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Plasmon-Enhanced Second-Harmonic Generation Nanorulers with Ultrahigh Sensitivities Shaoxin Shen,† Lingyan Meng,† Yuejiao Zhang,‡ Junbo Han,§ Zongwei Ma,§ Shu Hu,‡ Yuhan He,‡ Jianfeng Li,*,‡ Bin Ren,‡ Tien-Mo Shih,†,∥ Zhaohui Wang,‡ Zhilin Yang,*,† and Zhongqun Tian‡ †

Department of Physics, Xiamen University, Xiamen 361005, China State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China ∥ Institute for Complex Adaptive Matter, University of California, Davis, California 95616, United States ‡

S Supporting Information *

ABSTRACT: Attainment of spatial resolutions far below diffraction limits by means of optical methods constitutes a challenging task. Here, we design nonlinear nanorulers that are capable of accomplishing approximately 1 nm resolutions by utilizing the mechanism of plasmon-enhanced second-harmonic generation (PESHG). Through introducing Au@SiO2 (core@shell) shellisolated nanoparticles, we strive to maneuver electric-field-related gap modes such that a reliable relationship between PESHG responses and gap sizes, represented by “PESHG nanoruler equation”, can be obtained. Additionally validated by both experiments and simulations, we have transferred “hot spots” to the film-nanoparticle-gap region, ensuring that retrieved PESHG emissions nearly exclusively originate from this region and are significantly amplified. The PESHG nanoruler can be potentially developed as an ultrasensitive optical method for measuring nanoscale distances with higher spectral accuracies and signal-to-noise ratios. KEYWORDS: plasmon nanoruler, second-harmonic generation, surface plasmon resonance, core−shell nanoparticles, finite-difference time-domain

A

utilizing dark-field scattering spectra (DFSS), whose accuracy of average plasmonic responses appears nonideal.4−6,8 On the other hand, introducing molecules as a probe for plasmonic sensing, such as TERS, may result in unstable responses and strong backgrounds due to structural changes both in nanostructures and molecules.3 Therefore, alternatives that manifest ultrahigh resolutions far below diffraction limits are called for. Immediately recently, nonlinear responses, such as second-harmonic generation (SHG),13 third-harmonic generation,14 and four-wave mixing,15 under specific plasmonic configurations show excellent sensitivities for sensing near-field coupling effects. Several studies have been undertaken to prove that plasmon-enhanced SHG (PESHG) can be supersensitive to the geometry deformation in nanoscales both theoretically and experimentally.16−18 Primary reasons lie in that the PESHG depends not only on the symmetry breaking at the interface and diminishing on the structure but also on plasmon coupling modes.19−22 Therefore, PESHG may offer selective configuration information with ultrahigh sensitivities, avoiding interferences of probing molecules. Additional existing studies

mid the increasing development of nanotechnology, considerable attention has been paid to nanoscale characteristics less than a few nanometers. Over the past decade, a variety of super-resolution optical techniques, exemplified by near-field scanning optical microscope (NSOM),1 super-resolved fluorescence microscopy (the Nobel Prize in Chemistry 2014),2 and tip-enhanced Raman spectroscopy (TERS),3 have been proposed and developed. In recent years, linear plasmon nanorulers (PNRs) for resolving nanoscale distances have become increasingly prominent.4−8 Within the development of these PNRs, surface plasmon resonances are associated with collective oscillations of conduction electrons, and their coupling exhibits extreme sensitivities to the distance between two adjacent plasmonic components, especially in cases of gap modes.9−12 Reported nanostructures that function as linear PNRs include nanoparticle (NP) dimers,4,7 film-NP configurations,5,8 and plasmonic oligomers.6 In particular, linear PNRs based on coupled film-NP configurations with specific spacer layers (i.e., self-assembled monolayers) display the sensitivity in a wide gap range (subnm to 27 nm), and even attain spatial resolutions down to the angstrom level.5,8 However, optical characteristics of linear PNRs with tunable film-NP-gap sizes (g) continue to face challenges. One popular method is characterized in © 2015 American Chemical Society

Received: June 29, 2015 Revised: September 10, 2015 Published: September 15, 2015 6716

DOI: 10.1021/acs.nanolett.5b02569 Nano Lett. 2015, 15, 6716−6721

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Nano Letters

Measurements of SHG were performed by focusing a tunable Ti:sapphire laser onto SHINs facing the impinging beam at an incident angle (θ = 45°), whereas reflection-scattered SHG signals were collected by a CCD camera (see Methods and Figure S1 in Supporting Information (SI) for detailed SHG experiments). The coupling between particles can be notably decreased through the oblique incidence due to the reduction of parallel components of the incident electric field. Because the 40 μm-diameter incident-beam spot is much larger than the diameter of SHINs, experimentally observed PESHG signals represent the average performance of submonolayer-SHIN systems, enabling us to minimize signals’ deviations from shape variations of individual nonspherical nanoparticles. These measurements were taken on configurations of film-SHIN and film-NP as well as on different substrates with a pumping wavelength at 785 nm. As expected, measured signal intensities vary quadratically with the increasing excitation power (Figure 3a), while the emission peak position shifts from 370 to 445 nm when the excitation wavelength is tuned from 740 to 890 nm (Figure S2 in SI). Results obtained above signify characteristics of SHG.13,20 The configuration of SHINs (55@1) on the Au film exhibits the maximum PESHG intensity (possessing a weak and broad two-photon-excited luminescence background13) among all samples, including configurations of bare gold NPs (55@0) on the Au film, SHINs (55@1) on the Si film, bare Au film, and bare Si film (Figure 3b). The comparison of PESHG signals between SHINs and bare gold NPs on the Au film indicates that not only can the silica shell provide an innate nanogap between the upper NP and the lower film to confine the incident optical field through nearfield coupling effects, but also it can remarkably avoid the exchange of charges between upper and lower surfaces.25,26 The experiment of SHINs on the Si film shows a significant decrease of PESHG intensities when we exclude influences of the plasmonic substrate (Inset of Figure 3b). This fact implies that NP-gold film couplings, rather than interparticle couplings, play the leading role for observed PESHG signals in our PESHG system. Another evidence for SHIN-film couplings is that measured values of PESHG peak show a cosine-wave line-shape as a function of the incident polarization angle (Figure 3c, see Methods and inset of Figure S1 in SI for polarizationdependent SHG experiments), and the maximum PESHG intensity periodically emerges at p-polarization angles corresponding to n*π (n = 0, 1, 2). These results confirm that measured PESHG responses indeed almost originate from the

of PESHG have so far focused on all-optical switching and modulation as well as nonlinear plasmonic metamaterials.23,24 Experimentally introducing PESHG as a ruler to measure nanoscale distances has been unreported. In our laboratory, we propose to design Au@SiO2 (core@ shell) shell-isolated nanoparticle (SHIN) to supersensitively maneuver g values (Figure 1). The plasmon coupling within the

Figure 1. Schematic illustration of PESHG nanoruler. The silica shell of the SHIN forms an innate nanogap (g) between the upper NP and the lower Au substrate. The incident beam impinges with 45° (K) and p-polarization (electric field (E) is parallel to the incident plane). The drastic change of PESHG intensity can reflect variations of the plasmon coupling between the film and the NP. The color bar marks g based on the studied PESHG nanoruler.

film-NP nanogap can be effectively excited under the oblique ppolarization (E) incident beam (K), revealing an exponential decay with increased g (1, 2, and 6 nm). Moreover, because PESHG intensities strongly depend on gap sizes, they can be precisely tuned with changes of shell thicknesses, and in turn can be used to evaluate g. We synthesized a SHIN (Figure 2a) with a gold core (D = 55 nm) coated with silica shell of different thicknesses (g = 1, 2, 3, and 6 nm) for constructing PESHG nanorulers. For purposes of fabricating vast-area SHG-active substrate with uniform and repeatable responses and of avoiding interferences of plasmon coupling between adjacent SHINs, a submonolayer of SHINs (with average interparticle distance of 10 nm) on the smooth Au surface (film-SHIN configuration) was prepared (Figure 2b) with a bare (g = 0) counterpart (film-NP configuration, not shown here) for comparisons (see Supporting Information for a detailed preparation processes of SHINs/NPs and assembling method for constructing film-SHIN/NP configurations).

Figure 2. Characteristics of the film-SHIN configuration. (a) High-resolution transmission electron microscopy (HRTEM) images of SHINs with different silica shell thicknesses g = 1, 2, 3, and 6 nm (D = 55 nm). (b) Top view of the scanning electron microscopy (SEM) image of a submonolayer of SHINs (D = 55 nm, g = 1 nm) on a smooth Au surface. 6717

DOI: 10.1021/acs.nanolett.5b02569 Nano Lett. 2015, 15, 6716−6721

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Figure 3. Characteristics of SHG signals originating from the film-SHIN configuration. (a) Power-dependent SHG signals: the emission intensity at the SHW increases with increased average pumping powers (20−100 mW). Inset: Measured SHG intensities versus the square of pumping powers. (b) Comparisons of SHG signals for configurations of SHINs on the Au film (g = 1 nm) (magenta line), bare Au NPs on the Au film (g = 0 nm) (dark cyan line), SHINs on the Si film (g = 1 nm) (blue line), bare Au film (red line), and bare Si film (black line) with a given average power (80 mW), where TPEL denotes two-photon-excited luminescence. Inset: The SHG intensity distribution of SHINs on the Au film (g = 1 nm) (magenta stripe) and SHINs on the Si film (g = 1 nm) (blue stripe). The data are normalized by values of SHINs on the Au film. (c) Polarization-dependent SHG signals: measured SHG peak values (black dots) as a function of the incident polarization angle. The red line denotes the peak-valued fitting curve. (d) 3D-FDTD simulations of PESHG-EFs for the film-SHIN (55@1) configuration with 10 nm-NP-gap distance (in logarithmic scale): (up) FW mode (785 nm); (middle) SHW mode (393 nm); (low) total PESHG-EF.

film-NP nanogap since the multipolar interaction between the nanoparticle and the substrate can be notably excited using the oblique p-polarization incident beam.27,28 In light of SHG mechanisms, the SH field generated from film-NP configurations is connected with surface susceptibilities of plasmonic components and local field enhancements both at fundamental and harmonic frequencies. Regarding surface susceptibility, because of the centrosymmetry-breaking limit (χ(2) bulk = 0) according to electric-dipole approximations for isotropic materials (gold), different elements of the susceptibility tensor for film-NP configurations can be experimentally treated as a single nonvanishing element (surface normal 27,29,30 component χ(2) Therefore, we can define the PESHG nnn). enhancement factor (PESHG-EF) as M(ω , 2ω) = |L(2ω)|2 |L(ω)|4

reemission frequencies, respectively.31 The PESHG-EF represents the capability of nanostructures to amplify the SHG intensity. The relationship between reflected PESHG intensities and PESHG-EFs can be approximately given as31 ISHG ∝ M(ω , 2ω) = |L(2ω)|2 |L(ω)|4

(2)

which demonstrates that efficient PESHG intensities vary superlinearly with associated local-field distributions, differing from the SHG related to bulk SH crystals. Combining eq 1 with eq 2, we can calculate the magnitude distribution of PESHGEFs, and analyze how “hot spots” with huge local field enhancements contribute to PESHG performances at either the fundamental (excitation step) or the SH (reemission step) frequency.32 The two-step simulation resembles the processing method for evaluating the SERS EF.33 For evaluating the PESHG-EF, three-dimensional finite-difference time-domain (3D-FDTD) calculations (see Methods) have been conducted, and the result demonstrates that interparticle couplings can be very weak in the case of our film-SHIN configurations featuring

(1)

where L(ω) refers to the local field enhancement through L(ω) = Eloc(ω)/E(ω), in which Eloc(ω) and E(ω) are the local field amplitude and the electric far-field amplitude at excitation and 6718

DOI: 10.1021/acs.nanolett.5b02569 Nano Lett. 2015, 15, 6716−6721

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Figure 4. Measured PESHG intensities and PESHG-EFs of film-SHIN configurations at the specific wavelength. (a) PESHG intensities corresponding to SHINs (D = 55 nm) with different thicknesses of the silica shell laid on the smooth Au surface: g = 1 (black curve), 2 (red curve), 3 (blue curve), 4 (cyan curve), and 6 nm (yellow curve). Inset: The PESHG peak intensity as a function of the silica shell thickness. Error bars represent the deviation of the average PESHG intensity of submonolayer-SHIN systems over multiple acquisitions from different sample areas. Red line denotes the peak-valued fitting curve for PESHG nanoruler equation. (b)−(d) The 3D-FDTD simulations of film-SHIN configurations with different thicknesses of the silica shell (in logarithmic scale): (b) FW modes (785 nm); (c) SHW modes (393 nm); and (d) total PESHG-EFs. All data are normalized.

crucial to the film-NP coupling ability and relevant to material components and sizes of nanoparticles. The value of C represents the baseline, which can be corrected by the measuring range of PESHG nanoruler. It is worth noting that, the experimentally measured PESHG intensity is driven by the nonlinear polarization sources that integrates over the entire surface of the illuminated nanostructure,32 leading to the decaying trend of measured PESHG signals which appear smoother than that of previous theoretical simulations.27,35 We have also adopted DFSS (see Methods) to measure plasmon shifts as a function of g value. The configuration of DFSS with unpolarized white light source will inevitably lead to the undesirable inhomogeneous broadening of full-width at half-maximum (fwhm) because of film-NP and interparticle coupling effects (Figure 5a). Because this inhomogeneous broadening of fwhm can affect the acquisition of plasmon resonance peak positions, it is difficult to precisely convert plasmon shifts into nanoscale distances.8 We collect peak values of DFSS for various g to examine the sensitivity of nanoscale distance resolution (Figure 5b). For simplicity, wavelength-axis values corresponding to the top 20% altitude of the Peak-fwhm triangle in scattering curves are defined as the possible range of plasmon resonance peak positions in Figure 5b (detailed explanations of DFSS deviations please see Figure S5 in SI). The degree of the inhomogeneous broadening of fwhm increases as the decrease of shell thicknesses of SHINs, leading to the larger DFSS deviation. For comparisons, the PESHG peak distribution decays exponentially with lower standard deviations (approximately ±7%) as a function of g (Figure 5c). Above results demonstrate that PESHG nanorulers do exhibit their superiority in spectral accuracies and signal-to-noise ratios for obtaining the quantitative description of PESHG intensities against gap-sizes in the subtle range, enabling us to conveniently interpret the slight change of nanoscale distances. However, it should be pointed out that the quantitative relationship between PESHG intensities and nanoscale-gap sizes might become inaccurate with gap sizes in the angstrom range (0−1 nm) due to the emergence of quantum mechanical effects, that is, quantum tunneling and nonlocal effects.5,36 In summary, we have designed a nonlinear plasmon nanoruler with high sensitivities based on PESHG mechanism. Through the introduction of SHINs, the gap size between the film and NP can be accurately tuned. By analyzing the

10 nm average interparticle distances (Figure 3d). It can be observed clearly that “hot spots” are completely located in the nanogap between upper NPs and the lower film under the 785 nm laser excitation. More simulations have been performed (Figure S3 and S4 in SI) to further exclude influences of interparticle couplings. It should be pointed out that “hot spots” may transfer to other regions if different wavelength lasers are used as the pumping source.34 Calculation results also display that the electromagnetic field enhancement in the fundamental wavelength (FW) mode (|L(ω)|4) is larger than that in the SH wavelength (SHW) mode (|L(2ω)|2) by as high as 8 orders of magnitude, revealing that the fundamental resonance possesses dominating contribution to the total PESHG enhancement. To test the validity of utilizing PESHG nanorulers, we lay SHINs with different silica thicknesses (g = 1, 2, 3, 4, and 6 nm) on the smooth Au surface, respectively. The SHG intensities are recorded by manually choosing different sample areas to represent the deviation of the average performance of submonolayer-SHIN systems. A monotonic exponential attenuation trend of PESHG intensities with increasing shell thicknesses (up to 6 nm) can be observed (Inset in Figure 4a) with the pumping wavelength at 785 nm and an average power (80 mW). The largest SHG intensity occurs when NPs are placed very closely (i.e.,