Liping Shi

The formation of an intense filament controlled by interference of ultraviolet femtosecond pulses

We experimentally investigated the formation of a wavelength-scale photonic plasma grating induced by interference-assisted coalescence of two noncollinear ultraviolet femtosecond laser pulses. The period of the created plasma grating decreased with the crossing angle of the interacting laser pulses. For a proper small crossing angle, the noncollinear ultraviolet filaments were coalesced and an intense single ultraviolet filament was formed with a diameter of 5 μm which was below the focused limitation. This may provide a way to control ultraviolet femtosecond filamentation.

Enhanced ultraviolet pulse generation via dual-color filament interaction induced phase-matching control

We demonstrated efficient generation of intense vacuum ultraviolet pulses at 133 nm directly from a 400-nm filament in argon gas. The conversion efficiency was significantly enhanced by adding a coaxial 267-nm pump pulse to dramatically ameliorate the phase matching conditions, as the dispersion originated from neutral atoms was compensated by additional free electrons stemmed from enhanced ionization induced by dual-color fields. Such a robust all-optical phase-matching control was shown to be dependent upon the pump intensity and temporal delay between the dual-color fields.

Impact excitation of neon atoms by heated seed electrons in filamentary plasma gratings.

We demonstrate impact ionization and dissociative recombination of neon (Ne) atoms by means of seeded-electron heating and subsequent electron-atom collisions in an ultraviolet plasma grating, allowing for a substantial fraction of the neutral Ne atomic population to reside in high-lying excited states. A buffer gas with relatively low ionization potential (nitrogen or argon) was used to provide high-density seed electrons. A three-step excitation model is verified by the fluorescence emission from the impact excitation of Ne atoms.

Dissociative recombination in ultraviolet filamentary plasma gratings.

We investigated collisions of nitrogen and argon gas mixture with energetic electrons accelerated by Bragg incident intense infrared femtosecond laser pulses in ultraviolet filamentary plasma gratings. Significant decrease of fluorescence spectra of argon atoms were observed when a small amount of nitrogen gas was mixed with argon gas that facilitated observable argon-nitrogen collisions. We experimentally measured the fluorescence emission from the argon and nitrogen gas mixture under different driving pulse energies, the fluorescence decay dynamics after the impact excitation, as well as the fluorescence intensity dependence on the nitrogen and argon pressures. The experimental measurements were based on the electron acceleration and its subsequent impact with the gas mixture in the filamentary plasma gratings, which was essential for the observation of the dominant dissociative recombination in the gas mixture.

Electron impact excitation of helium and neon atoms in filamentary plasma gratings

We demonstrated a femtosecond pulse driven electron-impact method to efficiently enhance the intensity of filament-induced neon and helium fluorescence emission. Such an all-optical method holds the potential applications to improve the sensitivity of laser-induced breakdown spectroscopy.