Resonance Coupling in Silicon NanosphereâJ-Aggregate

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Resonance coupling in silicon nanosphere-J-aggregate heterostructures Hao Wang, Yanlin Ke, Ningsheng Xu, Runze Zhan, Zebo Zheng, Jinxiu Wen, Jiahao Yan, Pu Liu, Jun Chen, Juncong She, Yu Zhang, Fei Liu, Huanjun Chen, and Shaozhi Deng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02759 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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Resonance coupling in silicon nanosphere−Jaggregate heterostructures Hao Wang,†,‡,┴ Yanlin Ke,†,┴ Ningsheng Xu,† Runze Zhan,† Zebo Zheng,†,‡ Jinxiu Wen,†,‡ Jiahao Yan,‡ Pu Liu,# Jun Chen,† Juncong She,† Yu Zhang,† Fei Liu,† Huanjun Chen,†,* and Shaozhi Deng†,* †

State Key Lab of Optoelectronic Materials and Technologies, Guangdong Province Key

Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China ‡

School of Physics, Sun Yat-sen University, Guangzhou 510275, China

#

School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275,

China ┴

H. Wang and Y. L. Ke contributed equally to this work.

*

Corresponding authors: [email protected] (H. C.) and [email protected] (S. D.)

ABSTRACT Due to their optical magnetic and electric resonances associated with the high refractive index, dielectric silicon nanoparticles have been explored as novel nanocavities that are excellent candidates for enhancing various light−matter interactions at the nanoscale. Here, from both of theoretical and experimental aspects, we explored resonance coupling between excitons and magnetic/electric resonances in heterostructures composed of the silicon nanoparticle coated with molecular J-aggregate shell. The resonance coupling was originated

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from coherent energy transfer between the exciton and magnetic/electric modes, which was manifested by quenching dips on the scattering spectrum due to formation of hybrid modes. The influences of various parameters, including the molecular oscillation strength, molecular absorption line width, molecular shell thickness, refractive index of the surrounding environment, and separation between the core and shell, on the resonance coupling behaviors were scrutinized. In particular, the resonance coupling can approach the strong coupling regime by choosing appropriate molecular parameters, where an anti-crossing behavior with a mode splitting of 100 meV was observed on the energy diagram. Most interestingly, the hybrid modes in such dielectric heterostructure can exhibit unidirectional light scattering behaviors, which cannot be achieved by those in plexcitonic nanoparticle composed of metal nanoparticle core and molecular shell.

KEYWORDS: silicon nanospheres · magnetic resonances · excitons · resonance coupling · Jaggregates

Tailoring the local electromagnetic environment surrounding dye molecules can modify their optical absorption and emission behaviors. Of particular interest is the resonance coupling phenomenon where the molecules were placed closely to a nanostructure with strong optical resonances. In this scenario, the excitons can interact coherently with the optical resonances and form hybrid modes with intriguing optical properties. In the past decades the resonance coupling has been extensively cultivated in heterostructures composed of molecular J-aggregates and metal nanostructures with spectrally-overlapped excitonic and plasmonic resonances.1−18 These heterostructures were usually termed as plexcitons.4,8,12,17,19 In the plexciton the localized electromagnetic field adjacent to the surface of the metal nanostructures can remarkably polarize


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the J-aggregates, which in turn disturb the plasmon oscillations of the metal nanostructures. As a result, the optical responses of the heterostructures are dramatically modified and distinctly different from those of the separate counterparts. Depending on the optical properties of the Jaggregates, the occurrence of the resonance coupling is usually evidenced by mode splitting or quantized quenching dips on the optical extinction or scattering spectra of the plexcitons.4,7,11−13,20 In recent years high-refractive-index silicon nanostructures have been demonstrated to exhibit intriguing optical magnetic (SiMDR) and electric (SiEDR) dipolar resonances, which possess tunable frequencies and amplitudes amenable to the sizes and compositions of the nanostructures.21−24 Particularly, strong interference can occur between the SiMDR and SiEDR with spectral overlap and comparable oscillation strengths, giving rise to unidirectional light scattering behaviors which cannot be accessed in an individual plasmonic nanostructures.25−29 Such anisotropic scattering can benefit various optoelectronic applications, such as solar cells,30 ultrasensitive sensing,31 and light-emitting devices.32 Besides, in comparison with the plasmonic metals, the silicon nanostructures exhibit lower loss at optical frequencies and tunable carrier densities, whereby their optical response can be tailored by external doping. In view of these, it is foreseen that the silicon nanostructures will endow the aforementioned resonance coupling phenomena with more attributes in terms of magnetic hybrid states, anisotropic responses, and facile tunability. However, to the best of our knowledge, although previous study has employed the silicon nanostructures to tailor the emission of nanoemitters,33 the resonance coupling between the SiMDR in the silicon nanostructures and the excitons of molecular J-aggregates remains unexplored.


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In this work, we for the first time report the resonance coupling in heterostructures composed of silicon nanoparticle core coated with J-aggregate shell. We show from a theoretical aspect that due to the coherent energy transfer between the exciton and SiMDR/SiEDR modes, resonance coupling evidenced by quenching dips on the scattering spectra of the heterostructures can occur. Such an interaction is strongly dependent on various parameters, including the molecular oscillation strength, molecular absorption line width, refractive index of the surrounding environment, thickness of the shell, as well as separation between the core and shell. In particular, the resonance coupling can approach the strong coupling regime with specific molecular parameters, where hybridized modes are formed and give rise to an anti-crossing behavior on the energy diagram. The resonance coupling and anti-crossing behavior in the heterostructures are corroborated by experiments using single-particle dark-field scattering spectroscopy, where a mode splitting of 100 meV can be obtained. More importantly, the hybrid modes exhibit strong unidirectional light scattering behaviors, which are originated from the intrinsic attributes of the SiMDR modes and cannot be achieved in conventional plexcitonic structures composed of plasmonic metal nanoparticles coated with molecule shells. We start our discussion by inspecting the light scattering from a pristine silicon nanosphere of 140-nm diameter placed in water. According to the Mie theory, the scattering spectrum of a spherical nanoparticle can be calculated as34

σ scat =

(

2π ∞ 2 ( ) 2 ∑ 2n + 1 an + bn k n =1

2

)

(1)

where k is the wavevector and an and bn are the scattering coefficients standing for electric and magnetic multipolar contributions, respectively. Figure 1a shows the theoretical scattering cross section, which indicates that the scattering spectrum is dominated by the SiEDR (a1 term) and SiMDR (b1 term) centering at 500 nm and 586 nm, respectively. The contributions from the


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higher-order modes, i.e. electric and magnetic quadruple modes (SiEQR and SiMQR) can be neglected. The heterostructure is formed by coating the silicon nanosphere with a uniform Jaggregate shell of thickness t. The optical response of the molecular shell is described using a classical one-oscillator Lorentzian model with excitonic absorption line shape as

ε ex (ω ) = ε ∞ −

fω02 ω 2 − ω 02 + iωγ 0

(2)

where ε∞ is the high-frequency component of the dielectric function, which is set as the dielectric constant of the surrounding environment; ω0 is the exciton transition frequency of the Jaggregate; γ is the line width of the exciton transition; and f is the oscillator strength. The use of the homogeneously broadened Lorentzian oscillator to model the optical response of the Jaggregate layer has been proven to be successful in describing the resonance coupling between plasmonic nanoparticles and molecular J-aggregates.1,4,10−15 In our study, ε∞ is fixed as 1.7780 corresponded to that of the water. ω0 is set to be equal to the SiMDR energy of the silicon core to ensure degeneracy of the exciton and SiMDR. γ is set at 50 meV according to the experimental absorption spectra of conventional molecules.10,12,13 The scattering spectra of various heterostructures are then calculated using the Mie theory with fixed t (2 nm) and varied f. As shown in Figure 1b, the scattering spectrum of the core−shell heterostructure resembles that of the pristine silicon nanosphere for small f. For an f of 0.05, a slight dip at the same spectral location as the SiMDR can be observed on the scattering spectrum. Appearance of the dip indicates occurrence of the resonance coupling between the J-aggregate and silicon nanosphere. The dip becomes sharper and deeper when the f is increased, suggesting that the resonance coupling is strengthened for larger oscillator strengths. In particular, for an f of 0.4, the two shoulders separated by the dip evolve into a pair of discernible peaks. This is an indication of


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mode splitting.11,20 Besides, for f above 0.2 an additional dip can be observed close to the SiEDR, which also experiences a mode splitting when the f is increased to 0.4 (Figure 1b).

Figure 1. Theoretical scattering spectra of the pristine silicon nanosphere and heterostructures. (a) Scattering spectrum of the pristine silicon nanosphere (red). The contributions from the SiMDR, magnetic quadruple resonance (SiMQR), SiEDR, and electric quadruple resonance (SiEQR) are indicated by the lines of different colors, respectively. (b) Scattering spectra of the heterostructures composed of silicon core coated with J-aggregate shells. The shells have a fixed exciton line width of 50 meV and varied oscillation strengths as indicated. (c) Analysis of the scattering spectrum of the heterostructure with an f of 0.4, where the scattering spectrum is decomposed into contributions of the magnetic dipole resonance (HMDR, blue line), electric dipole resonance (HEDR, green line), magnetic quadruple resonance (HMQR, violet line), and electric quadruple resonance (HEQR, orange line). Calculation spectrum using the finite element method (FEM, gray circles) is also included. The various optical modes are indicated by the


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numbers. (d) Absorption spectrum of the pristine J-aggregate shell (red line), where the f and γ are set as 0.4 and 50 meV, respectively. The contributions from the magnetic dipole resonance (JMDR, blue line), electric dipole resonance (JEDR, green line), magnetic quadruple resonance (JMQR, violet line), and electric quadruple resonance (JEQR, orange line) are also included. The dashed black line indicates the scattering spectrum of the shell, which is magnified by 20 times for the purpose of comparison. The diameter of the silicon nanosphere is 140 nm and the thickness of the shell is 2 nm. The origins of the mode splittings can be understood by decomposing the scattering spectrum corresponding to f = 0.4 into contributions from various resonance modes. To make the discussion concise, the various resonance modes in the heterostructures are respectively represented by the acronyms HEDR (hybrid electric dipole resonance), HEQR (hybrid electric quadruple resonance), HMDR (hybrid magnetic dipole resonance), and HMQR (hybrid magnetic quadruple resonance). As shown in Figure 1c, the hybrid modes around the SiMDR are mainly contributed by both of a1 and b1, while those around the SiEDR are dominated by a1 (Figure 1c). Therewith the split modes around the SiMDR are of both HMDR and HEDR nature, while those associated with the SiEDR are mainly contributed by the HEDR. This finding seems incomprehensible because the exciton transition described by the one-oscillator model can solely couple with the SiMDR of spectral overlap and give only one pair of split modes. To tackle this issue, absorption spectra of the hollow J-aggregate shell with different f are calculated, whereupon the associated mode analyses can be performed. When the f is varied from 0.05 to 0.4, the real and imaginary parts of the dielectric functions of the molecular shell exhibit similar shapes (see Figure S1a, b in the Supporting Information), indicating that the optical response of the J-aggregate is still governed by one excitonic transition. The situation is however distinctly


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different in the hollow molecular shell. For an f larger than 0.1, high-energy absorption peaks emerges in addition to the excitonic transition (see Figure S1c in the Supporting Information). Such new peaks are attributed to the geometry of the shell and can be termed as cavity mode or void mode.13 The mode analysis suggests that the excitonic transition is contributed by the electric dipole (JEDR), electric quadruple (JEQR), and magnetic dipole (JMDR) modes, while the cavity mode is composed of both of the JEDR and JEQR (Figure 1d). On the basis of the above analyses, the HEDR can be attributed to coupling between the SiEDR and both of the JEDR and JEQR, while the HMDR is due to the coupling between the SiMDR and JMDR, JEDR, as well as JEQR. One should note that in the above analysis we choose to show the absorption spectra rather than the scattering spectra of the J-aggregate shells. This is because optical absorption spectrum is a straightforward indication of the excitonic transition in the molecules. We also calculated the scattering spectrum of the shell, which exhibits very similar shape to that of the absorption spectrum except the rather weak intensity (Figure 1d). Therefore the conclusion of our study will be the same by choosing either spectrum to demonstrate the response of the molecule shell. Further insights into the resonance coupling require knowledge of the respective near-field optical behaviors of the silicon core, J-aggregate shell, and heterostructure. To that end, electromagnetic simulations using finite-element method (FEM) was utilized to calculate the optical responses of the heterostructure. The simulated scattering spectrum matched well with that of the Mie theory (Figure 1c), which guaranteed the validity of our subsequent discussion. Figure 2a and b give the near-field electric field distributions and associated charge distributions of the heterostructure at the spectral peaks and dip of the HEDR (marked as 1−3 in Figure 1c). The high-energy mode at 492 nm is of electric dipole nature as manifested by its electric near-


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field distribution, which indicates field enhancements localizing at two apexes along the incidence polarization. The charge distributions suggest that this mode stems from the anti-phase coupling between SiEDR and JEDR, where the surface charges at the interface between the core and shell are stronger than those on the outer surface of the heterostructure (Maxima of the charge distributions at the interface and outer surface are 2.02 ×10−11 C/m2 and 1.21 ×10−11 C/m2, respectively.). In such a manner the charge distributions of the heterostructure give a net dipole moment and therefore the 492-nm resonance is a bright mode with strong far-field scattering intensity (Figure 1c).

Figure 2. Near-field optical responses of the heterostructure. (a) Electric field intensity enhancement contours on the central cross section of the heterostructure. (b) Charge density distributions of the heterostructure. The charge density distributions are inspected at the outer surface of the heterostructure (left panel) and interface between the core and shell (right panel), where the three-dimensional distributions are projected onto the two-dimensional circular disks. Both of the electric field intensity enhancement and charge density distribution contours are obtained at the spectral peaks (492 nm and 546 nm) and dip (526 nm) of the HEDR. The charge


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density at 526 nm has been scaled down to 1/5 of its original value for the purpose of comparison. The maximum and minimum values of the scale bar are chosen to give optimum color appearance. (c) Magnetic field intensity enhancement contours on the central cross section of the heterostructure. (d) Current density distributions of the heterostructure. Both of the magnetic field intensity enhancement and current density distribution contours are obtained at the spectral peaks (572 nm and 600 nm) and dip (586 nm) of the HMDR. For the spectral dip at 526 nm, the charge distributions on the interface and outer surface are of dipolar shape with comparable amplitude (Maxima of the charge distributions at the interface and outer surface are 1.48 ×10−10 C/m2 and 1.31 ×10−10 C/m2, respectively.). The anti-phase coupling between the SiEDR and JEDR will cancel each other and confine the electric field within the molecule shell, as indicated by the electric field intensity contour. Therefore the electromagnetic energy will be dissipated by the shell, giving rise to the dip on the scattering spectrum. The situation becomes more complex for the low-energy mode at 546 nm, where the interface and outer surface of the heterostructure both exhibit charge distributions of quadruplar patterns (Figure 2b). Appearance of such charge distributions can be understood by inspecting the resonance mode of the silicon nanosphere core and cavity mode of the pristine hollow molecule shell at 530 nm. The charge distribution of the pristine silicon nanosphere at 530 nm is of electric dipolar shape (Figure S2a in the Supporting Information). The cavity mode is contributed by both of JEDR and JEQR, which therefore give a charge distribution with asymmetric quadruplar patterns on the outer and inner surface of the cavity around 530 nm (see Figure S2b in the Supporting Information). In such a manner, when bringing the silicon core into the cavity to form the heterostructure, the silicon core will be polarized by the charge oscillations on the inner surface of the shell, whereby asymmetric quadruple patterns tend to be formed at


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546 nm (see Figure S2 in the Supporting Information). One should note that for this mode, the charge densities at the upper hemisphere of the outer surface are stronger than those at the interface (Maxima of the charge distributions at the outer surface and interface of the upper hemisphere are 2.78 ×10−11 C/m2 and 1.69 ×10−11 C/m2, respectively.), while at the lower hemisphere, the outer surface (maximum charge distribution of 2.22 ×10−11 C/m2) exhibits a weaker charge densities than those of the interface (maximum charge distribution of 2.52 ×10−11 C/m2). As a result, a net dipole moment with strong electric field enhancement located at the two apexes of the heterostructure can be obtained. This is the reason why the far-field scattering spectrum of the heterostructure around 546 nm is dominated by the dipolar contribution (Figure 1c). The HMDR of the heterostructure involves the interaction of the JMDR, JEDR, and JEQR with the SiMDR. The resonance coupling between the various modes in the silicon nanosphere and J-aggregate shell is mediated by the charge and current distribution associated with each mode. Therefore the coupling strengths between two specific modes are determined by their relative charge and current intensities. The circular current intensity associated with the JMDR is very weak in comparison with that of the SiMDR (see Figure S3 in the Supporting Information), suggesting that the coupling between the SiMDR and JMDR can be neglected. The charge distributions of the electric resonances are comparable with that of the SiMDR (see Figure S3 in the Supporting Information). In addition, the charge distribution in the inner surface of the Jaggregate shell is out-of-phase with that of the silicon core. As a result, the spectral features (marked by 4−6 in Figure 1c) of the HMDR are mainly induced by the resonance coupling of the SiMDR with the JEDR and JEQR. On one hand, the charge oscillations of the core and inner surface of the shell can interact with each other via the Coulomb interactions. On the other hand,


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the circular current associated with the SiMDR can electromagnetically couple with the charge oscillations of the JEDR and JEQR in the molecule shell. In such a manner, these three modes with spectral overlap can hybridize together to form the split peaks at 572 nm and 600 nm, which exhibit strong circular currents with strong optical magnetic fields localizing at the center of the silicon core (Figure 2c, d). On the other hand, the coupling between the circular current and charge oscillations also enables energy transfer from the core to the shell, which is dissipated by the molecule absorption. As a result, the magnetic field within the core as well as the scattering intensity of the heterostructure will be reduced, giving rise to the spectral dip at 586 nm. The coupling of the JEDR and JEQR with the SiMDR can also be manifested by the respective charge density distributions at the three spectral positions (see Figure S4 in the Supporting Information). For the split peak centering at 600 nm, the outer surface of the heterostructure and interface between the core and shell both exhibit dipolar-shaped charge distributions, while for the dip position and 572-nm split peak, relatively weak charge distribution with quadruple shape can be observed at the core−shell interface. To further reveal the resonance coupling behavior, we calculated the energy diagrams of the core−shell heterostructure by systematically varying the parameters of the molecule and surrounding environment, including the f, γ, separation between the core and shell (h), thickness of the shell (t), and the refractive index of the surrounding environment (n) (see Figure S5 in the Supporting Information). We first looked at the influence of the oscillator strength f (Figure 3a), where the other parameters are kept as those used to obtain the results of Figure 1. With an increasing f, the dip at the spectral position corresponding to the exciton absorption becomes deeper and wider and eventually evolves into two split peaks for f larger than 0.3. These spectral features are expected, because a higher f results in greater polarization of the molecule and


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therefore the stronger polarized electromagnetic field felt by the silicon core. As a result, the resonance coupling will become stronger and give rise to the observed mode splitting.

Figure 3. Calculated energy diagrams of the resonance coupling in the heterostructure. (a) Energy diagram when γ is kept at 50 meV while f is increased from 0 to 1. The thickness of the shell, t, is kept at 2 nm. The refractive index of the surrounding environment, n, is kept at 1.333. The separation between the core and shell, h, is 0. (b) Energy diagram when the γ is increased from 10 to 100 meV. The f, t, h, and n are kept at 0.4, 2 nm, 0, and 1.333, respectively. (c) Energy diagram when the t is varied from 0 to 30 nm. The f, γ, h, and n are kept at 0.4, 50 meV, 0, and 1.333, respectively. (d) Energy diagram when the h is increased from 0 to 50 nm. The f, γ, t, and n are kept at 0.4, 50 meV, 2 nm, and 1.333, respectively. The refractive index of the spacer is 1.5. (e) Energy diagram with different n. The f, γ, t, and h are kept at 0.4, 50 meV, 2 nm, and 0, respectively. (f, g) Resonance coupling energy diagrams of the heterostructure at the HMDR and


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HEDR. In the specific calculations the diameters of the silicon nanosphere core are varied from 130 nm to 160 nm to obtain different SiMDR and SiEDR energies. The f, γ, t, h and n are set as 0.4, 50 meV, 2 nm, 0, and 1.333, respectively. The two white lines represent the fittings using the coupled harmonic oscillator model to demonstrate anti-crossing behavior, where upper and lower branch modes can be observed. A mode splitting, hΩ , of 100 meV is obtained. The dashed blue lines indicate the exciton transition and cavity energies, respectively. (h) Exciton and SiMDR fractions for the upper and lower branches of the HMDR, respectively. On the other hand, if the f is kept at 0.4 and γ is varied from 10 to 100 meV, the dips separating the split peaks become shallower and finally disappear, where the split peaks around the SiMDR and SiEDR merge together and become two broad bands resembling those of the pristine silicon nanosphere (Figure 1a). Such a behavior suggests that there exist upper limits of γ for the resonance coupling to occur. The line width of the molecule usually characterizes its damping magnitude. A molecule shell with very large γ will rapidly dissipate energy transferred from the silicon core, whereby the coupling strength between the core and shell cannot compete with such dissipation. As a result, at large enough γ the total scattering spectrum of the heterostructure becomes that as if the SiMDR and SiEDR are weakened by a broad background. In real experiments, the thickness of the molecule shell is also an important parameter that can affect the resonance coupling in the heterostructure. On the one hand, usually molecule with different sizes are used to coat the inner core.4,7,35 On the second hand, molecules often form aggregates with size up to a few nanometers on the nanoparticles.4,5,7,8,36,37 Moreover, in situations where the heterostructures are formed by spin-coating the molecule layer onto the nanoparticle, the thickness of the shell can be up to dozens of nanometers. Figure 3c gives the dependence of the resonance coupling on the molecule shell thickness. For t larger than 2 nm, the split peaks around


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the SiMDR and SiEDR always present irrespective of the change in t. As the t is increased, the split magnitudes of the various modes first get larger and then stabilize for t over 5 nm. For the dips separating the split modes, they first get broader and deeper as t is raised, and then stabilize for t above 5 nm. These behaviors indicate that a thicker molecule shell can benefit the coupling between the core and shell because more molecules can be involved in the coupling, as long as they experience the near-field enhancements induced by the core (Figure 2). However, for a shell thicker than the penetration depth of the near-field around the core, the molecules locating at regions without field enhancement will not couple with the silicon core, whereupon stabilizations of the resonance coupling spectra will be observed. To sum up, the thickness of the molecule shell only affects the magnitude of the resonance coupling, while has little effect on the other features. The resonance coupling is also strongly dependent on the separation between the shell and core. As shown in Figure 3d, when a dielectric spacer with refractive index of 1.5 is inserted between the shell and core, the resonance coupling spectra can be modified according to the thickness of the spacer. Specifically, as the core−shell separation is increased from 0 to 50 nm, the mode splittings induced by the resonance coupling become weaker and are smeared out gradually, suggesting disappearance of the resonance coupling. The scattering spectrum is manifested by two broad bands with two small quenching dips associated with the shell absorption. This is reasonable because when the separation between the shell and core is larger than the penetration depth of the near-field around the core, the molecules will not feel the electromagnetic field from the core and therefore are decoupled from the core. As a result, the scattering spectrum of the heterostructure becomes that of the pristine silicon nanosphere, with two tiny quenching dips induced by the molecule shell absorption.


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In real applications the heterostructures usually will be placed in different environment. The effects of the surrounding refractive index n on the resonance coupling should therefore deserve to be investigated. To that end, the resonance coupling energy diagram is calculated with varied n from 1 (air) to 2.5. As the n is increased, more polarized charges can be induced at the interface between the heterostructure and surrounding dielectric. Such polarized charges can partially neutralize the charge density at the heterostructure surface, which can reduce not only the oscillation strength but also the resonance energy of the cavity mode of the molecule shell (see Figure S6 in the Supporting Information). As a result, the resonance coupling between the cavity mode and SiEDR will be weakened and fade away at large n (Figure 3e). Furthermore, around the SiEDR the dip between the two split modes red-shifts with increasing n. For the exciton transition of the molecule shell, its line width is widened while its resonance energy is invariant with n. Besides, the absorption magnitude becomes stronger at large n. Consequently the resonance coupling between the SiMDR and exciton transition of the shell is blurred out gradually for n above 2. In coupling between exciton and optical modes, usually anti-crossing behavior on the energy diagram can be observed by continuously tuning the energy of the optical mode across the exciton transition energy. Whereas such a phenomenon has been well-documented in the aforementioned plexciton systems, similar studies using dielectric nanoparticle remain completely unexplored. We then utilized the silicon nanosphere core−J-aggregate shell heterostructure as a model system to cultivate this effect. The resonance energy of the SiMDR can be facilely tuned by tailoring the diameters of the silicon nanospheres.23−26,28 In the situation of f = 0.4, as the SiMDR energy is varied across the exciton transition of the J-aggregate, the scattering spectra show a distinct anti-crossing behavior (Figure 3f). Two prominent branches


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can be observed on the energy diagram, identified as the high-energy upper-branch (UB) and low-energy lower-branch (LB). When the energy of the SiMDR is much smaller than that of the exciton transition, the LB dominates and redshifts with increasing SiMDR energy. As the SiMDR approaches and overlaps with the exciton transition, both of the LB and UB appear as two split modes. When the SiMDR moves away from the exciton transition to the high energy side, the LB vanishes slowly and only the UP is visible. These behaviors can be well-fitted using a coupled harmonic oscillator model (Figure 3f, solid white lines), which indicates a mode splitting of ħΩ = 100 meV (see Note 1 in the Supporting Information). Similar phenomena can be observed for the coupling between the cavity mode and SiEDR (Figure 3g). As the energy of the SiEDR is tuned across the cavity mode, two hybridized modes due to the coupling between the SiEDR and cavity mode appear on the scattering spectra, which are separated by a spectral dip corresponding to the energy of the cavity mode. In addition, there also exists spectral anticrossing effect, although it is less pronounced in comparison with that of the coupling between the SiMDR and exciton transition. One should note that the mode splitting can usually be induced by hybridizations between two optical modes. For the interactions between the exciton transition and optical modes, mode splitting induced by strong coupling will occur if the following two conditions are satisfied. The first one is the anti-crossing behavior on the energy diagram, which has been observed as shown above. The second one is that the splitting energy hΩ should fulfill the condition hΩ >

or hΩ >

γopt + γex 2

γopt − γex 2

, where γopt is the line width of the optical mode.12,18,38 In such a circumstance,

the energy exchange rate between the exciton and optical mode should exceed their respective decay rates, whereby the mode splitting induced by the coherent interaction between these two


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components becomes significant enough to be observed experimentally. Considering that the line widths of the SiMDR (γSiMDR) and exciton transition (γex) are respective 200 meV and 50 meV, the mode splitting energy fulfill the criterion of hΩ >

γSiMDR − γex . Therefore the resonance 2

coupling between the SiMDR and exciton transition enters the strong coupling regime. In this regime, hybridized modes with part-light and part-matter will be formed due to the energy exchange between the exciton and SiMDR. The coupled harmonic oscillator model is then utilized to calculate the contributions from exciton and SiMDR components for both branches. As shown in Figure 3h, by detuning the SiMDR to low-energy side of the exciton transition, the LB is more SiMDR-like while the UB more exciton-like, and vice versa for detuning to the high energy side. To verify the above theoretical calculations, we conducted experimental studies on the resonance coupling between the silicon nanosphere and molecule with strong exciton absorption. It is a great challenge for us to fabricate the silicon nanosphere core−J-aggregate shell heterostructures as proposed in our simulations. Instead we choose an alternative geometry for the experimental measurements, where the silicon nanospheres were deposited onto the indium tin oxide (ITO)-coated glass and coated with a J-aggregate film (see Figure S7 in the Supporting Information). In such a manner, the heterostructure were obtained with the silicon nanosphere embedded into a molecular film. The molecules have intimate contact with the nanosphere, which can guarantee the effective interactions between the SiEDR and SiMDR with the molecule resonances. We therefore believe that this type of heterostructure can capture the main features of the resonance coupling as those in the core−shell structures. In a specific measurement, the silicon nanospheres of different sizes were mixed to form colloidal solution, which was then drop-casted onto the ITO-coated glass substrate. After the


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deposit was dried naturally under ambient conditions various individual silicon nanospheres can be found on the substrate. A cyanine dye molecule, 5,6-Dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(4sulfobutyl)-benzimidazol-2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)-benzimidazolium hydroxide, inner sodium salt (TDBC, Figure 4a), was utilized in our study. The molecule was dissolved into the polyvinyl alcohol (PVA) solution to form the J-aggregates (1 mM), which exhibited strong oscillation strength and narrow line width (γex) of the exciton transition at room temperature. The heterostructures were formed by spin-coating the TDBC/PVA film on to the substrate with the silicon nanospheres. The thickness of the molecule film was controlled to 80 nm. Single-particle dark-field scattering spectroscopy was utilized to measure the scattering spectra of the various individual silicon nanospheres and heterostructures. A pattern-matching method was employed to ensure that spectra from the same silicon nanosphere, before and after coated with the molecule film, were measured and correlated.28

Figure 4. Observation of the resonance coupling on single silicon nanosphere embedded in TDBC/PVA molecule film. (a) Molecular structure of the TDBC. (b) Scattering spectrum of 80 nm-thick TDBC/PVA film coated onto a glass slide. (c) Experimental (solid lines) and calculated


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(dashed lines) scattering spectra of three typical silicon nanospheres deposited onto the ITOcoated glass substrate. (d) Experimental (solid lines) and calculated (dashed lines) scattering spectra of the corresponding heterostructures. (e) Resonance peaks as a function of the SiMDR energies. The red and blue dots are extracted peaks from experimental dark-field scattering measurements. The black solid lines are fitting results according to the coupled harmonic oscillator model, which trace the dispersion of both UB and LB and demonstrate the anticrossing behavior. A mode splitting energy of 100 meV can be obtained. The horizontal dashed line indicates the exciton transition energy while the diagonal one stands for the dispersion of the SiMDR of the pristine silicon nanosphere. Figure 4b illustrates the scattering spectrum of the TDBC/PVA film coated onto a glass slide. The exciton transition energy of the J-aggregate locates at 2.09 eV and the line width is 58 meV. The scattering spectra of the individual pristine silicon nanospheres are strongly dependent on their diameters. For each silicon nanosphere a low-energy resonance as well as a broad and highenergy resonance can be observed on its scattering spectrum, which are corresponded to the SiMDR and SiEDR, respectively. In addition, as the diameter of the silicon nanosphere is increased, the SiMDR and SiEDR both exhibit redshift behavior (Figure 4c and Figure S8a in the Supporting Information). The scattering spectra of the nanospheres coated with the TDBC/PVA layer are distinctly different from those of the pristine ones. Specifically, the SiMDR splits into two peaks that are dependent on the spectral overlap between the SiMDR and exciton transition (Figure 4d and Figure S8b in the Supporting Information). According to the above theoretical studies, the occurrence of the split peaks indicates that the SiMDR and the exciton transition experience resonance coupling and are strongly hybridized. The energies of the split peaks are then extracted from the scattering spectra using Lorentzian fitting and plotted versus the SiMDR


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energies. Both of the UB and LB can be clearly revealed from the dispersions (Figure 4e), which exhibit an unambiguous anti-crossing behavior. The UB and LB are then fitted using the coupled harmonic oscillator model, which gives a mode splitting energy of 100 meV. These experimental results are in good agreement with the above calculations. The splitting energy of 100 meV obtained in our study is not contradictory to the previous study with plasmonic systems, where such a large value can only be achieved in specially designed structures.12,39,40 Those studies aimed to cultivate the strong coupling involved only single and a few quantum emitters. Therefore coupled plasmonic structures were delicately designed to give strong enough electromagnetic field enhancements to obtain the mode splitting over 100 meV. In our study, the molecular layer contained a plenty of molecules. Therefore the ~ 100 meV mode splitting can be achieved in the simple structure with moderate electromagnetic field enhancements. On the other hand, one should also note that although the resonance coupling between the SiMDR and exciton transition has been successfully revealed by the experiments, the resonance coupling of the SiEDR counterpart cannot be observed in our measurements. We ascribe such discrepancy to the different geometries of the heterostructures used in the experiments, where the silicon nanospheres were embedded into the molecule continuous thin film. In such a manner, the molecule film cannot support the cavity mode as that predicted for a molecule spherical shell. Therefore no resonance coupling can happen for the SiEDR. We have also simulated the scattering spectra of the heterostructures using the configurations close to the experimental ones (see Figure S7a and Note 2 in the Supporting Information). The results agree well with the measured spectra, both for the pristine silicon nanospheres and heterostructures (dashed lines in Figure 4c, d).


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One exceptional merit of the silicon nanosphere is that its magnetic dipole resonance can strongly interfere with the electric dipole resonance and gives rise to unidirectional forward light scattering in an individual nanoparticle, a capability which cannot exist in the plasmonic metal nanoparticles.26 Considering that the resonance coupling in our heterostructure involves the magnetic dipole mode of the silicon nanosphere, it is anticipated that the heterostructure should also possess the characteristics of unidirectional light scattering. To reveal such an effect, we calculate the far-field scattering properties at the spectral positions associated with the exciton−SiMDR resonance coupling (Figure 5a). Both of the two split modes and the scattering dip exhibit distinct enhanced (suppressed) forward (backward) scattering. Such directional scattering properties are originated from the simultaneous interaction of the SiMDR with the SiEDR and the exciton transition of the molecule shell. The latter two modes are both of electric dipole nature. As a result, the directionality will be enhanced in comparison with that of the pristine silicon nanosphere (Figure 5b, c), where the unidirectional scattering is solely induced by the interaction between the SiMDR and SiEDR.


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Figure 5. Unidirectional scattering properties of the resonance coupling in the heterostructure. (a) Calculated scattering spectra of the silicon nanosphere core−J-aggregate shell heterostructure (sapphire). The geometrical and molecular parameters are the same with those in Figure 1c. Scattering spectrum of the individual silicon nanosphere is also included for reference (jacinth). The pristine SiMDR is marked by λ1. The two split peaks and scattering dip are indicated by λ2, λ4, and λ3, respectively. (b) Far-field scattering patterns of the pristine silicon nanosphere and heterostructure at the wavelength positions λ1−λ4. (c) Polar plot of the scattering patterns at λ1−λ4. (d) Calculated scattering spectra of the Au nanosphere core−J-aggregate shell heterostructure (sapphire). The diameter of the Au nanosphere is 108 nm. The molecular parameters are the same with those in (a). Scattering spectrum of the individual Au nanosphere is also included for reference (jacinth). The plasmon resonance wavelength of the pristine Au nanosphere is indicated by λ5. The two split peaks and scattering dip are indicated by λ6, λ8, and λ7, respectively. (e) Far-field scattering patterns of the pristine Au nanosphere and heterostructure at the wavelength positions λ5−λ8. (f) Polar plot of the scattering patterns at λ5−λ8. The two polar plots are given at the principal plane defined by the incidence wave vector and polarization vector. The directionality of the low-energy peak at 600 nm is larger than those of the high-energy peak (572 nm) as well as the scattering dip (586 nm) (Figure 5b, c). This better performance can be attributed to the stronger electric dipole moment of the low-energy peak (see Figure S4 in the Supporting Information), which can interfere more efficiently with the magnetic dipole resonance. The unidirectional scattering performance has also been evaluated for the resonance coupling between the SiEDR and cavity mode. The pristine SiEDR exhibits a far-field scattering pattern of doughnut-shaped (see Figure S9 in the Supporting Information). For the spectral


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positions corresponding to the SiEDR−cavity mode resonance coupling, the high-energy hybridized mode and spectral dip both show similar scattering patterns as that of the electric dipole resonance. Little unidirectional scattering can be observed. In contrast, the low-energy hybridized mode is a mixture of the electric and magnetic dipole resonances, which thereby exhibits improved unidirectional scattering with enhanced forward scattering. The predicted unidirectional scattering behaviors can be verified experimentally. The measurements were conducted using heterostructures similar to those in the Figure 4 (see Figure S10 in the Supporting Information). The results clearly indicate that both of the forward and backward scattering spectra exhibit the mode splitting around the SiMDR due to the resonance coupling (see Figure S11a−c in the Supporting Information). In addition, the forward scattering overwhelms the backward scattering in a broad spectral range, suggesting occurrence of the unidirectional scattering. These experimental results are consistent with the numerical simulations (see Figure S11d−f in the Supporting Information). However, two issues need to be considered carefully. First, the splitting peaks in the forward scattering spectra are red-shifted in comparison with those in the backward scattering. This is due to the red-shifted magnetic resonance in the forward scattering of the pristine silicon nanospheres.26 Second, there may be additional mechanisms responsible for the unidirectional light scattering observed in the experiments, which are associated with the geometry of the heterostructures used. On one hand, the molecular layer on top of the silicon nanosphere is ~ 12 nm thick (see Figure S7 in the Supporting Information), while the J-aggregate shell in the core−shell model is only 2 nm. The relatively thicker molecular layer can lead to additional light absorption and reduce the excitation intensity in the back-scattering measurements. On the other hand, previous studies have shown that when a nanoparticle is placed onto a dielectric substrate, the incidence light will be scattered


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preferentially into the substrate with a larger permittivity.41 The back-scattered light is therefore reduced in comparison with that of a free-standing nanoparticle. These two effects can suppress the backward scattering intensity observed in our experiments. At present we cannot differentiate them from the intrinsic unidirectional light scattering of the heterostructures, therefore the experimental results can be used for qualitative rather than quantitative evaluations of the unidirectional light scattering by the resonance coupling between the SiMDR and exciton transition. The unidirectional scattering behavior is unique for the silicon nanosphere and cannot be achieved using the plasmonic metal nanoparticles. As a comparison, we also calculated the farfield scattering patterns associated with the plexciton heterostructure. The inner core is a Au nanosphere with diameter of 108 nm, which exhibits plasmon resonance of spectral overlap with the exciton transition. Due to the resonance coupling between the plasmon and exciton, the scattering spectrum of the plexciton is manifested by similar split peaks at 572 nm and 608 nm, respectively (Figure 5d). These two peaks are separated by scattering dip at 586 nm. However, because the Au nanosphere can only support electric dipole plasmon mode, all of the scattering patterns at these three positions give conventional doughnut-shaped distributions as that of the dipolar plasmon resonance in pristine Au nanosphere (Figure 5e, f). The scattering intensities of the forward and backward scattering are equal to each other and therefore no unidirectional scattering can be observed. In summary, we for the first time show that resonance coupling, a phenomenon which has been widely studied in plexciton systems, can still exist in heterostructure composed of dielectric silicon nanosphere coated with J-aggregate shell. Our theoretical simulations clearly revealed that the resonance coupling, manifested by formation of split hybrid modes, is originated from


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coherent interactions of the exciton transition and cavity mode of the molecule shell with the SiMDR as well as SiEDR of the silicon nanosphere core. The influences of various geometrical and molecular parameters on the resonance coupling are systematically cultivated. In particular, the resonance coupling between the SiMDR and exciton transition has been verified experimentally. The strong coupling can be achieved where an anti-crossing behavior with a mode splitting of 100 meV is observed on the energy diagram. More importantly, the split modes and scattering dip exhibit unidirectional light scattering behaviors. This is a unique characteristic associated with the magnetic dipole resonance of the silicon nanosphere, which cannot be obtained using the plexciton nanostructures with pure electric dipole resonances. We believe that our study has opened up a new avenue for constructing organic–inorganic heterostructures with exceptional coherent optical responses, which will be promising for future nanophotonic applications. Besides, the dielectric heterostructure proposed can complement the traditional plexciton system and help to further our understanding of light–matter interactions at the nanoscale. ASSOCIATED CONTENT Supporting Information. Detailed numerical methods (Supplementary Texts); Experimental procedures (Supplementary Texts); Description of the coupled harmonic oscillator model (Supplementary Texts); Permittivities and absorption spectra of the J-aggregate shells with various molecule parameters (Figure S1); Surface charge distributions of the pristine silicon nanosphere, the hollow pristine Jaggregate shell, and the core–shell heterostructure (Figure S2); Charge and current density distributions of the silicon nanosphere core (left) and J-aggregate shell (right) at 586 nm (Figure S3); Charge density distributions of the heterostructure obtained at the spectral peaks and dip of


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the HMDR (Figure S4); Schematic of the heterostructure showing the core–shell heterostructure (Figure S5); Energy diagram of the pristine J-aggregate hollow shell at different n (Figure S6); Scheme and SEM images of the heterostructures used in the experimental measurements (Figure S7); Scattering spectra of various individual silicon nanospheres and heterostructures on the ITO-coated glass substrate (Figure S8); Additional unidirectional scattering data of the resonance coupling in the heterostructure (Figure S9); Scheme of the configurations used for the backward and forward dark-field scattering measurements (Figure S10); Experimental measurements and numerical simulations of the unidirectional scattering properties of the heterostructures (Figure S11). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Author Contributions H.C., S.D., and N.X. conceived the study and designed the experiments. H.W. and Y.K. conducted the theoretical calculations and single-particle measurements. R.Z., Z.Z., and J.W. participated in analysis of the theoretical and experimental data. J.Y. and P.L. synthesized the silicon nanospheres. J.C., J.S., Y.Z., and F.L. participated in the discussion of the data. H.W., Y.K., H.C., and S.D. co-wrote the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ┴These authors contributed equally. Notes


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The authors declare no competing financial interests. ACKNOWLEDGMENT We thank Professor Jun-Jun Xiao from the College of Electronic and Information Engineering at Shenzhen Graduate School of Harbin Institute of Technology for helpful discussions. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51290271, 11474364), the National Key Basic Research Program of China (Grant Nos. 2013CB933601, 2013YQ12034506), the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No. 2014A030306017), the Guangdong Special Support Program, and the Fundamental Research Funds for the Central Universities. ABBREVIATIONS SiMDR, magnetic dipolar resonance of the silicon nanosphere; SiEDR, electric dipolar resonance of the silicon nanosphere; SiMQR, magnetic quadruple resonance of the silicon nanosphere; SiEQR, electric quadruple resonance of the silicon nanosphere; HMDR, magnetic dipole resonance of the heterostructure; HEDR, electric dipole resonance of the heterostructure; HMQR, magnetic quadruple resonance of the heterostructure; HEQR, electric quadruple resonance of the heterostructure; JMDR, magnetic dipolar resonance of the J-aggregate shell; JEDR, electric dipolar resonance of the J-aggregate shell; JMQR, magnetic quadruple resonance of the J-aggregate shell; JMDR, electric quadruple resonance of the J-aggregate shell; FEM, finite-element method; UB, high-energy upper branch; LB, low-energy lower branch; ITO, indium tin oxide; TDBC, 5,6-Dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)-benzimidazol2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)-benzimidazolium hydroxide, inner sodium salt; PVA, polyvinyl alcohol.


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(39) Santhosh, K.; Bitton, O.; Chuntonov, L.; Haran, G. Nat. Commun. 2016, 7, 11823. (40) Chikkaraddy, R.; de Nijs, B.; Benz, F.; Barrow, S. J.; Scherman, O. A.; Rosta, E.; Demetriadou, A.; Fox, P.; Hess, O.; Baumberg, J. J. Nature 2016, 535, 127−130. (41) Mertz, J. J. Opt. Soc. Am. B 2000, 17, 1906−1913. Insert Table of Contents Graphic and Synopsis Here

The resonance coupling between the exciton and optical modes in dielectric nanoparticle has been systematically studied, both theoretically and experimentally, in heterostructures composed of silicon nanospheres coated with molecule J-aggregate shells.


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Resonance Coupling in Silicon NanosphereâJ-Aggregate

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