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Plasmon-Induced Magnetic Resonance Enhanced Raman Spectroscopy Shu Chen, Yuejiao Zhang, Tien-Mo Shih, Weimin Yang, Shu Hu, Xiaoyan Hu, Jian-Feng Li, Bin Ren, Bing-Wei Mao, Zhi-Lin Yang, and Zhong-Qun Tian Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04385 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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Plasmon-Induced Magnetic Resonance Enhanced Raman Spectroscopy Shu Chen,§,#,‡ Yuejiao Zhang,†,‡ Tien-Mo Shih,§ Weimin Yang,§ Shu Hu,† Xiaoyan Hu,† Jianfeng Li,*,§,† Bin Ren,† Bingwei Mao,† Zhilin Yang,*,§ and Zhongqun Tian† §

Department of Physics, Collaborative Innovation Center for Optoelectronic

Semiconductors and Efficient Devices, Xiamen University, Xiamen 361005, China. †

MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key

Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China.

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ABSTRACT: Plasmon-induced magnetic resonance has shown great potentials in optical metamaterials, chemical (bio)-sensing and surface enhanced spectroscopies. Here, we have theoretically and experimentally revealed (1) a correspondence of the strongest near-field response to the far-field scattering valley and (2) a significant improvement in Raman signals of probing molecules by the plasmon-induced magnetic resonance. These revelations are accomplished by designing a simple and practical metallic nanoparticle-film plasmonic system that generates magnetic resonances at visible-near infrared frequencies. Our work may provide new insights for understanding the enhancement mechanism of various plasmon-enhanced spectroscopies, and also helps further explore light-matter interactions at the nanoscale.

KEYWORDS: Plasmon-Induced Magnetic Resonance (PIMR), Fano Resonance, Hot Spots, Dark-field Spectrum, SERS, Surface Plasmons


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Artificial magnetic resonant modes have been vastly applied to negative refractive index, invisible clocking devices, perfect absorber, ultra-sensitive sensing among others.1-5 At microwave frequencies they can be achieved via the conduction electric current loop in the artificial split-ring resonator (SRR) or similar SRR micro-structures.6,7 However, transferring the concept of SRR from microwave to optical frequencies (particularly in visible and near-infrared ranges) poses difficulties, owing to the magnetic-response saturation led by the modified conduction properties in metals.5,8 Recently, with the aid of plasmon-hybridization effects, magnetic resonances at optical frequencies can be generated through the displacement current loop induced by the coupling among different plasmon modes of each structural-element in systems,5,8-10 such as fishnet,11 particle assemblies,9 and wire pair structures.12 For simplicity, the magnetic resonance based on plasmon-hybridization effects is herein termed as plasmon-induced magnetic resonance (PIMR). Currently, most of PIMR-related studies have placed the emphasis on the far-field optical properties,3,4,13-15 primarily catering to the field of optical metamaterials. Several investigations on near-field properties have theoretically demonstrated that the introduction of PIMR helps generate magnetic and electric “hot spots”, prompting the construction of simultaneous electric and magnetic near-field platforms.5,16,17 More importantly, some simulated results even imply that PIMR-based Fano resonance may embrace higher capabilities of near-field enhancements in comparison with plasmon electric modes.5,16 Until the present moment, however, comparative studies between experiment and theory on (a) electric- and magnetic-near fields, (b) 3 ACS Paragon Plus Environment

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relationships between far and near fields, and (c) corresponding molecular spectroscopies have been rarely reported. One of possible difficulties may lie in that most of currently PIMR-based plasmonic systems have been fabricated by using top-down methods including electron beam lithography and focus ion beam among others.7,17-19 Inevitably, these methods must face challenges in forming very small nanogap in samples, resulting in weak near-field enhancements. A few special fabrication methods (for example, self-assembly of metallic colloids or atomic force microscopy nanomanipulator)5,9 and some efficient plasmonic systems (for example, metallic nanoantenna-film systems)3,4,20,21 have shown great potentials in producing PIMR-based efficiently near-field enhanced platforms. As one of promising candidates, the metallic nanoantenna-film configuration, regarded as a simple and practical system, enables us to generate very narrow gaps that can be highly tuned.3,20-22 Through the assembly of molecular layers, atomic layer deposition or shell-isolated techniques,23-26 distances of these gaps can be controlled within nanometer precision or even sub-nanometer precision. Moreover, by randomly adsorbing chemically synthesized nanoparticles onto a metallic film,3,27 this system can be expanded to a large-scale platform. In the nanosphere-film system with a small diameter, it is difficult for the PIMR to be excited, and the involved modes (for example, the bonding plasmon dipolar mode) 27 are just electric modes. Herein for the first time, we have theoretically designed and experimentally fabricated a gold nanosphere-gold film system with a large sphere diameter (D > 150 nm) to investigate PIMR-based Fano resonance at optical frequencies and its surface enhanced Raman 4 ACS Paragon Plus Environment

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spectroscopy (SERS), in which optical characteristics exhibit significant differences in comparison with that with a small diameter. Using (a) single-particle dark-field scattering, (b) Raman spectroscopy, (c) nanoscale structural characterization scanning electron microscopy (SEM), and (d) finite difference time domain (FDTD) theoretical simulation methods, we explore far-field and near-field properties of PIMR, differences of near-field enhancements between PIMR and plasmonic electric modes, and quantitatively study contributions of PIMR to SERS. Our findings may offer new insights for surface-enhanced spectroscopies and thus facilitate the development of PIMR in SERS and other enhanced spectroscopies involving surface enhanced fluorescence and infrared spectroscopy.

Research model and principle of plasmon-induced magnetic resonance. In this study, our proposed model is represented by a single gold nanosphere situated on a gold film, spaced by monolayer molecules (Figure 1a). Experimentally, this plasmonic system can be constructed by simply dropping synthetic gold nanospheres onto the Au (111) single-crystal flat surface (Figure S1 and Note S1, Figure S2 and Note S2, Note S3). A mercapto benzoic acid (MBA) monolayer (thickness is about 1 nm) is sandwiched between the nanosphere and the Au (111) single-crystal flat surface (see Methods), as schematically enlarged in Figure 1a. Here, atomically flat Au (111) single-crystal surface, which replaces the conventional sputtering gold film, eliminates the roughness, so that it facilitates the consistency between experimental and theoretical situations. In simulations, this monolayer is represented by a 1-nm dielectric spacer with the refractive index of 1.4.28 5 ACS Paragon Plus Environment

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Figure 1. Schematic of studied model and principle of plasmon-induced magnetic resonance. (a) Schematic of research model. In experiments, the gold nanosphere is spaced from the Au (111) single-crystal flat surface by monolayer MBA denoted by a light-blue layer. Incident white light with nearly normal incidence illuminates samples. The inset shows the enlarged view of the nanogap, in which MBA molecules can also be observed as ball-and-stick models. (b) Schematic for the principle of plasmon-induced magnetic dipolar mode in this system, where L and C represent inductance and capacitance. a1 represents the coupling length between the sphere and the gold film for the magnetic dipolar mode.

Generally, when the light illuminates the single gold nanosphere-film system (Figure 1a), it may induce localized sphere-diameter-dependent electric dipole, quadrupole, and even higher order modes on the sphere surface.25,29,30 Meanwhile, similar electric modes with anti-phase distributions are also excited on the imaging sphere under the imaging-field effect.30 Owing to the anti-parallel distributed characteristic on real and imaging spheres, an electric current loop between both


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particles can possibly form, and thus induce magnetic modes (namely, PIMR mentioned above) under the plasmon-hybridization effect in the sphere-film system.3,21,31 Here, the magnetic dipolar moment of the excited PIMR parallels or anti-parallels with the incident magnetic field (H0), as presented in Figure 1a. With the aid of SRR concept, the principle of a magnetic dipolar resonance is presented schematically in Figure 1b. The gold nanosphere and its image can be regarded as two nanoconductors with optical impedance, whereas the dielectric spacer and the air domain surrounding the nanogap act as nanocapacitors.8 Thus, the sphere-film system can mimic the LC (inductance-capacitance) resonator to sustain the magnetic dipolar resonance, transplanting the SRR concept from microwave to optical frequencies. Specifically, the probability of PIMR-excitement and the spectral location of the PIMR critically depend on the gap size and the coupling length (denoted by a1 in Figure 1b) between the sphere and the metal film. Therefore, a1 becomes the very key factor because the gap size has been unaltered as 1 nm in the present system. Differing from the metal cube-film or the metal disk-film system.3,4,21,32 a1 is taken much smaller than the diameter of the nanosphere because of the spherical morphology. Thus, the excited PIMR can be located at higher frequencies in the nanosphere-film system comparing with that owns the same diameter in the metal disk-film and cube-film systems. In addition to PIMR, electric coupled modes, including dipole and quadrupole in the metal particle-film system, can also be excited.


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Figure 2. Analyses of magnetic and electric dipolar modes. (a) Scattering (black curve), absorption (red curve) spectra for 1 nm and scattering spectrum (blue-dashed curve) for 10 nm dielectric spacer of a nanosphere with 200 nm diameter on the gold film. (b). Displacement vector filling electric-field distributions (top row, 1-i and 2-i) and magnetic-field distributions (bottom row, 1-ii and 2-ii) corresponding to the marked “1” (left panel) and “2” (right panel) in the scattering spectrum of a. The electric and magnetic-field distributions have been respectively normalized by the same scale bar. (c) Surface-charge distributions of 2D xy-plane (left panel) and 3D nanosphere-film system (right panel) at 710 (“1”, top) and 780 nm (“2”, bottom)


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wavelengths. The selected area of the gold film in 3D surface-charge distribution is the same as the black circle in 2D distributions.

For a free-standing gold nanosphere with 200 nm diameter, a broad resonant peak of the simulated scattering spectrum centered at 610 nm (Figure S3a) originates from an electric dipole generated on the gold nanosphere (Methods).31 As this nanosphere approaches a gold film, the broad resonance will red-shift owing to the nanosphere-gold film plasmon coupling effect and the change of the substrate dielectric environment.31 For example, a 10 nm gap leads to a red shift with the wavelength changing from 610 nm to 740 nm (blue-dotted curve in Figure 2a). As a plasmon electric dipolar coupled mode, the broad band is instead replaced by a pronounced valley situated at 710 nm (black curve in Figure 2a) when the gap diminishes to 1 nm. This asymmetrical curve is characterized by Fano resonance that generally originates from the interference between a bright broadband mode and a dark narrow band mode.5,9,31,33 Interestingly, a peak of the absorption spectrum is observed near the scattering Fano dip (red curve in Figure 2a), implying that a new mode may have been excited. Meanwhile, a non-conspicuous peak at ~ 780 nm can also be observed in the absorption spectrum (the blue dot and arrow in Figure 2a and Figure S3b). To facilitate explaining the mechanism that governs Fano resonance, we present near-field distributions that include electric displacement vectors filling electric fields, magnetic fields (Figure 2b and Figure S3d) and surface charges (Figure 2c) of the proposed system at five selected wavelengths. First, we focus on the electric and 9 ACS Paragon Plus Environment

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magnetic response in the vicinity of nanogap corresponding to the scattering valley at 710 nm wavelength (marked 1). In addition to the extremely localization of electric and magnetic fields at the nanogap (Figure 2b-1-i and ii), vectors recirculate, showing an excited magnetic dipolar mode.5,8,15 Meanwhile, surface charges behave like a binary LC resonator, which is induced by reverse polarity surface charges on surfaces of the nanosphere and gold film, indicating the existence of a magnetic dipolar mode (Figure 2c-1-right).3,21 Vector and surface-charge distributions show a magnetic moment that is aligned with H0. As a result of the near-field coupling effect between the nanosphere and the gold film, propagating surface plasmons (PSPs) are excited, as evidenced by the standing wave on the film surface (Figure 2c-1-left).25,34 Electric and magnetic responses corresponding to 780 nm wavelength (marked 2) near the main scattering peak differ from those corresponding 710 nm wavelength. The electric field and the displacement current density on left and right nanosphere surfaces, instead of near the nanogap, intensify and behave like an electric dipole, as clearly observed from vectors filled in electric-field (Figure 2b-2-i) and 3D surface-charge distributions (Figure 2c-2-right). Meanwhile, near the nanogap the current loop has greatly weakened, and has been accompanied with much less degree in localization of electric and magnetic near fields, as confirmed by the magnetic near-field (Figure 2b-2-ii) and 2D surface-charge distributions (Figure 2c-2-left). Because of this dominant electric response, this mode can be regarded as belonging to an electric dipolar coupling mode (here abbreviated as an electric dipole) of the nanosphere-film system.29,32 To make further comparisons, we also investigate near-field properties corresponding to two 10 ACS Paragon Plus Environment

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scattering peaks at 765 and 670 nm wavelengths (marked 3 and 4). Near-field characteristics show that 765 nm wavelength corresponds to an intermediate state between electric and magnetic dipolar modes (Figure S3d-3-i and ii), and that 670 nm wavelength can be regarded as a non-resonant state of magnetic dipolar mode (Figure S3d-4-i and ii). In addition, comparing scattering and absorption characteristics (Figure 2a), we have unintentionally discovered a new mode near the second scattering valley ~ 620 nm wavelengths. As evidenced by (a) vectors filling electric-field (Figure S3d-5-i), (b) magnetic-field distribution (Figure S3d-5-ii) and (c) surface-charge distributions (Figure S3c), this new mode can be regarded as belonging to a higher gap-plasmon mode (Note S4).21,32 Normalized magnitudes of magnetic-field distributions for these five wavelengths (Figure 2b and Figure S3d) indicate that the strongest magnetic near field of the magnetic dipolar mode reaches as highly as 84 enhancement of H0, which is approximately 5.0 × 107 in the format of |H/H0|4, relatively to 3.0 × 106 for the electric dipolar mode. Herein, Fano resonance at wavelengths larger than 670 nm is induced by the interference of electric and magnetic dipolar modes. It should be noted that the physical nature of the plasmon-induced magnetic mode is the same as the plasmon-induced electric mode. Both of the two modes belong to bounded surface charge density wave originating from the collective oscillation of free electrons induced by the incident light. The electric dipolar resonance is regarded as the super-radiative mode owing to the largeness of total electric dipole moment that is proportional to the nanosphere volume,30 as depicted by the surface-charge 11 ACS Paragon Plus Environment

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distribution (bottom row in Figure 2c). Although antiphase electric dipole distributions on the gold film surface weaken the electric dipolar moment on the nanosphere surface, the latter dwarfs the former, leading to the fact that the net dipolar moment remains large, particularly for nanosphere with large diameter (for example, D > 150 nm). By comparison of the electric dipolar mode, most of surface charges for the magnetic dipolar mode (top row in Figure 2c) are confined within a very small volume near the nanogap, resulting in the significant reduction of the total electric dipolar moment. Although magnetic dipolar moment also contributes to the radiative of magnetic dipolar mode, it serves as a higher-order radiative channel with a weak efficiency relative to the electric dipole.5 Therefore, the magnetic dipolar mode acts as a sub-radiative mode (a magnetic “dark mode”) relative to the electric dipole. At the gap region, Fano interference between the magnetic and electric dipolar modes happens when specific frequency and phase conditions are met. The physical picture of this can be built with the help of the two coupled oscillators model.35, 36 The other Fano dip at 620 nm originates from the interference of electric dipolar and higher order modes, of which the latter behaves as the sub-radiative mode. The magnetic dipolar mode critically depends on the effective coupling length (a1) between nanosphere and gold film, but not the diameter of nanosphere (more discussions later). If we increase a1 to ~ 28 nm by theoretically dissecting a cross-sectional plane (~ 625 nm2) at the bottom of nanosphere (Figure S4 and Note S5), the magnetic dipolar mode with small scattering efficiency significantly red-shifts to 918 nm,


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accompanied by the great decay of Fano resonance of the electric and magnetic dipolar modes.

Figure 3. The relationship between the scattering and near-field spectra. (a) The scattering spectrum (black curve) and average electric-field enhancement spectrum at the nanogap (red curve) for nanosphere-film system with 200 nm diameter and 1 nm dielectric spacer. The average electric-field enhancement is estimated from the volume average of nanogap with |Eloc/E0|4. (b) Electric-field enhancement distributions on the side view (xz-plane, top row) in the center plane of nanosphere and on the top view (xy-plane, bottom row) for the magnetic (left panel) and the 13 ACS Paragon Plus Environment

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electric dipolar mode (right panel). Enhanced electric-field distributions have been normalized by the maximum enhanced values. The insets show the enlarging schematics for field distributions at the nanogap.

For most of plasmonic systems, to obtain optimally-enhanced near fields, we select appropriate experimental laser lines near scattering peak positions, which generally are regarded as resonant positions of plasmon modes.37 Herein, it is extremely worthwhile to unveil the relationship between PIMR-based near-field enhanced and far-field scattering spectra to help understand the potential mechanism of plasmon-enhanced spectroscopies.38-40 One conspicuous peak and the other less-conspicuous peak near two scattering Fano dips exist in the electric-field spectrum and appear differently from scattering spectral features with several peaks and Fano lineshapes (Figure 3a). Only one peak with symmetry lineshape near the main Fano dip is observed in the magnetic-field spectrum (Figure S5). In correlation with aforementioned analyses on plasmon modes, electric and magnetic near-field enhancement peaks are generated by the magnetic dipolar mode that is located at the scattering Fano dip, neither at the main scattering peak (765 nm) nor at electric dipolar mode (780 nm). The novel relationship of near-field and scattering spectra for the nanosphere-film system with 200 nm diameter exhibits the correspondence of near-field peaks to scattering valleys. It significantly differs from that of most of conventional nanosystems,37 even including the nanosphere-film system with small diameters.41 Quantitatively, the average electric-field enhancement, which is presented in the format of |E/E0|4 taking the evaluation of SERS electromagnetic 14 ACS Paragon Plus Environment

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enhancement into consideration,39,41 reaches 1.6×109 for the magnetic dipolar mode, but only 1.6×108 and 4×107 for the scattering peak and the electric dipolar mode (Figure 3a). To distinguish differences of near-field enhancements under two dipolar modes in details, we present corresponding electric-field distributions (Figure 3b). The enhanced field distributions corresponding to the magnetic dipolar resonance shows strong electric “hot spots” at the nanogap, enjoying over 1×1012 of maximum enhancement. For the other electric dipolar mode, the maximum enhanced field remains at the nanogap, but enjoys only approximately 1×1010. It should be noted that the “hot spots” with huge field enhancement play a dominant role in SERS. In general, most of SERS signals are from the probing molecules in the “hot spots” area, although molecules in other area also contribute to total SERS signals. The better electric and magnetic near-field enhancements of PIMR may originate from several aspects. First, in comparison with the electric dipolar mode, which primarily couples with the electric-field component of the incident light, PIMR can simultaneously couple with both electric- and magnetic-field components of the light, resulting in a higher light coupling efficiency.3,8 Second, since PIMR involves smaller electric dipolar moments, it incurs lower radiation loss, thus leading to stronger near-field enhancements.5,31 Meanwhile, simultaneous electric and magnetic surface currents, of which the latter is induced by the displacement current loop, also help suppress the radiation loss, particularly under a balancing condition between both currents,3 hence further allowing more energy to be stored within the near-field space.


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To experimentally demonstrate above-mentioned optical properties, we perform single-particle dark-field scattering and Raman spectra for the proposed system, in which D ranges from 160 nm to 250 nm with 1 nm sphere-film gap distance controlled by monolayer MBA molecules (see detailed measurements, devices for dark-field scattering spectra in Methods and Figure S6). Pronounced dips and peaks, observed from experimental (Figure 4b) and theoretical scattering spectra (Figure 4c), gradually redshift as the nanosphere diameter increases (Figure 4a). Moreover, the asymmetry of Fano lineshapes becomes inconspicuous and the full width half maximum of the main peak broadens with the increase of the nanosphere diameter. Slightly significant deviations of simulated peak and dip positions from experimental counterparts for D ≥ 235 nm (Figure 4d) may originate from the fact that the morphology of idealized nanospheres used in simulations inevitably differs from that of realistic experimental nanospheres. Moreover, little difference in refractive indices of the spacer and the light incident situations (Figure S7 and Note S6) between experimental and simulated cases also contributes to deviations of spectral lineshapes, positions and depths between the experimental and theoretical results. Note that the detailed influences of the light incident situations on the magnetic dipolar mode, electric dipolar mode and Fano resonance are presented and discussed in Figure S7 and Note S6. In addition, different slopes for peaks and dips (Figure 4d) correspond different redshift degrees with the increasing of D from 180 nm to 250 nm. For example, the former shows 155 nm red-shifts (892 nm-737 nm) but only 87 nm (780 nm-693 nm) for the latter. With the help of simulated absorption spectra (magenta 16 ACS Paragon Plus Environment

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dashed-curves in Figure 4c), we can better understand aforementioned spectral behaviors (involving peak shifts and width) relevant to sphere-diameter changes. Notably, the resonant wavelength for the magnetic dipolar mode does not happen near the scattering dip at all studied nanosphere diameters. Moreover, the deviation for the resonant wavelength between electric and magnetic dipolar modes varies with the increasing of D. For the smallest nanosphere (Figure 4a-i), the magnetic resonance is excited at ~ 697 nm with a large deviation from the scattering valley (Figure S8a and Note S7). Incidentally, the resonant position of the electric dipolar mode occurs at the blue side of the magnetic resonance. For D = 180 nm (Figure 4a-ii), both modes exhibit redshifts, and are nearly fused to establish a broadband ranging from 676 nm to 725 nm. As D continues to increase to 210 nm (Figure 4a-iii), the resonant wavelength for the magnetic dipolar mode nearly matches the scattering dip at 711 nm, whereas that for the electric dipolar mode occurs at the red side (~ 790 nm). For D ≥ 240 nm (Figure 4a-iv and v), the resonant wavelength of the magnetic dipolar mode stays near the scattering dip, whereas that for the electric dipolar mode occurs near the scattering peak, accompanied by a broad band (Figure S8b and Note S7). Based on these discussions, magnetic and electric dipolar modes exhibit different sensitivities to D, and the former is much less sensitive to sphere diameter relative to that of the latter. This trend is attributed to that the dipolar moment of the magnetic resonance is intimately related to a1 value, which varies minutely within the given D range, whereas the electric counterpart primarily depends on D itself.31 Thus, magnetic modes cannot be observed at our interested frequencies regions (the visible 17 ACS Paragon Plus Environment

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or lower frequencies) when D is small in the gold nanosphere-film system because a1 is too small to excite the magnetic mode.41 As D increases, Fano resonance diminishes due to such non-synchronization, and the main scattering peak width broadens when the gradually-dominant radiative damping of the electric dipolar resonance increases.

Figure 4. Dark-field scattering characterization of the single nanosphere-gold film systems. (a) SEM images for sphere-film systems with D = 160 nm (i), 180 nm (ii), 18 ACS Paragon Plus Environment

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210 nm (iii), 240 nm (iv), and 250 nm (v) on the Au (111) single-crystal flat surface. (b) Dark-field scattering spectra of single particle on the Au (111) single-crystal flat surface with i: black, ii: red, iii: blue, iv: green, and v: red-brown. (c) Computed scattering spectra (the solid curves) corresponding to the systems in b and absorption spectra (the dotted curves) with FDTD. (d) The plot of theoretical peak (black line) and dip (blue line) positions, experimental peak (red dots) and dip positions (pink dots). Here, experimental spectral values are evaluated through measurements of multiple particles with the approximate D, and theoretical spectra values are evaluated in a 10 nm interval of varying D ranging from 160-250 nm. Dashed vertical lines in b and c denote the laser line at 785 nm (black) and Raman Stokes lines of MBA at 1078 cm-1 (red) and 1592 cm-1 (blue). Arrows in c denote the spectral positions of the magnetic (magenta) and electric (blue) dipolar modes.

Raman spectra of MBA probing molecules (SEM images in Figure 4a) are measured to explore the enhancement effect of PIMR. As above-discussed, the coherent excitation of the magnetic mode in the nanosphere-film system will trap the incidence light in the near field, and incur a large electromagnetic field enhancement at the gap. The field enhancement magnifies the intensity of the incident light, which will excite the Raman modes of the MBA molecule, therefore increasing the signal of the Raman scattering. The Raman signal is then further amplified by the system with the same mechanism that excited by the incident light, resulting in a greater increase in the total output. Thus, the significant electric-field enhancement of the PIMR which


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owns very low radiation efficiency can be explored by the enhanced Raman signal of MBA molecules. As D increases from 160 nm to 250 nm, (a) the excitation laser (785 nm, black dashed line), (b) Raman Stokes lines of MBA at 1078 cm-1 (named as α line at 856 nm, red dashed line) and 1592 cm-1 (named as β line at 897 nm, blue dashed line), and (c) resonant regions of magnetic and electric dipolar modes exhibit various degrees of tuning (Figure 4b and c). Consequently, Raman signals for five sphere-film systems significantly change (Figure 5a). For D = 160 nm (Figure 4a-i), 785 nm laser line and two Stokes lines lie beyond resonant regions of electric and magnetic modes, leading to the weakest signal (black curve in Figure 5a). As D increases to 180 nm (Figure 4a-ii), the laser line and two Stokes lines remain outside both resonant regions but nearer to resonant regions, leading to a slightly-increased signal. For D = 210 nm (Figure 4a-iii), the laser line locates in the resonant region of the electric dipolar mode and has slightly passed its resonant position with α and β lines near the resonant region of the electric dipolar mode, accompanied by a further slightly-increased signal (blue curve in Figure 5a). For D = 240 nm, the 785 nm laser line approaches the resonant region of the magnetic dipolar mode, and the α line has passed the resonant position for the electric dipole. Meanwhile, the β line appears at the resonant region of the electric dipole, accompanied by a strongly increasing of Raman signal. As the laser and α lines further approach the resonant region of the magnetic dipolar mode, and the β line further moves closely to the resonant region of the electric dipole at D = 250 nm, the Raman signal exhibits further enhancement. Finally, at D = 250 nm, 20 ACS Paragon Plus Environment

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Raman peaks become the tallest, reaching 225, 40, and 14 times those at D = 160 nm, 180 nm, and 210 nm for α line. Clearly, this Raman-signal amplification indicates that the magnetic dipolar mode can sustain the maximal SERS response, coinciding with our theoretical predictions. Furthermore, the Raman signal is dominated by the electric dipolar mode at D = 210 nm, while it is decided by both electric and magnetic dipolar modes at D = 250 nm. Thus, the Raman enhancement (~14 times) from the systems with D = 210 nm to D = 250 nm originates from the magnetic dipolar mode, as the contribution of the electric dipolar mode to Raman signals at D = 210 nm and 250 nm remains almost the same. In addition, our simulated SERS electromagnetic mappings (Figure 5b) show a qualitative agreement with experimental results, and hot spots at D = 250 nm (Figure 5b-v) that provide the majority of SERS signal show the largest magnitude of SERS EFs,42 reaching about 745, 156, and 16 times those at D = 160 nm, 180 nm and 210 nm for α line, similarly to that for β line (see Methods and Figure S9 for SERS mapping details). Based on aforementioned theoretical and experimental SERS results, the gradual increase of SERS signals indeed originates from the match of various degrees among the excited laser line, two Stoke emission lines and the magnetic dipolar mode. Slight deviations of theoretical SERS EFs from experimental results may be attributed to the fact that the former and the latter are respectively estimated by maximum EFs within hot spots and average values from monolayer molecules. Moreover, the non-local quantum effect may also contribute to such deviations, yielding a slight reduction of experimental results,34 whereas the quantum tunneling effect can be neglected due to its existence in a narrow gap smaller 21 ACS Paragon Plus Environment

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than 0.5 nm.43 Additionally, at 633 nm laser wavelengths, we also investigate Raman signals for various D’s ranging from 160 nm to 250 nm (Figure S10 and Note S8). The strongest Raman signal is acquired when the laser line and the Raman Stokes lines lie within the resonant region of the magnetic dipolar mode, with the acquisition of the weakest one when these three lines lie far beyond the region, further confirming significant contributions of PIMR to SERS enhancements.

Figure 5. SERS enhancements of single sphere-gold film systems. (a) Raman signals of MBA adsorbed on the Au (111) single-crystal flat surface of the single nanosphere-gold film systems that correspond to SEM images of Figure 4a. The height of the dual-arrow black line is 1500 counts. (b) SERS electromagnetic enhancement mappings for 1078 cm-1 Stokes band under the 785 nm excitation wavelength. They are normalized by maximum SERS enhancement factors of D = 250 nm. SERS are defined as log10 (|Eex/E0|2∙|EStokes/E0|2), where Eex/E0 denotes the electric field enhancement at the excitation wavelength and EStokes/E0 denotes the electric field enhancement at the Stokes band of interest for the detected molecule.


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In conclusion, we have built a simple and practical PIMR-based platform made of the metallic nanosphere and the metallic film. Single particle dark-field scattering characterizations and in-depth theoretical analyses for the proposed system have revealed a magnetic-based Fano resonance as a result of interactions of electric and magnetic dipolar modes, of which the latter behaves as a dark magnetic mode appearing near the Fano dip. Our Raman experimental results have further disclosed that PIMR significantly enhances the SERS response of probing molecules due to large electric-field enhancements generated by the PIMR. High capabilities of PIMR on electric and magnetic near-field enhancements originate from (a) low electromagnetic radiative losses and (b) efficient coupling between PIMR and both electric- and magnetic-field components of the incident light. Notably, via the correlation between the dark-field and Raman spectra, we have uncovered the rarely reported connection of near-field peaks to scattering valleys. Our findings of PIMR serve as a new insight for surface enhanced Raman spectroscopy and non-Raman spectroscopies such as Fluorescence and Infrared absorption. Furthermore, design of the large-scale plasmonic platform based on PIMR can be applied to work for magnetic-based optical metamaterials. Methods. Sample synthesis and Au (111) single-crystal preparation. Gold nanospheres were prepared by sodium citrate reduction and seeded growth methods (see Note S1 and Figure S1). Au (111) single-crystal flat surface were prepared according to a described procedure (see Note S2 and Figure S2). For purpose of assembling probing molecules, the gold single crystal was immersed in a 0.1 mM 23 ACS Paragon Plus Environment

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ethanol solution of mercapto benzoic acid (MBA) for 0.5 h. Then the gold crystal was rinsed byethanol and dried with nitrogen.

Dark-field scattering spectroscopy. All optical measurements in this work were carried out prior to SEM characterizations to avoid possible carbon contamination and molecular breaking. Dark-field imaging and spectroscopy on the single Au nanosphere were carried out with a Leica upright microscope coupled with a color digital camera (Q-image EXI) and spectral CCD (Renishaw inVia) (Figure S6). Samples were vertically shined using an unpolarized white light source from a 100W halogen lamp. Scattered light was collected through a 50×, 0.55 NA objective (Leica) and directed to a spectrograph or a color camera to record scattering spectra and dark-field color images. All scattering spectra were background subtracted and wavelength response-corrected using the spectrum obtained from the gold substrate that contains no nanospheres. For each sphere, dark-field spectra were acquired 3 times, without observable differences between each time.

Raman spectroscopy. Prior to dark-field and Raman measurements, MBA molecules were adsorbed on the Au (111) single-crystal surface. For each sphere, Raman and dark-field detections were carried out simultaneously. All Raman detections were performed using a Renishaw inVia Raman microscope (the same instrument as dark-field characterization in Figure S6). The power of 785 nm and 633 nm laser were set to values as low as possible to avoid the laser-induced morphological changes during measurements. The detailed power was changed according to realistically-required conditions to obtain high-signal-to-noise-ratio 24 ACS Paragon Plus Environment

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spectra. Finally, we normalized these Raman spectra to the same power value. For each sphere, Raman spectra were acquired 3 times without observable differences between each time.

SEM and TEM characterization. SEM images for the spheres were taken with a Hitachi S-4800. Correlating SEM images with dark-field images, we located the targeted nanosphere according to step features and some man-made signs on the Au (111) single-crystal flat surface. High-resolution TEM images for gold nanospheres with different diameters in Figure S1 were taken with a JEM 2100 TEM microscope. TEM images were taken at an accelerating voltage of 200 KV and magnifications of ×50,000.

Numerical simulations. Simulations were carried out by a commercial software package (Lumerical Company) that adopted the 3D - FDTD method. The dielectric function of gold was taken from a multi-coefficient fitting model offered by Lumerical FDTD. In simulations, a total-field scattered-field source acts as a linearly polarized light normally incident on the nanosphere. Although the simulation source slightly deviates from dark-field scattering measurements, experimentally most of the non-polarization white light nearly tends to impinge normally. For the purpose of estimating influences of oblique incidence on focused plasmonic modes, we theoretically compared deviations of scattering spectra under different incident angles of 0, 25, and 33 degrees. In all cases, deviations on focused electric and magnetic modes can be neglected (Figure S7 and Note S6). Perfectly matched layer boundary conditions were adopted in order to avoid unphysical reflections around structures. 25 ACS Paragon Plus Environment

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The simulation time was set to 1000 fs, which sufficiently warranted the convergence. For computation-time saving, nonuniform mesh method was generated. For the narrow dielectric gap, 0.25 nm × 0.25 nm × 0.25 nm Yee cell size was set, whereas 1 nm × 1 nm × 1 nm was used in other regions. The average electric-field enhancement 4

(Figure 3A) was obtained by ∫∫∫V |

Eloc | dV / ∫∫∫ dV V E0

, where V is the volume of nanogap

between the sphere and the gold film, with regions of nanogap being 80 × 80 nm2 in xy-plane and 5 nm height in z-direction. Eloc and E0 represent the localized electric field and the incident electric field, respectively. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Preparation process of gold nanospheres and Au (111) single-crystal flat surface. Optical and SERS measure details and the related numerical simulation results. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Present Address #

(S. C) Nanooptics, CIC nanoGUNE, Donostia-San Sebstián, Spain

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. S. C., Z.-L.Y., J.-F. L., Z.-Q. T., B. R. and B.-W. M. conceived the idea and discussed the results. S. C., Z.-L.Y. and T.-M. S. wrote the manuscript. S. C. and W.-M. Y. performed the theoretical simulations. S. C. and Y.-J. Z. carried out the experimental characterization. Y.-J. Z. and X.-Y. H. prepared the samples for experiments. S. H. guided the dark field characterization. ‡These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge support from the National Science Foundation of China (No.11474239, No.21673192) and MOST (2016YFA0200601, 2017YFA0204902).

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(b)

(a) E0

H0

a1

k

C L

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(a)

(c)

1

+

+ 3

2

-

4 5

-

2

+

+

1

-

2-i

1-ii

2-ii

Lg|E/E0|4

1-i

7 5 3 1


Lg|H/H0|4

(b)

strong

-

weak

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a) .

. .

.

.

.

.

.

. .

.

(b)

12 8 4

Lg|E/E0|4

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0 12 8 4 ACS Paragon Plus Environment

0

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Nano Letters

(a)

(b) i

Lg(|Eex/E0|2•|EStokes/E0|2)

ii

10 8 6

iii

v

4 2 0

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