Classical mechanisms of recollision and high harmonic generation
- Series
- Other Talks
- Time
- Monday, December 3, 2018 - 15:00 for 1 hour (actually 50 minutes)
- Location
- Howey N110
- Speaker
- Simon Berman – Georgia Tech (Physics)
Thesis defense:
Advisors: Turgay Uzer and Cristel Chandre
Summary:
Thirty years after the demonstration of
the production of high laser harmonics through nonlinear laser-gas
interaction, high harmonic generation (HHG) is being used to probe
molecular dynamics in real time and is realizing its
technological potential as a tabletop source of attosecond pulses in the
XUV to soft X-ray range. Despite experimental progress, theoretical
efforts have been stymied by the excessive computational cost of
first-principles simulations and the difficulty of
systematically deriving reduced models for the non-perturbative,
multiscale interaction of an intense laser pulse with a macroscopic gas
of atoms. In this thesis, we
investigate first-principles reduced models for HHG using
classical mechanics. On the microscopic level, we examine the
recollision process---the laser-driven collision of an ionized electron
with its parent ion---that drives HHG. Using nonlinear dynamics, we
elucidate the indispensable role played by the ionic
potential during recollisions in the strong-field limit. On the
macroscopic level, we show that the intense laser-gas interaction can be
cast as a classical field theory. Borrowing a technique from plasma
physics, we systematically derive a hierarchy of
reduced Hamiltonian models for the self-consistent interaction between
the laser and the atoms during pulse propagation. The reduced models
can accommodate either classical or quantum electron dynamics, and in
both cases, simulations over experimentally-relevant
propagation distances are feasible. We build a classical model based on
these simulations which agrees quantitatively with the quantum model
for the propagation of the dominant components of the laser field.
Subsequently, we use the classical model to trace
the coherent buildup of harmonic radiation to its origin in phase
space. In a simplified geometry, we show that the anomalously high
frequency radiation seen in simulations results from the delicate
interplay between electron trapping and higher energy recollisions
brought on by propagation effects.