Defect-assisted wavelength dependence of one photon and multiphoton absorptions in a composite nanofiber of polyvinylpyrrolidone and hexagonal boron nitride

Abstract

Polyvinylpyrrolidone/hexagonal boron nitride (PVP/hBN) composite nanofibers were produced by the electrospinning method at 4, 6, and 8 wt% concentration of hBN in an effort to examine the effects of defect distribution and density on the nonlinear absorption and optical limiting behavior. Open aperture (OA) Z-scan experiments were performed at 1064, 532 and 355 nm under various input intensities. While scanning electron microscopy revealed homogeneous and cylindrical nanofibers for each composition, the average diameter of the cylindrical nanofibers decreased from 349 nm to 296 nm with increasing hBN concentration. The linear absorption results showed an increase in the defect states and distribution in the energy band gap with the addition of hBN. All composite nanofibers have a positive nonlinear absorption (NA) signal and decrease with increasing input density at 1064 and 532 nm due to the filling effect of defect states with one photon absorption (OPA). While an increase in β_eff and I_SAT values is obtained with increasing hBN concentration at all excitation wavelengths, a more pronounced NA behavior is observed at 1064 nm. OPA is the dominant mechanism due to the defect distribution inside the band gap of PVP at 1064 nm with significant contribution from sequential and simultaneous two photon absorption. Although similar NA mechanisms occurred at 532 and 355 nm, excited state absorption from defect states of PVP to defect states of hBN in the band gap led to higher nonlinear absorption behavior compared to 532 nm. The lower onset optical limiting threshold was obtained to be 0.58 mJ cm⁻² at 1064 nm for PVP nanofibers with 8 wt% of hBN. The broad absorption spectrum, strong NA behavior and low limiting threshold properties make PVP/hBN composite nanofibers good candidates as effective optical limiters in a wide spectral region.