Ultrafast Optical Nonlinearities In Novel Polycyclic Aromatic Hydrocarbons And Its Optical Limiting Application

Polycyclic aromatic hydrocarbons(PAHs),as a branch of organic materals,possess vast potentials in field-effect transistors(FETs)and organic light emitting diodes(OLEDs)due to their excellent capability of electron transport and fluorescence.However,optical nonlinearity in PAHs originating from their ? conjugation are rarely studied systematically.Via researching the time-resolved spectra exicted by a tunable femto-laser source and pico-/nano-lasers,we investigate the optical nonlinearity in a series of PAHs.The structure-property relations are discussed with the help of numerical simulation and theoretical calculation.The research contains several aspects:Broadband singlet-state absorption with a long lifetime in a fluorescence emitting PAHs molecule is investigated.Base on the results,we design and synthesize a series of linear shape PAHs with symmetric molecular structures.The novel molecules inherit the original broadband singlet state absorption and have their two-photon absorption(TPA)enhanced via extending ? conjugated system s.The singlet-state absorption is successfully induced by TPA in experiment.Comparing to those optical limiting materials based on triplet-state absorption,the newly designed materials have faster responding time,which make the broadband reverse saturable absorption feasible under femtoseconds,picoseconds and nanoseconds laser sources.Employing the instant response of broadband singlet state absorption,a broadband optical limiting material is developed.The optical limiting material has symmetric linear molecular structure and can response to both femto-and pico-seconds laser pulses.Experiment results indicate that the material combine broadband response,high transmittance and strong optical limiting and each of the three properties is comparable to those most advanced optical limiting materials.Optical limiting behavior of a multi-branch molecule is tested for comparison,the difference between linear and multi-branch molecules is discussed.A couple of molecules with strong acceptor are designed and synthesized to investigate the influence of intramolecular charge transfer in the reported symmetric molecules with linear shape.The effect of charge transfer is studied via Z-scans,transient absorption/refraction,nonlinear optical mechanism is discussed with the help from theoretical calculation.Based on the knowledge of the effect of intramolecular charge transfer on nonlinear optical response.Two asymmetric pyrene derivatives are designed and synthesized.Via structure twisting between donor and acceptor group,nonlinear refraction is enhanced while nonlinear absorption is suppressed.Multi-wavelength Z-scan experiments give large figure of merits in 600 nm~1030 nm,indicating this pyrene derivative a novel broadband nonlinear refractive material.The nonlinear optical mechanism is investigated via transient refraction under on-/off-resonant excitation,providing reference for future research.