Impact of Plasmon-Induced Atoms Migration in Harmonic Generation

Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Impact of Plasmon-Induced Atoms Migration in Harmonic Generation Liping Shi,*,†,‡,∥ Rana Nicolas,§,∥ Jose R. C. Andrade,† Willem Boutu,§ Dominik Franz,§ Torsten Heidenblut,⊥ Carsten Reinhardt,# Uwe Morgner,†,‡ Hamed Merdji,*,§ and Milutin Kovacev*,†,‡ †

Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, 30167, Hannover, Germany Centre for Quantum Engineering and Space-Time Research, 30167, Hannover, Germany § LIDYL, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191, Gif-sur-Yvette, France ⊥ Institut für Werkstoffkunde, Leibniz Universität Hannover, An der Universität 2, 30823, Garbsen, Hannover, Germany # Hochschule Bremen City University of Applied Sciences, Neustadtswall 30, 28199 Bremen, Germany ‡

ABSTRACT: Under illumination of a Ti:sapphire femtosecond oscillator, amplification of third harmonic generation by subwavelength plasmonic apertures is observed. However, the harmonic yield efficiency decays rapidly over time. In this work we investigate the physical phenomena behind the temporal attenuation of the harmonic signal. From high-resolution scanning electron micrographs and two-dimensional energy dispersive X-ray maps, we conclude that the attenuation of the third harmonic is attributed to trapping of a low-density carbon layer inside the plasmonic apertures. Furthermore, we show that the profile of the carbon deposit follows the enhanced electric near-field distribution, which indicates that the carbon atoms are transported to the field hotspot by the plasmonically enhanced optical tweezer effect. From the measurement of linear transmission spectra, we find that the dielectric constant inside the nanoholes is increased by the carbon deposit. However, numerical simulations demonstrate that the increase of dielectric constant does not reduce the electric near-field enhancement factor. Therefore, the decay of third harmonic radiation is primarily due to the strong reabsorption by the carbon deposit inside the gold-free aperture. KEYWORDS: metasurface, carbon contamination, optical trapping, nonthermal ablation, nonlinear frequency upconversion

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In order to generate detectable frequency conversion, the plasmonic nanostructures have to be exposed to an intense femtosecond laser field. Although amplification by up to 1 order of magnitude of third and even nonperturbative higher order harmonic11 generation by plasmonic metasurfaces has been widely observed, an open question remains, that is, the underlying mechanisms responsible for the progressive decay of the harmonic conversion efficiency in the strong femtosecond laser.11,12 Thermal damage is often considered as the main mechanism responsible for the harmonic signal decay.12 However, in addition to the thermal effects, the strong field induced nonthermal damage channels are also of concern, such as field ion evaporation13 and surface lattice weakening, as well as nonthermal melting.14 In our previous work we have demonstrated that the surface plasmons enhanced electron emission can result in electrostatic ablation of metallic atoms

onlinear frequency upconversion of light through harmonic generation has been extensively investigated and applied in various areas such as atto-science,1 biomedical imaging,2 and quantum information processing.3 The harmonic generation is a highly nonlinear process, which can benefit from boosting the driver intensity. The plasmonic metasurface, consisting of tailored metallic nanostructures and a supporting dielectric substrate, can significantly confine the incident light in their proximity and strongly enhance the electric field in a subwavelength dimension.4 Therefore, the plasmonic effect provides a promising platform to bring the nonlinear optics to the nanometer scale. For instance, the amplification of third harmonic generation (THG) by single gold (Au) nanoparticles, dimer Au nanoantennas and hybrid plasmonic nanostructures have been widely observed.5−8 Such nanoscale coherent sources are attractive for applications in microscopy for fast tracking label-free biological molecules and sensitive sensing of small variations in the local field enhancement, as the nonlinear optical process offers a much higher sensitivity when compared to the linear response.9,10 © XXXX American Chemical Society

Received: December 19, 2017 Published: February 8, 2018 A

DOI: 10.1021/acsphotonics.7b01560 ACS Photonics XXXX, XXX, XXX−XXX

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ACS Photonics

Figure 1. (a) Schematic of third-harmonic generation from a periodic array of nanoholes. The Au nanoholes are fabricated on a substrate of ZnO. Inset: SEM image of a unit cell of the array, showing that the nanoholes have a diameter of d = 400 nm, periods along the x- and y-axes of px = py = 600 nm. (b) Experimental measurement of third harmonic spectra and (c) signal dependence on the pumping intensity emission from bare ZnO crystal (green curve) and from the nanohole array (blue curve). (d) Numerical simulations of third harmonic spectra considering the following: bare ZnO crystal (green curve); only ZnO χ(3) with the field enhancement inside the Au-free nanohole (blue curve); and Au and ZnO χ(3) with field enhancement (red curve). (e) Temporal evolution of the third harmonic emission from the nanoholes. (f) SEM image of a nanohole array after illumination of 30 min. The red arrow indicates the location, which has been irreversibly modified by the laser illumination. Scale bar: 5 μm. Fundamental wave (FW) in (g) and third harmonic (h) transmission map of the sample as shown in (f). Note that the signal intensity in (g) is detected by decreasing the FW intensity by 5 orders of magnitude.

around the curved surface,15 which in return influences the local field enhancement. In this Letter, we report an unpresented phenomenon that contributes to the decay of the third harmonic yield. Using high resolution scanning electron microscopy, we find that the Au nanostructures are not modified by laser. Alternatively, a layer of low-density carbon is built up in the optical hotspot, which is attributed to plasmon-enhanced optical trapping, accompanying with strong field-induced dissociation and nonthermal ablation of carbon-contained molecules adsorbed on the sample surface. As a consequence, the decrease of third harmonic signal is mainly attributed to the strong reabsorption by the deposited carbon atoms.

of THG may originate from two different sources: (i) the nonlinear polarization of ZnO within the nanohole where the field is enhanced and (ii) the nonlinear polarization of the bare Au.16 In order to understand the origin of harmonic enhancement, we employ the open source implementation of MEEP to perform a nonlinear FDTD simulations of the nearfield THG radiating from the bare ZnO crystal (green curve, Figure 1d) and from the Au nanoaperture array. The nonlinear response of Au is considered with its intrinsic χAu(3) = 2 × 10−19 m2/V2 (ref 17); ZnO is characterized with a dielectric constant ε = 4 and χZnO(3) = 2 × 10−22 m2/V2 (ref 18). We consider two conditions, that is, without (assuming χAu(3) = 0) and with the nonlinear response of Au.8 We find that when assuming χAu(3) = 0, THG emission from the ZnO surrounded by the nanoholes is 3× stronger than that from the bare ZnO crystal (blue curve, Figure 1d), which already approaches to the experimental result. The case including the contribution of Au results in 1 order of magnitude higher near-field THG intensity (red curve, Figure 1d), exceeding the experimental measurement. This implies that detected far-field radiation of THG from Au may be not as strong as predicted from the near-field nonlinear simulation, which can be attributed to the following two reasons. First, in our experimental conditions, strong twophoton (1.55 + 1.55 = 3.1 eV) absorption of the intense driver will efficiently excite interband transition of electrons from the 5d- to the 6sp- bands. It has been demonstrated that the strong two-photon absorption can render the third-order nonlinear response predominantly imaginary, effectively quenching THG which is linked to the real part of χ(3) (ref 19). Second, THG generated in Au can be reflected by the Au film, giving rise to a much bigger far-field radiation angle20 with respect to the acceptance angle (∼11°) of our detection system. The THG signal rapidly decays over an irradiation time span of 30 min, as shown in Figure 1e. The attenuation of THG indicates that the plasmonic nanostructures have been modified by the strong femtosecond electric fields. Figure 1f evidence this modification of the nanostructures. We can see from the

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EXPERIMENTAL RESULTS AND NUMERICAL SIMULATIONS The laser pulses from a Ti:sapphire femtosecond oscillator are tightly focused onto a ZnO substrate partly covered with gold nanohole arrays (Figure 1a). The nanoholes have a diameter of 400 nm and periods of px = py = 600 nm (inset, Figure 1a). The nanohole array is designed to have a broad extraordinary optical transmission between 700 and 900 nm, which matches well our laser spectrum ranging from 650 to 950 nm. The forward THG transmission is collected and measured by a toroidal grating in combination with a photon multiplier. Figure 1b plots the experimentally measured photon counts of THG emission from a bare ZnO crystal (green curve) and from the nanohole array (blue curve). Please note that the intensity of THG emitted from the bare ZnO substrate is calibrated from the ratio (r) of the gold-free aperture area (π × 200 × 200 nm2) with respect to the unit cell (600 × 600 nm2) of this array, that is, r ∼ 35%. From Figure 1b one can see that the intensity of THG emission from the ZnO covered by Au nanoapertures is ∼3.5× higher than from the bare ZnO crystal. The detected photon counts scale with the cube of the pump intensity (Figure 1c), confirming that indeed the signals arise from a third order nonlinear optical process. The enhancement B


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between them clearly reveals that the Au aperture is filled by an ellipsoidal nanostructure through the interaction with the femtosecond laser. To gain further insight into this laser produced nanostructure, we analyze its top-view SEM image, that is, in the x−y plane shown in Figure 2c. This high resolution SEM image indicates that the deposit might have a relatively lower density with respect to the surrounding material. Moreover, Figure 2d depicts a cross-sectional view of the nanostructure topography in the y−z plane. The crosssectional view is made accessible by milling the sample with a Ga+-based focused ion beam (FIB). The nanohole is milled along the dashed line as shown in Figure 2b. From the crosssectional view, we find that the deposited nanostructure is not a hollow bubble. In fact, the aperture region is entirely filled by certain material. More strikingly, it is shown that the profile of the deposited material excellently complies with the electric near-field distribution. Figure 2e and f show the FDTD-based numerical simulation of the electric field distribution of a nanohole in the top- (x−y plane) and cross sectional-view (y−z plane), respectively. Localized surface plasmons result in the field enhancement inside the aperture, forming a bone-shaped optical hotspot in the x−y plane, and a cone-shaped one in the y−z plane. These shapes agree with the experimentally observed profiles of the deposited material, as qualitatively compared in Figure 2c,e and d,f. This consistency in our observations implies that the shape of the material deposit is organized by the inhomogeneous strong electric fields. We further carry out energy dispersive X-ray spectroscopy (EDS) to determine the chemical elements constituting this plasmon-induced atoms migration. The two-dimensional scan of element maps in the x−y plane (left row) and the y−z plane (right row) are shown in Figures 3. The dashed curves delineate the outline of the Au nanohole. The high-resolution EDS map reveals that the dominant chemical elements in the laser-induced nanostructure within the aperture is carbon (C, Figure 3a,b). It should be pointed that the amount of C atoms peaks in the center of the nanohole (Figure 3a). A small amount of silicon (Si, Figure 3c,d) and oxygen (O, Figure 3e,f) are also observed on the top of the carbon. Au (Figure 3g,h) and Zn (Figure 3i,j) remain confined to the original nanostructure and substrate. The absence of Au and Zn in the deposit indicates that there is no ablation of the Au nanostructure and optical breakdown of the ZnO substrate.

overview of the scanning electron microscopy (SEM) image that a ∼5 μm wide (scale bar) region of the nanohole array is modified, as indicated by the red arrow. Furthermore, we perform a far-field optical transmission scan of the attenuated near-infrared fundamental wave (FW) and the deep-ultraviolet third harmonic generated by the substrate across the entire nanohole array. For the FW scan, we decrease the peak intensity by 5 orders of magnitude, while for the THG scan we drive the process as normal, but due to the low illumination time, the nanoholes morphological changes are negligible. At the upper right corner, there exists a lower transmission area for both the FW (Figure 1g) and the THG (Figure 1h) spectral regions. Due to the diffraction limit, the optical maps appear as having a larger damaged area with respect to the SEM image (Figure 1f). In this area, the transmittance of the FW photons is reduced by 40% compared to the pristine nanoholes. The decay of the THG is slightly stronger, with the attenuation factor up to 65%. The SEM image as well as the optical transmission maps confirms that, indeed, structural modification has occurred in the nanoholes due to the illumination of an intense femtosecond laser. Figure 2a shows a high resolution perspective-view SEM picture of an unused nanohole, while Figure 2b displays another nanohole after 30 min of irradiation. The comparison

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DISCUSSIONS We attribute the Si, O and C elements to the possible inorganic as well as organic contaminations on the Au film, because a typical metal film is frequently not pure enough, given that a high amount of inevitable contaminant sources are present during the sample fabricating process, as well as within experimental environments.21 The hydrocarbon molecules, such as oil vapors from the machining process or vacuum pump oil vapors will adsorb on the metal surface. Although the samples have been carefully cleaned prior to measurements, it is not possible to completely remove these contaminants. During the interaction with an ionizing radiation source such as the strong laser field, carbon atoms are generally produced and stick onto the metal surface when the hydrocarbons encounter a dissociation process.22 The dissociation process is strongly related to the photon flux; therefore, most of the carbon atoms will be generated at the curved surfaces of the Au nanohole, where the electric near-field is enhanced.

Figure 2. Perspective-view (tilt angle 45°) SEM images of a representative nanohole as prepared (a) and after femtosecond laser illumination (b). Apparent structural modification within the aperture is shown. (c) Top-view in the x−y plane and (d) cross-sectional view in y−z plane SEM images of the laser-modified nanohole. Please note that the cross-sectional image is obtained by milling the nanohole along the dashed line in (b), that is, along the y-axis (at x = 0). The interface between Au film and ZnO substrate is defined as z = 0. FDTD-based numerical simulation of electric near-field distribution of the nanohole in (e) the x−y plane at z = 20 nm and in (f) the y−z plane at x = 0. C


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mechanisms, the ablation rates logarithmically scale with the laser field strength. On plasmonic nanostructures, the electric field is highly concentrated near the surface layer. The skin layer effect, therefore, cannot be directly employed to interpret the ablation at the plasmonic metasurface. Alternatively, the near-field effect has to be considered in this case. Figure 4a plots the

Figure 4. (a) Carbon deposit volume vs the electric near-field strength. Insets (I−V): cross-sectional and side-view SEM images of the nanoholes illuminated by different field strengths. (b) Numerical simulation of the potential energy distribution of the plasmonically enhanced optical tweezer. Zone I shows two minimum potential wells near the Au curved surface and zone II depicts a local minimum potential surface in the center of the hole. (c) Experimentally measured extraordinary optical transmission of the pristine (blue curve) and the carbon-deposited (red curve) nanoholes.

Figure 3. Two-dimensional EDS maps of a laser-illuminated nanoaperture in top-view (a), (c), (e), (g), and (i) and in crosssectional view (b), (d), (f), (h), and (j). The corresponding elements of the rows from top to bottom are C, Si, O, Au, and Zn, respectively. The dashed curves depict the profile of the Au structure. The EDS maps are in different color scales.

experimentally measured volumes of the carbon deposit inside the nanohole, that is, the ablation rate versus the near-field strengths. It is shown that the deposit size exponentially increases with the field strength. This already excludes the femtosecond ablation mechanisms established for the bulk material as the primary ablation channel in our experimental conditions. The exponential dependence of the ablation rate on the field strength implies that the near-field ablation might be triggered by the near-field enhanced electron emission. As predicted by the Fowler-Nordheim equation, the electron emission probability exponentially scales with the field strength,25

Subsequently, the laser ablation effect will drive a portion of the formed carbon atoms to escape from the metal surface. Although the mechanisms of femtosecond laser-induced bulk material ablation have been well established in the past decades,23 the mechanisms of the nanoscale ablation at the plasmonic nanostructure surface are still under investigation.12 Femtosecond ablation mechanisms of bulk material are closely related to the averaged electron energy in the skin layer within the electron−ion energy transfer time (a few picoseconds). Two ablation channels, thermal effects at a low electron temperature and nonequilibrium electrostatic effects at higher temperatures, are frequently observed.24 For both ablation D


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ACS Photonics ⎛ ⎞ 8 2qeme 3/2⎟ Φ ⎟ t(E) = exp⎜⎜ − ⎝ 3 ℏβE0 ⎠

transmission from the pristine nanoholes (blue curve) is observed. For the nanoholes with carbon deposit, the optical transmittance (red curve) is attenuated. Meanwhile the plasmonically resonant wavelength is slightly red-shifted with respect to the pristine one. This indicates that the produced carbon behaves as a lossy dielectric medium, which increases the dielectric constant inside the nanoholes.29 To investigate the influence of dielectric constant variation on field enhancement, we simulate the near-field distribution around the gold nanohole including a semiellipsoid-shaped carbon nanostructure. The center of the ellipsoid is located at the interface between gold film and ZnO substrate. The major-axis (a) and minor-axis (b) of these ellipsoids are defined as a = 50 nm, b = 50 nm (Figure 5a), a = 100 nm, b = 80 nm (Figure 5b), a = 200

(1)

Here me and qe are electron mass and charge, respectively, ℏ is the Planck constant, and Φ denotes the work function of a given material. In our experimental conditions, the applied electric field E0 is enhanced by the localized surface plasmons with a peak enhancement factor of β = 11.4 (Figure 2e). The ejection of numerous electrons results in a positively charged surface. Thus, the resultant Coulomb repulsive force can drive the adsorbed carbon ions to escape from the Au film.14 This process is termed as a nonthermal electron-driven ablation.26 Assuming the ablation rate is proportional to the electron emission probability, we find that the volume(v) of the carbon deposit (dots in Figure 4a) can be well fitted (blue curve in ⎛ 8 2q me ⎞ Figure 4a) by eq 1, that is, v(E0) = α ·exp⎜ − 3 ℏβEe φ3/2⎟. ⎝ ⎠ 0 Here the free parameters α denotes the normalization factor and φ denotes the work function of carbon, which is evaluated to be 3 eV by the nonlinear fitting procedure. After ablation, new carbon containing molecules present in the residual gas in the chamber will adsorb on the surface again, and the molecules on the Au film will also diffuse to the field enhancement area. Therefore, the dissociation and ablation represent a continuous process over the laser irradiation time. The ablated carbon atoms will not randomly deposit on the ZnO substrate surface, as the surface plasmons enhanced optical tweezer effect will subsequently act on these atoms.27 Figure 4b shows the two-dimensional potential energy distribution resulting from the illumination of an Au nanohole. The atoms would preferentially move to the minimum point at the potential surface,28 which is close to the Au surface (zone I, Figure 4b). However, the charged surface induced electrostatic repulsive force also peaks in this area, which strongly restrains the accumulation of atoms. Therefore, the carbon atoms would rather start to accumulate from the center of the nanohole, where exists a local minimum potential energy (zone II, Figure 4b), allowing for stable trapping of the atoms. Indeed, from the high resolution SEM images (inset I, Figure 4a), one can see that the carbon deposit starts to appear in the center of nanohole. The two-dimensional EDS map (Figure 3a) shows that the amount of carbon atoms peaks in the central region. After long-term laser illumination, the entire optical hotspot inside the nanohole is finally filled by carbon (Figure 2c and inset V in Figure 4a). Due to the high optical potential in zone III (Figure 4b), carbon atoms are not observed in these regions (Figures 2c and 3c). Although the accumulated carbon layer has a thickness of ∼120 nm (Figure 2d), we can still clearly see underlying Zn from the EDS map (Figure 3e). This further confirms that the carbon deposit has a rather low density so that the electron beam of the EDS can pass through the carbon layer and enter the ZnO substrate. In the nanometer regime, the nonlinear harmonic conversion process greatly depends on the near-field enhancement, which is significantly sensitive to the local conditions, including the geometry and the surrounding dielectric constant. From the high-resolution SEM images and EDS maps we already showed that the geometry of the Au nanostructure is not modified by the laser exposure. In order to investigate the influence of the carbon deposit to the local field enhancement, we compare the linear transmission spectra of the pristine and carbon-filled nanoholes, as shown in Figure 4c. An extraordinary optical

Figure 5. FDTD-based numerical simulations of the near-field enhancement around the nanohole with carbon deposit. The shape of carbon nanostructure is defined as a semiellipsoid, with its center at the interface between gold film and ZnO substrate. The major-axis and minor-axis of these ellipsoids are defined as (a) 50 nm, 50 nm; (b), 100 nm, 80 nm; (c) 200 nm, 150 nm; and (d) 300 nm, 200 nm, respectively.

nm, b = 150 nm (Figure 5c), and a = 300 nm, b = 200 nm (Figure 5e), respectively. The effective dielectric constant of carbon at 800 nm is set to 2 (ref 30). When the carbon volume is very small (Figure 5a), we find that the field enhancement factor (β = 11.6) approaches to the case without carbon deposit (β = 11.4, Figure 2e). With the gradual accumulation of carbon, the field enhancement factor slightly increases from 11.4 to 12.6 (Figure 5d). Therefore, the carbon deposit does not reduce the field enhancement factor and thus THG conversion efficiency. Nevertheless, the progressive increase of carbon atoms inside the nanoholes will strongly raise the reabsorption of THG radiation from ZnO, which is attributed to be the main reason for the temporal decay of detected harmonic signal.

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CONCLUSION AND OUTLOOK In this work, we have demonstrated the trapping of carbon atoms into the plasmonic nanoapertures resulting from the illumination with a strong femtosecond laser. The underlying mechanism of carbon deposit is attributed to carbon containing molecules cracking and electron-driven nonthermal ablation combined with the optical tweezer effect. The progressive decay of the THG emission is ascribed to the strong absorption of E


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ACS Photonics carbon atoms in a broad spectral region. Our findings might provide a reasonable explanation of the open question in ref 11 raised by Vampa et al. While the near-field enhanced atom migration limits the applications of extremely nonlinear nanophotonics, the surprisingly observed optical tweezer effect enables a high resolution mapping and visualization of the three-dimensional nanoscale near-field distribution. Our findings also provide a method for self-organized injection of materials into the plasmonic hotspot, which opens new perspectives for the fabrication of nanoscale metasurfaces. As a future prospect, we foresee applications in atoms trapping using shaped and controlled plasmonic fields.

interface. During the THG simulations, after the source has turned off, it keeps running for an additional 50 time units until the square amplitude has decayed by 10−8 from its peak, to ensure that the Fourier transforms have converged. SEM Images and EDS Analysis. The SEM images were performed by Focused Ion Beam-Scanning Electron Microscopes (FIB-SEM, Zeiss Auriga). Note that the ablated carbon atoms ejected from the metal surface fill up the whole aperture progressively with accumulated laser irradiation. However, due to the stronger secondary electrons emission at the edges of material, the outlines are seen to be much brighter. The EDS element maps were performed on the same device, equipped with an Oxford X-Max80-detector.

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METHODS Experimental Setup. Using focused ion beam (at CSNSM, Orsay, France), subwavelength apertures are fabricated by patterning a 40 nm Au film deposited on a ZnO crystal. The thickness of the ZnO substrate is 500 μm. The nanoholes have a diameter of d ∼ 400 nm, which are arranged in square arrays of 50 μm × 50 μm, with a spacing period of px = py = 600 nm in both x- and y-direction. The sample is placed in a chamber filled with atmospheric nitrogen. The laser pulses from a femtosecond oscillator with a repetition rate of 100 MHz, average power of 100 mW, and duration of 8 fs are tightly focused either onto the Au nanoholes or onto the ZnO crystal. A pair of fused silica wedges combined with double-chirped mirrors are utilized to compensate the dispersion introduced by the air, the substrate and the 2 mm calcium fluoride optical window of the chamber. Using z-scan method, the beam waist at the focal spot of the incident laser on nanoholes is measured to be ∼6.5 μm. The laser peak intensity is ∼4 × 1011W/cm2. The laser is polarized along the x-axis. Linear Extinction and THG Spectroscopy. The ultrabroadband Ti:sapphire femtosecond oscillator is utilized as a white light source to enable an in situ linear transmission measurement. Two neutral filters are used to attenuate the power by 5 orders of magnitude. Two lenses are used to collect and focus the transmitted laser beam on a fiber spectrometer (Ocean Optics maya 2000). The transmission of the laser beam from a large gold-free area is defined as a reference signal. The forward transmission of the THG signal is monochromatized and focused by a toroidal grating on a photon multiplier (PMT) capable of single photon counting (Hamamatsu H8259-09), used in combination with a photon counter (Scientific Research SR400) and an exit slit. FDTD Simulations. We perform 3D-FDTD simulations using the open source implementation of MEEP. The size of the simulated lattice is 600 nm × 600 nm × 400 nm (x × y × z), with a spatial resolution of 5 nm. Periodic boundary conditions in the x- and y-direction mimic the nanohole array used experimentally are adopted to reduce the computation time. Perfectly matched layers (30 nm) in the z-direction are used to avoid numerical reflections of the incident field. The geometry of the nanostructure is designed to best match the experimental case. A broadband (700−1000 nm) Gaussian light source centered at 830 nm, polarized in x-axis and propagated along the z-direction is placed in the ZnO substrate with a distance of 100 nm (effective skin depth) below the Au−ZnO interface. The electric field vector is monitored using 2D monitors both in the x−y (at position of 20 nm above the substrate) plane and in the y−z (x = 0) plane. The near-field THG spectra are monitored in the transmission geometry, that is, in air, with a distance of 100 nm above the Au−ZnO

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liping Shi: 0000-0002-1998-6805 Author Contributions ∥

These authors contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the funding supports from Deutsche Forschungsgemeinschaft (DFG; KO 3798/4-1), Centre for Quantum Engineering and Space-Time Research (QUEST), from Lower Saxony through “Quanten-und Nanometrologie” (QUANOMET, Project Nanophotonik), from the European Union through the VOXEL FET Open, from the French Ministry of Research through the ANR grants “NanoImagine”, “IPEX”, “HELLIX”, PACHA and from the C’NANO research program through the NanoscopiX grant, and the LABEX “PALM” through the grants “Plasmon-X” and “HILAC”. We acknowledge the financial support from the French ASTRE program through the “NanoLight” grant and the support from the DGA RAPID program through the “SWIM LASER” grant. L.S. is grateful to Dr. Juemin Yi from Oldenburg University for stimulating discussion.

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Impact of Plasmon-Induced Atoms Migration in Harmonic Generation

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