Deep-ultraviolet Femtosecond Laser Filamentation And Its Application In Ultrafast Spectroscopy

The propagation of ultra-short intense800nm Ti:sapphire laser pulses in the transparent media is rich in nonlinear physics and may have a broad range of applications, induced supercontinuum genereation, pulse self-compression, conical emission, harmonics generation, chemical reaction, population inversion and trapping, remote sensing of chemical and biological agents, electric discharge, remote lasing action, single attosecond pulse generation, etc. Ultraviolet femtosecond pulses show the advantages of lower filamentation threshold power, more excellent focusing ability, and higher critical plasma density; however, the investigations of ultraviolet filamentation in transparent media are limited by the lack of intense ultraviolet femtosecond laser.This paper introduces and discusses the main aspects and potential applications of267nm deep-ultraviolet femtosecond laser filamentation in gaseous media including air, argon, neon and helium. The nonlinear phenomena of femtosecond filamentation such as spectral modulation, intensity clamping, mode self-healing, pulse splitting are described. Self-guiding is shown to subsist in low gas pressure. The various techniques consist of fluorescence imaging, external electric field induced transient current and in-line holographic imaging provide a comprehensive image of the ultraviolet femtosecond filamentation process and its characteristics. It is shown that higher free electrons density, longer plasma channels, and smaller beam profile accompany the ultraviolet filamentation, allowing for a better application in all-optical devices, fluorescent spectroscopy, and frequency up-conversion.The interference between two almost parallel267nm ultraviolet filamentary pulses produced periodic wavelength-scale plasma channels surrounding with air molecules. Such a periodic refractive index modulation can be functioned as effective plasma grating exhibits a nonuniform photonic band gap that support the efficient Bragg diffraction of probe laser pulse. As a result of the nonlinear interaction between these two ultraviolet filaments, enhanced ionization is produced, which represent waveguide walls for laser beams that can suppress their divergence, resulting in a dramatic increase of the ionization rate. It is shown that the modulation depth of plasma grating was enhanced by one order of magnitude under the irradiation of intense800nm laser pulses with the peak intensity of1014W/cm2. This is ascribed to laser driven electrons impact ionization, which increased the peak plasma density up to1019cm-3. The unique feature of plasma grating such as ultrahigh damage threshold and low cost enables its application in compression of ultrashort intense positively or negatively chirped pulses.Due to the higher multi-photon ionization rate and lower electron temperature inside the ultraviolet filaments, its potential application in remotely nonlinear fluorescence spectroscopy is proposed. The method of ultraviolet+infrared dual-color filamentation in mixed gaseous media was introduced to enhance the plasma fluorescence intensity and signal-to-noise ratio. A semiclasscal three-step-excitation model was verified by the fluorescence emission from the impact excitation of high potential atoms such as neon and helium. Firstly, some free electrons were liberated from buffer gases through multi-photon absorption of ultraviolet photons due to its low ionization potential. Secondly, as a subsequent intense infrared pulse was tightly coupled and guided into the preformed plasma channels, the well-confined free electrons were driven by the laser electric field along its polarization direction. A host of free electrons were further peeled through these energetic electrons collision ionization. Thirdly, some neutral neon or helium atoms in excited states approaching ionization potential were kicked into ionic states through the collisions of these hot electrons, facilitating the impact ionization of neon atoms. As a result, more high-lying excited states were populated through ion conversion and dissociative recombination. Generation of ultrashort vacuum ultraviolet pulses by using nonlinear frequency conversion through filamentation was experimentally investigated. Gently focusing267nm and400nm pulses into argon efficiently generated multicolor vacuum ultraviolet pulses including133nm,114nm,100nm, and89nm generated by sum-frequency four-wave mixing, which are the shortest wavelength generated through filamentation in gaseous media. The pressure dependence of the conversion efficiency can be explained by phase matching induced by dynamical balance among dispersion originated from neutrally atomic atoms, plasma and Gouy phase shift. The pulse breakup effect was observed during ultraviolet filamentation. The generated multicolor vacuum ultraviolet pulses with the energies exceeding10μJ are available for time-resolved studies of atomic and molecular ultrafast dynamics.In the following, the propagation of ultraviolet femtosecond pulses in solid and liquid media will stimulate the studies of underlying filamentation physics and potential allpications, such as filamentation induced chemical reaction, filamentation induced micro/nano precision machining, few-cycle extreme ultraviolet pulses generation, femtosecond resolved dynamics research of atomes and molecules, remote sensing by filamentation induced lasing action, and even long distance laser weapon, etc.