Femtosecond Laser Filamentation Interaction And Characterization

Intense laser pulses experience dramatic self-focusing, multi-photon ionization, spectral broadening and phase modulation when propagating in atmosphere. Counterbalance between Kerr self-focusing and plasma-defocusing in neutral media, bringing about robust self-guided channels that facilitate abundant self-action nonlinear processes. Filamentary propagation of intense femtosecond laser pulses has been so far demonstrated quite useful for light detection and atmosphere remote sensing, atmosphere pollutant detection, lightening guiding, laser induced water condensation and pulse self-compression down to a few cycles in duration. As a result, the filament characteristics, including electron density, peak intensity, core size, length, plasma lifetime and pulse duration are significantly important in filament manipulation and application.This paper focus on the spatiotemporal modulation, refractive index variation and phase modulation induced by fs filament and periodic plasma micro-channels, summarized as follows.1. We show that coalescence of interfering non-collinear intense femtosecond pulses assisted periodic wavelength-scale self-channeling with encircling air molecules. Such a periodic modulation can be functioned as effective plasma grating that support diffraction of the incident pulse, experimentally confirmed by the co-axis propagated third harmonic in the filament. The THG diffraction dependence on the crossing angle of the grating forming filament as well as the temporal evolution of the plasma grating were studied in details. Compared with traditional grating, such plasma grating exhibit novel features like ultrahigh damage threshold, low cost, long lifetime, period tunablity and convenient multi-dimension extension.2. Significant fluorescence enhancement in the interaction region was observed, We proposed an effective high density free electron generation method based on filament interaction that avoid filament split and modulation instability. The filament propagation, split and evolution as well as the electron density within the plasma micro-channels were studied by the in-line holographic recording technique. Ten times electron density enhancement within the plasma channels was experimentally confirmed as compared with that in the plasma channels forming filament.3. The time characteristic of air filament fluorescence emission and electron density were studied. Different time emission feature of molecule, molecule ion, plasma supercontinuum and atom spectra lines were distinguished, which were quite dissimilar to the fluorescence induced by ns time scale laser pulses. In addition, by calibrating fluorescence emission from single filament and plasma micro-channels, we conclude that filament interaction provides an efficient way to break the well-known clamping intensity and a120TW new clamping intensity value is given. Potential applications of plasma micro-channels in atmospheric sensing are preliminary explored, which are anticipated to be used in atmospheric pollution monitoring and warning with high sensitivity.4. Field-free alignment of gaseous molecules could function as an ultrafast polarization optical gating with periodic revivals originated from quantum wakes of the impulsively excited molecular wave-packets. The M-XFROG technique employs the impulsive transient alignment of gaseous molecules as a gate function to characterize the ultrshort pulse and exhibits advantages of no phase-matching constraint and applicability to pulses at any wavelength ranging from ultraviolet to far-infrared. Ultrashort pulse meaurements of ultraviolet, supercontinuum and sub-10fs pulses were experimentally performed by using the M-XFROG technique.5. We compared the role of the electronic Kerr effect and molecular alignment induced birefringence in molecular N2on the measurement of ultra-short laser pulse. Our result indicated that either Kerr or molecular alignment could be utilized to characterize an unknown laser pulse, while the molecular alignment was the dominative one for molecular gas and showed a very high signal-to-noise ratio. Moreover, the dissimilarity between such two phenomena as well as their independent contributions to the gate function was clarified by simply comparing the measured FROG traces and retrieved gate, consistent with the recently revealed molecular alignment direct measurement by spatial (de)focusing effects.