Tunable nonlinear optical properties in polyaniline-multiwalled carbon nanotube (PANI-MWCNT) system probed under pulsed Nd:YAG laser

Abstract

The investigation of the nonlinear optical (NLO) properties of polyaniline (PANI) and polyaniline doped with multi-walled carbon nanotube (PANI-MWCNT) in both solution and film forms was conducted using the open-aperture z-scan approach. A Q-switched resonant Nd: YAG laser with a wavelength of 532 nm and two different fluences was utilized in this study. Based on the z-scan experiments, it was observed that the NLO behaviour of PANI and PANI-MWCNT composites exhibited a transition from reverse saturable absorption (RSA) to saturable absorption (SA) when the samples changed from a liquid to a film state. Two photon absorption (TPA) and thermally induced nonlinear scattering are the main mechanisms behind the RSA behaviour of the materials in solution form. The reason for SA behaviour of these materials in film form is attributed as the ground-state free electron bleaching in the conduction band due to the increased laser intensity. Due to increased structural disorder and the defect states or trap levels created by the dopants in the PANI system, the NLO properties of the PANI-MWCNT composite in both solution and film forms have significantly improved compared to those of pure PANI. Urbach tail analysis and carbon cluster determination revealed the presence of defect states in the composites system. The composite of PANI and MWCNT was verified using Fourier transform infrared (FTIR) spectroscopy. The functional elements present in the composites are verified by X-ray photoelectron spectroscopy. The NLO parameters of the samples, viz., the nonlinear absorption coefficient (β), the imaginary part of the nonlinear susceptibility ${\chi }_i^{\left(3\right)}$ , and the figure of merit (FOM) of PANI-MWCNT, exhibited a notable enhancement in their values over PANI. Moreover, the PANI-MWCNT film demonstrated superior SA performance and the PANI-MWCNT solution demonstrated better OL performance compared to that of the PANI film and solution respectively. Also the effects of dopant induced structural modifications and lattice disorder on the NLO behaviour of these materials are correlated with the obtained NLO parameters. The tunable NLO properties accomplished by controlling the structural phase and dopant-induced defects enhance the possibility of these nanostructures for applications in optical limiting, optical switching, optical modulation, optical pulse compression and laser pulse narrowing.