Seed-mediated Growth of Silver Nanoplates and Investigation on Their Nonlinear Optical Behaviors

Silver nanoplates have obvious advantages compared with other silver structural nanoparticles due to their wide tunability of the local surface plasmon resonance (LSPR) and significant local field enhancement around the sharp corners. Nonlinear optical absorption (NLA) and nonlinear optical refraction (NLR) are widely employed to characterize the nonlinear optical properties due to their simply testing requirement and comparable valuable results. Z-scan method is widely used in optical characterization of nonlinear properties of different materials. Here, a seed-mediated growth method of the silver nanoplates with well controlled size and local surface plasmon wavelength is presented. The nanoparticle size increases gradually and becomes more uniform distribution as the red-shift of the plasmon resonance wavelength from 650 nm to 950 nm. In particular, we show the dependence of NLA and NLR of the silver nanoplates on the LSPR. The absolute value of effective NLA coefficient increases from 1.38 to 4 cm/GW and that of the NLR index increases as large as 3 times when the LSPR wavelength changes from 728 nm to 898 nm, which is attributed to the strong plasmon absorption and local filed enhancement of larger size silver nanoplates. These findings have great potentials in the explorations of functional non-linear devices.


Introduction
Plasmon, which describes collective oscillation of electrons on the metal surface, has potential applications in plasmon enhanced photoluminescence [1][2][3], Raman scattering [4,5], and plasmon enhanced light absorption [6][7][8]. Aside from the nanoparticle materials, the plasmon resonance wavelength is strongly dependent on the physical geometrical shape, size and morphology [9][10][11][12][13]. Among the metal nanoparticles plasmonic materials, silver is one of the best candidates due to its lower optical loss in the visible and near infrared ranges. Silver nanoplates have obvious advantages compared with other silver structural nanoparticles due to their wide tunability of the local surface plasmon resonance (LSPR) and significant local field enhancement around the sharp corners, which is attributed to their higher aspect ratio between the two in-plane dipolar and the quadrupolar plasmon modes [14][15][16][17][18][19][20][21][22].
Nonlinear optical absorption (NLA) and nonlinear optical refraction (NLR) are widely employed to characterize the nonlinear optical properties due to their simply testing requirement and comparable valuable results. Z-scan method is widely used in optical characterization of nonlinear properties of different materials. The nonlinear properties of the metal nanoparticles have attracted much attention in recent years due to their potential application in biology sensors and many other areas. Huge works focus on the nonlinear properties of different composite nanoparticles, such as metal nanoparticles with various kinds of geometric shape, metal-semiconductor nanocomposite film and many other plasmon doped materials [23][24][25][26][27][28][29][30][31][32][33].
Here a seed-mediated synthesis method is adopted to fabricate silver nanoplates with widely tunable longitude LSPR wavelength ranging from 650 nm to 950 nm. A systematical investigation on the NLA and NLR dependence on the LSPR of the silver nanoplates is displayed. The NLR and NLA coefficients of silver nanoplates increase with the red-shift of LSPR, which is attributed to the increase of the strong plasmon absorption and local filed enhancement accompanied with the growing size of silver nanoplates. The absolute value of effective NLA coefficient β increases from 1.38 to 4 cm/GW and that of NLR index γ rises from 1.4 to 3.71 cm 2 /GW. These findings have great potentials in the explorations of functional non-linear devices.

Experimental
A two-step seed-mediated growth method was applied to synthesize the silver nanoplatelets. In the first step, the suspension of seeds was prepared as follows: 147 ml distilled water was mixed with 9 ml of sodium citrate (Na 3 CA) (30 mmol/ml) under magnetic stirring. Then, 9 ml polyvinyl pyrrolidone (MW~40000, 20.3 mg/ml), 0.36 ml hydrogen peroxide (30%), 3 ml silver nitrate (AgNO 3 ) (0.85 mg/ml), and 1.5 ml sodium borohydride (3.78 mg/ml) were added into the solution in sequence. The second step was beginning with taking out 10 ml as-prepared silver nanoplate seeds into a flask. Then 10 ml of aqueous solution containing Lascorbic acid (1.2 mmol/ml) and Na 3 CA (0.4 mmol/ml) were inserted into the seeds under magnetic stirring. Following was the injection of AgNO 3 solution (0.6 mmol/ml) using a syringe pump at a rate of 10 mL/h. The silver nanoplates with different LSPR wavelength were pulled out at different times.
The absorption spectra were measured using a UV-VIS-NIR spectrophotometer (Varian Cary 5000). The NLA and NLR of the samples were measured by Z-scan methods. The excitation source was a mode-locked Ti/sapphire pulsed laser (Mira 900, Coherent) with pulse width approximately 150 fs and a repetition rate of 76 MHz. The optical length of the sample is 1 mm. Figure 1 shows the absorption spectra of the silver nanoplates with different sizes. They are characterized by the linear absorption spectra during the growth of the silver nanoplates at different times. There are two in-plane dipolar and quadrupolar plasmon modes of silver nanoplates. The dipolar resonance is extremely sensitive to the height and edge length of the silver nanoplates (the peak from 650 to 950 nm). The quadrupolar resonance is weaker and also presents smaller red-shift compared to the dipole resonance.

Results and Discussion
A third plasmon resonance around 330 nm without shifting, which is contributed to the out-of-plane excitation, is also observed. The LSPR linearly red-shift and the intensity increased as the growth time goes by. While the full width at half maximum decreases with the growth time, which indicates a more uniform size of silver nanoplates as the growth time. The inserts of figure 1 present the two typical SEM images of the silver nanoplate with LSPR 650 and 950 nm, respectively. We can observe that the silver nanoplates with LSPR 950 nm display larger edge length and more uniform size distribution.   is the wave vector of the laser radiation and z 0 is the Rayleigh length of the Gaussian incident beam for typical two photon absorption (TPA) process.
is the effective thickness of the samples, with L being the length of the sample. As the sample moved into the focused beam, the transmittance is found to increase, which shows an optically induce transparency. The saturated absorption (SA) behavior is attributed to the strong ground state plasmon bleaching. Figure 2b presents the normalized CL OP T T curves of the samples, the saturated phenomenon is also obviously observed. And the NLR index γ can be calculated by using the equation (2).
The third-order NLA coefficient β and NLR index γ of silver nanoplates are calculated, and the values are plotted as a function of excitation wavelength in figure 3. The absolute value of effective NLA coefficient β is increased from 1.38 to 4 cm/GW (figure 3a). Figure 3b presents the tendency of NLR index γ with different excitation wavelengths. With the increase of excitation wavelength from 728 nm to 898 nm, the absolute value of NLR index γ rises from 1.4 to 3.71 cm 2 /GW. The increase absolute value of the NLR and NLA coefficients for different silver nanoplates attributes to the strong plasmon absorption and local filed enhancement of larger size silver nanoplates. Further studies are constructing to extent the LSPR of the samples to find the saturated absorption point since their higher aspect ratio between the two in-plane dipolar and the quadrupolar plasmon modes.

Conclusions
In conclusion, we adopt a seed-mediated synthesis method to fabricate silver nanoplates with widely tunable LSPR ranging from 650 nm to 950 nm. Moreover, we give a system investigation on the nonlinear absorption and refraction spectra dependence on the LSPR of the silver nanoplates.
The absolute value of effective NLA coefficient β is increased from 1.38 to 4 cm/GW. And the absolute value of NLR index γ rises from 1.4 to 3.71 cm 2 /GW. These findings will have potential applications in the biology sensors, solar cells and many other areas.