Excitation Wavelength and Colloids Concentration-Dependent Nonlinear Optical Properties of Silver Nanoparticles Synthesized by Laser Ablation

Abstract

We reported experimental results from investigations that employed the Z-scan method to explore the dependence of silver nanoparticles' (AgNPs) nonlinear optical properties on the excitation wavelength, AgNP concentration, and size. Using a 532 nm Nd: YAG laser beam at 100 mJ/pulse for different ablation times, AgNPs were synthesized from a silver target immersed in distilled water. UV-Vis spectroscopy and an atomic absorption spectrometer are used to characterize the optical properties of laser-synthesized AgNPs as well as their concentrations. The AgNPs' size and shape are determined using a transmission electron microscope (TEM). The laser-synthesized AgNPs are spherical, with an average particle size of 12 to 13.2 nm. Whatever the ablation time, the AgNP colloids exhibit reversed saturable absorption and a negative nonlinear refractive index (n₂). Both n₂ and the nonlinear absorption coefficient (α₃) increase as the AgNP concentration increases. As the excitation wavelength and average size of the AgNPs increase, n₂ and α₃ decrease.

Keywords: Z-scan; laser ablation; nonlinear optical properties; silver nanoparticle.

Figure 1

Schematic diagram of laser ablation experimental setup. L, convex lens; M1, M2, and M3, highly reflecting mirrors.

Figure 2

Setup for Z-scan experiment.

Figure 3

UV–Vis absorbance spectra of AgNP colloids at constant ablation energy of 100 mJ/pulse and at different ablation times.

Figure 4

Size distribution of AgNPs synthesized at various ablation times. Insets show TEM images of AgNPs. (a) 5 min laser ablation time (b) 10 min laser ablation time (c) 15 min laser ablation time.

Figure 5

Tauc’s plot extrapolation to determine the bandgap of AgNP colloids for different ablation times.

Figure 6

Dependence of bandgap energy on average size of AgNP colloids.

Figure 7

Dependence of AgNP concentration and size on ablation time.

Figure 8

OA Z-scan measurements for AgNP colloids at constant excitation power of 1 W, different excitation wavelengths ranging from 740 to 820 nm, and at different AgNP concentrations of (a) 3.35 mg/L, (b) 6.74 mg/L, and (c) 7.38 mg/L.

Figure 9

Dependence of α₃ on excitation wavelengths at constant AgNP concentrations and at constant excitation power of 1 W.

Figure 10

CA Z–scan transmission for AgNPs at 1 W excitation power, different excitation wavelengths between 740 and 820 nm, and various AgNP concentrations of (a) 3.35 (mg/L), (b) 6.74 (mg/L), and (c) 7.38 (mg/L).

Figure 11

Variation of n₂ as a function of excitation wavelength at different AgNP concentrations.

Figure 12

Dependence of α₃ and n₂ on AgNP colloid concentration at constant excitation power and wavelength of 1 W and 800 nm, respectively.

Figure 13

Dependence of n₂ and α₃ on average particle size at constant excitation power and wavelength of 1 W and 800 nm, respectively.

Figure 14

Dependency of three-photon absorption cross-section of AgNP colloids on excitation wavelength at constant AgNP concentrations and constant excitation power of 1 W.