Femtosecond Pulse Propagation Assisted By Molecular Alignment

Intense femtosecond laser pulses propagating in transparent nonlinear media will undergo several physics effects such as optical Kerr self-focusing, plasma defocusing, plasma and multiphoton absorptions as well as material diffraction and dispersion. The interaction between these effects will lead to the formation of filamentation. Periodic revivals of molecular alignment through nonadiabatic (field-free) rotational Raman excitation, which can be used to manipulate the orientation of molecules, have already been reported. In this dissertation, we showed that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation induced by an advancing femtosecond pump pulse. The following studies have been performed.1.The phase modulation of femtosecond probe pulse induced by molecular alignment was studied both experimentally and theoretically. The molecular alignment led to a spectral red-or blueshift of the probe pulses when they were properly delayed around the rising or falling edge of the half-revival period of molecular alignment, respectively. These results open the way to remote probing of the molecular alignment and tuning the central wavelength of femtosecond pulse.2. Based on the spatial-temporal dependent refractive index modulation induced by the pre-aligned molecules, cross-(de)focusing assisted polarization spectroscopy technique was developed to distinguishing the transient orientation of prealigned molecules. Moreover, the dynamics of multiple filamentation of the succeeding probe pulse can be controlled by the molecular alignment induced cross-(de)focusing effect.3. The filamentaion dynamics of succeeding probe pulse was enhanced by the cross-focusing effect induced by parallel aligned CO2 molecules. The output spectrum of probe pulse was significantly broadened to form supercontinuum, which has a controllable cutoff extension in ultraviolet spectral region as well as excellent beam quality. The supercontinuum generation through femtosecond filamentation in prealigned diatomic molecules of O2 and N2 were also studied.4. The filamentation dynamics of high energy probe pulse through molecular alignment of diatomic molecules in air was studied. The focusing condition was loosened by the cross-defocusing effect induced by perpendicularly orientated molecules in air, which led to broadened spectrum of probe pulse and elongated plasma length of probe filament. The pulse energy of the generated supercontinuum was up to 1.8 mJ. It is one of the effective methods for generating high energy supercontinuum with good spatial pattern and manipulating the plasma channel length of filament.5. An approach to explore and control nonlinear interactions between two orthogonally polarized femtosecond filaments launched parallel in air was proposed. The self-phase and cross-phase modulations due to the Kerr effect and cross-(de)focusing induced by the plasma and molecular alignment were distinctly identified resulting in attraction and repulsion of parallel filaments with different spatiotemporal proximities.