Gap Induced Giant Third-Order Optical Nonlinearity and Long Electron

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Surfaces, Interfaces, and Applications

Gap Induced Giant Third-Order Optical Nonlinearity and Long Electron Relaxation Time in Random-Distributed Gold Nanorod Arrays Xia Wang, Linhua Yao, Xiaodie Chen, Hongwei Dai, Mingshan Wang, Luman Zhang, Yun Ni, Lixia Xiao, and Junbo Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08935 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Materials & Interfaces

Gap Induced Giant Third-Order Optical Nonlinearity and Long Electron Relaxation Time in RandomDistributed Gold Nanorod Arrays Xia Wang,† Linhua Yao, ‡ Xiaodie Chen, ‡ Hongwei Dai, ‡ Mingshan Wang, ‡ Luman Zhang, ‡ Yun Ni, † Lixia Xiao,† and Jun-Bo Han*,‡ †School

of Mathematics and Physics, Wenhua College, Wuhan, 430074, P. R. China

‡Wuhan

National High Magnetic Field Center and School of Physics, Huazhong University of

Science and Technology, Wuhan, 430074, P. R. China * Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT：The third-order optical nonlinearities and the hot electron relaxation time () of random-distributed gold nanorods arrays on glass (R-GNRA) have been investigated by using Zscan and optical Kerr effect (OKE) techniques. Large third-order optical susceptibility ((3)) with the value of 2.5x10-6 esu has been obtained around the plamsonic resonance peak under the excitation power intensity of 0.1 GW/cm2. Further decrease of the excitation power intensity down to 0.3 MW/cm2 will lead to the significant increase of (3) up to 6.4x10-4 esu. OKE results show that the relaxation time of R-GNRA around the plasmonic peak is 13.90.4 ps, which is more than 4 times longer than those of the individual gold nanostructures distributed in water solutions. FDTD simulations demonstrate that this large enhancement of (3) and slow down of  are caused by the gap induced large local field enhancement of GNRs dimers in R-GNRA. These significant results offer plasmonic nanostructures great opportunities in applications of photonic and photocatalytic devices.

TOC GRAPHICS:

Table of Contents Graphic KEYWORDS: Gold nanorods; Surface plasmon resonance; local field enhancement; third-order optical nonlinearity; hot electron relaxations


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■ INTRODUCTION Noble

metallic plasmonic nanostructures have attracted intensive attentions more than fifteen

years, due to their important applications in enhancement and imaging substrates,1-5 nonlinear and quantum optics,6-11 ultrafast switches,12-14 and photovoltaic devices.15-17. Therefore, many works have been taken to design and fabricate different noble metallic nanostructures to meet the requirement of different applications. Strong local field enhancement and wide tunability of surface plasmon resonance (SPR) are two basic requirements for all the above-mentioned application. While large optical susceptibility and suitable slow relaxation time of hot electrons are of particular importance for the applications in nonlinear optics and photovoltaic devices. For nonlinear optical materials, large optical susceptibility could be usually achieved in metaldielectric composites with high threshold and anisotropic nanostructures with high particle densities.18-21 While for photovoltaic applications, including photovoltaic solar cells and photocatalysis, suitable long hot electron relaxation time is crucial for improving the quantum efficiencies of the energy conversions. Because it usually takes several picoseconds for hot electrons to transit or tunnel through the Schottky barrier of metal-semiconductor interfaces. The slower the hot electrons decay, the higher the conversion efficiency to be achieved.22,23 However, it is challenging to extend the time scales of hot electrons involved in plasmonic structures. So far, people know that structure shapes (size and shape) and excitation conditions (excitation wavelength and pump power intensity) are two factors that can modulate the relaxation time of hot electrons.7,21 Systematical investigations of Au triangular-prisms (GTP),19 Au nanobipyramids (GNP),20 Au nanorods (GNRs) with different diameters and aspect ratios,21 and Au-Ag core-shell nanorods (Au-Ag GNRs),8,12 demonstrate that the above mentioned parameters can only modulate the hot electron relaxation time from hundreds of femtoseconds (482 fs) to several picoseconds


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(3.8 ps).21 How to further extend the relaxation time of hot electrons in a larger time scale is still not obtained. To solve this problem, the best way is to seek for new materials or design new plasmonic structures that possess longer electron relaxation times. Recently, plasmonic dimers and trimers come into people’s eyes due to their outstanding additional tunability of optical properties compared to their individual counterparts. 24-29 By tuning the gap and geometry configuration of the elements in plasmonic dimers or trimers, the SPR wavelength and the local electromagnetic field could be modulated significantly.24-26 Furthermore, optical Fano resonance, magnetic resonance, as well as surface lattice resonances could also be realized, thus result in large optical nonlinearities. 30-35 However, limited by the small area of the functional structures in samples (hundreds of micro square meters) and the expensive fabrication cost of plasmonic arrays prepared by electron beam etching or ion etching methods, the third-order optical nonlinearity and the corresponding hot electron relaxation times of high density plasmonic dimers or trimers (coupling plasmonic nanostructures) were seldom investigated. In this work, large area (tens of square centimeters) random-distributed gold nanorods arrays (R-GNRA) deposited on glass were used to investigate the third-order optical nonlinearities and the corresponding response times of the coupling gold nanorods structures. Z-scan measurement and optical Kerr effect (OKE) results demonstrate that the third-order optical susceptibility of the R-GNRA is extremely huge under weak laser excitations, and the OKE response time is more than 4 times slower than those of the single gold nanostructures which have been characterized by the same setup previously. These results are exciting, which may bring plasmonic nanostructures much more opportunities in applications of nonlinear optics and photo catalysis.

■ EXPERIMENTAL AND NUMERICAL SIMULUATIONS


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The random-distributed gold nanorods arrays on glass (R-GNRA) was obtained from a commercial company (NanoSeedz). The synthetic process of Au nanobipyramids can be found in previous articles.24-26 Scanning electron microscopy (SEM) images were acquired by using a Nova NanoSEM 450 microscope. Optical absorption spectra were measured on an UV-VIS-NIR spectrophotometer (PE Lambda 950). The third-order optical nonlinearities were performed by using a home-made standard Z-scan setup with power set as 3 mW and lower (see Figure S1). The OKE was performed by using femtosecond (fs) optical Kerr effect technique. The schematic diagram of the OKE setup can be found in Figure S2. A Ti:sapphire laser (Coherent, Mira 900) with output pulse duration of 130 fs and repetition rate of 76MHz was used as light source. The laser beam was divided into pump and probe beams with an intensity ratio of 10:1. The pump power was set to be 200 mW (1.5 GW/cm2) for all the OKE measurements. Finite-Difference Time-Domain (FDTD) solutions were used to calculate the extinction spectra and the electric field distribution of the nanorods. The source was full field scattered light. The cell size was 2 × 2 × 2 nm3. The boundary condition here was perfectly matched the layers. In the calculations, the radius ends length of the Au nanorods was 6 nm. The Au nanorods with 37 nm diameter and total 64 nm length lied flat on the glass dielectric layer and were surrounded by the air. The dielectric constants of the materials were obtained from the built-in parameters in the FDTD software.

■ RESULTS AND DISCUSSION Figure 1 shows the scanning electron microscopy (SEM) images and the absorbance spectra of R-GNRA and GNRs solutions. Figure 1a and b present the SEM images of R-GNRA with scale bars of 2 m and 200 nm, respectively. A single layer of GNRs distributed on the surface of glass compactly without any order, and the GNRs density is about 77 rods per m2. The average length


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and diameter of the GNRs are 743 nm and 423 nm, respectively, with the average spacing of 95 nm between neighboring GNRs. Slightly aggregation of GNRs occurs and forms dimers (see the highlight area in Figure 1b) or trimers in the R-GNRA. This is of particular important, since the coupling of nanorods and the gap-induced large local field enhancement could modulate the thirdorder optical nonlinearities and hot electron relaxation processes of Au nanorods greatly. Which offer them new opportunities in plasmonic applications.

Figure 1. SEM images and absorption spectra of R-GNRA. (a) SEM image of R-GNRA on glass, the scale bar is 2 m. (b) SEM image of R-GNRA on glass with the scale bar of 200 nm, GNRs dimers and trimers could be observed clearly. (c) Absorbance and transmission spectra of RGNRA on glass. (d) Absorbance spectrum of well-dispersed GNRs in water solution.


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Figure 1c gives the absorbance spectra together with the transmission spectrum of R-GNRA distributed on glass (detailed information about the sample characterization is given in part 1 of supporting information), while Figure 1d is the corresponding absorbance spectrum of the welldispersed GNRs in water solution. From Figure 1c, three absorption peaks with peak positions of 503 nm, 583 nm, and 780 nm could be observed. Peaks 1 and 2 originate from the transverse and longitudinal SPR of the Au nanorods, respectively, which are corresponding to the two peaks shown in Figure 1d. The slightly blue-shift of the SPR peaks on glass compared to those of Au nanorods solution (see Figure 1d) is caused by the change of the dielectric constants of the surroundings.24 Peak 3 only appears in R-GNRA, indicates that the SPR peak around 780 nm is formed by the aggregation and coupling of Au nanorods on glass,24-26 this agrees quite well with the observation of GNRs dimers or trimers shown in SEM images. To check the origination of peak 3 theoretically, Finite-Difference Time-Domain (FDTD) software was used to calculate the extinction spectra of the GNRs dimers (see the data given in Figure S1). The results demonstrate that the SPR peak around 780 nm comes from the coupling of GNRs. Third-order optical nonlinearities of optical materials have broad applications in frequency conversion, optical information processing, and optical limiting, etc. Large value of third-order optical susceptibility ((3)) is a key parameter for nonlinear optical materials. Figure 2 demonstrates the third-order optical nonlinear properties of R-GNRA. Figure 2a and b show the wavelength dependent imaginary part (Im(3)) and real part (Re(3)) of (3) extracted from the nonlinear abortion (NLA) and nonlinear refraction (NLR) data of R-GNRA taken by Z-scan measurements (the setup is illustrated in Figure S2).36 The typical Z-scan curves are shown in the insets of Figure 2a and b, where the normalized transmittance T is plotted as a function of sample position z. The dots are experimental data, the solid lines are fittings. Both positive and negative


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NLA and NLR have been obtained.37 The valley-type and peak-type NLA curves represent positive and negative signs of Im(3), respectively. While the valley-peak type and peak-valley type of NLR curves represent positive and negative Re(3). By using the equations given in the part 3 of the supporting information,18 Im(3), Re(3) and (3) could be calculated. As shown in the figures, both Im(3) and Re(3) change signs around the SPR wavelengths. Im(3) is positive at the longer wavelength (lower energy) side and negative at the shorter wavelength (higher energy) side. While Re(3) is negative at the longer wavelength side and positive at the shorter wavelength side. The overall dispersion (wavelength dependent) behaviors of Im(3) and Re(3) agree quite well with the previous calculations of Im(3) and Re(3) in Ag-glass composites, and the alternations of signs in R-GNRA could also be attributed to changes of the dielectric constant induced by the creation of hot electrons.38 Furthermore, similar crossing zero behavior was also reported in coupled epsilon-near-zero (ENZ)-nanoantenna system (Au nanorod-ITO, ITO is indium in oxide), where the sign alternation is also addressed to change of efficient refraction index of ITO layer.39 It is also worth noticing that the excitation power for all the wavelength dependent Z-scan measurements was kept at 3 mW, corresponding to the peak power intensity (I0) of 0.1 GW/cm2. Such weak power intensity together with the positive NLR obtained at short wavelength side, demonstrate that thermal effect induced nonlinear optical effect could be neglected. Figure 2c presents the absolute value of (3) against the excitation wavelength around SPR peak. The value of (3) varies between 7.9x10-7 esu and 3.2 x10-6 esu, and the minima occurs around 800 nm due to the sign alternation of both Im(3) and Re(3) around SPR wavelength. Figure 2d gives the power dependent third-order optical nonlinearities of R-GNRA measured at 760 nm, 780 nm, 840 nm and 860 nm, respectively. The value of (3) are inverse proportion to the excitation power


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Figure 2. Third-order optical nonlinearities of R-GNRA. (a) Wavelength dependent Im(3) of RGNRA. The insets are NLA curves taken at 740 nm and 880 nm, respectively. The valley-type and the peak-type curves represent positive and negative NLA and Im(3), respectively. (b) Wavelength dependent Re(3) of R-GNRA. The valley-peak type and peak-valley curves represent positive and negative NLR and Re(3), respectively. (c) Wavelength dependent (3) of R-GNRA. The excitation powers for all the wavelength dependent Z-scan measurements are kept at 3 mW. (d) Power dependent (3) of R-GNRA with the excitation power varies from 10 W to 3 mW. for the curve obtained at all the four wavelengths. To understand this behavior, the data taken at 780 nm (as an example) are demonstrated and discussed in detail (see part 3 and 4 of the supporting information and the data in Fig. S3). As the excitation power decreases from 3 mW to 10 W, the


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absolute value of (3) ((3) = Im(3) + Re(3)) increases monotonically from 2.5x10-6 esu to 6.4x10-4 esu, while both q0 (Im(3)  q0/I0) and 0 (Re(3)  0/ I0) extracted from the Z-scan curves don’t change much with the variation of the power. This indicates that 10 W is sufficient to excite the third-order optical nonlinearities of R-GNRA, and further increase of the excitation power could not enhance the third-order optical nonlinearities anymore due to the saturation effect of excitation.39 When the excitation power is increased to 10 mW, higher-order optical nonlinearities or thermal effect begin to emerge (see in Z-scan curves in Fig. S4), so we only demonstrate the Z-scan results with the power no more than 3 mW in this work.

Figure 3. Time-resolved OKE signal of R-GNRA. (a) OKE curve measured at 780 nm, 800 nm, and 740 nm, respectively. The pump power is 200 mW. (b) Summarization of the relaxation decay times of different gold nanostructures, including GNRs, Au nanotriangular prisms (GTP), Au nanobipyramids (GNP) and R-GNRA. Relaxation time is another important parameter for nonlinear optical materials, especially for noble metal nanostructures, whose relaxation time is key to their applications in photovoltaic solar cells and photo-catalysis. Therefore, OKE technique was used to characterize the response decay time of hot electrons in R-GNRA. The OKE setup used here is a standard one as reported in the


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literature (see Figure S5),21 and the measurement configuration is the same to those performed in our previous works.9,12,19-21 The excitation wavelengths were chose as 800 nm, 780 nm and 740 nm, and the excitation power was fixed at 200 mW. Figure 3a presents the decay curves of RGNRA measured at different wavelengths. By using the fitting through exponential decay function (y=y0+Ae-t/), the relaxation time at the SPR wavelength (780 nm) is extracted to be 13.9±0. 4 ps, much slower than those obtained at the wavelength away from the SPR. This could be attributed to the higher electrons temperatures induced by SPR at resonance wavelength.20 Figure 3b summarizes the relaxation times of different gold nanostructures, including GNRs, Au nanotriangular prisms, Au nanobipyramids and R-GNRA. The relaxation time of R-GNRA is much slower than those of the other nanostructures (also see Table S1). Implying that R-GNRA is a good candidate of plasmonic structures for their applications in photovoltaic devices, such as plasmonic solar cells, H2 dissociation devices and photocatalysis agents, where long decay time of hot electrons are required.22,23

Figure 4. Maximum electrical field intensity enhancement factor fmax and local electrical field distributions of GNRs dimers and single GNRs. (a) Gap size dependent fmax of gold nanorod dimers assembled end-to-end with the exaction polarization parallel to the longitudinal direction of GNRs. The insets are electrical field distribution maps of GNRs dimers with the gap size of 1 nm. E and


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K represent the electrical field and wave vector of excitation light. The electrical field distribution maps of single GNR are also given for comparison. (b) Gap size dependent fmax of gold nanorod dimers assembled side-by-side, the insets are electrical field distribution maps of GNR dimers (the gap size of the map is 1 nm) with the excitation polarization parallel and perpendicular to the longitudinal direction of GNRs. To understand the physical mechanism of the generations of larger third-order optical susceptibility and the slower OKE response time obtained in R-GNRA compared to other individual gold nanostructures, electrical filed distribution and the corresponding enhancement factor fE ( fE=|E|2, where E is the maximum value of the electrical field component) of GNRs dimers were calculated by using FDTD simulations. As shown in Figure 4a, when the polarization of the excitation light is parallel to the longitudinal direction of GNRs dimers assembled end-to-end, large electrical field enhancement could be achieved. The maximum electrical enhancement factor fE_max is about 426, much larger than those obtained in single GNRs, which is only 16 at 780 nm and 57 at 580 nm (SPR wavelength). Similar results are obtained in GNRs dimers assembled sideby-side, when polarization of excitation laser is perpendicular to the longitudinal direction of GNRs, the maximum electrical field enhancement factor reaches 602, see Figure 4b. For thirdorder optical susceptibility of plasmonic materials, the effective (3) is positively related to enhancement factor fE, that’s why larger (3) could be obtained in R-GNRA which contains large amounts of GNRs dimers. While for hot electron relaxation time obtained from OKE measurements, the decay time is proportional to the initial temperature of electrons heated by Landau damping, which is also closely related to the SPR induced electrical field enhancement of GNRs.21


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Hereinbefore, we have presented the large third-order optical nonlinearity and the moderate long hot electron relaxation time of R-GNRA. To better see the significant properties of R-GNRA, third-order optical susceptibility χ(3), figure of merit χ(3)/, excitation power intensity I0, and relaxation time  for different gold and silver nanostructures are summarized in Table S1. One can see that χ(3) varies from 1.6x10-13 to 6.4x10-4 esu, χ(3)/ varies from 1.2x10-14 esucm, while  ranges from 0.5 ps to 13.9 ps for different nanostructures. Although the measurement conditions, such as laser wavelength, pulse duration, excitation peak irradiation and details of the nanostructures can influence the above-mentioned optical properties very much,40 R-GNRA demonstrates the record large values of χ(3) (maximum value of 6.4x10-4 esu), χ(3)/ (7.1x10-9 esucm), and  (longest relaxation time of 13.9 ps) within the given nanostructures, thanks to the gap-induced large local field enhancement.

■ CONCLUSIONS We have investigated the third-order optical nonlinearities and the optical Kerr response times () of R-GNRA deposited on glass. Plasmonic gap enhanced large third-order optical susceptibility χ(3) with the value of 3.9x10-6 esu has been obtained under the excitation power intensity of 0.1 GW/cm2, and huge χ(3) with the value of 6.4x10-4 esu could be obtained under the excitation of 0.3 mW/cm2. Beside, relative slow response of OKE which is related to decay of hot electrons was achieved to be 13.90.4 ps, four times slower than those of other gold nanostructures as demonstrated. These results are significant, which enable plasmonic structures have more opportunities in the application of photonic and photocatalytic devices.

■ ASSOCIATED CONTENT


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Supporting Information. Calculated extinction spectra of GNRs dimers. The scheme of Z-scan setup. Calculation of Im(3), Re(3) and (3) from NLA and NLR Z-scan curves. Power dependent third-order optical nonlinearities of R-GNRA taken at 780 nm. The schematic of OKE setup. Comparison of χ(3) and hot electron relaxation time  for different gold nanostructures.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. H.) ORCID Junbo Han: 0000-0002-5072-4897 Notes The authors declare no competing financial interests.

■ ACKNOWLEDGMENT This work was partially supported by the Innovation team of Wenhua University (2019T02), Fundamental Research Funds for the Central Universities (2018KFYXKJC011), and the Project of Scientific and Technological Innovation Team of Hubei Province (T201531). The SEM measurements were supported by the Huazhong University of Science and Technology (HUST) Analytical and Testing Center.


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