טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentFleischer Avner
SubjectHigh-Harmonic Generation from Atoms Subjected to
Two-Color laser Field
DepartmentDepartment of Chemistry
Supervisor Professor Emeritus Nimrod Moiseyev
Full Thesis textFull thesis text - English Version


Abstract

The nonlinear interaction of electrons inside matter with an intense (I ≈ 1014-1015W/cm2) laser pulse, with monochromatic carrier frequency in the visible or IR region of the electromagnetic spectrum, can lead to the coherent emission of high-energy photons that are odd-integer harmonics of the driver frequency, with frequencies extending into the extreme ultraviolet (XUV) and X-ray region, by high harmonic generation (HHG). HHG stands as one of the most promising methods to produce X-ray sources of coherent radiation. In addition, it has been observed that upon the collection of the highest-energy photons and filtering out the low-energy ones, a train of sub-fs pulses (1fs=10-15sec) is obtained. The duration of each pulse in the train is determined by the time duration of the HHG event itself, as well as by the spectral properties of the filter.


Motivated by the possibility of exploiting Fourier's uncertainty principle to reduce the duration of the pulses in the train to the attosecond scale (1as=10-18sec), or to produce a single isolated as-pulse, the extensive research of HHG over the last 20 years has concentrated on finding ways to generate higher-energy photons from HHG, and to make the interaction time shorter and shorter. Attosecond pulses are the key component which will enable real-time analysis and control of fundamental electronic processes in atoms, molecules and solid state with attosecond temporal resolution.


So far, using state-of-the-art HHG experiments, it has become possible to produce a single sub-fs pulse of duration 250as by driving the system with a strong, short fs driver pulse supporting just 1 or 2 optical cycles, with a fixed value of the carrier envelope phase. In this thesis a theoretical scheme for producing as-pulses using two-color (bichromatic) HHG instead of monochrmoatic-driven HHG, will be presented. Instead of using short fs driver pulses, this scheme allows the conversion of much longer and easier-to-obtain ps-pulses (1ps=10-12sec) to as-pulses. By using two colors with close frequencies it is possible to create an amplitude gating such that the system will produce a single pulse instead of a train. In addition it will be shown that a second (even weak) high-frequency field which is added to the strong IR field, may act as a seed for the production of new high harmonics, thus increasing the support of the HHG spectra.