Enhanced Directional Fluorescence Emission of Randomly Oriented

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Enhanced Directional Fluorescence Emission of Randomly Oriented Emitters via a Metal−Dielectric Hybrid Nanoantenna Song Sun,*,†,‡ Taiping Zhang,†,‡ Qing Liu,§ Li Ma,†,‡ Qingguo Du,∥,⊥ and Huigao Duan§

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†

Microsystem and Terahertz Research Center, China Academy of Engineering Physics, No. 596, Yinhe Road, Shuangliu, Chengdu, China 610200 ‡ Insititute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, China 621999 § State-Key Laboratory of Advanced Design and Manufacturing of Vehicle Body, Colledge of Mechanical and Vehicle Engineering, Hunan University, Changsha, China 410082 ∥ School of Information Engineering, Wuhan University of Technology, 122 Luo Shi Road, Wuhan, Hubei, China 430070 ⊥ Hubei Key Laboratory of Broadband Wireless Communication and Sensor Networks, Wuhan University of Technology, Wuhan, Hubei, China 430070 S Supporting Information *

ABSTRACT: This paper reports a hybrid nanoantenna consisting of an inner metal nanodisk and an outer dielectric ring cavity with superior fluorescence enhancement performance. Based on the multipole decomposition analysis and the coupled oscillator model, it is found that the surface plasmon resonance of the metal nanodisk could interact with the magnetic dipole mode of the dielectric ring more strongly than the electric dipole mode, improving the excitation rate in the gap region by more than 1 order of magnitude compared to those from the pure metal or dielectric constituent. The hybrid structure partially inherits the low loss advantage of the dielectric component, dramatically boosting the radiative decay rate and generating a decent quantum yield. The enhancement factor for a single emitter could reach more than one order of magnitude higher than those from the pure metal and dielectric counterparts, depending on the emitter’s location and orientation. The overall emission intensity of multiple randomly oriented emitters are estimated via the reciprocity principle, which can reach more than 1500 than that in free space and nearly 2 orders of magnitude higher than those from pure metal or dielectric counterparts. On top of that, an excellent directional radiation pattern is obtained in the hybrid configuration with a maximum directivity enhancement >3000, and 74% of total power propagates upwardly. This robust hybrid structure shows the possibility to leverage the advantages from both metal and dielectric, which could be useful in fluorescence sensing, nonlinear optics, and quantum photonics. to enhance the fluorescence intensity and manipulate its emission directivity is urgently needed. Nanoantenna is a delicately designed nanostructure that functions to mediate the optical energy transfer from the near field to the far field and vice versa. Such a unique capability provides a promising solution to manipulate the fluorescence emission at the deep subwavelength regime.10,11 On one hand, the nanoantenna is able to confine the incident optical radiation at the hotspot region, generating an extraordinary local electromagnetic field that is orders of magnitude higher than that of the incident light. When the fluorescence emitter is placed inside the hotspot region, its excitation rate could be dramatically enhanced.12,13 On the other hand, the resonant mode of the nanoantenna alters the local density of optical states (LDOS), creating more decay channels than that in the free space. Consequently, the radiative decay rate as well as the resultant far field intensity can be significantly improved.14,15

1. INTRODUCTION Fluorescence spontaneous emission is the ultimate foundation for a broad range of applications such as light-emitting device, analytical chemistry, spectroscopy, sensing and imaging, among the others.1,2 The most prominent feature of fluorescence emission relies on its independent radiation characteristic (both spatial and temporal) upon optical excitation, thereby naturally breaking the Abbe diffraction limit that is notorious in the convention microscopy and resulting in an unrivalled super-resolution.3 In addition, the fluorescence emitter is able to interact with the target substance at the molecular or atomic scale, rendering it an ideal choice as a fingerprint-tag in many advanced technologies such as gene sequencing,4,5 neuron mapping,6 early diagnosis,7 and food and drug safety monitoring,8 among others.9 Nevertheless, the intrinsic emission of the fluorescence emitter in the free space is too weak to be detected at a low concentration, and its omnidirectional radiation characteristic brings a further challenge in the collection efficiency of signal. To fulfill the requirements of these emergent applications, a viable method © XXXX American Chemical Society

Received: July 2, 2019 Revised: August 4, 2019 Published: August 7, 2019 A

DOI: 10.1021/acs.jpcc.9b06280 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

plays a vital role in manipulating the emission of the fluorescence emitter. Not only the resonant modes affects the spatial distribution of the local electromagnetic field, which determines the fluorescence excitation rate, but also it would selectively couple to the emitter and alter its decay rate depending on the emitter orientation.40,41 To understand the nature of the resonant modes, the multipole decomposition technique is adopted.42,43 The scattering of light by a nanostructure can be equivalently seen as a process where the incident light excites in the nanostructure polarization and conduction currents that radiate, which can be decomposed into a series of current multipoles. The lth-order current multipole moment is expressed as

Furthermore, the presence of nanoantenna breaks the symmetrical condition of free space. A properly designed nanoantenna could engineer the directivity of fluorescence emission, boosting the collection efficiency.16,17 At optical frequency, noble metals (e.g., Au, Ag, etc.) are commonly adopted to build nanoantenna devices. The collective oscillations of electrons under the resonant condition, so-called the surface plasmon resonances (SPR), could produce a large electromagnetic field enhancement in the proximity of the metallic surface, which could benefit the excitation rate of the fluorescence emitter.18−20 However, the metal materials possess severe dissipations when the fluorescence emitter get too close to the metallic surface, which would cause a quenching problem that is detrimental for the fluorescence emission.21−23 The inevitably high dissipation of metal material drives the development of high permittivity dielectric nanoantenna (e.g., Si, etc.),24 whose low loss characteristic naturally suppresses the nonradiative decay of fluorescence emission and yields a higher quantum efficiency.25−27 Depending on the displacement current generated inside the nanostructure, the dielectric nanoantenna could provide both electric and magnetic resonances and utilize their multipolar interference to flexibly control the directivity of fluorescence emission.28−30 Nevertheless, the optical confinement capability of the dielectric material is relatively weaker than that of metal.31 To achieve all-round improvements in the fluorescence emission, a possible way is to synergistically combine the advantages of both metal and dielectric, so as to benefit from the large electromagnetic field enhancement and the low radiation loss simultaneously.14,32,33 Some early works have proven the feasibility to form hybrid configurations with excellent fluorescence enhancement performance,34−36 and its potential to surpass the pure metallic or dielectric counterpart.37−39 Nevertheless, the full potential of the hybrid nanoantenna is yet to be realized, and the interaction between the metal SPR and the dielectric electric/magnetic resonances requires further investigation. In this contribution, a radically novel hybrid nanoantenna is proposed, which consists of an inner metal nanodisk and an outer dielectric ring cavity, to dramatically enhance the fluorescence emission and achieve a directional radiation pattern. The spectral response of the individual metal/ dielectric constituent is analyzed via the multipole decomposition technique, to elucidate the respective contributions from various electric and magnetic multipolar resonances. Next, the metal SPR and dielectric electric/magnetic resonances are judiciously designed to spectrally overlap with each other to investigate the coupling mechanism between metal and dielectric. With the finite element method (FEM), the complete fluorescence enhancement process, including the excitation rate, radiative/nonradiative decay rate, quantum yield, and enhancement factor, is systematically studied under the weak coupling framework. To better mimic the practical situation, the overall fluorescence emission of multiple randomly oriented fluorescence emitters is explored by means of the reciprocity principle. Combining these results, the robustness of the hybrid configuration could be revealed at a glance.

i (l − 1) ! ω

M(l) =

rr ... r d3r ∫ J(r) ß

(1)

l − 1 terms

where M is a tensor of rank l and ω is the angular frequency. For l = 1, 2, and 3, the respective current multipole moments are dipole, quadrupole, and octupole moments. J(r) = −iωε0[εr(r) − εm]E(r) is the scattering current density, where ε0, εr(r), and εm are the permittivities of vacuum, nanoantenna, and environment medium, respectively. E(r) is the total electric field inside the nanostructure generated by the incident light. From eq 1, the vector potential of the system A(r), which satisfies (∇2 + k2)A(r) = −μ0J(r) in the Lorentz gauge, can be obtained as (l)

A(r ) = ×

1 k3 ω 4πε

l−1 l−a−1

∞

∑∑∑ ∑ l=1

x

a=0

M′ (l)(x , a , b)x

b=0

(− 1)l − 1(l − 1)! d a db dl − (a + b + 1) (1) h0 (kr ) a! b![l − (a + b + 1)]! dw a dw*b dz l − (a + b + 1)

(2)

where M′ (x, a, b) is the element of tensor M in the circular coordinate system, w, w*, and z are the unit vector of the system, and x could be one of w, w*, or z. The electric and magnetic fields outsides the nanostructure follow E(r) = iω{A(r) + 1/k2∇[∇·A(r)]} and H(r) = 1/μ0∇ × A(r), from which the electric and magnetic multipole coefficients aE(l,m) and aM(l,m) can be described as (l)

aE(l , m) =

(l)

(− i)l + 1kr hl(1)E0[π(2l

2π

1/2

+ 1)l(l + 1)]

∫0 ∫0

π

* (θ , φ) Y lm

(3)

× r·E(r)sin θ dθ dφ (− i)l + 1ηkr

aM (l , m) =

hl(1)E0[π(2l

+ 1)l(l + 1)]1/2

2π

∫0 ∫0

π

* (θ , φ) Y lm

(4)

× r·H(r)sin θ dθ dφ

hl(1)

where Y*lm is the spherical harmonics and is the spherical Hankel function of the first kind. Finally, the overall extinction cross-section (CS) of nanostructure can be rigorously obtained as the summation of multipolar contributions as Cext = −

π k2

∞

+l

∑∑ l = 1 m =−l

(2l + 1)Re[maE(l , m) + am(l , m)]

(5)

Nanoantenna-Enhanced Fluorescence for a Single Emitter. The interaction between a single fluorescence emitter and the nanostructure is essentially a quantum mechanical process.44 Nevertheless, nanoantenna-enhanced fluorescence fulfills the weak coupling assumption (e.g., the excitation and emission processes are independent, and there is no energy backtransfer between the emitter and the nanostructure) and

2. METHODS Resonant Mode Analysis Based on Multipole Decomposition Technique. The resonant mode of the nanoantenna B


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The Journal of Physical Chemistry C can be simply described by the classical electromagnetic theory, which gives the same results as that of the full quantum electrodynamic (QED).25,30,45 In this case, the emitter is commonly treated as an oscillating dipole. Upon optical incidence, the fluorescence excitation rate can be expressed as γext ∝ |E·p|2, where p is the dipole momentum and E is the nanoantenna enhanced local electric field at the position of the emitter. For the fluorescence emission process, the quantum yield q is used to balance the radiative decay rate Γr and nonradiative decay rate Γnr of the emitter, which represents the probability that a photon can radiate to the far field instead of being absorbed by the environment: q=

emission I(ϑ, φ) and normalized by that in the free space, the overall improvement in the fluorescence emission intensity for multiple randomly oriented emitters can be obtained as FEtot FE0

(6)

where the superscript “0” represents the respective quantity in the free space. Emission from Multiple Randomly Oriented Emitters Based on Reciprocity Principle. In reality, multiple emitters are presented in the proximity of the nanoantenna. Since the radiations from different emitters are completely independent,3 it is legitimate to treat multiple emitters as an assembly of incoherent dipole sources, and the overall far field emission is governed by the reciprocity principle.46,47 Briefly, the electric field emitted by a dipole source toward a certain direction (θ, φ) can be equivalently seen as the electric field generated by another dipole source located along the same direction of the emission angle (θ, φ) at the far field, which can be further treated as a combination of the S- and P-polarized plane waves incident from (θ, φ). The overall far field intensities can thus be obtained via integrating the contributions from all emitters: |p|2 3

∭ |Es(r)|2 + |EP(r)|2 d3r

|p|2 3

∭ 13 γext(r)·q(r)·(|Es(r)|2 + |EP(r)|2 )d3r

2π

=

2π

2π 2I 0

3. RESULTS Schematic of Hybrid Nanoantenna. Figure 1 illustrates the schematic of the proposed hybrid nanoantenna that is

(8)

where ES(r) and EP(r) are the electric fields generated by Sand P-polarized plane waves, respectively. p denotes the individual dipole momentum and the factor 3 in the denominator takes into account the randomness of the emitter orientation during the emission process. In the free space, the radiation pattern from multiple randomly oriented emitters should be omnidirectional, which is verified in Figure S1 in the Supporting Information (SI). With slight modification, the effect of the excitation rate and quantum yield can also be included, and the overall fluorescence intensity can be estimated as Itot(ϑ, φ) =

π

∫0 ∫0 I 0(ϑ, φ)·dθ·dφ

π

∫0 ∫0 Itot(ϑ, φ)·dθ·dφ (10)

where q0 is the intrinsic quantum yield of the emitter, and it is assumed = 1 to objectively evaluate the performance of the nanoantenna regardless of the choice of the emitter. The enhancement factor ηem/η0em for a single emitter is defined by multiplying the excitation rate γext/γ0ext and quantum yield q/ q0. ηem γ q = ext · 0 0 0 ηem γext q (7)

Iem(ϑ, φ) =

2π

where I0 is the angular fluorescence emission in the free space and is a constant due to the omnidirectional radiation pattern. Note that FEtot/FE0 in eq 10 is different from the conventional definition ηem/η0em in eq 7. ηem/η0em mainly considers the excitation rate γext/γ0ext upon the incident light and the quantum yield q/q0, which indicates the probability of an emitted photon decay radiatively. In addition to these two factors, FEtot/FE0 also accounts the increment in the total emitted photon rate Iem/I0em induced by the nanoantenna. Numerical Analysis via Finite Element Method. To utilize eqs 1−8, a finite element method (FEM) using COMSOL Multiphysics is adopted to supply the raw input data, including (1) the electric field E(r) generated inside the nanostructure to facilitate the multipole decomposition analysis; (2) the scattering power Psca and absorption power Pabs to calculate the radiative (nonradiative) decay rate as Γr/ Γ0 = Psca/P0 (Γnr/Γ0 = Pabs/P0); and (3) the electric fields ES(r) and EP(r) generated upon S- and P-polarized plane wave incidences that are required in the reciprocity principle calculation. More detail can be found in the SI.

Γr /Γ 0 Γr /Γ 0 + Γnr /Γ 0 + (1 − q0)/q0

=

π

∫0 ∫0 Itot(ϑ, φ)·dθ·dφ

Figure 1. Schematic of hybrid nanoantenna that consists of an inner metal nanodisk and an outer dielectric ring cavity.

composed of an inner metal nanodisk and an outer dielectric ring cavity. For illustration purposes, Au and Si are used as the metal and dielectric materials, respectively, while other metal (e.g., Ag, Al, etc.) or dielectric (e.g., GaP, GaAs, etc.) materials can also be adopted. The optical properties are taken from the experiments.48 The resonant wavelength of the Si ring cavity can be flexibly tuned over a large spectral scale by adjusting the geometrical parameters, whereas the plasmonic resonance of the Au nanodisk is relatively stable against its geometrical variations. More details can be found in Figures S2 and S3 in the SI. Therefore, it is more convenient to match the dielectric electric/magnetic resonance with the metal SPR via controlling the geometry of the Si ring cavity. The fluorescence emitters

(9)

where γext and q are the associated excitation rate and quantum yield, respectively, and the additional factor 1/3 takes into account the randomness of the emitter orientation during the excitation process. By integrating the angular fluorescence C


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Figure 2. Multipole decomposition of (a) Au nanodisk and (b) Si ring cavity, where metal SPR and dielectric MD are designed to align at 530 nm. (c) Extinction cross-section and (d) near-field distribution of hybrid nanoantenna. Interaction between metal SPR and dielectric MD produces a hybrid resonant mode that is red-shifted to 580 nm, associated with a strong field enhancement in the gap region.

are assumed to be homogeneously distributed among the gap region between the metallic and the dielectric constituents. Coupling Mechanism between Metal SPR and Dielectric MD. To start with, the coupling between a Rm = 50 nm, H = 120 nm Au nanodisk and a Rin = 60 nm, Rout = 90 nm, H = 120 nm Si ring cavity is investigated. The multipole decomposition results for the isolated Au nanodisk and Si ring cavity are presented in Figure 2a and b, respectively, and are confirmed with the FEM full wave simulations. The electric dipole mode (ED), magnetic dipole mode (MD), electric quadrupole mode (EQ), and magnetic quadrupole mode (MQ) are primarily contributed to the extinction spectra. Higher order modes are thus neglected. Clearly, the SPR of Au nanodisk is dominated by the ED, while the Si ring possesses comparable and separated ED and MD. This particular design aligns the metal SPR and dielectric MD at the same wavelength, ∼530 nm, and the resultant extinction CS of the hybrid configuration is shown in Figure 2c; the detail parameter studies are shown in Figures S2 and S3 in the SI. It is observed that the coupling between the SPR and MD lead to a red-shifted resonant peak at 580 nm, and a blue-shifted shoulder at 510 nm. The dielectric ED remains almost unchanged at 430 nm, indicating a weak interaction between the SPR and ED. Figure 2d illustrates the corresponding nearfield distributions for the three resonant peaks, respectively. The resonant peak at 580 nm yields a strong field enhancement in the gap region of the hybrid nanoantenna, which is much larger than those of other two peaks.

The strong near-field enhancement of the hybrid resonant peak can be understood via examining the vector electric field distribution of the dielectric MD and metal SPR. The dielectric MD results in a rotational electric field in the central cavity region, while the metal SPR shows a typical electric dipolar distribution. When the metal nanodisk is surrounding by the dielectric ring, an attractive force could be established between these two components, according to the equivalent charge distributions at the boundaries. Such an attractive force could lead to a large electrical field enhancement in the gap region. More details can be found in Figure S4 in the SI. Coupling Mechanism between Metal SPR and Dielectric ED. Similarly, the interaction between metal SPR and dielectric ED is investigated by aligning these two modes at the same wavelength ∼520 nm based on the multipole decomposition results, which correspond to optimal parameters of Rm = 50 nm, H = 160 nm Au nanodisk and Rin = 60 nm, Rout = 110 nm, H = 160 nm Si ring cavity, as shown in Figure 3a and b, respectively. From the extinction CS of the hybrid nanoantenna, as shown in Figure 3c, it is observed that the interaction between the metal SPR and dielectric ED leads a minor split in the extinction CS of the hybrid nanoantenna, with one peak that red-shifts to 550 nm and the other peak blue-shifts to 505 nm. In addition, another strong resonant peak appears at 700 nm, which is attributed to the interaction between the metal SPR and the tail of dielectric MD. Again, the coupling mechanism can be revealed by studying the vector electric field distributions of the metal SPR and dielectric ED. D


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Figure 3. Multipole decomposition of (a) Au nanodisk and (b) Si ring cavity, where metal SPR and dielectric ED are designed to align at 520 nm. (c) Extinction cross-section and (d) near-field distribution of hybrid nanoantenna. Interaction between metal SPR and dielectric ED is generally much weaker than that between SPR and MD. Coupling between SPR and ED leads to a split of resonant modes in extinction spectra. l x (t ) + γ x (t ) + w 2 x (t ) + g x (t ) + g x (t ) = F (t ) o ̈ ̇ ̇ ̇ o SPR SP SPR SPR SPR SPR o 1 ED 2 MD o o o o 2 o m x ED̈ xEḊ (t ) + wEDx ED(t ) − g1xSPR ̇ (t ) + g3xMD ̇ (t ) = FED(t ) o o o o o 2 o o ̈ (t ) + γMDxMD ̇ (t ) + wMDxMD(t ) − g2xSPR ̇ (t ) − g3x EḊ (t ) = FMD(t ) o xMD n

This time, the dielectric ED yields an electric dipolar field in the central cavity region instead of the rotational field distribution pattern for the dielectric MD case. Once the Au nanodisk is placed inside the Si ring cavity, an attractive force could be established at the top part of the gap region, whereas a repulsive force would occur at the bottom part. The attractive and repulsive forces work together and cause a split in the extinction CS of the hybrid nanoantenna. More details can be found in Figure S5 in the SI. Note that the spectral split due to the SPR-ED interaction can be observed more clearly by tuning the relative magnitude of the dielectric ED contribution. Another case is presented in Figure S6 in the SI. Figure 3d shows the electric field enhancement features for the three resonant peaks of the hybrid nanoantenna. Clearly, the interaction between the metal SPR and dielectric ED is relatively weaker than that between the metal SPR and dielectric MD. A strong electric field enhancement is presented over the entire gap region at 700 nm, even if the metal SPR and dielectric MD are not completely aligned in this case. In contrast, the two resonant peaks at 505 and 550 nm due to the SPR-ED interaction only produce weaker electric field enhancements in a limited space. To further understand the main spectral characteristics of the hybrid nanoantennas in Figures 2 and 3, a phenomenological coupled oscillator model is invoked, where the metal SPR, dielectric ED, and dielectric MD are approximated as the damped harmonic oscillators. The equations of motions of the coupled systems are thus

(11)

where xSPR, xED, and xMD represent the displacements of metal SPR, dielectric ED, and MD, respectively, g1, g2, and g3 account for the SPR-MD, SPR-ED, and ED-MD coupling strengths. For simplicity, the internal ED-MD interaction is assumed to be negligible, with g3 ≈ 0 since both ED and MD are modes from the same dielectric nanoparticle. FSPR, FED, and FMD represent the external forces driving the system, and are assumed to be harmonic (e.g., FSPR(t) = Re(FSPRe−iw0t), where w is the angular frequency of the incident light). wSPR, wED, and wMD and γSPR, γED, and γMD are the corresponding resonant frequencies and line widths, respectively, whose values can be retrieved from the multipole decomposition results. The resultant extinction CS can be obtained as the total work ̇ done by external forces as Cext(w) ∝ ⟨FSPR(t)xSPR(t) + FED(t) ̇ ̇ xED(t) + FMD(t)xMD(t)⟩. The detail derivation can be found in the SI. By adjusting g1 and g2, the coupled oscillator model reproduces the main spectral features of the hybrid nanoantennas, which qualitatively agree with the FEM simulations as shown in Figure 4. The results confirm that the extinction CS of the hybrid nanoantennas are primarily associated with the interactions between metal SPR and dielectric ED/MD. It further proves that the SPR-MD coupling strength is generally E


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from the pure metal or dielectric constituent, regardless of the emitter locations. In particular, at the top and middle parts of the antenna gap, the excitation rates from the hybrid nanoantenna could be 1 order of magnitude higher than those from the individual components. Even at the bottom part, where the excitation rate is relatively weaker compared to other locations of the hybrid nanoantenna, it is still more than two times those from the individual components. The presence of nanoantenna also manipulates the local density of optical states (LDOS), thereby introducing more radiative decay channels for the fluorescence emission. Figure 6 below confirms this point by analyzing the radiative decay rate Γr/Γ0 for emitters located at (a−c) the top/bottom and at (d−f) the middle part of the nanoantenna gap. Due to the symmetrical conditions, the emitters at the top or bottom part of the gap are equivalent. Three typical emitter orientations (x-, y-, and z-oriented emitter) are investigated. The results show that the hybrid nanoantenna could provide a universal enhancement on the radiative decay rate over a broad spectral range, for example, Γr/Γ0 > 1. The radiative decay rates for xoriented emitters are 1−2 orders of magnitude higher than those of the other two orientations, indicating a selective coupling between the nanoantenna and the dipole emitter.23,26,31 It also implies that the overall fluorescence emission feature from multiple randomly oriented emitters would be primarily contributed by the x-oriented emitter. In addition, for the x-oriented emitter the radiative decay rates of the hybrid nanoantenna could reach 1 order of magnitude higher than those from the individual constituents. While for the yand z-oriented emitters, the hybrid configuration also yields comparable radiative decay rates to those from the pure metal or dielectric component. Note that the spectral feature of the radiative decay rate is different to that under plane wave excitation in Figure 2, where some new peaks (e.g., 490 and 700 nm) occurs in the spectra other than the SPR-MD hybrid peak around 580 nm. This can

Figure 4. Extinction spectra for (a) SPR-MD and (b) SPR-ED hybrid nanoantenna based on coupled oscillator model, which qualitatively agree with FEM simulations. SPR-MD coupling strength is generally stronger than SPR-ED coupling strength (e.g., g2 > g1).

larger than the SPR-ED coupling strength (e.g., g2 > g1), which is also consistent with the results in Figures 2 and 3. Note that the coupled oscillator model is only a phenomenological approximation for the hybrid nanoantenna system. The discrepancy between the rigorous FEM and the coupled oscillator model might be attributed to the following: (1) the effects of higher order multipole modes are neglected and (2) the ED, MD, and SPR from the multipole decomposition results are not in the standard Gaussian shapes. In other words, they are not perfect harmonic oscillators. Fluorescence Emission from a Single Emitter. Combining the results from Figures 2 and 3, it is concluded that the SPR-MD interaction can produce a strong electric field enhancement in the gap region of the hybrid nanoantenna, which is much stronger than that from the SPR-ED interaction. Such a strong field enhancement could be utilized to boost the excitation rates of the fluorescence emitters. For illustration, Figure 5 records the excitation rates γext/γ0ext of a single emitter located at (a) top, (b) middle, and (c) bottom parts of the gap region of the SPR-MD hybrid nanoantenna with the same geometry as in Figure 2. Clearly, the excitation rates generated by the hybrid nanoantenna are universally larger than those

Figure 5. Fluorescence excitation rate γext/γ0ext for a single emitter located at the (a) top, (b) middle, and (c) bottom part of the hybrid nanoantenna gap. Excitation rates from hybrid nanoantenna are universally larger than those from pure metal or a dielectric constituent, regardless of emitter locations, in particular, at top and middle parts of the antenna gap. F


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Figure 6. Radiative decay rate Γr/Γ0 for a single emitter located at the (a−c) top/bottom and (d−f) middle parts of the hybrid nanoantenna gap. Three typical emitter orientations (x-, y-, and z-oriented emitter) are investigated. Nanoantenna selectively couples to x-oriented emitters with much stronger radiative rates than those of the other two orientations. Hybrid configuration could generate a much stronger radiative decay rate than those from pure metal or dielectric component for x-oriented emitter, and comparable radiative decay rate for y- and z-oriented emitters.

Figure 7. Quantum yield q/q0 for single emitter located at (a−c) top/bottom, and (d−f) middle parts of hybrid nanoantenna gap. Three typical emitter orientations (x-, y-, and z-oriented emitter) are investigated. Hybrid nanoantenna could generate a comparable quantum yield to that from the pure dielectric component, in particular, for the x-oriented emitter, which indicates that the hybrid nanoantenna could partially inherit the low loss advantage from its dielectric constituent.

be explained via invoking the nature of the dipole radiation. The radiation of a dipole emitter can be treated as the summation of multiple plane waves with different propagation directions and polarizations. Therefore, the resultant extinction spectra should also be the assembly effect of multiple plane wave incidences. The other peaks (e.g., at 490 and 700 nm) are indeed due to the plane wave illumination from the horizontal directions. Details can be found in Figure S7 in the SI. Figure 7 illustrates the quantum yields q/q0, which takes into account both the radiative and nonradiative decay rates. In

general, the pure Si ring has the largest quantum yield in all cases due to its intrinsic lossless nature, while the pure Au nanodisk always possesses the smallest quantum yield due to its large dissipation. In addition, the hybrid nanoantenna is capable of generating a quantum yield that is higher than that from the pure metal nanodisk. In particular, for the x-oriented emitter, the quantum yield from the hybrid nanoantenna is comparable or even greater than that of the pure Si ring, which indicates that the hybrid configuration could partially inherit the low loss advantage from its dielectric constituent. G


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Figure 8. Fluorescence enhancement factor ηem/η0em for single emitter located at the (a−c) top, (d−f) middle, and (g−i) bottom parts of the hybrid nanoantenna gap. Hybrid nanoantenna could generate a universally larger fluorescence enhancement factor than those from the pure metal or dielectric component in almost every case. x-Oriented emitter located at the top part of the gap region offers the strongest enhancement factor due to its large excitation rate and decent quantum yield.

Combining the results of the excitation rate (Figure 5) and the quantum yield (Figure 7), the enhancement factor ηem/ η0em for a single emitter is illustrated in Figure 8. Clearly, the hybrid nanoantenna is able to generate a universally larger enhancement factor than those from the pure metal or dielectric component in almost every case, showing the robustness of the hybrid configuration. Depending on emitter’s location and orientation, the enhancement factor of the hybrid nanoantenna could achieve more than 1 order of magnitude higher than those from the pure metal or dielectric component. The enhancement factors for the emitters located at the top part of the gap region is larger than the other locations due to the greater excitation rates there, and the enhancement factors for the x-oriented emitters are larger than the other orientations owing to their relatively larger quantum yields. Therefore, it is reasonable to speculate that the overall emission feature of multiple randomly oriented emitters would be dominant by that of x-oriented emitter located at the top part of the gap region. Overall Fluorescence Emission from Multiple Emitters. So far, the fluorescence enhancement has been studied for a single emitter. To mimic the practical situation, the overall emission characteristics from multiple emitters need to be investigated. Figure 9a estimates the overall enhancement in the fluorescence intensity FEtot/FE0. As expected, the highest FEtot/FE0 occurs around the SPR-MD hybrid resonance at 600 nm, where the excitation rates and the radiative decay rates reach maximum. Compared to that in the free space, the hybrid nanoantenna could improve the overall fluorescence

Figure 9. (a) Overall enhancement in fluorescence intensity FEtot/FE0 and (b) angular radiation pattern at 600 nm from multiple randomly oriented emitters, which are assumed to be distributed homogeneously in the gap region. Hybrid nanoantenna could generate a tremendous total enhancement, which is nearly 2 orders of magnitude higher than those from pure metal or dielectric component. Emission directivity is also improved with a hybrid configuration, with 74% of total radiation propagating upwards.

intensity by 3 orders of magnitude (FEtot/FE0 > 1500). Compared to those from the pure metal or dielectric component, the FEtot/FE0 for the hybrid nanoantenna can achieve nearly 2 orders of magnitude higher. In addition to the giant enhancement in the fluorescence intensity, the hybrid configuration also results in a directional radiation pattern, as shown in Figure 9b. The maximum directivity enhancement can reach more than 3000 than that in the free space. Around 74% of the total radiation propagates upward, indicating an excellent directivity. The evolution of the angular radiation patterns is presented in Figure S8 in the SI. H


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could potentially relax the fabrication difficulties. However, altering the height will change the resonant wavelength, and it requires the codesign of other geometrical parameters to align the metal SPR and dielectric MD at the same wavelength. As long as the metal SPR and dielectric MD are strongly coupled, an enhanced directional fluorescence emission should be achieved. Effects of Higher Order Multipole Modes. Although this work focuses on the effects of the primary dipolar modes (e.g., metal SPR, dielectric ED/MD), the effects of other higher order multipole modes are worth discussing. Different multipolar modes have different electromagnetic field distributions and polarizations, which potentially offer better flexibility to manipulate the optical response of the nanoantenna.24,26 In principle, a properly designed nanostructure could utilize the interference between various multipolar modes (e.g., kerker’s condition, anapole etc.) to engineer the local electric field enhancement and the far field scattering pattern.31,49 Therefore, it is reasonable to speculate that the fluorescence enhancement performance as well as the emission directivity could be further improved via involving other higher order multipole modes of the nanoantenna. This might require a more complex design where the relative contributions from higher order multipole modes are comparable to the primary dipolar modes. Effect of Substrate. In reality, the nanoantenna will be placed on a certain substrate. The previous work has shown that a substrate with a low refractive index will have little effect on the nanoantenna resonance and the fluorescence emission. On the contrary, a high refractive index substrate would be detrimental to the nanoantenna resonance due to the image charge generated inside the substrate, which will cause a red shift in the resonant wavelength and a decrement in the resonant strength.50,51 Besides, a high refractive index substrate will also direct the fluorescence emission toward the substrate side, which will reduce the directivity of the fluorescence emission. Therefore, it is recommend using a low refractive index substrate such as SiO2. Other Potential Applications. Besides the promising fluorescence enhancement features, the hybrid nanoantenna proposed in this work might also be useful in other applications. In particular, the coupling mechanism between metal SPR and dielectric ED/MD could provide a solid guidance in the design and optimization of photonic devices. For example, the strong field enhancement in the gap region due to the SPR-MD interaction can be directly exploited for nonlinear optics,32 since the conversion efficiencies of higher order harmonics obey the general power law with respect to the electrical field enhancement. Another potential application could be the quantum photonics. The strong field enhancement could be used to improve the plasmonic hot electron generation for photocatalysis and photodetection applications,52 or it can be used to enhance the coupling coefficient between the emitter and the nanoantenna in the cavity quantum electrodynamics (CQED) for the strong coupling applications. Lastly, the improved radiation directivity of the hybrid nanoantenna could also be beneficial for a directional quantum light source.53,54

Such a high directivity could be explained by three reasons: (1) Intuitively, the emitters are sandwiched by the metal nanodisk and dielectric ring, thereby confining the lateral radiations. In contrast, the pure Au nanodisk and Si ring could not generate such directional radiation pattern. More details are shown in Figure S9 in the SI. (2) The excitation rates are distributed nonuniformly in the gap region, where the excitation rates at the top part are stronger (Figure 2). Therefore, the emitters at the top part of the gap should contribute more to the overall fluorescence emission, resulting in an unsymmetrical radiation pattern. More details are shown in Figure S10 in the SI. (3) As it is proven in the radiative decay rate (Figure 6) and enhancement factor (Figure 8) for a single emitter, the xoriented emitter couples strongest to the nanoantenna, whose emission features should also be reflected in the overall radiation pattern for randomly oriented emitters. The xoriented emitter generates a radiation pattern where majority of the emission power propagates upwardly, which is consistent with our results. More details can be found in Figure S11 in the SI.

4. DISCUSSION Effect of Gap Distance. Like other composite antenna structures,10,20,33 the coupling strength between the metal and dielectric constituents of the hybrid nanoantenna depend primarily on the gap distance. In our case, the gap distance can be conveniently tuned by adjusting the radius of the Au nanodisk while maintaining the spectral alignment of the metal SPR and dielectric MD. It is found that a larger gap size significantly reduces the SPR-MD coupling strength, leading to a minor red-shifted peak in the extinction spectrum and giant decrements in the excitation rates and radiative decay rates. Although the quantum yield could be improved with a larger gap size due to the suppression of the nonradiative loss, it is not enough to compensate for the decrement in the excitation rates and radiative decay rates. Details can be found in Figures S12−S16 in the SI. As a result, the overall enhancement in the fluorescence intensity FEtot/FE0 for the hybrid nanoantenna with a 20 nm gap drops by one order of magnitude more than that with a 10 nm gap, as shown in Figure 10a. In addition, the radiation directivity is also lost for a larger gap size, with only 52% propagating upward, as shown in Figure 10b, since the xoriented emitter at the top part of the gap no longer possesses the dominant contribution. Note that a small gap is relatively difficult to fabricate, and reducing the height of the structure

Figure 10. (a) Overall enhancement in fluorescence intensity FEtot/ FE0 for different gap sizes, and (b) angular radiation pattern at 550 nm for nanoantenna with 20 nm gap. Larger antenna gap significantly reduces coupling strength between metal and dielectric constituents, resulting in diminished fluorescence intensity and reduced radiation directivity.

5. CONCLUSION By leveraging the advantages from both metal and dielectric, a robust hybrid nanoantenna with superior fluorescence enhancement performance has been reported. Via the multiple I



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decomposition technique, the surface plasmon resonance (SPR) of the inner metal nanodisk and the electric dipole/ magnetic dipole mode (ED/MD) of the outer dielectric ring cavity have been rigorously aligned at the same wavelength to reveal the coupling mechanism. Both the full wave simulation and the phenomenological coupled oscillator mode indicate that the SPR could interact with the MD more strongly than the ED to generate a giant excitation rate in the nanoantenna gap region, which can be 1 order of magnitude higher than those from the pure metal or dielectric constituent. In addition, the hybrid structure could partially inherit the low loss advantage of the dielectric constituent, dramatically increasing the radiative decay rate and generating a decent quantum yield. As a result, the enhancement factor for a single emitter could reach more than 1 order of magnitude higher compared to those from the pure metal and dielectric counterparts, depending on the emitter’s location and orientation. Finally, the overall improvement on the fluorescence emission intensity for multiple randomly oriented emitters are studied based on the reciprocity principle, which could reach more than 1500 compared to that in the free space and nearly 2 orders of magnitude higher than those from pure metal or their dielectric counterpart. An excellent directional radiation pattern has been obtained at the resonant condition with a maximum directivity enhancement >3000, and 74% of the total power propagates upwardly. The coupling strength between metal and dielectric constituent depends primarily on the gap distance. Increasing the gap size could result in a significant reduction on the overall fluorescence emission intensity as well as the radiation directivity. This hybrid structure could be, in principle, fabricated with the “sketch and peel” self-aligned fabrication process,55−58 which would be beneficial in fluorescence sensing, nonlinear optics, and quantum photonics.

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REFERENCES

(1) Wu, D.; Sedgwick, A. C.; Gunnlaugsson, T.; Akkaya, E. U.; Yoon, J. Y.; James, T. D. Fluorescent chemosensors: the past, present and future. Chem. Soc. Rev. 2017, 46, 7105−7123. (2) Li, C. K.; Kuang, C. F.; Liu, X. Prospects for fluorescence nanoscopy. ACS Nano 2018, 12, 4081−4085. (3) Betzig, E.; Patterson, G. H.; Sougrat, R.; Lindwasser, O. W.; Olenych, S.; Bonifacino, J. S.; Davidson, M. W.; Lippincott-Schwartz, J.; Hess, H. F. Imaging intracellular fluorescence proteins at nanometer resolution. Science 2006, 313, 1642−1645. (4) Stehr, F.; Stein, J.; Schueder, F.; Schwille, P.; Jungmann, R. Flattop TIRE illumination boosts DNA-paint imaging and quantification. Nat. Commun. 2019, 10, 1268. (5) Ray, S.; Widom, J. R.; Walter, N. G. Life under microscope: single-molecule fluorescence highlights the RNA world. Chem. Rev. 2018, 118, 4120−4155. (6) Ecker, J. R.; Geschwind, D. H.; Kriegstein, A. R.; Ngai, J.; Osten, P.; Polioudakis, D.; Regev, A.; Sestan, N.; Wickersham, I. R.; Zeng, H. K. The brain initiative cell census consortium: lessons learned toward generating a comprehensive brain cell atlas. Neuron 2017, 96, 542− 557. (7) Park, S. J.; Kim, B.; Choi, S.; Balasubramaniam, S.; Lee, S. C.; Lee, J. Y.; Kim, H. S.; Kim, J. Y.; Kim, J. J.; Lee, Y. A.; et al. Imaging inflammation using an activated macrophage probe with S1c18b1 as the activation-selective gating target. Nat. Commun. 2019, 10, 1111. (8) Andersen, C. M.; Mortensen, G. Fluorescence spectroscopy: a rapid tool for analysing dairy products. J. Agric. Food Chem. 2008, 56, 720−729. (9) Fothergill, S. M.; Joyce, C.; Xie, F. Metal enhanced fluorescence biosensing: from ultra-violet towards second near-infrared window. Nanoscale 2018, 10, 20914−20929. (10) Novotny, L.; van Hulst, N. Antenna for light. Nat. Photonics 2011, 5, 83−89. (11) Vasa, P.; Lienau, C. Strong light-matter interaction in quantum emitter/metal hybrid nanostructure. ACS Photonics 2018, 5, 2−23. (12) Francisco, A. P.; Botequim, D.; Prazeres, D. M. F.; Serra, V. V.; Costa, S. M. B.; Laia, C. A. T.; Paulo, P. M. R. Extreme enhancement of single-molecule fluorescence from porphyrins induced by gold nanodimer antennas. J. Phys. Chem. Lett. 2019, 10, 1542−1549. (13) Alam, M. S.; Karim, F.; Zhao, C. L. Single-molecule detection at high concentrations with optical aperture nanoantennas. Nanoscale 2016, 8, 9480−9487. (14) Doeleman, H. M.; Verhagen, E.; Koenderink, A. F. Antennacavity hybrids: matching polar opposites for purcell enhancements at any linewidth. ACS Photonics 2016, 3, 1943−1951. (15) Koenderink, A. F. Single-photon nanoantennas. ACS Photonics 2017, 4, 710−722. (16) Choudhury, S. D.; Badugu, R.; Lakowicz, J. R. Directing fluorescence with plasmonic and photonic structures. Acc. Chem. Res. 2015, 48, 2171−2180. (17) Tian, J.; Li, Q.; Yang, Y.; Qiu, M. Tailoring unidirectional angular radiation through multipolar interference in a single-element subwavelength all-dielectric stair-like nanoantenna. Nanoscale 2016, 8, 4047−4053. (18) Singha, S.; Kim, D.; Seo, H.; Cho, S. W.; Ahn, K. H. Fluorescence sensing systems for gold and silver species. Chem. Soc. Rev. 2015, 44, 4367−4399. (19) Crick, C. R.; Albella, P.; Kim, H. J.; Ivanov, A. P.; Kim, K. B.; Maier, S. A.; Edel, J. B. Low-noise plasmonic nanopore biosensors for single molecule detection at elevated temperatures. ACS Photonics 2017, 4, 2835−2842. (20) Xin, L.; Lu, M.; Both, S.; Pfeiffer, M.; Urban, M. J.; Zhou, C.; Yan, H.; Weiss, T.; Liu, N.; Lindfors, K. Watching a single fluorophore molecule walk into a plasmonic hotspot. ACS Photonics 2019, 6, 985−993. (21) Li, J. F.; Li, C. Y.; Aroca, R. F. Plasmon-enhanced fluorescence spectroscopy. Chem. Soc. Rev. 2017, 46, 3962−3979. (22) Wu, W. D.; Shi, N.; Zhang, J.; Wu, X. D.; Wang, T.; Yang, L.; Yang, R.; Ou, C. J.; Xue, W.; Feng, X. M.; et al. Electrospun

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b06280.

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Article

Detailed information for the finite element modeling, geometrical effects of the nanostructure, vector electric field distributions of various resonant modes, derivation of the coupled oscillator model, extinction spectra for other incident directions, the detail angular radiation patterns, and the results for different gap sizes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Song Sun: 0000-0003-2382-6481 Huigao Duan: 0000-0002-7016-6343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge funding support from the Science Challenge Project, PRC China (Grant No. TZ2016003), and the National Natural Science Foundation Committee (NSFC) of China (Grant No. 11574078). The authors also thank Dr. Shen Zhou and Dr. Xiaojia Luo for the fruitful discussion on the coupled oscillator model. J


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The Journal of Physical Chemistry C fluorescent sensors for the selective detection of nitro explosive vapors and trace water. J. Mater. Chem. A 2018, 6, 18543−18550. (23) Anger, P.; Bharadwaj, P.; Novotny, L. Enhancement and quenching of single-molecule fluorescence. Phys. Rev. Lett. 2006, 96, 113002. (24) Kuznetsov, A. I.; Miroshnichenko, A. E.; Brongersma, M. L.; Kivshar, Y. S.; Luk’yanchuk, B. Optically resonant dielectric nanostructures. Science 2016, 354, 2472. (25) Yang, X. G.; Li, Y. C.; Li, B. J. Silicon particle as a nanocavity for stable emission of quantum dots. ACS Photonics 2017, 4, 2669− 2675. (26) Rutckaia, V.; Heyroth, F.; Novikov, A.; Shaleev, M.; Petrov, M.; Schilling, J. Quantum dot emission driven by Mie resonances in silicon nanostructures. Nano Lett. 2017, 17, 6886−6892. (27) Shibanuma, T.; Albella, P.; Maier, S. A. Unidirectional light scattering with high efficiency at optical frequencies based on low-loss dielectric nanoantennas. Nanoscale 2016, 8, 14184−14192. (28) Staude, I.; Pertsch, T.; Kivshar, Y. S. All-dielectric resonant meta-optics lightens up. ACS Photonics 2019, 6, 802−814. (29) Krasnok, A. E.; Miroshnichenko, A. E.; Belov, P. A.; Kivshar, Y. S. All-dielectric optical nanoantennas. Opt. Express 2012, 20, 20599− 20604. (30) Feng, T. H.; Zhang, W.; Liang, Z. X.; Xu, Y.; Miroshnichenko, A. E. Isotropic magnetic purcell effect. ACS Photonics 2018, 5, 678− 683. (31) Sun, S.; Wu, L.; Bai, P.; Png, C. E. Fluorescence enhancement in visible light: dielectric or noble metal? Phys. Chem. Chem. Phys. 2016, 18, 19324−19335. (32) Shibanuma, T.; Grinblat, G.; Albella, P.; Maier, S. A. Efficient third harmonic generation from metal-dielectric hybrid nanoantennas. Nano Lett. 2017, 17, 2647−2651. (33) Harats, M. G.; Livneh, N.; Rapaport, R. Design, fabrication and characterization of a hybrid metal-dielectric nanoantenna with a single nanocrystal for directional single photon emission. Opt. Mater. Express 2017, 7, 834−843. (34) Sun, S.; Li, M.; Du, Q. G.; Png, C. E.; Bai, P. Metal-dielectric hybrid dimer nanoantenna: coupling between surface plasmons and dielectric resonances for fluorescence enhancement. J. Phys. Chem. C 2017, 121, 12871−12884. (35) Lan, Y.; Wang, S. Q.; Yin, X. P.; Liang, Y.; Dong, H.; Gao, N.; Li, J.; Wang, H.; Li, G. T. 2D-ordered dielectric sub-micron bowls on a metal surface: a useful hybrid plasmonic-photonic structure. Nanoscale 2016, 8, 13454−13462. (36) Rusak, E.; Staude, I.; Decker, M.; Sautter, J.; Miroshnichenko, A. E.; Powell, D. A.; Neshev, D. N.; Kivshar, Y. S. Hybrid nanoantennas for directional emission enhancement. Appl. Phys. Lett. 2014, 105, 221109. (37) Liu, J. N.; Huang, Q. L.; Liu, K. K.; Singamaneni, S.; Cunningham, B. Nanoantenna-microcavity hybrids with highly copperative plasmonic coupling. Nano Lett. 2017, 17, 7569−7577. (38) Sun, S.; Li, R.; Li, M.; Du, Q. G.; Bai, P.; Png, C. E. Hybrid Mushroom Nanoantenna for Fluorescence Enhancement by Matching the Stokes-Shift of the Emitter. J. Phys. Chem. C 2018, 122, 14771− 14780. (39) Deng, Y. H.; Yang, Z. J.; He, J. Plasmonic nanoantennadielectric nanocavity hybrids for ultrahigh local electric field enhancement. Opt. Express 2018, 26, 31116−31128. (40) Schmidt, T. D.; Lampe, T.; Sylvinson, D. M. R.; Djurovich, P. I.; Thompson, M. E.; Brutting, W. Emitter orientation as a key parameter in organic light-emitting diodes. Phys. Rev. Appl. 2017, 8, 037001. (41) Zalogina, A. S.; Savelev, R. S.; Ushakova, E. V.; Zograf, G. P.; Komissarenko, F. E.; Milichko, V. A.; Makarov, S. V.; Zuev, D. A.; Shadrivov, L. V. Purcell effect in active diamond nanoantennas. Nanoscale 2018, 10, 8721−8727. (42) Ma, C. R.; Yan, J. H.; Wei, Y. M.; Liu, P.; Yang, G. W. Enhanced second harmonic generation in individual barium titanate nanoparticles driven by Mie resonances. J. Mater. Chem. C 2017, 5, 4810−4819.

(43) Grahn, P.; Shevchenko, A.; Kaivola, M. Electromagnetic multipole theory for optical nanomaterials. New J. Phys. 2012, 14, 093033. (44) Novotny, L.; Hecht, B. Principles of Nano-Optics; Cambridge University Press: Cambridge, England, 2006. (45) Bharadwaj, P.; Novotny, L. Spectral dependence of single molecule fluorescence enhancement. Opt. Express 2007, 15, 14266− 14274. (46) Janssen, O. T. A.; Wachters, A. J. H.; Urbach, H. P. Efficient optimization method for the light extraction from periodically modulated LEDs using reciprocity. Opt. Express 2010, 18, 24522− 24535. (47) Vaskin, A.; Mashhadi, S.; Steinert, M.; Chong, K. E.; Keene, D.; Nanz, S.; Abass, A.; Rusak, E.; Choi, D. Y.; Fernandez-Corbaton, I.; et al. Manipulation of magnetic dipole emission from Eu3+ with Mieresonant dielectric metasurfaces. Nano Lett. 2019, 19, 1015−1022. (48) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1997. (49) Renaut, C.; Lang, L.; Frizyuk, K.; Timofeeva, M.; Komissarenko, F. E.; Mukhin, I. S.; Smirnova, D.; Timpu, F.; Petrov, M.; Kivshar, Y.; et al. Reshaping the second-order polar response of hybrid metal-dielectric nanodimers. Nano Lett. 2019, 19, 877−884. (50) Valamanesh, M.; Borensztein, Y.; Langlois, C.; Lacaze, E. Substrate effect on the plasmon resonance of supported flat silver nanoparticles. J. Phys. Chem. C 2011, 115, 2914−2922. (51) Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A. Substrate effect on refractive index dependence of plasmon resonance for individual silver nanoparticles observed using darkfield micro-spectroscopy. Opt. Express 2005, 13, 2668−2677. (52) Besteiro, L. V.; Kong, X. T.; Wang, Z. M.; Hartland, G.; Govorov, A. O. Understanding hot-electron generation and plasmon relaxation in metal nanocrystals: quantum and classical mechansims. ACS Photonics 2017, 4, 2759−2781. (53) Ren, J. J.; Gu, Y.; Zhao, D. X.; Zhang, F.; Zhang, T. C.; Gong, Q. H. Evanescent-vacuum-enhanced photon-exciton coupling and fluorescence collection. Phys. Rev. Lett. 2017, 118, 073604. (54) Zhang, F.; Shan, L. X.; Ren, J. J.; Duan, X. K.; Li, Y.; Zhang, T. C.; Gong, Q. H.; Gu, Y. A photonic interface of chiral cavity quantum electrodynamics. arXiv:1808.07210 [physics.optics] 2018, na. (55) Chen, Y.; Bi, K.; Wang, Q.; Zheng, M.; Liu, Q.; Han, Y.; Yang, J.; Chang, S.; Zhang, G.; Duan, H. G. Rapid focused ion beam milling based fabrication of plasmonic nanoparticles and assemblies via sketch and peel strategy. ACS Nano 2016, 10, 11228−11236. (56) Chen, Y.; Xiang, Q.; Li, Z. Q.; Wang, Y. S.; Meng, Y. H.; Duan, H. G. Sketch and peel” lithography for high-resolution multiscale patterning. Nano Lett. 2016, 16, 3253−3259. (57) Zhang, S.; Li, G. C.; Chen, Y.; Zhu, X.; Liu, S. D.; Lei, D. Y.; Duan, H. G. Pronounced fano resonance in single gold split nanodisks with 15 nm split gaps for intensive second harmonic generation. ACS Nano 2016, 10, 11105−11114. (58) Droulias, S.; Jain, A.; Koschny, T.; Soukoulis, C. M. Novel lasers based resonant dark states. Phys. Rev. Lett. 2017, 118, 073901.

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Enhanced Directional Fluorescence Emission of Randomly Oriented

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