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Ultrafast Third-Order Optical Nonlinearity in Au Triangular Nanoprism with Strong Dipole and Quadrupole Plasmon Resonance Zixuan Li,†,§ Ying Yu,‡,§ Ziyu Chen,‡ Tianran Liu,† Zhang-Kai Zhou,*,†,§ Jun-Bo Han,*,‡ Juntao Li,† Chongjun Jin,† and Xuehua Wang† †

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China ‡ Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *

ABSTRACT: Au triangular nanoprisms have been prepared by the wet chemical method. By using absorption measurements and finite difference time domain (FDTD) calculations, the dipole and quadrupole plasmon resonances of Au triangular nanoprisms are investigated experimentally and theoretically. Calculations show that large electric fields are confined at the tips of the Au prisms, leading to large thirdorder optical nonlinearities. The Z-scan measurements show a third-order optical susceptibility of about 1.25 × 10−11 esu at 1240 nm, which is 19 times larger than that at 800 nm. The ultrafast light response time is about 482 fs measured by optical Kerr effect technique at 800 nm. The distinct third-order optical nonlinearities and the ultrafast response time enable the Au triangular prisms to be a good candidate for future all-optical switches and ultrafast optical information manipulators. have been widely studied;30−37 however, the ultrafast nonlinear optical property is seldom reported. Previous works on metallic nanoparticles and thin films have demonstrated that metallic nanostructures have great potential applications in all-optical manipulators, fast light response switches, and other nonlinear functional devices that benefit from their excellent third-order optical nonlinearities and ultrafast response time.38−41 Therefore, Au nanotriangular nanoprisms are also expected to exhibit excellent nonlinear optical properties in the IR region. In this article, we investigated the dipole and quadrupole plasmon modes of Au triangular nanoprisms using absorption measurements and finite difference time domain (FDTD) calculations. In addition, the third-order optical nonlinear absorptions, nonlinear refractions, and the optical response time of the samples are revealed by utilizing the Z-scan and optical Kerr effect (OKE) techniques. The results show that Au triangular prism nanostructures are good nanostructures which could be used in ultrafast optical switches in the IR wavelength region.

1. INTRODUCTION Tremendous momentum has been gained in the study of metallic nanostructures because of their ability to support surface plasmons which are collective oscillations of electron gas in a metal surface.1−5 With the ability to concentrate and propagate electromagnetic fields in nanoscale, plasmonic metallic structures are promising for the applications in negative refractive index metamaterials,6,7 photonic waveguide,8−11 and surface-enhanced Raman scattering (SERS) devices.12−15 These intriguing research discoveries not only broaden our fundamental understanding of photon−matter interaction within nanoscale systems but also help to build applications in the manipulating of light.16−18 In particular, triangular nanoprisms are a class of nanostructures that have attracted intense enthusiasm due to their unique optical properties.19−21 For example, unlike nanoparticles and nanorods whose surface plasmon resonances (SPR) wavelengths usually locate at the visible wavelength region, Au triangular nanoprisms can present their SPR peaks at near-infrared (IR) band which enable Au nanoprisms to exploit their applications in the IR cancer hyperthermia and optical communication devices.22,23 Furthermore, with the ability to confine electric fields at the sharp tips which can generate large local-field enhancements,24−26 the triangular nanoprisms are promising materials for optical waveguides and plasmonic nanoantennas.27−29 To the best of our knowledge, the linear optical properties as well as the synthetic methods of Au nanotriangular nanoprisms © 2013 American Chemical Society

2. EXPERIMENTAL METHODS Synthesis of Gold Triangular Nanoprisms. The seeded growth method was adopted to synthesize Au nanoprisms.42 The Au nanoparticle seeds were prepared by the reduction reaction in the solutions with the presence of sodium citrate Received: April 3, 2013 Revised: September 4, 2013 Published: September 5, 2013 20127

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(10 mM, 1 mL), HAuCl4 aqueous solution (10 mM, 1 mL), NaBH4 (100 mM, 1 mL), and deionized water (36 mL). To hydrolyze the unreacted NaBH4 completely, the mixed solution was reacted for a whole night. Three groups of growth solutions were prepared for the seed-mediated growth steps. Solutions A and B were identical, containing NaOH (0.1 M, 0.05 mL), ascorbic acid (0.1 M, 0.05 mL), KI (0.1 M, 0.075 mL), CTAB (0.05 M, 9 mL), and HAuCl4 (10 mM, 0.25 mL). Solution C contained NaOH (0.1 M, 0.50 mL), ascorbic acid (0.1 M, 0.50 mL), KI (0.1 M, 0.075 mL), CTAB (0.05 M, 90 mL), and HAuCl4 (10 mM, 2.5 mL). All these solutions were prepared by adding chemicals step by step in the sequence listed above. Au particle formation started from adding 1 mL of seed solution to growth solution A. The mixture was gently shaken, and then 1 mL of this mixture was added to growth solution B. After being gently shaken, the whole mixture was added to solution C. The color of the final mixed solution changed from clear to purple red in about 20 min. The water in our experiments was purified with a Millipore Milli-Q water system to 18.2 MΩ·cm. Instrumentation and Measurements. The scanning electron microscopy (SEM) images were performed by using a Zeiss Auriga-39-34 operated at an accelerating voltage of 20.0 kV. The transmission electron microscopy (TEM) graphs were performed by using a JEOL 2010HT TEM machine operated at 200 kV. The absorption spectra were recorded by an ultraviolet−visible−near-infrared (UV−vis−NIR) spectrophotometer (PerkinElmer Lambda950). The nonlinear optical measurements were performed using the femtosecond (fs) optical Kerr and Z-scan techniques. For the OKE measurement, a Ti:sapphire laser (Coherent, Mira 900) with the pulse duration of 130 fs and repetition rate of 76 MHz was used, and the wavelength was centered at 800 nm. While for the Z-scan measurements, both Ti:sapphire laser and its optical parametric oscillator (OPO, Coherent, APE OPO) were used as laser sources to obtain the laser emissions from 800 to 1240 nm. Both the open- and closed-aperture Z-scan measurements were done to get the third-order optical nonlinear absorption coefficient and refraction index. Computational Simulations. The computational simulations were performed by using the FDTD method with perfectly matched layers (PML) boundary condition. The cell size was 1 × 1 × 0.25 nm3. The edge length and thickness of the Au triangular nanoprisms were set to be 170 and 7 nm, respectively. The complex dielectric constants of gold were taken from the literature of Johnson and Christy.43

Figure 1. SEM and TEM images of Au triangular nanoprisms. (a) SEM image of Au triangular nanoprisms, showing that the average edge length is about 170 nm. (b) High-resolution TEM micrograph and selected area electron diffraction (SEAD) patterns. The lattice spacing is calculated to be 2.60 Å, and the d-spacing in SEAD patterns is measured to be 1.47 Å.

absorption spectrum of the Au triangular nanoprisms to some certain degree, but the fundamental plasmon resonance property of Au triangles cannot be changed (see results in Figure 2). Therefore, it is believed that the influence of impurity in our investigation for the properties and applications of Au triangular nanoprisms can be neglected.

3. RESULTS AND DISCUSSION The SEM image of Au triangular nanoprisms is shown in Figure 1a. The edge length of Au nanoprisms is measured to be 170 ± 30 nm, and the thickness is about 7 nm (see the side view of Au triangular nanoprisms in Figure S1 in Supporting Information). The inset picture of Figure 1a shows a more vivid TEM image of a single Au triangle nanoprism with the edge length of about 155 nm. Although the edge length of this inset Au nanoprism is smaller than the average length, it clearly illustrates a typical geometric shape of our prepared samples. Also, one can note that together with the Au triangular nanoprisms there exist a small number of Au hexagons, prisms with irregular shape prisms, and spherical particles, but the yield of Au triangular nanoprisms is estimated to be 50−70%. Furthermore, the Au particles and irregular shape prisms can only broaden the

Figure 2. Absorption spectra of Au triangular nanoprisms. The red and blue curves are experimental and simulation data, respectively.

Further details of the Au triangular nanoprisms are obtained by the high-resolution TEM micrograph and selected area electron diffraction (SEAD) patterns (see Figure 1b). The highresolution TEM results indicate that the Au triangular nanoprisms have the same crystallographic structures. The fringes are separated by 2.60 Å, which can be attributed to the 20128

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(1/3) {422} reflection that is generally forbidden for a facecentered cubic (fcc) lattice.30 The related SEAD patterns (inset picture) exhibit the hexagonal symmetrical diffraction spots, which strongly confirm the single-crystalline structure of Au nanoprisms. As the d-spacing in SEAD patterns is measured to be 1.47 Å, the hexagonal symmetry of these pattern spots indicates that these nanoprisms are single crystal bounded mainly by (111) facets.30−32 Figure 2 is the absorption spectra of Au triangular nanoprisms, and the red line is the experimental absorption data recorded by a UV−vis−NIR spectrophotometer (PerkinElmer Lambda950). Two distinct bands can be observed in the spectra: the one appearing at 532 nm comes from the dipole resonance of spherical Au particles, while the one located at 1243 nm is originated from the dipole plasmon resonance of Au triangular nanoprisms. The much larger absorption intensity at 1243 nm than that of at 532 nm implies the high yield of Au nanoprisms. Also, it should be noted that the small absorption band located at about 800 nm comes from the quadrupole plasmon mode of Au triangular nanoprisms.41 Since the uniformity of Au nanoprism size distribution is varied from batch to batch, an absorption spectrum with a more obvious quadrupole plasmon absorption band can be found in Figure S2. The blue curve in Figure 2 is the simulated absorption data of a single Au triangular nanoprism with the edge length and thickness of 170 and 7 nm, respectively. The computational simulations were performed out by using the FDTD method with commercial software of FDTD Solutions 8.0. Two absorption peaks corresponding to dipole and quadrupole plasmon modes of Au triangular nanoprisms are demonstrated at 1240 and 775 nm, respectively. From Figure 2, one can notice that the simulation data agrees well with the experiment ones. It should also be noted that the peak located at 700 nm indicates the octapole plasmon mode of Au triangular nanoprism. Because of the nonuniform size distribution of Au triangular nanoprisms and the huge difficulty to sustain high order plasmon mode, the octapole plasmon mode of Au nanoprism is hardly proved by experimental results. Thus, we focus our investigation on the application of dipole and quadrupole plasmon modes. In order to clearly indentify the plasmon mode in Au triangular nanoprisms, the electric field distributions of Au nanoprism at 1240 nm (Figure 3a) and 775 nm (Figure 3b) were calculated. During the calculation, the excitation laser beam was set to be perpendicular incidence (z-axis) and the electric field was parallel (y-axis) to Au nanoprism. From Figure 3a (at 1240 nm), field maximum points are located at the tips of the Au nanoprism while the minimum field points appear at the center of the side edges. This field distribution shows the presence of dipole plasmon mode and suggests that large field enhancements appear at the tips. From Figure 3b (at 775 nm), four field maximum points along the two side edges (b and c) can be seen, which implies the existence of quadrupole plasmon mode. A more detailed demonstration of plasmon resonance mode is illustrated by surface charge distributions which can be found in Figure S3 (including the description of octapole plasmon mode at 700 nm). Since Au triangular nanoprisms display large electric field confinement, which may lead to large third-order optical nonlinearities. Thus, the measurements of third-order optical nonlinear absorption, nonlinear refraction, and response time of the samples were carried out by the Z-scan and OKE

Figure 3. Electric field distributions of Au triangular nanoprism. (a) The dipole plasmon mode with an incident wavelength of 1240 nm. (b) The quadrupole plasmon mode at 775 nm.

measurements, respectively. During the following OKE and Z-scan measurements, a 1 mm thick quartz cuvette was used to hold the Au triangular nanoprism solution with a concentration of about 24 nmol/mL, and the absorption coefficient is 0.6 cm−1 at 1240 nm. Figure 4a shows the schematic of the OKE setup, where an ultrafast Ti:sapphire laser source with a pulse duration of 130 fs was splitted into two branches in order to generate the pump pulse and the probe light. After the operations of beam collimating, pump pulse delaying, and power adjusting, the two laser beams with pump and probe powers ratio of 10:1 were focused on the sample with a small angle. The polarization of the probe beam was rotated 45° with respect to the linear polarization of the pump beam. In addition, a lock-in amplifier with a dual slots chopper was used to improve the signal-to-noise ratio of the data (Figure 4a). A typical temporal behavior of the optical Kerr signal observed for Au triangular nanoprisms is shown in Figure 4b. The hollow dots are experimental data, while the red bold solid line is single-exponential decay fitting. The decay time (response time) of the sample is about 482 fs, which is comparable to most of the previous reports.44−46 The third20129

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Figure 4. (a) Schematic of the OKE experimental setup. M = mirror, PBS = polarized beam splitter, L = lens, D = detector, S = sample, P = polarizer, and LIA = lock-in amplifier. (b) Time-correlated OKE signal of Au triangular nanoprisms. The hollow dots are experimental data, and the red bold solid line is exponential decay fitting. The incident wavelength is centered at 800 nm.

Figure 5. Third-order optical nonlinear absorption (a) and nonlinear refraction (b) curves of Au triangular nanoprisms measured by the Zscan techniques. The open dots are experimental (Exp) data while the solid lines are theoretical fittings (Fit). The irradiation (I0) of the laser was fixed at 0.43 GW/cm2.

order optical nonlinear susceptibility χ(3) for the sample can be calculated using the equation47 ⎛ I ⎞1/2 ⎛ n ⎞2 l χs(3) = χr(3) ⎜ s ⎟ ⎜ s ⎟ r f (α) ⎝ Ir ⎠ ⎝ nr ⎠ ls

in which the valley type of curves implies that third-order nonlinear absorption here is two-photon-induced absorption. Figure 5b displays the normalized results of the closed-aperture Z-scan divided by the corresponding open-aperture one, where the peak−valley profile indicates that the nonlinear refraction is negative. The dots are experimental data, and the solid lines are theoretical fittings. By using the fitting results and the formulas in ref 48, the imaginary (Im) and real parts (Re) of χ(3) can be calculated, and the results are listed in Table 1. It is worth

(1)

where I is optical Kerr intensity, n is the refractive index, l is the interaction length, and f(α) is the absorption correction factor. Here subscripts r and s refer to the Au triangular nanoprisms and the CS2 reference, respectively, and f (α ) =

αl (1 − e

−αl

)e−αl/2

(2)

Table 1. Nonlinear Third-Order Optical Susceptibility χ(3) and Response Time of Au Triangular Prisms Obtained by the OKE and Z-Scan Measurements, Respectively

where α is the linear absorption coefficient of the Au triangular nanoprisms. The values of f(α) are calculated from the spectra data. The values of the parameters of reference (CS2) are χ(3) r = 1.0 × 10−13 esu, nr = 1.62, and lr = 1 mm. Ir is measured to be 0.48 at the same conditions as those of Au triangular nanoprisms (see Figure S4). Therefore, the third-order nonlinear susceptibility of the sample is calculated to be 1.87 × 10−14 esu at 800 nm. For the limitation of output power from OPO system, the OKE measurements cannot be taken at 1240 nm, where stronger Kerr signal may occurs. However, it is possible to use the Z-scan measurements to respectively calculate the third-order nonlinear susceptibilities χ(3) of Au nanoprisms at the wavelengths of 1240 and 800 nm so as to predict the ultrafast response property of Au nanoprism at the IR region. Figure 5 shows the normalized third-order optical nonlinear absorption and nonlinear refraction curves of Au triangular nanoprisms measured at 1240 and 800 nm, respectively.48,49 Figure 5a presents the normalized open-aperture Z-scan curves,

technique OKE (800 nm) Z-scan (800 nm) Z-scan (1240 nm)

Im(χ(3)) (esu)

−13

Re(χ(3)) (esu)

−13

χ(3) (esu)

response time (fs)

1.87 × 10−14

482

−13

1.21 × 10

6.37 × 10

6.48 × 10

1.03 × 10−13

1.25 × 10−11

1.25 × 10−11

noticing that the irradiation (I0) of the laser in the Z-scan measurements was chosen to be as low as possible (∼0.43 GW/cm2) so as to minimize the heat effect caused by the laser. That is why the transmission differences are small for the given data in Figure 5. By using samples with higher Au triangular nanoprisms concentrations and increasing the irradiation intensity, a much larger signal can be obtained (see Figure S5). 20130

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ACKNOWLEDGMENTS We thank Prof. Huanjun Chen and Ms. Yinyin Li for helpful discussions. This work was supported in part by NSFC (11204385), National Basic Research Program of China (2010CB923200, 2011TS105), the Fundamental Research Funds for the Central Universities (Grant 12lgpy45), and Fund of Education department of Guangdong Province (2012LYM_0011).

From Table 1, one can calculate that ratio of the third-order nonlinear susceptibilities at 1240 nm to that at 800 nm is about 19 under the same measuring conditions by the Z-scan measurements. This result reveals the fact that Au nanoprisms can present better optical nonlinear property in the IR region and suggests their promising applications in the design of functional IR ultrafast response nonlinear materials. It should also be noticed that the value of Re(χ(3)) measured by the Zscan technique is 1 order of magnitude larger than that of OKE method, and this can be explained by the fact that the Z-scan result originates from not only optical Kerr effect but also laserinduced self-defocusing effect.50−52 To evaluate the performance of the sample for optical-switching application, onephoton figure of merit (W) and two-photon figure of merit (T) have been calculated according to our previous reported method.49 The results show that W = 1.15 and T = 0.19 for the wavelength of 1240 nm, which satisfy the demand of W > 1 and T < 1 very well. In all, the third-order optical nonlinearity of Au triangular nanoprisms at 1240 nm makes it well-reasoned that the Au nanoprisms can be a good near-infrared ultrafast response material.

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ASSOCIATED CONTENT

S Supporting Information *

The side SEM view, surface charge distribution, and more detailed optical Kerr effect and Z-scan measurements of Au triangular nanoprisms. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSION In conclusion, we have prepared Au triangular nanoprisms with the side edge and thickness of 170 ± 30 nm and 7 ± 1 nm, respectively. From UV−vis−NIR spectrophotometer spectra and FDTD calculations, we experimentally and theoretically achieved the dipole and quadrupole plasmon resonance in the Au triangular nanoprisms, which is located at about 775 and 1240 nm and caused large electric field enhancements. It is believed that the strong plasmon field enhancements bring Au triangular nanoprisms good third-order optical nonlinear property, and make us achieve the optical Kerr response of Au nanoprisms as fast as 482 fs at 800 nm with quadrupole plasmon resonance. Moreover, the Z-scan results demonstrate that χ(3) at 1240 nm is about 19 times larger as that at 800 nm due to the large dipole plasmon resonance, which implies that the Au nanoprisms can display much better ultrafast light response at near-infrared region. The significant third-order nonlinearities and the ultrafast response time observed in this letter suggest that the Au triangular nanoprisms can greatly help to build various all-optical manipulating, fast light response switches, and other nonlinear functional devices working at the near-infrared and communication wavelength region.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86-020-84113723. *E-mail [email protected]; Tel +86-027-87792267. Author Contributions §

Z.L., Y.Y., and Z.-K.Z. contributed equally.

Notes

The authors declare no competing financial interest. 20131

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