Second harmonic generation and two photon absorption assisted power limiting mechanism of l-Phenylalanine (LPA) - poly-ethylene-oxide (PEO) electrospun nanofibers

Abstract

Nanofibers consisting of organic nonlinear optical l-Phenylalanine (LPA) nanocrystals embedded in poly-ethylene-oxide (PEO) polymer were synthesized by the electrospinning technique. The preferred crystallographic orientation of the organic molecules that crystallized inside the nanofibers was demonstrated through X-ray diffraction and calculations show that the average crystallite size is around 28 nm. SEM images clearly show the formation of smooth fibers with average diameter of 246 nm. The emission of green light from the nanofiber powder sample approved the second harmonic generation (SHG) under the excitation of Nd: YAG laser (1064 nm). Further the generation of second harmonic from the fibers was confirmed by nonlinear microscope imaging and power dependent SHG studies, performed using femtosecond fiber laser (1040 nm). The nonlinear absorption and its power limiting characteristics of LPA-PEO electro-spun nanofibers were studied utilizing the Z-scan method employing Q switched Nd: YAG laser as an excitation source (532 nm, 5 ns, 10 Hz). Open aperture Z-scan measurement (with intensity 1.23 × 1012 W/m2 and 2.4623 × 1012 W/m2) revealed the reverse saturable absorption property of LPA-PEO nanofibers. The existence of a sequential two-photon absorption process involving real resonant intermediate states has been demonstrated by the intensity dependent two-photon absorption coefficient. The usefulness of LPA optical fibers as power limiters is ensured by their lower optical limiting threshold than other optical limiting materials.