Optimized Multi-color Laser Pulses For Extending High-harmonic Cutoff

When an intense laser pulse interacts with atoms, a bound electron first tunnel-ionizes into the continuum; then it is accelerated by the laser field; and finally it recombines with the parent ion on reversal of the laser field, emitting a high-energy photon. This high-order harmonic generation(HHG) has been intensively studied for more than two decades, due to its good coherence and wide wavelength spectral regions from the visible to the vacuum ultraviolet and soft X-ray. Numerous applications of HHG sources have been demonstrated.At present the main limitation that prevents the application of HHG is its low conversion efficiency. In this thesis we will study how to simultaneously increase both the cut-off frequency and the HHG efficiency. The main work is as follows:1. The waveform of a five-color laser field is determined by using a genetic algorithm to optimize the maximum electron recollision energy without increasing total laser power. By comparing the harmonic yields of single-atom and macroscopic high harmonic spectra, we find that on the single-atom level, longer-wavelength component of combined field can increase effectively maximum electron recollision energy. On the other hand, the HHG yields for long-wavelength driving lasers under the same experimental conditions appear quite unfavorable. In the optimization we must balance the pros and cons of the longer-wavelength component.2. We demonstrate a more practical way to increase cutoff energy by optimally synthesizing "only" two-color laser fields. The harmonic cutoff generated by the optimized laser field can be extended by about 2 times without losing the harmonic yield, and without increasing total laser power. What’s more, this kind of waveform is easy to be generated in the laboratory.