Experimental Investigation of the Optical Nonlinearity of Laser-Ablated Titanium Dioxide Nanoparticles Using Femtosecond Laser Light Pulses

Abstract

In this report, the nonlinear optical (NLO) properties of titanium dioxide nanoparticles (TiO₂ NPs) have been explored experimentally using femtosecond laser light along with the Z-scan approach. The synthesis of TiO₂ NPs was carried out in distilled water through nanosecond second harmonic Nd:YAG laser ablation. Characterization of the TiO₂ NPs colloids was conducted using UV-visible absorption spectroscopy, transmission electron microscopy (TEM), inductively coupled plasma (ICP), and energy-dispersive X-ray spectroscopy (EDX). The TEM analysis indicated that the size distribution and average particle size of the TiO₂ NPs varied from 8.3 nm to 19.1 nm, depending on the laser ablation duration. The third-order NLO properties of the synthesized TiO₂ NPs were examined at different excitation laser wavelengths and incident powers through both open- and closed-aperture Z-scan techniques, utilizing a laser pulse duration of 100 fs and a high repetition rate of 80 MHz. The nonlinear absorption (NLA) coefficient and nonlinear refractive (NLR) index of the TiO₂ NPs colloidal solutions were found to be influenced by the incident power, excitation wavelength, average size, and concentration of TiO₂ NPs. Maximum values of 4.93 × 10⁻⁹ cm/W for the NLA coefficient and 15.39 × 10⁻¹⁵ cm²/W for the NLR index were observed at an excitation wavelength of 800 nm, an incident power of 0.6 W, and an ablation time of 15 min. The optical limiting (OL) effects of the TiO₂ NPs solution at different ablation times were investigated and revealed to be concentration and average size dependent. An increase in concentration results in a more limiting effect.

Keywords: Z-scan approach; femtosecond laser; laser ablation; nonlinear absorption coefficient; nonlinear optics; nonlinear refractive index; optical limiting effect; titanium dioxide.

Figure 1

Laser ablation setup for preparing TiO₂ NPs colloids via a 532-nm Nd:YAG laser.

Figure 2

Z-scan experimental setup. L, convex lens; A, attenuator; I, Iris; S, TiO₂ NPs sample; PM, power meter.

Figure 3

Spectral absorption of TiO₂ NP colloidal solutions as a function of wavelength at different ablation times.

Figure 4

(a–c) show the energy band gaps that were obtained by extrapolating the straight line of Tauc’s plot of TiO₂ nanocolloids at various ablation times.

Figure 5

(a–c) depict the size distributions of TiO₂ NPs colloids that were synthesized using various ablation times of 5 min, 10 min, and 15 min, respectively.

Figure 6

EDX spectra of the TiO₂ NP colloid and inset ZAF Method Standardless Quantitative Analysis of TiO₂ NPs.

Figure 7

(a–c) OA Z-scan measurements of TiO₂ NP colloids with different ablation times and incident powers at an 800 nm excitation wavelength. (d) Dependence of the NLA coefficient on the incident laser power at an 800 nm excitation wavelength.

Figure 8

(a–c) OA Z-scan experimental data of TiO₂ NP colloidal solutions with different ablation times and excitation wavelengths at 1 W incident power. (d) Relationship between the excitation wavelength and NLA coefficient at 1 W incident laser power.

Figure 9

(a) OA Z-scan experimental data of TiO₂ NP colloidal solutions with different ablation times and a constant excitation wavelength of 800 nm and an incident power of 1 W. (b) Dependence between the ablation time and the NLA coefficient.

Figure 10

(a–c) CA Z-scan measurements for TiO₂ NP colloids at different incident powers and ablation times at an excitation wavelength of 800 nm. The symbols represent the experimental data, and the solid curves are the fits obtained via Equations (6) and (7). (d) Relationship between the NLR index and incident power at each ablation time.

Figure 11

(a–c) CA Z-scan transmission of TiO₂ NP colloids at different excitation wavelengths and ablation times; (d) relationship values of n₂ for TiO₂ NP colloidal solutions with different ablation times at 1 W incident power. The dots represent the experimental data, and the solid curves are linear fits.

Figure 12

(a) Plot of CA Z-scan measurements of different ablation times of 5 min, 10 min, and 15 min at 800 nm excitation wavelength and 1 W incident power. (b) Dependence between the measured n₂ and ablation time at a 1 W incident power and 800 nm excitation wavelength. The dots represent the experimental data, and the solid lines are linear fits.

Figure 13

Optical limiting of the TiO₂ NP colloids at ablation times of 5 min, 10 min, and 15 min and an 800 nm excitation wavelength.