Femtosecond nonlinear optics in gases and solids


This thesis describes three experiments on nonlinear optical interactions in materials using high-energy, femtosecond laser pulses as well as several applications of these experiments. The first one make use of linear and nonlinear optical techniques to study ultrafast laser-induced disordering in gallium arsenide. The pump-probe experiment is performed on both the (100) and (110) GaAs crystalline surfaces with 165-fs, 620-nm optical pulses. The second-harmonic generation monitors the electronic disordering induced by the high-energy pump pulses. The linear reflectivity, on the other hand, gives information on the variation of the dielectric constant in the highly- excited region. Experimental results show that the second-harmonic signal vanishes with a decay time of 90 fs, indicating that a centrosymmetric structure is established within the pulse width. The linear reflectivity rises to a metallic value with a rise time of about 200 fs. Theoretical study shows that nonlinear optical absorption processes are important for laser pulses shorter than the electron-phonon interaction time. When a critical free carrier density is excited, the average bonding force is weakened and cannot maintain the crystal structure. The dielectric constant extracted from the high reflectivity value indicates a less conducting liquid phase than equilibrium liquid GaAs. Second-harmonic generation is also used to study Auger recombination at high carrier density for pump pulse fluence below the disordering threshold. The Auger recombination time is measured to be 400 fs, which requires a screening model for explanation. The long-time lattice heating by the pump pulse can be investigated by both linear and nonlinear optical techniques, which give the same lattice heating time of 60 ps in GaAs. In the second experiment, self-phase modulation in a single-mode fiber is used to provide a synchronized 200-nm supercontinuum source generated from the 165-fs laser pulses. Because of the Gaussian spatial profile from the single-mode fiber and the profile-preserving amplifier cells, high-quality, high-energy laser pulses are produced by a 10-Hz dye amplifier. A grating pair then compresses the pulse width to 45 fs. This design provides synchronized, tunable, femtosecond laser pulses with a Gaussian profile, suitable for high-energy two-color pump-probe experiment and ultrafast nonlinear laser spectroscopy. The third experiment demonstrates the effects of self-phase modulation and self-focusing on stimulated Raman scattering in high-pressure hydrogen using subpicosecond laser pulses. Theoretical study of transient stimulated Raman scattering is performed to take into account pump depletion due to self-phase modulation. The experimental results show that the behavior of the Raman gain falls into three input energy regimes. The Stokes radiation production in the low input energy region can be predicted by the theory of transient stimulated Raman scattering without pump depletion. In the medium input energy region, strong self-phase modulation suppresses the Stokes radiation output. This behavior is further confirmed by the addition of argon gas. Strong self-focusing, on the other hand, breaks up the laser beam and suppress self-phase modulation in the high input energy region. The Stokes radiation, therefore, recovers partially.
Last updated on 07/24/2019