We present an experimental investigation of a filamentation-assisted fourth-order nonlinear optical process in KTP crystals pumped by intense 1.53 eV （807 nm） femtosecond laser pulses. Femtosecond light pulses at 2.58 eV （480 nm） are generated by the fourth-order nonlinear polarization （p（4） （ω2） = X（4） （ω2, ω, ω, ω, -ω1）E3 （ω）E＊ （ω1）, where E（w） corresponds to the pump frequency and E（wl） to the supercontinuum generated through filamentation）. If the system is seeded by a laser beam at ω1 or ω2 and there are spatial and temporal overlaps with the pump beam, E（ω1） and E（ω2） are simultaneously amplified. When the intensity of the seed laser beam exceeds a certain intensity threshold, the contribution of p（4） （ω） = X（4） （ω, ω1, ω2, -ω, -ω）E（ω1）E（ω2）（E＊ （ω））2 becomes non-negligible, and the amplification weakens. The conversion efficiency from the pump to the signal at 2.58 eV （480 nm） attains to 0.1%.
We investigate the ionization dynamics of atoms by chirped attosecond pulses using the strong field approximation method. The pulse parameters are carefully chosen in the regime where the strong field approximation method is valid. We analyse the effects of the chirp of attosecond pulses on the energy distributions and the corresponding left-right asymmetry of the ionized electrons. For a single chirped attosecond pulse, the ionized electrons can be redistributed and the left-right asymmetry shows oscillations because of the introduction of the chirp. For time-delayed double attosecond pulses at different intensities with the weaker one chirped, exchanging the order of the two pulses shows a relative shift of the energy spectra, which can be explained by the different effective time delays of different frequency components because of the chirp.
This paper studies the intramolecular photoinduced electron-transfer （PET） of covalent bonded azobenzene-perylene diimide （AZO-PDI） in solvents by using steady-state and time-resolved fluorescence spectroscopy together with ultrafast transient absorption spectroscopic techniques. Fast fluorescence quenching is observed when AZO-PDI is excited at characteristic wavelengths of AZO and perylene moieties. Reductive electron-transfer with transfer rate faster than 10^11 s^-1 is found. This PET process is also consolidated by femtosecond transient absorption spectra.
This paper studies the type-I phase-matched second harmonic generation using 25-fs input laser pulses in a thick BBO crystal. The harmonic signal exhibits a narrow spectrum bandwidth, even though the input pulse has a broad bandwidth. The energy transfer efficiency and modulation of the fundamental spectrum are investigated.
The propagation of picosecond deep ultraviolet laser pulse at wavelength of 193 nm in air is numerically investigated. Long plasma channel can be formed due to the competition between Kerr self-focusing and ionization induced defocusing. The plasma channel with electron density of above 10^13/cm^3 can be formed over 70 m by 50-ps, 20-mJ laser pulses. The fluctuation of laser intensity and electron density inside ultraviolet （UV） plasma channel is significantly lower UV laser by air is considered in the simulation and it the limit of the length of plasma channel. than that of infrared pulse. The linear absorption of is shown that the linear absorption is important for the limit of the length of plasma channel.
