure can itself become chiral in the presence of chiral molecules, forming an induced nanostructure CD.

We

quantitatively compare these inherent and induced CDs to identify which dominates surfaceenhanced CD spectroscopy. RESULTS AND DISCUSSION Inherent and induced CD signals. Figure 1a shows a schematic of typical surface-enhanced CD spectroscopy. An array of nanostructures is prepared on a substrate, and chiral molecule analytes are adsorbed on the surface of the nanostructures. The CD signals of the chiral molecule/nanostructure coupled system can be measured by the differential transmittance ∆T = T+ − T− , where T± is the transmittance of left (+) and right (-) circularly polarized light through the coupled system. When chiral molecules are adsorbed on the surfaces of nanostructures and illuminated by left and right circularly polarized light, not only can their inherent CD be enhanced by the strong local fields (Figure 1b), but the chiral molecules can also change the electromagnetic responses of the nanostructures (Figure 1c) asymmetrically. This causes an asymmetric absorption of circularly polarized light by nanostructures termed induced CD. This induced CD can make a significant contribution to the total CD signals measured by CD spectroscopy. The ratio of the strengths of the inherent CD and induced CD in chiral molecule/nanostructure coupled systems depends on the specific geometry of the nanostructure, the handedness of chiral molecules adsorbed, and the wavelength of the incident light.


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To explain the two microscopic origins behind surface-enhanced CD, we analyze energy % and H % dissipation in a chiral molecule/nanostructure coupled system. For the harmonic fields E

with the angular frequency w in a lossy medium, the energy dissipation in a volume V is given

(

)

% ⋅D % * − B% ⋅ H % * dV ′ from Poynting’s theorem (see Supporting Information for by Pabs = (ω / 2 ) ∫ E V

% are determined by the electromagnetic constitutive relations of each % and B details). Here, D medium in the chiral molecule/gold nanodisk coupled system. Additionally, chirality is introduced to Poynting’s theorem using the electromagnetic constitutive relations of the chiral % = µH % − iκ E% , where ε , µ , and κ are the electric permittivity, the % = ε E% + iκ H % and B media, D magnetic permeability, and the chirality parameter of the chiral medium, respectively. The chirality parameter κ completely describes the chiroptical responses of chiral molecules. Note that the real and imaginary parts of the chirality parameter κ represent the optical rotatory dispersion (ORD) and the CD, respectively19. ORD is the wavelength-dependent rotation of the polarization plane of the linearly polarized beam on passing through the chiral molecule sample. The

energy

(

dissipation

)

( )

can

( )

be

(

decomposed

into

three

terms

)

% ,H % =P % % % % ( Pabs E abs ,E E + Pabs , M H + Pabs ,C E, H ):

( )

( )

% ≡ ( ω / 2 ) Im ( ε ) E% 2 dV ′ = ω {Im ( ε ) / Re ( ε )} u E% dV ′ , Pabs ,E E E ∫ ∫ V

V

( )

( )

(2)

)

(3)

% ≡ (ω / 2 ) Im ( µ ) H % 2 dV ′ = ω {Im ( µ ) / Re ( µ )} u H % dV ′ Pabs , M H M ∫ ∫ V

(

)

V

(

)

(

% ≡ ω Im (κ ) Im E % ⋅H % * dV ′ = 2ω 2 c 2 Im (κ ) h E %,H % dV ′ , Pabs ,C E% , H ∫ ∫ V

(1)

V


4

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( )

( )

(

)

% , P % % % where Pabs ,E E abs , M H , and Pabs ,C E, H are the electric, magnetic, and chiral parts of the dissipation,

respectively.

( )

% = Re ( ε ) E % 2 /2 uE E

(

)

The

energy

( )

% = Re ( µ ) H % 2 /2 . uM H

and

(

corresponding

densities

The

are

optical

defined

helicity

by

density,

)

% ,H % = (1/ 2ωc2 ) Im E % ⋅H % * , represents the projection of the spin angular momentum density h E of light onto the electromagnetic momentum density1,15, where c is the speed of light in vacuum. When left and right circularly polarized lights σ + and σ − excite chiral molecule/nanostructure % ± and magnetic fields H % ± are induced differently coupled systems, their local electric fields E inside the lossy materials. Thus, the differential energy dissipation, that is, the CD of the chiral

(

)

(

)

% +,H % + −P E % −,H % − , also decomposes molecule/nanostructure coupled system, ∆Pabs ≡ Pabs E abs to three components: the electrically induced CD ∆Pabs , E , the magnetically induced CD ∆Pabs ,M , and the inherent CD ∆Pabs ,C given by

( ) ( ) ( H% ) − P ( H% ) , ( E% , H% ) − P ( E% , H% ) .

%+ −P %− , ∆Pabs ,E ≡ Pabs , E E abs , E E +

∆Pabs , M ≡ Pabs , M ∆Pabs ,C ≡ Pabs ,C

−

abs , M

+

+

−

(4)

−

abs ,C

For chiral molecule/nanostructure coupled systems, CD signals result from both the absorptive chiral molecule and the lossy nanostructure. The volume integration in each term in Eq. (4), over the regions enclosing the chiral molecules and the nanostructures, reveal the local microscopic origin of the CD signals from a chiral molecule/nanostructure coupled system. Note that the inherent CD ∆Pabs ,C is proportional to the differential optical helicity h in Eq. (4). Many efforts have been made to enhance the optical helicity h in the vicinity of optical resonators without paying attention to other contributions1,7,11,15,16. Eq. (4) shows that the inherent CD alone cannot


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fully explain the total CD signals of the chiral molecule/nanostructure coupled system and in fact the induced CD makes a significant contribution to CD signals as we demonstrate below. Surface-enhanced CD in a chiral molecule/plasmonic nanostructure coupled system. This paper analyzes surface-enhanced CD signals in a chiral molecule/gold nanodisk coupled system. The gold nanodisk array was chosen as an example system because it is the simplest plasmonic nanostructure with mirror symmetry, as shown in Figure 2a. When measuring the effects on CD of chiral molecule adsorption, the achirality resulting from this mirror symmetry eliminates the complication of CD signals originating from geometrical nanostructure chirality. In Figure 2b, the gold nanodisk array exhibits strong localized surface plasmon resonance (LSPR) at the visible wavelength of 620 nm when the thickness, the radius, and the lattice constant of the gold nanodisk array are H disk =30 nm, Rdisk =45 nm, and Λ=350 nm, respectively. Therefore, we can study the effect of the dipolar LSPR mode being perturbed by chiral molecules on CD signals. Adsorbed chiral molecules are modeled as a homogeneous chiral medium of relative permittivity

ε mol =1.92+0.56i, and thickness H mol =3 nm. The chirality parameters are given by κ mol =0.001 in Figure 2c&e and κ mol =0.001i in Figure 2d&f. These chiral molecule layer parameters are chosen to the typical value of chiral liquids6,16. We calculate the circular dichroic response of the chiral molecule/gold nanodisk coupled system using the numerical solver of the electromagnetic wave equation in chiral media (see Supporting Information for details). Figure 2c shows CD spectra macroscopically measured by the differential reflectance ∆R ≡ R+ − R− , the differential transmittance ∆T ≡ T+ − T− , and the differential absorbance ∆A ≡ A+ − A− when the chirality parameter of the adsorbed molecules is purely real-valued, i.e.,

κ mol = 0.001 . The subscripts ± denote the handedness of the incident plane waves. In Figure 2c,


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the chiral molecule/gold nanodisk coupled system shows nonzero ∆T = −∆A at the LSPR wavelength, while the differential reflectance ∆R vanishes. Therefore, we find that the CD signal of the coupled system originates from asymmetric absorption according to the polarization of the incident light. We also note that the differential reflectance ∆R = 0 for a homogeneous chiral film under the normal incidence of light20 (see Supporting Information for details). For conventional CD measurements using a spectropolarimeter, a chiral molecule with a purely realvalued chirality parameter κ mol shows neither differential absorption signals ∆A nor differential transmittance signals ∆T because it has an ORD response only19.

Therefore, the peak of

∆T = −∆A in Figure 2c is clear evidence of surface-enhanced CD signals. Figure 2e shows the CD spectra obtained by microscopic calculation of the differential energy dissipation ∆ Pabs inside the volume V of the chiral molecule/nanodisk coupled system and the

(

)

differential energy flux ∆ ∫ Re S% ⋅ n dA through the surface S enclosing the total volume V. S

(

)

Figure 2e, shows that the calculations of ∆ Pabs and ∆ ∫ Re S% ⋅ n dA give identical results S

(

)

because they are related by the chiral Poynting’s theorem, ∆Pabs = ∆ ∫ Re S% ⋅ n dA . Comparing S

the

spectra

in

(

Figure

2c&e,

we

find

that

the

locally

obtained

CD

spectra

)

( ∆Pabs = ∆ ∫ Re S% ⋅ n dA ) are identical to the differential transmittance spectra ( ∆A = −∆T ). S

This implies that the far-field measurement of the differential transmittance ∆T can be completely described by the near-field calculation of local CD ∆ Pabs .


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For the chiral molecule layer with a purely imaginary κ mol = 0.001i , we are also able to confirm

(

)

the equivalence of ∆A = −∆T (Figure 2d) and ∆Pabs = ∆ ∫ Re S% ⋅ n dA (Figure 2f). The CD S

lineshapes of the chiral molecule/nanodisk coupled system are different for the purely real chirality parameter κ mol = 0.001 in Figure 2c&e and the purely imaginary κ mol = 0.001i of the adsorbed molecules in Figure 2d&f. The differences between the two spectral lineshapes are due to the different origins of the CD. For the adsorbed molecules with the purely real chirality parameter κ mol = 0.001 in Figure 2c&e, the induced CD ∆ Pabs , E only contributes to the total CD ∆ Pabs because there is no inherent CD ∆ Pabs ,C that is proportional to the imaginary part of the

chirality parameter Im (κ mol ) . In contrast, when the adsorbed molecules have the purely imaginary-valued chirality parameter κ mol = 0.001i in Figure 2d&f, both ∆ Pabs , E and ∆ Pabs ,C can contribute to the total CD ∆ Pabs . We also note that the differential transmittance ∆T in Figure 2d&f represents a 3-fold enhancement compared to the ∆T = 4π L Im ( κ ) / λ = 3.16 × 10−6 of a chiral thin film of thickness L = π H mol ( R / Λ ) =0.156 nm at the wavelength λ =620 nm19. 2

To quantitatively analyze the circular dichroic behaviors of the chiral molecule/gold nanodisk coupled

systems,

we

decompose

the

total

CD

∆ Pabs

into

∆Pabs = ∆Pabs , E ( Au ) + ∆Pabs , E ( mol ) + ∆Pabs ,C( mol ) in each region in Figure 3. Figure 3a shows the

induced CD in gold ∆Pabs , E ( Au ) , the induced CD in the molecules ∆Pabs , E ( mol ) , and the inherent CD in the molecules ∆Pabs ,C( mol ) for the chiral layer with the real κ mol = 0.001 . In Figure 3a, we find that ∆Pabs , E ( Au ) dominates the total CD ∆Pabs , while ∆Pabs , E ( mol ) only slightly contributes to the CD signals because the Ohmic loss in gold is much stronger than that in the chiral molecule layer.


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∆Pabs ,C( mol ) vanishes for the chiral layer with κ mol = 0.001 because the inherent CD ∆Pabs ,C( mol ) is

only proportional to the imaginary part of the chirality parameter Im ( κ mol ) . In contrast, for the chiral layer with the imaginary κ mol = 0.001i shown in Figure 3b, the inherent CD in the chiral molecule layers ∆Pabs ,C ( mol ) play a significant role because the imaginary part of the chirality parameter is nonzero ( Im (κ mol ) ≠ 0 ). In Figure 3b, the sign of ∆Pabs ,C ( mol ) changes around the LSPR wavelength. At the dipolar LSPR, the gold nanodisk can be described by the % induced by the incident fields E of polarizability α% . In the electric dipole moment p% = α% E 0, ± 0

% p% and H % =E % + ω 2 µG % = m iE% / c vicinity of the gold nanodisks, the local fields are E ± 0, ± NF ± ± 0, ± % 21. E% and H % with the dyadic Green’s function in the near field zone G NF 0,± 0,± are the circularly polarized incident fields. Near the LSPR, the differential optical helicity density at the top of the gold nanodisks, r = Hzˆ , is given by (See Supporting Information for detailed derivation):

∆h ( r = Hzˆ, ω ) ≈ The

differential

optical

helicity

µω 1 % 2 . Re (α% ) E 0, ± 3 2 2 4π c H k H

density

∆h ( r , ω )

determines

(5) the

inherent

CD

∆Pabs ,C ( mol ) = 2ω 2 c 2 ∫ Im (κ ) ∆hdV ′ , and is easily found to be dependent on the real part of the V

polarizability Re (α ) . Here, the resonant polarizability of the gold nanodisks α can be described by the Drude model, and thus the real part of the polarizability Re (α ) changes its sign at the resonance wavelength, resulting in the sign flipping of ∆h ( r , ω ) . Due to the sign flipping of ∆Pabs ,C ( mol ) , the signs of ∆Pabs , E ( Au ) and ∆Pabs ,C ( mol ) are the same at wavelengths larger


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than the LSPR wavelength but opposite at shorter wavelengths. Note that because the sign flipping of the inherent CD ∆Pabs ,C ( mol ) can cause the quenching of CD signals, nanostructures for the surface-enhanced CD spectroscopy should be carefully designed. For example, the sign flipping in the optical helicity density can be avoided and optical helicity can be greatly enhanced with nanostructures such as double fishnet negative index metamaterials5 or diagonal slits on a mirror22. Surface-enhanced CD spectroscopy of dispersive chiral molecules. Chiral molecules in the real world have a dispersive permittivity and a chirality parameter. The previous section found that the sign flipping of the inherent CD ∆Pabs ,C ( mol ) can compete with the induced CD ∆Pabs , E ( Au ) . Considering the competition between the induced and inherent CD in the chiral molecule/nanostructure coupled system, the spectral matching between the nanostructure resonance and the molecular absorption band warrants study. Next is an investigation of the competition between induced and inherent CD when the nanostructure resonance and the molecular absorption are on- and off-resonance. The dispersion of chiral molecules can be modeled by ε mol = ε solv + Nd ω0 { f ( ω ) + ig (ω )} and κ mol = Nrω { f (ω ) + ig (ω )} with the relative permittivity of solvent ε solv , the number density of molecules N , the electric dipole strength d , the rotatory strength r , and the molecular absorption band frequency ω 0

19,22

.

Dispersion and absorption lineshape functions f ( ω ) and g (ω ) are given by19 f (ω ) =

g (ω ) =

ω02 − ω 2

(ω02 − ω 2 ) + ω 2Γ2 2

ωΓ

(ω

2 0

− ω 2 ) + ω 2Γ2 2

,

(6)

,

(7)


10

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where Γ is the damping factor that determines the bandwidth of the lineshape functions f ( ω ) and g (ω ) . We set the parameters ε solv =1.332, Nd = 0.8 × 1014 s −1 , Nr = 0.8 × 1012 s −1 , and Γ = 2π c / ( 500 nm ) = 3.77 × 1014 s −1 . To show the effect of spectral matching between the

nanostructure resonance and the molecular absorption band, we set the molecular absorption band

frequency ω0 = 2π c / ( 650 nm ) = 2.90 × 1015 s −1 for

on-resonance

molecules,

and

ω0 = 2π c / ( 500 nm ) = 3.77 × 1015 s −1 for off-resonance molecules. The respective resulting chirality parameters κ mol are plotted in Figure 4a&b. The circular dichroic response of the chiral molecule/gold nanodisk coupled system when the chiral molecules are on-resonance (off-resonance) with respect to the LSPR of the gold nanodisk is illustrated in Figure 4c&e (Figure 4d&f). We find that CD in on-resonance molecules is stronger than CD in off-resonance molecules. In Figure 4e, the induced CD ∆Pabs , E ( Au ) in gold nanodisks also changes its sign at the LSPR wavelength due to the sign flipping Lorentzian lineshape of the real part of the chirality parameter Re (κ mol ) . Therefore, the signs of ∆Pabs , E ( Au ) and ∆Pabs ,C ( mol ) can change simultaneously, resulting in CD enhancement. Figure 4d&f shows CD signals of the coupled system when the adsorbed molecules are offresonance with respect to the dipolar LSPR of gold nanodisks. In Figure 4d&f, the CD signal peaks at 500 nm and 630 nm correspond to the molecular absorption and the dipolar LSPR, respectively. The sharp dips at 480 nm originate from the quadrupolar LSPR. At the dipolar LSPR wavelength at 650 nm, the real part of the chirality parameter Re (κ mol ) is much larger than the imaginary part Im ( κ mol ) because it is far from the molecular absorption band frequency, and thus the CD signals at the LSPR wavelength of 630 nm are mainly induced by Re (κ mol ) . At


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the LSPR wavelength, the inherent CD ∆Pabs ,C ( mol ) barely contributes to the total CD ∆Pabs , whereas the electrically induced CD in the gold nanodisks ∆Pabs , E ( Au ) make a dominant contribution to total CD ∆Pabs . The effect of the induced CD ∆Pabs , E ( Au ) on the total CD ∆Pabs in Figure 4f is important for surface-enhanced CD spectroscopy because many biomolecules exhibit CD signals of ultraviolet wavelength. We also note that recent literature on surfaceenhanced CD spectroscopy only considers the enhancement of the inherent CD ∆Pabs ,C ( mol ) in the vicinity of nanostructures5,7,16.

CONCLUSIONS We analyzed CD signals in the chiral molecule/gold nanodisk coupled system because they are the simplest plasmonic nanostructure with mirror symmetry. Although a full discussion of CD enhancement optimization is beyond the scope of this paper, some important characteristics of the CD of chiral molecule/nanostructure coupled systems to consider when designing nanostructures for surface-enhanced CD spectroscopy follow. First, the CD signals measured in the differential transmittance experiments have multiple origins, and they are the sum of the induced CD in the plasmonic nanostructure ∆Pabs , E ( Au ) , the induced CD in the adsorbed chiral molecules ∆Pabs , E ( mol ) , and the inherent CD in the adsorbed chiral molecules ∆Pabs ,C ( mol ) . We note first, that although the enhancement of optical helicity density h ( r , ω ) has recently been the subject of intense interest in the surface-enhanced CD spectroscopy field, enhanced optical helicity density h ( r , ω ) only affects ∆Pabs ,C ( mol ) . Therefore, nanostructure design for surfaceenhanced CD spectroscopy should aim for not only a large asymmetry between the local fields


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induced by the two oppositely circularly polarized light sources but also a strongly enhanced optical helicity density ∆h . Second, the chiral molecule/nanostructure can exhibit CD signals even when the adsorbed chiral molecules have only ORD, i.e. Re (κ mol ) ≠ 0 and Im (κ mol ) = 0 . Third, the sign of ∆Pabs ,C ( mol ) flips at the LSPR wavelength if plasmonic nanostructures are coupled to circular dichroic chiral molecules with a nonzero chirality parameter, that is, if Im ( κ mol ) ≠ 0 .

CD is the non-radiative (absorptive) process of differential decay between two excited states of opposite helicities in chiral molecules. Although chiral fluorophores exhibit differential radiative decay, namely fluorescence-detected CD (FDCD)23, this paper only considers non-radiative CD signals coupled to nanostructures. Recently, it has been reported that the coupling of molecular FDCD signals to optical resonators can be explained by a theory of the chiral Purcell effect15. The chiral Purcell effect describes the enhancement of molecular FDCD signals in terms of the quality factor Q which represents the temporal confinement of energy, and the chiral mode volume FC which represents the spatial confinement of the optical helicity density inside the optical resonator. The chiral Purcell effect theory only considers optical helicity density enhancement by the optical resonator. Considering the splitting of electromagnetic modes by chiral perturbation, an important parameter describing optical resonators for FDCD enhancement is the sum of the differential Purcell factor ∆FP and the chiral Purcell factor ∆FC . By analogy between the radiative and the non-radiative process, the differential Purcell factor ∆FP and the chiral Purcell factor ∆FC correspond to the electrically induced CD ∆ Pabs ,E and the enhanced inherent CD ∆Pabs ,C , respectively.


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ASSOCIATED CONTENT Supporting Information. Supporting Information includes the details for the full derivation of the Poynting’s theorem for chiral media, the optical helicity density near plasmonic nanostructures, the optical response of a homogeneous chiral slab, and numerically solving wave equations for chiral media.

AUTHOR INFORMATION Corresponding Author * E-mail: (Q.H.P) [email protected]. Author Contributions ‡These authors contributed equally. S.J.Y established a theory for the surface-enhanced CD spectroscopy using the chiral Poynting theorem; S.J.L developed the numerical algorithm for chiral media and performed numerical calculations; S.J.L, S.J.Y, Q.H.P analyzed data and wrote the manuscript. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the Samsung Science and Technology Foundation under Project No. SSTF- BA1401-05, a Korea University Grant, and Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future Planning (2017R1A6A3A11034238)


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Figure 1. A schematic drawing of (a) typical surface-enhanced CD spectroscopy, and (b, c) the microscopic origins of surface-enhanced CD spectroscopy; (b) the inherent CD of chiral molecules is enhanced by the strong near fields on the nanostructure, (c) the induced CD of nanostructures — the optical response of the nanostructures coupled to chiral molecules becomes chiral. Note that the chiroptical responses of the enhanced inherent CD and the induced nanostructure CD can vary according to specific nanostructure design, the handedness of the adsorbed molecules, and the wavelength of light.


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Figure 2. (a) Schematic and (b) macroscopic optical response (reflectance R, transmittance T, and absorbance A = 1 − R − T ) of the chiral molecule/gold nanodisk coupled system. Circular dichroism measured by the differential reflectance ∆R (green lines), transmittance ∆T (red lines), and absorptance ∆A (blue lines) associated with chiral molecules with (c) κ mol = 0.001 and (d) κ mol = 0.001i . Circular dichroism calculated by the differential energy dissipation ∆Pabs

(

)

and flux ∆ ∫ Re S% ⋅ n dA associated with chiral molecules with (c) κ mol = 0.001 and (d) S

κ mol = 0.001i . CD spectra in Fig. 2e&f are normalized by the electromagnetic energy per unit % 2 / 2 = 2ε Λ 2 Z 2 with the vacuum impedance Z . cell of area Λ 2 , U unit = ε 0 Λ 2 E 0 0 0


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Figure 3. Components of the total CD ∆Pabs of the chiral molecule/gold nanodisk coupled system; the electrically induced CD in the gold nanodisks ∆Pabs , E ( Au ) (black lines), the electrically induced CD in the chiral molecule layers ∆Pabs , E ( mol ) (red lines), and the enhanced inherent CD in the chiral molecule layers ∆Pabs ,C ( mol ) (blue lines) for (a) κ mol = 0.001 and (b) κ mol = 0.001i .


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Figure 4. Dispersive chirality parameters (Insets: refractive indices) of (a) the on-resonance molecules ( ω0 = 2π c / ( 650 nm ) ) and (b) the off-resonance molecules ( ω0 = 2π c / ( 500 nm ) ). The differential transmittance ∆T , reflectance ∆R , and absorptance ∆A of the chiral molecule/gold nanodisk coupled system for (c) the on-resonance molecules and (d) the offresonance molecules. Spectra of the total CD ∆Pabs (black lines) and its components ∆Pabs , E ( Au ) (red lines), ∆Pabs , E ( mol ) (blue lines), and ∆Pabs ,C ( mol ) (green lines) for (e) the on-resonance molecules and (f) the off-resonance molecules.


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Table of Contents (TOC) image


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Microscopic Origin of Surface-Enhanced Circular Dichroism - ACS

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