Nonlinear Light - Matter Interactions of Ultrafast High Intensity Laser Pulses
This thesis focuses on the key nonlinear optical effects that arise from the interactions of intense ultrafast laser pulses with various states of matter. These interactions involve electronic and molecular states and yield new information on the underlying fundamental processes that govern the molecular world. Modern day lasers offer ultrashort pulses, high intensities, and complex polarizations and wavefronts. These extreme conditions have profound effect on the optical properties and behaviors of electronic and molecular states within a material. The changes in these mechanisms effect generation of nonlinear optics, such supercontinuum (SC), stimulated Raman (SRS), self-focusing and filamentation, conical emission (CE), and the various components of the Kerr effect. The ultrashort nature of modern laser pulses also allows for these ultrafast processes to be measured in time using the pump-probe technique. Explored in the thesis are several topics concerning novel light - matter interactions. The first topic to be discussed focuses on the behavior of nonlinear optical effects, such as SRS and filamentation, produced from nano and femtosecond complex vector light pulses, which carry orbital angular momentum. It is shown that complex light and especially the Majorana sub-class, have a large effect on self-focusing and the stability and noise in the produced filaments. This in turn affects the expression of other nonlinear effects such as SRS. The second topic concerns conical emission generated from ultrashort 50 femtosecond laser pulses in calcite and Bk-7 glass. Experimental data is compared to three competing mathematical models in both the degenerate (~0 cm-1) and anti-stokes non-degenerate (2000 - 9000 cm-1) emission regimes. Degenerate results yield a direct measurement of the nonlinear index n2 and illustrate how ultrashort pulses can drastically reduce the molecular contributions to the Kerr index through transience. The third and fourth topics use the pump-probe technique to investigate pico and femtosecond material processes associated with the Kerr effect. The third topic looks at the effects of extreme optical intensities on Kerr induced birefringence in CS2. At high input intensities, the material will act similar to a waveplate and can create multiple phase rotations in a probing beam. This effect can be studied by looking at the oscillatory structure it forms in a Kerr Gate. The oscillatory structure reported here has never been reported in CS2 and is backed up by theoretical simulations. The fourth topic covers the first ever time-resolved measurements of the thermalization of an optically pumped “hot” refractive index arising from the electronic cloud and molecular motions. This was observed in acetone and methanol by measuring the forward scattering of a spectrally broadened probe beam correlated to the frequency of a chopped pump. The scattering signal reveals a 4-stage index thermalization process, in which “mother” states decay into a series of “daughter” non-resonate states with different response times. Electron cloud - vibrational coupling is also recorded in the “mother” states, which can be used to produce the Raman spectrum. Each n(t) thermalization stage is fitted to theoretical decay equations that give their rise and fall times.
Nonlinear Optics, Ultrafast Lasers, Time-Resolved Systems, Light Scattering, Kerr Effect, Complex Light, Atomic, Molecular and Optical Physics, Electromagnetics and Photonics
City University of New York - City College
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