Large Third-Order Optical Susceptibility with Good Nonlinear Figures

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Large Third-Order Optical Susceptibility with Good Nonlinear Figures of Merit Induced by Octupole Plasmon Resonance of Asymmetric Au−Ag Core−Shell Nanorods Xiao-Li Liu,†,* Fan Nan,‡ Yun-Hang Qiu,‡ Da-Jie Yang,§ Si-Jing Ding,‡ and Qu-Quan Wang‡,§,* †

School of Physics and Physical Engineering, Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Qufu Normal University, Qufu 273165, P. R. China ‡ Department of Physics, School of Physics and Technology, and §The Institute for Advanced Studies, Wuhan University, Wuhan 430072, P. R. China S Supporting Information *

ABSTRACT: The Au−Ag core−shell nanorods with asymmetric transverse cross section are synthesized, and their tunable linear and nonlinear optical responses are investigated. After the overgrowth of the asymmetric Ag shell on the Au nanorods, (i) a strong transverse octupole (TO) plasmon resonance at 390 nm is observed; (ii) the nonlinear refractive index γ at the TO resonance wavelength significantly increases from −0.15 × 10−4 to −0.85 × 10−4 cm2/GW, and the γ at the longitudinal dipole (LD) resonance wavelength slightly increases from −0.42 × 10−4 to −0.58 × 10−4 cm2/GW; (iii) but the nonlinear absorption coefficient β induced by the TO resonance is approximately 1 order of magnitude smaller than the one induced by the LD resonance. Consequently, the asymmetric Au−Ag core−shell nanorods at the TO resonance wavelength demonstrate a very large nonlinear refractive index as well as excellent nonlinear figures of merit (W = 4.4 > 1 and T = 0.14 < 1), which satisfy the demands for waveguide all-optical switching.

1. INTRODUCTION

Although gold and silver are usually used as singlecomponent materials owing to their different linear and nonlinear optical properties, the configurations of Au−Ag heteronanostructures have also been widely used to broaden the range of the SPR response of the single element.49−52 These Au−Ag core−shell nanostructures can also support multipole plasmon resonances by adjusting their size and shape.31 The strong multipole plasmon resonance leads to the enhancement in the nonlinear optical responses, such as second harmonic generation, two-photon luminescence, saturation absorption, and self-defocusing effect,38,39 which can be used for bioimaging and nonlinear optical information processing.45 In this paper, we synthesized Au−Ag core−shell nanorods with asymmetric transverse cross section in aqueous solution by controlling the pH value with cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride

The noble metal nanomaterials have developed rapidly because of their unique optical properties and a wide range of applications, such as plasmon-enhanced spectroscopies,1−4 bioimaging and therapeutics,5−7 chemical sensing,8−15 and optoelectronic devices.16−21 A large variety of noble material systems have been used to study the optical properties of the surface plasmon resonance (SPR).22−25 Among them, silver and gold nanostructures are most often considered owing to their strong SPR with tunable resonance wavelength. For instance, the SPR wavelength of the gold nanorods can be adjusted continuously ranging from visible to near-infrared regions by adjusting their aspect ratio. The Ag nanostructures also present fascinating SPR properties which are suitable for target functionalities in the violet region. Besides, the multipole plasmon resonance modes will appear as the size of the nanostructures increases.26−31 These properties of gold and silver nanostructures are of great research interest and have wide applications.32−48 © XXXX American Chemical Society

Received: August 5, 2017 Revised: January 30, 2018 Published: January 31, 2018 A

DOI: 10.1021/acs.jpcc.7b07801 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

calculated from the equation ωc = λF/πω0, where F = 76 MHz is the repetition frequency.

(CTAC) as the surfactant agent. We controlled the thickness of the Ag shell by adjusting the volume of silver nitrate (AgNO3). The strong transverse octupole (TO) plasmon resonance at 390 nm is observed after the overgrowth of the Ag shell on the Au nanorods. The nonlinear refractive index γ enhanced by the TO resonance is slightly larger than the one at the longitudinal dipole (LD) resonance wavelength, but the nonlinear absorption (NLA) coefficient β induced by TO resonance is approximately 1 order of magnitude smaller than the one induced by LD resonance. Therefore, the asymmetric Au−Ag core−shell nanorods at the TO resonance wavelength demonstrate a very large nonlinear refractive index as well as excellent nonlinear figures of merit (FOMs; W = 4.4 > 1 and T = 0.14 < 1).

3. RESULTS AND DISCUSSION 3.1. Growth and Plasmon Resonances of Asymmetric Au−Ag Core−Shell Nanorods. We synthesized Au−Ag core−shell nanorods by controlling the pH value of the growth solution. Within the alkaline environment, the Au−Ag core− shell nanorods exhibit asymmetric transverse cross section. The shell thickness of overgrown Ag is controlled by adjusting the amount of AgNO3. To describe the asymmetry of the cross section of the Au−Ag nanorods, we define an aspect ratio of the cross section as ρC‑S = Ly/Lx, where Lx and Ly represent the length of the Au−Ag nanorod along X and Y directions (Ly ≥ Lx). The ratio ρC‑S = 1 for the initial Au nanorods, ρC‑S > 1 for the asymmetric Au−Ag nanorods, and ρC‑S increases with the Ag shell thickness. Figure 1a shows the TEM image of the Au− Ag nanorods with asymmetric transverse cross section. The inset TEM image of a single Au−Ag nanorod shows that the Ag shell thickness is about 6.5 ± 1.0 nm on the side and 2.7 ± 1.0 nm on the end of the Au core, with a longitudinal length (Lz) of 58.0 ± 2.0 nm and a diameter of 9.5 ± 1.0 nm of the Au core. For this sample, the ratio ρC‑S is about 2.0. This result was obtained by the fast growth speed of the Ag shell because of the alkaline growth environment, higher concentration of AgNO3, and CTAC as the surfactant agent.54−56 The higher pH value making a rapid growth of the Ag shell was necessary for the growth asymmetric structure.54 In addition, the adsorption affinity of CTAC on gold surfaces was weaker than that of CTAB. The rapid formation of silver shells on Au nanorods probably originated from the dynamic and loosely packed properties of the CTAC layers on nanorod surfaces.55 Figure 1b shows the extinction spectra of the asymmetric Au−Ag core−shell nanorods with different LD wavelengths with the same sizes of Au nanorod cores. When the volume of the added AgNO3 (10 mM) is 30 μL, the LD plasmon wavelength (labeled λLD) of the Au−Ag nanorods locates at 850 nm, blue shifting about 80 nm compared with the Au core nanorods. The λLD of Au−Ag nanorods further blue shifts to 742 nm when the volume of AgNO3 increases to 100 μL. This hypsochromic shift of the LD mode compared with the Au core (i.e., λLD bule shifts from 930 to 742 nm) is due to the different dielectric functions of Ag and Au.57 The Ag shell thickness is about 2.0 ± 0.5 nm on the side of Au nanorods with ρC‑S = 1.4, when the volume of AgNO3 (10 mM) is 30 μL. When the volume of AgNO3 (10 mM) increases to 100 μL, the Ag shell thickness is about 10.5 ± 1.5 nm with ρC‑S = 3.2. With increasing the Ag shell thickness, the extinction intensity of the Au−Ag core−shell nanostructure raised continuously. There are three peaks in the extinction spectrum of the Au−Ag core− shell nanorods, which are assigned to the transverse dipole, octupole, and higher multipole plasmon resonances and are labeled TD, TO, and TM, respectively.35,58 The TD resonance intensity of the symmetric Au−Ag nanostructure is stronger than that of the TO mode.58 However, the TO resonance is stronger than that of the TD mode of the asymmetric Au−Ag nanorods.54 Figure 1c presents the linear absorption α and resonance wavelength of the TO and LD mode varying with the volume of the AgNO3 solution (i.e., the thickness of the Ag shell). As the thickness of the Ag shell increases, the λLD blue shifts from 930 to 742 nm but the TO wavelength almost remains constant; the ratio of the linear absorption at the TO and LD resonance wavelength increases from 0.10 to 0.56. This

2. EXPERIMENTAL SECTION 2.1. Synthesis of Au and Au−Ag Nanorods. The assynthesized Au nanorods were prepared using a seed growth method.53 Au seeds were synthesized by adding 0.6 mL of icecooled NaBH4 aqueous solution (10 mM) into 10 mL aqueous solution which contains HAuCl4 (0.25 mM) and CTAB (100 mM), generating a brownish solution. The seed solution was kept undisturbed for 1 h at 37 °C to ensure complete decomposition of NaBH4 remaining in the solution. The growth solution was prepared as follows: 0.247 g of sodium oleate was dissolved in 30.8 mL of water. After the sodium oleate completely dissolved, we added 19.2 mL of CTAB aqueous solution (0.2 M) and 3.6 mL of AgNO3 solution (4 mM) into it. Until the solution changed to a colorless clear liquid, we added 50 mL of HAuCl4 solution (1 mM) into the mixture at 37 °C under magnetic stirring for 90 min. A certain amount of hydrochloric acid (HCl) solution which was diluted 10 times by water and 80 μL of ascorbic acid solution (10 mM) were added into 5 mL of growth solution and mixed uniformly. Generally, the longitudinal resonance wavelength of Au nanorods changes from 872 to 1054 nm with the volume of HCl solution increasing from 180 to 300 μL. Next, 5 μL of seed solution was added into the mixture solution. The resultant mixture solution was mixed by gentle inversion for 10 s and then left undisturbed overnight. For the growth of Au−Ag nanorods, Au nanorods (2 mL), CTAB (200 μL, 0.2 M), and CTAC (300 μL, 0.2 M) were mixed and heated at 60 °C under magnetic stirring. Typically, NaOH solution (10 μL, 0.2 M) was added to adjust the pH value for the growth of asymmetric Au−Ag nanorods. After 20 min, a certain volume of AgNO3 solution (10 mM) mixed with 1 mL of water and another 1 mL of mixture solution containing ascorbic acid (50 mM) and CTAC (40 mM) was simultaneously injected drop by drop, and the mixture was stirred for 4 h. The volume of AgNO3 changed from 5 to 100 μL with its concentration about 10 mM to control the thickness of the Ag shell. 2.2. Characterization of the Samples. Transmission electron microscopy (TEM) observations were performed with a JEOL 2010 HT TEM operated at 200 kV. The extinction spectra of the samples were measured using a TU-1810 UV−vis spectrophotometer (Purkinje General Instrument Co. Ltd.). The nonlinear optical responses of the Au−Ag core−shell solution with an optical length of 1 mm were measured by open and closed aperture Z-scanning. The excitation laser of 150 fs pulse with 76 MHz repetition rate was generated by a modelocked Ti:sapphire laser (Coherent, Mira 900). The excitation laser beam waist radius before focusing is ω0 = 4.34 mm. In addition, the excitation laser beam waist radius at focus is B


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Figure 2. 3-D asymmetric Au−Ag core−shell nanostructure for FDTD simulation. Yellow and gray colors represent gold and silver, respectively. (a) cross section of the design structure. The blue line represents the surface we observe the charge distribution. (b) Longitudinal section of the design structure. (c) Calculated extinction spectrum of T-SPR of Au−Ag nanorods. (d) Calculated extinction spectrum of LD of Au−Ag nanorods. (e) Charge distribution of peak TO which indicates λTO at ∼385 nm is the octupole plasmon mode. (f) Charge distribution of λLD which indicates λLD at ∼760 nm is the LD plasmon mode.

nanorods red shifts from 872 to 1054 nm (Figure S1a). The TEM images of Au nanorods with the λLD at 800, 930, and 1150 nm which are uniform and monodispersed in aqueous solution with CTAB as the stabilizer attached on the surface are shown in Figure S1b−d. By adjusting the amount of AgNO3 in the process, we can precisely control the λLD of Au−Ag nanorods with different Au cores focused at 800 nm shown in Figure S2. 3.2. Simulations of Asymmetric Au−Ag Core−Shell Nanorods. To investigate the origination of the multipole plasmon resonances of the Au−Ag core−shell nanorods, we designed a similar structure model for simulation analysis by the finite-difference time-domain (FDTD) simulations, which are performed using FDTD Solutions 8.6 (Lumerical Solutions, Inc.). The dielectric constants of silver and gold are taken from “Handbook of optical constants of solids” edited by Palik.59 The refractive index of the environment medium is taken to be 1.33. According to the TEM images, we design an asymmetric core−shell three-dimensional (3-D) structure composed of an Au cylindrical core with a longitudinal length of 77 nm and a transversal diameter of 12 nm and an Ag shell. The Ag shell is composed of an elliptic cylinder with a longitudinal length of 78 nm and a transversal diameter of 20 nm and a cuboid with the side lengths of 78, 20, and 20 nm (see Figure 2a,b). Figure 2c−f shows the extinction spectra and charge distribution of the 3-D model. The structure exhibits three transverse peaks shown in Figure 2c. This is consistent with the results of the experiment. From the charge distribution in Figure 2e, we attribute the strongest peak of TO at 385 nm to the octupole mode. The electric transverse dipole, quadrupole, and octupole are shown in Figure S3. The mostly investigated mode is the electric

Figure 1. (a) TEM image of asymmetric Au−Ag core−shell nanorods with the λLD of the Au nanorod at 887 nm. The scale bar of the TEM image is 50 nm. Inset: A TEM image of a single Au−Ag nanorod with a scale bar of 10 nm. (b) Extinction spectra of the asymmetric Au−Ag core−shell nanorods with different thicknesses of the Ag shell. The λLD of Au−Ag nanorods at 742, 796, 826, and 850 nm with the same Au nanorod cores. (c) Linear absorption α and resonance wavelength of the TO and LD mode vary with the volume of the AgNO3 solution (10 mM).

increased absorption ratio represents stronger TO resonance induced by asymmetric Ag shells on the Au nanorods. We also prepared the Au core nanorods with a variety of sizes by adjusting the volume of HCl in the process using the seed method in the aqueous solution. With increasing the volume of HCl (from 180 to 300 μL), the λLD of Au core C


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Figure 4. (a) NLA coefficient β of different Au−Ag core−shell nanorods. (b) Nonlinear refractive index γ of different Au−Ag core− shell nanorods. The excitation wavelength λexc = λTO and λexc = λLD, respectively. The red curves represent the change of β and γ under λexc = λTO varying with the linear absorption α of TO and LD of the Au− Ag nanorods. The black curves represent the change of β and γ under λexc = λLD varying with the linear absorption α of TO and LD of the Au−Ag nanorods.

Figure 3. NLA spectra of asymmetric Au−Ag core−shell nanorods are measured by using the open-aperture Z-scan technique. The excitation power is 10 mW. (a) Excitation wavelength λexc = λTO and (c) excitation wavelength λexc = λLD. Normalized closed-aperture Z-scan transmittance divided by the open aperture of asymmetric Au−Ag core−shell nanorods. (b) Excitation wavelength λexc = λTO and (d) excitation wavelength λexc = λLD.

where q0 = βI0Leff and Δϕ0 = γkI0Leff. I0 is the peak irradiance at the focus (z = 0). Leff is the effective thickness of the samples, k = 2π/λ is the wave vector of the laser radiation, and z0 is the Rayleigh length of the Gaussian incident beam. Figure 3 shows the nonlinear transmittance recorded by the Z-scan technique. Figure 3a shows the NLA response of the core−shell nanorods using the open-aperture Z-scan technique with the excitation wavelength (defined as λexc) being equal to the TO resonance wavelength (λTO) and the excitation power being fixed at 10 mW. All the samples exhibit saturated absorption, and the transmittance of these NLA signals increased with the linear absorption α of the TO resonance. This is due to the bleaching effect of the ground state, which indicates the stronger saturable absorption (i.e., a larger |β|) of the Au−Ag nanorods. Figure 3b presents the normalized closed-aperture Z-scan transmittance divided by the open aperture (TCA/TOA) of asymmetric Au−Ag core−shell nanorods. With the increase of the linear absorption α of the TO and LD resonances, this intensity of the nonlinear signal also increased. The NLA curves are shown in Figure 3c, and the normalized TCA/TOA of the asymmetric Au−Ag core−shell nanorods at the TO and LD resonances are shown in Figure 3d. We calculated the NLA coefficient β and NLR index γ of the composite nanostructure shown in Figure 4 with different linear absorptions α. As the linear absorption α of TO and LD

dipole mode, which carries different charges at different halfsides of the rod. The octupole mode has two more nodes around the rod surface than the quadrupole mode. Because of the none-zero pure electric moment, it is observed in our investigation. The peak at 760 nm is a LD plasmon resonance according to the charge distribution shown in Figure 2f. 3.3. NLA and Refraction Properties of Asymmetric Au−Ag Nanorods. The multipole plasmon resonance of these asymmetric nanostructures induces strong nonlinear optical response.38 Nonlinear optical properties of the obtained nanorods are characterized by using a femtosecond Z-scan experiment. The excitation laser of 150 fs pulse with a 76 MHz repetition rate was generated by a mode-locked Ti:sapphire laser (Coherent, Mira 900). By a lens of 150 mm focal length, the laser pulses are focused onto a 1 mm thick quartz cuvette which contained the sample solution. The effective NLA coefficient β and the nonlinear refraction (NLR) index γ are calculated by the following relationships60 ∞

TOA =

∑ m=0

( −q0)m (1 + z 2/z 0 2)m (1 + m)3/2

4Δϕ0z /z 0 TCA =1+ 2 TOA [(z /z 0) + 9][(z /z 0)2 + 1]

(1)

(2) D


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results indicate that the Au−Ag nanorod has a stronger nonlinear effect under the TO mode excitation than that of the LD mode. This is because the local electric field of the Au−Ag nanorod is stronger at the TO mode than at the LD mode. 3.4. Nonlinear Figures of Merit. There are two parameters termed the nonlinear FOMs. One is one-photon FOM W = |γ|I/αλexc and the other is two-photon FOM T = βλexc/|γ|, where I is the laser power. The FOMs W > 1 and T < 1 are essential for the applications in waveguide all-optical switching.44,65 The asymmetric Au−Ag nanorods have a larger γ and lower β when λexc = λTO. As shown in Figure 5, the linear absorption α induced by the TO resonance is smaller than the one induced by the LD resonance, which leads to a better onephoton FOM W = 4.4; the NLA coefficient β induced by the TO mode is also smaller than the one induced by the LD mode, which results in a better T = 0.14. Therefore, two FOMs of the Au−Ag nanorods at λexc = λTO satisfy the demands W > 1 and T < 1 for optical switching.

4. CONCLUSIONS We synthesized asymmetric Au−Ag core−shell nanorods by controlling the pH value of the growth environment in aqueous solution. The asymmetric Au−Ag core−shell nanorods exhibit a strong TO plasmon resonance at 390 nm. The linear absorption α of the TO resonance increased with the Ag shell thickness. The asymmetric Au−Ag core−shell nanorods also have strong third-order susceptibity at the resonance wavelength of the TO mode. Owing to a higher NLR index and a lower NLA coefficient, the asymmetric Au−Ag core−shell nanorods at the TO resonance wavelength demonstrate excellent nonlinear FOM (W = 4.4 > 1 and T = 0.14 < 1), which satisfy the demands for waveguide all-optical switching.

Figure 5. (a) One-photon nonlinear FOM W values of asymmetric Au−Ag core−shell nanorods. (b) Two-photon nonlinear FOM T values of asymmetric Au−Ag core−shell nanorods. The excitation wavelength λexc = λTO and λexc = λLD, respectively. The red curves represent the change of W and T under λexc = λTO varying with the linear absorption α of TO and LD of the Au−Ag nanorods. The black curves represent the change of W and T under λexc = λLD varying with the linear absorption α of TO and LD of the Au−Ag nanorods. The black dotted lines represent the point of W = 1 and T = 1 in the graphs, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07801. Extinction spectra and TEM images of Au nanorods with different λLD, extinction spectra of Au−Ag nanorods that their λLD are fixed at 800 nm with different Au cores, and schematic illustration of the electric transverse dipole, quadrupole, and octupole mode (PDF)

resonance increases, the absolute value of β and γ also increased gradually. Figure 4a clearly shows that the β value at the TO resonance is much smaller than the one at the LD mode. The NLA coefficient β induced by TO resonance is approximately 1 order of magnitude smaller than the one induced by LD resonance. This weakened nonlinear saturate absorption can be explained by the more efficient excitation energy transfer between the gold core and the Ag shell in the octupole mode, which shorten the effective lifetime of excited electrons,61,62 thus causing the drop off of β. However, Figure 4b exhibits a large NLR index γ of the asymmetric Au−Ag nanorods. As the linear absorption α of TO and LD increases, the nonlinear refractive index γ increases gradually under both resonance excitation λexc = λTO and λexc = λLD. The NLR coefficient γ = −0.85 × 10−4 cm2/GW (λexc = λTO) when the linear absorption α induced by the TO resonance is about 0.46.63,64 It is 5.3 times compared to the value (γ = −0.15 × 10−4 cm2/GW) when α at the TO resonance is 0.036. The γ at the LD resonance wavelength slightly increases from −0.42 × 10−4 to −0.58 × 10−4 cm2/GW. When the λLD of the Au−Ag nanorods is 742 nm, the γ at the TO resonance wavelength is −0.85 × 10−4 cm2/GW. In addition, it is 1.46 times compared with the γ at the LD resonance wavelength (γ = −0.15 × 10−4 cm2/GW). These

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-L.L.). *E-mail: [email protected] (Q.-Q.W.). ORCID

Qu-Quan Wang: 0000-0003-0399-0612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Program on Key Science Research of China (2013CB632805), the National Natural Science Foundation of China (11604180, 11174042, and 11374039), China Postdoctoral Science Foundation (2016M602338), and the Science and Technology Plan Projects of Qufu Normal University (xkj201522). E


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