We report on controllable pulse shaping in a Yb-doped stretched-pulse fiber laser followed by a high-power chirped pulse amplifier. We demonstrate that the pulses after an extra-cavity grating pair change their intensity profile from Lorentz to Gaussian and then to sech2 shapes by adjusting the intra-cavity polarization through a quarter-wave plate inside the fiber laser cavity. The laser pulses with different pulse shapes exhibit pulse-to-pulse amplitude fluctuation of -- 1.02%, while the sech2-shaped pulse train is provided with a more stable free-running repetition rate as a result of the stronger self-phase modulation in the fiber laser cavity than Lorentz- and Gaussian-shaped pulse trains.
We demonstrate spectral modulation of third-harmonic generation from molecular alignment effects. The third harmonic spectrum is broadened or narrowed under different influences of cross-phase modulations originating from various molecular alignment revivals. Furthermore, the spectrum and spatial distribution of the generated third harmonic pulse change dramatically in the presence of a preformed plasma. Under the influence of a preformed plasma, a narrower third harmonic spectrum is observed, and the conical third-harmonic pulse increases while the axial part decreases. The investigation provides an effective method to modulate the spectral characteristic and spatial distribution of third-harmonic generation from intense femtosecond filament.
Terahertz generation driven by dual-color filaments in air is demonstrated to be remarkably enhanced by applying an external electric field to the filaments. As terahertz generation is sensitive to the dual-color phase difference, a preformed plasma is verified efficiently in modulating terahertz radiation from linear to elliptical polarization. In the presence of preformed plasma, a dual-color filament generates terahertz pulses of elliptical polarization and the corresponding ellipse rotates regularly with the change of the preformed plasma density. The observed terahertz modulation with the external electric field and the preformed plasma provides a simple way to estimate the plasma density and evaluate the photocurrent dynamics of the dual-color filaments. It provides further experimental evidence of the photo-current model in governing the dual-color filament driven terahertz generation processes.
Response theory is used to investigate one- and two-photon absorption（TPA） as well as the emission properties of a series of potential zinc ion and pH sensitive materials containing 2-（2＇-hydroxyphenyl）benzoxazole（HPBO） end groups.Special emphasis is placed on the evolution of their optical properties upon combining with zinc ions or deprotonation.Our calculated results indicate that upon combining with zinc ions or deprotonation,these HPBO derivatives show drastic changes in their one-photon absorption（OPA）,emission,and TPA properties.Moreover,the mechanisms of the probes are analyzed and found to be an intramolecular charge transfer.These compounds are thus proved to be excellent candidates for two-photon fluorescent zinc and pH probes.
Recent progress in filament-induced atmospheric sensing is reviewed. Self-guided propagation of ultrashort laser pulses in air induces laser filamentation. All molecules in the path of a filament can be dissociated into highly excited fragments, resulting in emission of characteristic fluorescence spectra. The fluorescence spectra provide information about the various molecules in the filaments. By using a filament-induced ＂fingerprinting＂ fluorescence technique, molecules in the atmosphere can be identified.
In this paper, we demonstrate a carrier envelope phase-stabilized Yb-doped fiber frequency comb seeding by a nonlinear-polarization-evolution（NPE） mode-locked laser at a repetition rate of 60 MHz with a pulse duration of 191 fs.The pump-induced carrier envelope offset frequency（ f0） nonlinear tuning is discussed and further explained by the spectrum shift of the laser pulse. Through the environmental noise suppression, the drift of the free-running f0 is reduced down to less than 3 MHz within an hour. By feedback control on the pump power with a self-made phase-lock loop（PLL）electronics the carrier envelope offset frequency is well phase-locked with a frequency jitter of 85 m Hz within an hour.
The vector properties of reaction O（1D）＋HBr→OH＋Br on the potential energy surface （PES） of X1A＇ ground singlet state are studied by using the quasi-classical trajectory （QCT） theory. The polarization-dependent differential cross sections （PDDCSs）, the average rotational alignment factor （P 2 （j＇- k）〉, as well as the distributions reflecting vector correlations are also computed. The analysis of the results shows that the alignment and the orientation distribution of the rotation angular momentum vector of product molecule OH is influenced by both the effect of heavy-light heavy （HLH） type mass combination and the deep well of PES.
