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Magnetic Plasmon-Enhanced Second-Harmonic Generation on Colloidal Gold Nanocups Si-Jing Ding, Han ZHANG, Da-Jie Yang, Yun-Hang Qiu, Fan Nan, Zhong-Jian Yang, Jianfang Wang, Qu-Quan Wang, and Hai-qing Lin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00020 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Magnetic Plasmon-Enhanced Second-Harmonic Generation on Colloidal Gold Nanocups Si-Jing Ding,†,‡,‖,§ Han Zhang,‡,§ Da-Jie Yang,‖,┴,§ Yun-Hang Qiu,‖ Fan Nan,‖ Zhong-Jian Yang,# Jianfang Wang,*,‡ Qu-Quan Wang,*,‖ and Hai-Qing Lin*,┴ †School

of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan 430074,

Hubei, China ‡Department

of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

‖Department

of Physics, The Institute for Advanced Studies, Wuhan University, Wuhan 430072,

Hubei, China ┴Beijing #Hunan

Computational Science Research Center, Beijing 100193, China

Key Laboratory of Super Microstructure and Ultrafast Process, School of Physics and

Electronics, Central South University, Changsha 410083, Hunan, China

ABSTRACT: The magnetic plasmons of three-dimensional nanostructures have unique optical responses and special significance for optical nanoresonators and nanoantennas. In this study, we have successfully synthesized colloidal Au and AuAg nanocups with a well-controlled asymmetric geometry, tunable opening sizes and normalized depths (h/b, h: depth, b: height of the templating PbS nanooctahedrons), variable magnetic plasmon resonance, and largely

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enhanced second-harmonic generation (SHG). The most efficient SHG of the bare Au nanocups is experimentally observed when the normalized depth h/b is adjusted to ~0.78–0.79. We find that the average magnetic field enhancement is maximized at h/b = ~0.65 and reveal that the maximal SHG can be attributed to the joint action of the optimized magnetic plasmon resonance and the “lightning rod effect” of the Au nanocups. Furthermore, we demonstrate for the first time that the AuAg heteronanocups prepared by overgrowth of Ag on the Au nanocups can synergize the magnetic and electric plasmon resonances for nonlinear enhancement. By tailoring the dual resonances at the fundamental excitation and second-harmonic wavelengths, the far-field SHG intensity of the AuAg nanocups is enhanced 21.8 folds compared to that of the bare Au nanocups. These findings provide a strategy to the design of nonlinear optical nanoantennas based on magnetic plasmon resonances and can lead to diverse applications ranging from nanophotonics to biological spectroscopy.

KEYWORDS: asymmetric metal nanostructures, bimetallic nanostructures, gold nanocups, magnetic plasmon resonance, plasmon resonance, second-harmonic generation

Noble metal nanocrystals exhibit rich plasmonic properties. Their surface plasmon resonances can enhance many linear and nonlinear optical signals.1–3 The combination of plasmonics and nonlinear optics gives rise to a new research field called nonlinear plasmonics.2,3 Nonlinear optical processes involving multi-photon excitations are much more sensitive to the enhanced local field than linear processes. Various nonlinear optical processes, including second- and third-harmonic generation,4–15 four-wave mixing,16,17 and multi-photon luminescence,18–20 have been observed and measured on plasmonic metal nanostructures.


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Second-harmonic generation (SHG) is a second-order nonlinear optical process, whereby two photons of an incident laser at the fundamental frequency (ω0) are absorbed simultaneously and then one photon at the second-harmonic frequency (2ω0) is emitted. SHG offers an approach for photon upconversion and has applications in photonic devices and biological spectroscopy.3 SHG is forbidden in centrosymmetric materials within the electric dipole approximation.2,3 Therefore, great efforts have been made to design and fabricate plasmonic metal nanostructures with various noncentrosymmetric geometries, such as lithographically fabricated L-,21 T-,22 U-,23 V-,24 and G-shaped nanostructures,10–12 chemically-synthesized colloidal nanorods,25 triangular nanoprisms,26 and nanocups.27 So far, most studies on SHG enhancements have focused only on electric plasmon resonances. Many properties and technologies based on electromagnetism require magnetic resonances, such as negative index materials and cloaking.28,29 Magnetic plasmon resonances are usually supported in highly asymmetric nanostructures. Up to date, several types of metal nanostructures have been demonstrated to exhibit magnetic plasmon resonances, for example, split rings,23,30 coupled slit-holes,31 fish nets,32 nanosandwiches,33 horseshoes,34 open shells,35–44 and split balls.45 They have mainly been fabricated by physical methods. Only a few types of metal nanostructures have so far been synthesized by chemical methods to show strong magnetic plasmons.44 Magnetic plasmon resonances have been successfully used to enhance the SHG of symmetry-broken metal nanostructures fabricated by physical methods, which permit high excitation powers and give high SHG efficiencies.46 However, magnetic plasmon resonances in three-dimensional (3D) metal nanostructures fabricated by physical methods are hard to optimize and have not been used in dual-resonance antennas for nonlinear enhancement.24,47,48


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In this letter, we have successfully optimized the magnetic plasmon resonance of colloidal Au nanocups, created nonlinear dual-resonance antennas (DRAs) using Au–Ag heteronanocups for the first time, and demonstrated strikingly enhanced SHG. The colloidal Au and Au–Ag nanocups with controlled asymmetric geometry are synthesized by facile chemical methods. The magnetic plasmon resonance and the correspondingly enhanced SHG are optimized by adjusting the normalized depth of the nanocups. The maximal SHG of the Au nanocups is observed and revealed to be caused by the joint action of the magnetic plasmon resonance and the “lightning rod effect”. Furthermore, the Au–Ag heteronanocups with Ag nanoparticles attached on the edge of the Au nanocups are prepared. They own dual resonances respectively at the excitation and second-harmonic wavelengths and exhibit 21.8-fold enhancement of SHG compared to the bare Au nanocups. This largely enhanced SHG is caused by the cooperative magnetic and electric plasmon resonances at the same hot spot. The colloidal Au nanocups were prepared using PbS nanooctahedrons as sacrificial templates in three steps,44 as detailed in the Supporting Information and illustrated in Figure 1a. Briefly, the colloidal single-crystalline PbS nanooctahedrons with height b were synthesized at the first step (Figure S1, Supporting Information). The following Au overgrowth started preferentially at one vertex of each PbS nanooctahedron to form Au/PbS Janus nanostructures. The hollow Au nanocups were subsequently produced by selectively dissolving the PbS components off the Janus nanostructures. The depth h and the opening size d of the Au nanocup were adjusted to optimize the magnetic plasmon resonance by varying the added amount of the PbS nanooctahedrons in the Au growth solution, which has not been carefully examined in the previous studies.27,44


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Figure 1. Gold nanocups with different h/b values. (a) Schematics illustrating the growth of the Au nanocups on the PbS nanooctahedron templates and the adjustment of the normalized depth (0 < h/b < 1) of the Au nanocups. (b) SEM images of the Au nanocups with different h/b values (b = 105 ± 4 nm). The normalized depth h/b and lateral diameter D of each nanocup sample are given above the corresponding SEM image. The Au nanocups have faceted inner surface and


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relatively rough outer surface with the largest opening occurring around h/b = 0.50 for the nanocups.

Figure 1b presents a set of representative scanning electron microscopy (SEM) images of the Au nanocups with h/b = 0.45 ± 0.03, 0.49 ± 0.03, 0.53 ± 0.03, 0.69 ± 0.03, 0.78 ± 0.03 and 0.85 ± 0.04 (b = 105 ± 4 nm). With increasing h/b values, the lateral diameter D along the direction perpendicular to the symmetry axis of the nanocup increases, but the opening size decreases when h/b > 0.5. The inner surface of the synthesized Au nanocups is faceted,44 but the outer surface is nearly spherical and relatively rough, with the roughness increasing with h/b. The similar nanostructures were also observed in another series of Au nanocups with a larger b = 141 ± 6 nm and h/b = 0.28 ± 0.02, 0.31 ± 0.02, 0.44 ± 0.03, 0.71 ± 0.03, 0.79 ± 0.03 and 0.86 ± 0.04 (Figure S2, Supporting Information). The prepared Au nanocups exhibit two resonance peaks in the extinction spectra (Figure 2a and b). The plasmon modes of the Au nanocups are highly dependent on the incidence and polarization directions of excitation light.44 The extinction spectra of the Au nanocups suspended in aqueous solutions with random orientations were recorded. The major resonance peak can be assigned to the magnetic dipole (MD) mode excited under transverse polarization and the minor peak at the high-energy side of the major peak can be partially attributed to the electric quadrupole (EQ) mode.27 The relative intensity of the minor peak prominently increases with h/b. The Au nanocups with different b values show similar dependence of the MD resonance wavelength (MD) on h/b, but the ones with a larger b have higher tunability of MD in the longerwavelength region. MD redshifts from 707 to 779 nm as h/b is increased from 0.45 to 0.85 when b = 105 nm (Figure 2a), and it increases from 764 to 873 nm as h/b is increased from 0.28 to


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0.86 when b = 141 nm (Figure 2b). From the dependences of MD on h/b for b = 105 and 141 nm, we can roughly estimate the MD value of the prepared Au nanocups with the given values of b and h/b.

Figure 2. h/b dependence of the plasmon modes of the Au nanocups. (a, b) Measured extinction spectra of the Au nanocups with b = 105 ± 4 and b = 141 ± 6 nm, respectively. MD monotonously increases with h/b owing to the increased depth h as well as the lateral size D. (c) Calculated extinction spectra of the Au nanocups with h/b = 0.25, 0.50, 0.65, 0.70 and 0.75, respectively, with D fixed at 180 nm. MD increases with h/b when h/b ≤ 0.65 and then slightly decreases with h/b when h/b > 0.65. (d) Charge distributions of the MD and EQ plasmon modes


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of the Au nanocup with h/b = 0.75. During the calculations, the excitation is along the y axis and polarized along the x axis, and the b value of the Au nanocup is 141 nm.

Figure 2c displays the calculated extinction spectra of the Au nanocups with different h/b values and a fixed lateral diameter D. The calculations were performed using COMSOL Multiphysics (see the Supporting Information for the details). The excitation light is along the y axis and polarized along the x axis, where the x and y axes are defined along the two crossed edges of the nanocup. The calculation results clearly reveal the MD and EQ resonances (Figure 2d) and well reproduce the key spectral features observed in the experiments with random polarization (when h/b ≤ 0.65). The calculated MD resonance wavelength MD of the Au nanocup with a fixed D = 180 nm increases with h/b when h/b ≤ 0.65 but slightly decreases when h/b is further increased. In the experiments, the lateral size D of the synthesized Au nanocups increases with h/b, which results in the monotonous redshift of MD with h/b. The weak peak around 680 nm in Figure 2c is a mixture of the electric and magnetic plasmon resonances attributed to the edge of the nanocups (Figure S4, Supporting Information). The SHG of the Au nanocups was investigated using a wavelength-tunable femtosecond laser with a pulse width of ~150 fs (see the Supporting Information for the details). Figure 3a and b show the SHG spectra of the Au nanocups with b = 105 and 141 nm, respectively. The spectra are all excited at L = MD. The SHG intensity is strongly dependent on the fundamental excitation wavelength L. As shown in Figure 3c, all nanocup samples with b = 105 and 141 nm exhibit maximal SHG intensities when L = MD. This indicates that the observed SHG signals of the Au nanocups are enhanced by the MD resonance. As h/b is varied from 0.28 to 0.79, the SHG intensity of the nanocups monotonously increases. When h/b is adjusted to 0.79 ± 0.03 for


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the nanocups with b = 141 nm, the intensity reaches the maximum. The very similar h/b dependence of SHG is also observed on the Au nanocups with the smaller b = 105 nm (Figure 3d), where the SHG intensity reaches the maximum at h/b = 0.78. This suggests that the h/b value for the optimized SHG signal is almost independent on the b value.


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Figure 3. Measured far-field SHG intensities and calculated near-field H(0), E(0) and PSHG(20) of the Au nanocups. (a, b) Normalized SHG spectra of the Au nanocups with b = 105 and 141 nm and different h/b values, respectively. The Au amount was fixed and the excitation laser wavelength was adjusted to their corresponding MD values in the measurements. (c) Excitation wavelength dependence of the far-field SHG intensities of the Au nanocups (b = 105 nm, MD = 760 nm and b = 141 nm, MD = 850 nm). The SHG reaches the maximum at L =

MD. (d) Normalized SHG intensities of the Au nanocups with b = 105 and 141 nm and different h/b values. Both samples demonstrate the maximal SHG intensity at h/b = ~0.78–0.79. (e) Calculated h/b dependences of the magnetic field H(0), electric field E(0), and near-field SHG PSHG(20) of the Au nanocups on the xz plane (0 = MD). H(0) reaches the maximum at h/b = ~0.65 and E(0) reaches the maximum at h/b = ~0.70 owing to the joint action of the magnetic plasmon resonance and the “lightning rod effect”. During the calculations, the excitation light is along the y axis and polarized along the x axis, and the Au nanocups have b = 141 nm and D = 180 nm.

To reveal the physical origin of the h/b-dependent SHG of the Au nanocups, the field distributions of the magnetic field H(0), electric field E(0) and near-field SHG PSHG(20) were calculated at the MD resonance 0 = MD = 2c/MD (b = 141 nm, D = 180 nm) (Figure 3e and Figure S3, Supporting Information). They are all strongly dependent on h/b. The average H(0) intensity as a function of h/b exhibits three features (Figure S5, Supporting Information): (i) H(0) reaches the maximum around h/b = ~0.65; (ii) H/(h/b) has a maximum at h/b = 0.50; and (iii) The magnetic dipole resonance strength HMD(0) = H(0) –


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H0(0) → 0 when the opening size d → 0. H0(0) is the magnetic field induced by the pure electric plasmon resonance at d = 0. The analytic relationship of the magnetic resonance with h/b for the three-dimensional nanocups is hard to be theoretically deduced, but we find that the three key features of the magnetic dipole strength can be well described by (see the Supporting Information for the details) 𝐻MD(0) ∝ 𝑑𝑚(ℎ/𝑏)2𝑚

(1)

where 0.5 ≤ m ≤ 3 for the Au nanocups (Figure S5, Supporting Information). A group of functions xm = dm(h/b)2m with different m values and weight factors can well fit the entire curve of HMD(0) with h/b for both two-dimensional and three-dimensional magnetic resonantors.34,45 The empirical formula of the magnetic resonance strength is highly valuable for predicting the magnetic extremum parameters of plasmonic open shells, including two-dimensional horseshoes34 and three-dimensional split balls.45 Compared to the maximal H(0) around h/b = ~0.65, E(0) reaches the maximum at a smaller opening size (a larger h/b = ~0.70), which is caused jointly by the strong magnetic resonance in the hollow cavity and the “lightning rod effect” around the edge of the fundamental laser beam. Similarly, the “lightning rod effect” also affects the emission field of SHG. Figure 3e shows that the strong magnetic field is located in the hollow cavity, and the hot spots of E(0, rHS) and PSHG(20, rHS) are approximately located at the same hot spot position rHS around the edge of the nanocup. The nonlinear hot spot PSHG(20, rHS) plays a critically important role on the SHG emission.10,11 In addition, the rough outer surface of the nanocups can enhance the local electromagnetic field intensity and the far-field SHG emission owing to the broken symmetry.49,50


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Ag was further overgrown on the Au nanocups to obtain Au–Ag heteronanocups and the synergetic action of the magnetic and electric plasmon resonances on the nonlinear enhancement was investigated (Figure 4a). To clearly reveal the configuration of the overgrown Ag, the Au nanocups with a large opening (h/b = 0.52 ± 0.03, b = 170 ± 8 nm) were employed for Ag overgrowth. The Au–Ag heteronanocups were produced by carefully adjusting the Ag+ concentration, pH value and ligand. In general, a high deposition rate of silver, which was realized by adding NaOH and supplying AgNO3 at a high concentration, was found to be favorable for the preferential overgrowth of Ag nanoparticles on the edge of the Au nanocups. Most Au–Ag heteronanocups have a single Ag nanoparticle (NP) on each Au nanocup, and very few ones have two Ag NPs but with largely different sizes. Figure 4b displays a typical SEM image of the Au–Ag heteronanocups. The Ag NPs are attached on the edge of the Au nanocups (Figure 4b, inset). The diameter of the overgrown Ag NPs varies in the range of 30–50 nm. The elemental mapping images reveal a thin layer of Ag on the Au nanocup with a thickness of approximate 6 nm (Figure S6, Supporting Information).


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Figure 4. Linear and nonlinear optical responses of the Au–Ag heteronanocups. (a) Schematics illustrating the overgrowth of a silver nanoparticle on the opening edge of the Au nanocup. (b) SEM image of the Au–Ag heteronanocups (h/b = 0.52 ± 0.03 for the initial Au nanocups). The inset is a zoomed-in SEM image, showing a silver NP overgrown on a gold nanocup. (c) Extinction spectra of the initial Au nanocups and the resultant Au–Ag heteronanocups. The magnetic and electric plasmon resonances of the Au–Ag heteronanocups match the fundamental


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and SHG wavelengths of 866 and 433 nm, respectively. (d) Far-field SHG intensities of the Au– Ag heteronanocups and the initial Au nanocups. The SHG of the heteronanocups is enhanced by 21.8 times. (e) Electromagnetic field H(0), E(0) and SHG PSHG(20) contours of a bare Au nanocup, a Au@Ag nanocup (with a continuous Ag shell) and a Au–Ag nanocup (with a Ag NP). (f) Surface charge distribution of a Au–Ag nanocup at the wavelength of 420 nm. For the calculation, the excitation light is along the y axis and polarized along the x axis. The size parameters are h/b = 0.50, b = 170 nm D = 195 nm for the bare Au nanocup, the Ag shell thickness tAg shell = 6 nm and the Ag NP diameter dAg NP = 29 nm for the Ag component.

The prepared Au–Ag heteronanocups exhibit three plasmon resonance modes in the wavelength regions of 800–950 nm (MD), 520–590 nm (EQ) and 400–450 nm (EM), respectively, as shown in Figure 4c. EM represents a complex electric multipole plasmon mode, which is caused by the coupling of the small Ag NP on the edge and the Au nanocup, as discussed below. After the overgrowth of Ag on the Au nanocups, the resonance width of the MD mode around 870 nm is considerably broadened, the peak wavelength MD is slightly shortened, the EQ peak is blueshifted from 590 to 550 nm, and a new resonance EM peak appears around 433 nm. By tuning the fundamental laser wavelength to 866 nm, the SHG from the Au–Ag heteronanocups is largely enhanced compared to that of the initial Au nanocups. The corresponding SHG enhancement factor reaches 21.8 (Figure 4d). To reveal the physical mechanism of the observed SHG enhancement of the Au–Ag heteronanocups, we calculated the local field distributions of the linear and nonlinear responses of two model heterostructures, the Au nanocup with a uniform continuous Ag shell (labeled as Au@Ag nanocup) and the Au nanocup carrying a single Ag NP on the edge (labeled as Au–Ag


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nanocup), as shown in Figure 4e. The MD resonance of the Au nanocup greatly enhances the magnetic and electric field around the Ag NP (h/b = 0.50, b = 170 nm, D = 195 nm, dAg NP = 29 nm), i.e., a large electric field enhancement factor |f(0, rHS)| around the Ag NP hot spot is induced by the magnetic plasmon resonance when 0 = MD (Figure 4e). On the other hand, the charge distribution on the surfaces of the Ag NP and the Au nanocup at the resonance wavelength of 420 nm is displayed in Figure 4f. The interaction between the Ag NP and the Au nanocup causes a large perturbation on the charge distributions on the Ag NP and the edge of the Au nanocup around the junction region. A large electric field enhancement |f(20, rHS)| in these local regions is therefore induced by the electric plasmon resonance when 20 is in the wavelength range of 400–450 nm. As a result, the cooperative magnetic and electric plasmon resonance enhancements |f(0, rHS)| and |f(20, rHS)| at the same hot spot leads to a very large SHG intensity enhancement factor at rHS 2

𝐹SHG(20,𝐫HS) = |𝑓(20,𝐫HS)| ∙ |𝑓(0,𝐫HS)|

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(2)

Therefore, extremely strong near-field SHG around the Ag NP and the adjacent edge of the Au nanocup is obtained, as shown in Figure 4e. In comparison, the local electric field of the Ag shell is much smaller than that of the Ag NP on the Au nanocup at the MD and the corresponding EM resonances. This indicates that the Ag NP hot spot plays a crucial role on the largely enhanced SHG of the Au–Ag heteronanocups. In summary, we have synthesized colloidal Au nanocups and Au–Ag heteronanocups with faceted inner surface and investigated their optimized magnetic plasmon resonance and largely enhanced SHG. For the bare Au nanocups, the maximal magnetic field enhancement is found to occur at h/b = ~0.65, while the most efficient SHG is experimentally observed at h/b = ~0.78–0.79. The maximal SHG can be attributed to the strongest electric field around the edge


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induced jointly by the magnetic plasmon resonance and the “lightning rod effect”. Moreover, the Au–Ag heteronanocups with overgrown Ag NPs synergize magnetic and electric plasmon resonance enhancements on the same hot spot with double mode matching for SHG, which induces 21.8-fold enhancement of SHG compared to the bare Au nanocups. These findings provide a new strategy for the design of nonlinear plasmonic antennas based on magnetic plasmon resonance. Such nonlinear antennas integrate the highly desired multi-functionalities of multi-frequency resonances and high scattering/absorption ratios. They will find diverse promising applications ranging from nanophotonics to biological spectroscopy. On the other hand, the measurements of SHG signals at the single-level have remained challenging on our measurement system, but they can provide rich information about the dependences of the SHG signal on the excitation and emission polarization directions and therefore opportunities for bettering understanding the relationship between SHG and different plasmon modes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxx. Synthesis of the colloidal Au nanocups and Au–Ag heteronanocups, numerical calculations and theoretical analysis of the charge distributions and electromagnetic field enhancement contours of the Au nanocups, TEM and element mapping images of the Au–Ag heteronanocups (PDF) The authors declare no competing financial interest. AUTHOR INFORMATION


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Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID Jianfang Wang: 0000-0002-2467-8751 Author Contributions §These authors contributed equally. S.J.D. prepared the samples and analyzed the experimental data. H.Z. helped with the sample preparations and characterization. D.J.Y. was responsible for the theoretical modeling and numerical simulations. Y.H.Q. performed the SHG measurements and data analysis. F.N. and Z.J.Y. helped with the SHG measurements and numerical simulations, respectively. J.F.W., Q.Q.W. and H.Q.L. were responsible for the project. S.J.D., J.F.W. and Q.Q.W. wrote the manuscript with help from all authors. ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2017YFA0303402), the National Natural Science Foundation of China (91750113, 11674254, 11704416), Hong Kong Research Grants Council (GRF, 14306817), NSAF (U1530401), the Ministry of Science and Technology of China (2017YFA0303404) and the computational resources from Beijing Computational Science Research Center. The authors thank Zhenyu Zhang for stimulating discussion. REFERENCES (1)

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