|M.Sc Student||Leonid Rybak|
|Subject||Femtosecond Coherent Phase Control of Resonance-Mediated|
|Department||Department of Chemistry||Supervisor||Professor Amitay Zohar|
|Full Thesis text|
The present research presents femtosecond phase control of resonance-mediated third-harmonic generation (THG). The control is over the total yield as well as over the spectrum of the third harmonic (TH). The former is studied both experimentally and theoretically, while the latter is studied only theoretically. The studies focus mainly on the weak-field regime, where the photo-excitation can be described within 3rd-order time-dependent perturbation theory.
The physical model system is the Na atom, initially populating the 3s state. The TH is emitted due to the build-up of a time-dependent matter polarization corresponding to a dipole moment between various states. Depending on the excitation pulse spectrum and intensity, the THG is either (a) predominantly due to a single excited state (the 7p state) or (b) due to a manifold of excited states accessed off-resonantly. The intermediate state of the resonance-mediated excitation is the 4s state accessed from the 3s state by non-resonant two-photon transition. The amplitude of an emitted TH frequency, wTH, results from interferences between all the multi-photon pathways that lead to a total excitation energy of wTH. In the weak-field regime, all the pathways are of three photons and each of them is either on-resonance or near-resonance with the 4s state. Beyond the weak-field regime, pathways of higher numbers of photons are also involved. By controlling the spectral phases of the shaped excitation pulse, these interferences are manipulated and the resulting THG is effectively controlled.
When the 7p state gets populated [cases (a) above], most of the TH emission results from the time-dependent dipole moment associated with the created coherent superposition of the 7p and 3s states, existing also after the pulse is over. The resulting TH emission is quasi-CW with a narrow spectrum around the 7p-3s transition frequency, and its duration is determined by the superposition’s decoherence. The corresponding THG yield is proportional to the product of the final 7p and 3s populations. In the weak-field regime, enhancement of the THG yield beyond the yield generated by the transform-limited pulse is achieved using simple pulse shaping.
When the TH is generated off-resonantly via a manifold of p-states [cases (b) above], the emitted TH is broadband with duration comparable to the excitation pulse’s duration. Additional to controlling the THG yield, control is also shown to be possible over the TH spectrum. Notably, the spectral characteristics of the shaped excitation pulse can be directly transferred to the emitted TH pulse.