Linear and Nonlinear Optical Properties of Silver-Coated Gold Nanorods

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Linear and Nonlinear Optical Properties of Silver Coated Gold Nanorods Hongwei Dai, Luman Zhang, Zhiwei Wang, Xia Wang, Junpei Zhang, Hong-Mei Gong, Junbo Han, and Yi-Bo Han J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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The Journal of Physical Chemistry

Linear and Nonlinear Optical Properties of Silver Coated Gold Nanorods Hongwei Dai, † Luman Zhang,† Zhiwei Wang,‡ Xia Wang,

¶

Junpei Zhang,† Hongmei

Gong,§ Jun-Bo Han,*,† Yibo Han*,† †

Wuhan National High Magnetic Field Center and Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China ‡

School of Material Science and Engineering, Nanyang Technological University, 639798, Singapore. ¶

§

Wenhua College, Wuhan 430074, P. R. China

Department of Applied Physics, Nanjing University of Science & Technology, Nanjing 210094, Jiangsu, Peoples R China

ABSTRACT: Silver-coated Gold nanorods (GNRs) with large longitudinal surface plasmon resonance (SPR) wavelength tunability were fabricated by depositing silver (Ag) on the surface of GNRs. Linear and third-order optical nonlinear properties together with the ultrafast response time of these nanorods were investigated. The results demonstrate that the longitudinal SPR wavelength of GNRs is very sensitive to the thickness (tAg) of the Ag coating layer, which changes the dielectric constant of the environment. As tAg increases from zero to 15 nm, the SPR wavelength decreases 1 ACS Paragon Plus Environment


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dramatically from 840 nm to 520 nm, the corresponding wavelength dependent third-order optical susceptibility changes dependently with the changing of the SPR absorption curve, while the one-photon and two-photon figures of merits required for the optical switching applications, and the ultrafast response time also changes continuously with the varying SPR wavelength. These observations are important for the applications of plasmonic structures in ultrafast wavelength division multiplexing devices.

1. INTRODUCTION

Gold (Au) and Silver (Ag) nanostructures have received considerable attentions because of their unique properties known as localized surface plasmon resonance (SPR).1-8 This character has greatly motivated the research efforts in their applications such as ultrafast optics,9 nonlinear optics,3 photovoltaic devices,10-11 and biological sensors.12-15 In order to realize broadband applications, the SPR wavelength from near-ultraviolet to middle infrared regions is tuned by changing the geometry of the nanostructures.1, 16-17 However, it is still not easy to control the purity, homogeneity as well as the SPR modes of the nanostructures when changing the geometry of the structures in a large scale for obtaining widely tunable SPR wavelength. Recently, people observed that hybrid structures have unique advantage in controlling the SPR characters. Through changing the dielectric functions of the environment (the dielectric of the outer layer or the surroundings), introducing extra SPR modes at interfaces, and combining the SPR modes of multi-components 2 ACS Paragon Plus Environment

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together, the linear and nonlinear optical responses of Au or Ag nanostructures could be designed and improved.18-19 Some successes have been achieved in improving the third-order optical nonlinearity of nanostructures through constructing hybrid structures. Li et al. have studied the optical nonlinearity of Au/Ag core/shell nano-shuttles, and the results show that the extinction cross section and nonlinear refraction of the Au/Ag nano-shuttles could be enhanced to be 1.5 and 8.0 times of those of the original Au nanorods, respectively.20 Papagiannouli et al. have reported the nonlinear optical response of AuxAg1-x nanoalloys under 532 nm picosecond laser excitation and observed that the nonlinear response of the nano-alloys could be controlled by changing their composition.21 Kirubha and Palanisamy have synthesized Au-Ag core-shell nanoparticles by using a facile and complete green method and obtained improved third-order optical nonlinearity in the nanoparticles.22 Recently, with the technical progress of the synthetic methods for nanostructures, high uniform Au nanorods with controllable Ag coatings could be synthesized. This makes the Au/Ag core/shell systems more attractive, because the modification of Au nanorods (GNRs) by Ag coating layer or Ag nanorods by Au coating layer could result in a large tuning range of SPR modes. Several works have been done to obtain the dipole and quadrupole modes of Au-Ag hybrids recently.12,

23-28

However, most of the

researches were focusing on the linear absorption and Raman scattering applications. Few works have been done to study the nonlinear optical properties of these structures, which are very important for their applications in nonlinear optical devices.

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To find out the key factor that can dramatically affect the nonlinear optical properties of Au or Ag hybrids, Ag coated GNRs with different thickness of Ag coating layer have been investigated in this paper. The linear absorption and the SPR properties of the GNRs were characterized by using the transmission measurement. The third-order nonlinear optical susceptibility (χ(3)), as well as the ultrafast optical response of the samples were investigated by using femtosecond laser based Z-scan and Optical Kerr effect (OKE) experiments. The results show that both the linear and the nonlinear optical properties, including third-order optical nonlinearity, optical response as well as one-photon- and two-photon- figures of merit, could be tuned dramatically in the wavelength range of 840-520 nm as the thickness of Ag coating layer increases from 0 to 15 nm. The obtained broad wavelength tunability, large third-order optical nonlinearity, and ultrafast response time, make the coated GNRs have great potential applications in wavelength dependent ultrafast optics devices.

2. EXPERIMENTAL SECTION

The GNRs were prepared using a seed-mediated growth method in aqueous solutions.29-30 Four sets of the GNRs solutions were mixed separately with sodium citrate solution used as seeds to synthesize Ag coated GNRs. By adding different amount (10, 25, 50 and 120 µl) of AgNO3 solution into these mixed solutions, four sets of growth solutions were prepared. After that, ascorbic acid (AA) was adding to these solutions to start the deposition. After stirring for about 5 minutes, four sets of Ag coated GNRs solutions were synthesized.25 By comparing the absorption spectra 4 ACS Paragon Plus Environment

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taken before and after a long time of laser irradiation, the photothermal stability of the samples were tested. Transmission electron microscopy (TEM) images were acquired by using an FEI Tecnai Spirit microscope. Optical absorption spectra were recorded through a UV−Vis−NIR spectrophotometer (PE Lambda950). The third-order optical nonlinearities of these samples were obtained by using femtosecond Z-scan technique (see schematic in Figure S1), and the response time was performed by using femtosecond optical Kerr effect (OKE) technique (see schematic in Figure S2). A Ti:sapphire laser (Coherent, Mira 900) with output pulse duration of 130 fs and repetition rate of 76 MHz was used as the light source in both Z-scan and OKE setups. RESULTS AND DISCUSSION



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Figure 1. (a) Typical TEM image of the Ag coated GNRs samples. The inset shows pictures of the 5 samples. (b) Absorption spectra of the GNRs (S1) and Ag coated GNRs (S2~S5). The solid lines are experimental data and the dash lines are FDTD simulation results.

GNRs with the average length of 66±0.5 nm and diameter of 15±0.2 nm have been investigated in our experiment. The thickness of the Ag layers was controlled by adding different amount of AgNO3 in the synthesize process. A typical TEM image for the coated sample was demonstrated in Figure 1a, in which the Ag coated GNR structure can be clearly seen. Pictures of the five samples were shown in the inset of Figure 1a. The color of sample changes dramatically as the thickness of the Ag coating layer changes. The absorption spectra of the GNRs coated with different thickness of Ag layer are showed in Figure 1b (solid lines). As seen from the curves, the longitudinal SPR band blue-shifts from 840 nm to 520 nm as the thickness of Ag shell increase.25 The bandwidth of the coated GNRs became slightly narrower compared to pure Au NRs due to the plasmon focusing.31 The dash lines of figure 1b shows the simulate absorption spectra of these samples calculated by finite-difference time-domain (FDTD) solutions. In the calculation, the thickness of the Ag layer is set to be 0 nm, 2.5 nm. 5 nm. 7.5 nm and 15 nm, respectively. As demonstrated from the figure, the simulation absorption profiles are in good agreement with the experimental ones, except that a shoulder at the lower wavelength side of the longitudinal SPR band could be observed from the simulated spectrum. This is because the calculated result is based on the assumption that all the nanorods are homogeneous. Thus, the 6 ACS Paragon Plus Environment

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two longitudinal SPR bands originating from the outer Ag coating layer and the interface between the core/shell structure could be recognized. While for the experimental results, the two absorption peaks are broadened and overlapped, which make them hard to be distinguished.31

Figure 2. Electric field enhancement profiles calculated by FDTD solution along the longitudinal direction for GNR (black line), Ag coated GNRs with 5 nm (red line), and with 10 nm (blue line) Ag shells. The inset shows the resonant enhancement of local fields of these samples with mesh size of 0.2 nm. The excitation wavelength is chosen as their respective LSPR peak position.

To investigate the local electric field enhancements of GNRs, FDTD simulations were performed for all the samples. Figure 2 shows the electric field enhancement profiles of ANRs with the thickness of Ag coating 0 nm, 5 nm and 10 nm, respectively. The inset shows the distribution of local electric field of these samples.



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As shown in the figure, the coated GNRs with 5 nm Ag thickness shows the maximum local electric field enhancement, which is about 2.4 times larger than the pure GNRs and 7.0 times larger than that of GNRs with 10 nm coating layer. Therefore, large local electric field enhancement could be obtained when the Ag shell thickness is around 5 nm. As the Ag layer gets thicker, the effect of the Au core gets weaker. 32 These results are also confirmed by the Z-scan results shown in table 1. The value of increases and then decreases as the thickness of Ag coating layer increase from zero to a larger value.


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Figure 3. Three-dimensional view of Charge distribution of three typical samples. (a) GNRs without coating layer, (b) GNRs with 5 nm coating layer, (c) GNRs with 10 nm coating layer. To get an insight look on the interfaces between GNRs and Ag coating layer, the charge distribution is simulated by FDTD solution, the calculation results are shown in Figure.3. The three-dimensional charge distributions of the GNRs with coating layers (see Figure 3b and 3c) show that most of the charges distribute at the interface between NRs and surroundings, but still a few charges distribute at the interface between Au core and Ag coating layer, which indicates that the third-order optical nonlinearity and ultrafast response time of the coated GNRs structure would be different from the pure GNRs, which is experimentally studied as follows.



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Figure 4. (a) Z-scan nonlinear refraction experimental data (dots) and fitting curves (lines) of sample 2 at different excitation wavelength. The inset shows a typical nonlinear absorption data of sample 2 at 680 nm. (b) The wavelength dependent real part of the third order nonlinear optical susceptibility of the sample 2. The third-order optical nonlinearity as well as one-photon- and two-photon- figures of merit (FOM) of these samples were investigated using Z-scan measurements. To minimize the heating effect, the excitation power used in this experiment is very low. At 800 nm, the pulse energy density is around 8.2 µJ/cm2, the peak power density is about 0.06 GW/cm2. Z-scan data of all the five samples are acquired, the experimental data were given in Figure S3. The third-order nonlinear refraction data of sample 2 was given in Figure 4a. The inset shows a typical nonlinear absorption data of sample 2 at 680 nm. The hollow dots are experimental data and the solid lines are fittings. The

third-order

nonlinear

optical

susceptibility

is

expressed

as

= Re + Im , where Re comes from third-order nonlinear refraction index γ (Re = 2 γ, where is the linear refraction index, is permittivity of vacuum), Im is contributed by the third-order nonlinear absorption coefficient β (Im = β/ω, where ω = 2πc/λ). For sample 2, the real and imaginary part of at 750 nm are calculated to be 1.45 × 10 ! esu and 3.35 × 10

%%

esu. The data of both real and imaginary part of at different

wavelength of all the five samples were given in table 1. The absolute value of were given in table S1 for reference. Figure 3b shows the value of Re as a function of the excitation wavelength. As shown in the figure, the maximum peak to 10 ACS Paragon Plus Environment

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valley difference as well as the maximum value of Re appears at 750 nm, at the longitudinal LSPR wavelength (see Figure 2b) of this sample.

Figure 5. Value of Re of the samples at different wavelength. The maximum values of Re for all the samples appear around their own LSPR wavelength. Additionally, of all the five samples as functions of excitation wavelength are shown in Figure 5, the dots are calculated results based on experimental data and the dotted line is guide to the eyes. As shown in the figure, the maximum values of of the five samples appear at 800 nm, 750 nm, 730 nm, 620 nm and 580 nm, respectively, which are close to their longitudinal SPR wavelength (840 nm, 750 nm, 700 nm, 615 nm and 520 nm, see Figure 1b). It is easy to understand, because the SPR induced electric field enhancement effect could result in large third-order optical nonlinearity at their SPR wavelength.17 Therefore, by modifying the thickness of the Ag coating layer, one can control the longitudinal SPR wavelength, and then tailor the third-order optical nonlinearity of GNRs. All the real and imaginary parts of are 11 ACS Paragon Plus Environment


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presented in table 1 for reference，the maximum values of for all the five samples are marked with gray shade. Table 1. The real and imaginary parts of of sample S1~5 at different excitation wavelength. S1 (10-11 esu)

570

S2 (10-11 esu)

S3 (10-11 esu)

S4 (10-11 esu)

S5 (10-11 esu)

Re

Im

Re

Im

Re

Im

Re

Im

Re

Im

-13.6

-0.70

-8.71

-0.27

-8.04

-0.54

-32.6

-9.93

-21.9

-0.77

-9.68

-0.45

-9.5

-0.50

-15.6

-4.2

-96.60

-2.64

-0.03

-0.07

-19.5

-7.21

-48.9

-0.66

-49.6

-1.45

-0.04

-0.004

-4.48

-0.23

-38.3

-1.06

-120

-0.73

-55.1

-19.4

-13.9

-0.40

-4.14

-0.15

-37.9

-1.80

-137

-3.16

-35.9

-1.50

-10.1

-0.30

-4.08

-0.26

-109

-2.58

-122

-2.56

-44.3

-1.60

-16.0

-1.51

-5.36

-0.70

-75.2

-1.83

-46.1

-0.95

-15.5

3.20

-18.0

-0.70

-5.64

-0.12

-78.2

-4.94

-2.85

-0.002

-20

-1.48

-2.01

1.27

-3.19

-0.37

nm 620 nm 680 nm 730 nm 750 nm 800 nm 850 nm 900 nm

Also, to evaluate the advantages of Ag coated GNRs hybrids, the comparison of the optical nonlinear response between Ag coated GNRs and other published results of different nanostructures are summarized in table 2. The laser source, repetition rate, pulse duration as well as the laser intensity are also included in the table for reference. Although the experimental data of a given sample measured using KHz laser source may be much smaller than that obtained by using MHz laser source due to possible


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optical thermal effect, the comparison of the results under similar experimental conditions are still valuable. 33-34 As seen from the table, by deposing a thin layer of Ag to GNRs, the sample (sample 2 as an example) shows larger maximum Re than other kinds of Au or Ag nanostructures at their LSPR band under similar excitation conditions (Ti:sapphire laser, ~130fs, ~76MHz).

Table 2. Comparison of the nonlinear optical properties of different nanostructures measured by Z-scan techniques. Samles

Laser source

Ag coated GNRs (S2)

Ti:Sapphire laser 130 fs

GNRs (S1)

76 MHz

Au triangular35

At SPR wavelength

Ag triangular36 Au20

Ti:Sapphire laser

Au/Ag nanoshuttle20

Intensity

Reχ(3)

Imχ(3)

χ(3)

(GW/cm2)

(10-11 esu)

( 10-11 esu)

(10-11 esu)

0.06

-137

-3.16

137

0.06

-109

-2.58

109

0.43

1.25

0.01

1.25

0.4

7.3

5.8

7.5

-

-

-

9.4

-

-

-

72

-

-

-

74.2

0.23

0.0012

0.0023

0.0026

0.10

0.0007

0.0001

0.0006

2.5 ps 76 MHz 860 nm, 760 nm

Au-Ag core-shell 22

nanoparticles

Ti:Sapphire laser 160 fs 80 MHz 800 nm

21

Au nanoparticles

Au0.5Ag0.5 composite nanoalloy21

Nd:YAG laser 35 ps 1-10 Hz 532 nm

To evaluate the properties of the samples for the application of all-optical switching, two parameters, one-photon-FOM (W) and two-photon-FOM (T) can be extracted from the Z-scan experimental data. Where & = γI/αλ and ( = βλ/γ, I is the light intensity at focus, α is the linear absorption index, W>1 and T1 and T

Linear and Nonlinear Optical Properties of Silver-Coated Gold Nanorods

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