Second harmonic generation and reverse saturable absorption in electrospun picric Acid-PMMA nanofibers

Abstract

Picric acid (PA) – polymethyl methacrylate (PMMA) nanofibers were synthesised using electrospinning technique by optimising all the required parameters. The nonlinear optical behaviour of the synthesised nanofibers was investigated along with their preliminary characterisations like morphological, structural, thermal, mechanical, dielectric and linear optical studies. The nanofibers with average diameter around ∼800 nm, were found to crystallise in Pca21 space group under the orthorhombic crystal system. The incorporation of Picric acid nanocrystals inside the PMMA nanofibers enhanced their tensile strength. Second harmonic generation imaging of picric acid embedded PMMA nanofibers was done by nonlinear microscopy and the emission was confirmed by power dependence as well as Kurtz – Perry powder technique. Nonlinear absorption studies by Z – scan technique were conducted with a Q – switched Nd: YAG laser and the nanofibers exhibited reverse saturable absorption nature at all the input intensities (0.616 × 1012 W/m2, 1.23 × 1012 W/m2 and 2.46 × 1012 W/m2) and the value of the nonlinear absorption coefficient increased corresponding to an increase in input intensity (0.98 × 10−10 m/W, 1.39 × 10−10 m/W and 1.61 × 10−10 m/W) owing to a sequential two photon absorption. Using the transmittance data optical limiting properties of the nanofibers were also analysed and the limiting threshold values (1.92 × 1012 W/m2, 1.80 × 1012 W/m2 and 1.54 × 1012 W/m2) at respective intensities were extracted. The low optical limiting threshold obtained in the present case confirms that picric acid – PMMA nanofibers can be an assured contender as optical limiters in laser safety devices.