Characteristics Of Sub-cycle Pulsed Beam And Its Interaction With Atoms

In recent years, the great progress in the generation of ultrashort laser pulses has made the shortest electromagnetic pulses which consist of one or less optical cycles (measured full-width at half-maximum of the pulse’s envelope) possible. From the terahertz band, to the visible spectrum and the extreme ultra-violet band, the single-cycle pulses start a new era for ultrafast science. Despite of this great progress in experiments, the theory for single-or sub-cycle pulses is still in its baby phase. Some fundamental problems like the description of the sub-cycle pulses have not been well solved yet. Both of the carrier-envelope model and the former vector-potential definition have intrinsic problems. Nonphysical results will be inevitable, if they are applied to the study of laser-matter interaction.In this thesis, the description of sub-cycle pulses and their interaction with matters have been studied systematically. An accurate description of a nonparaxial subcycle pulsed beam (SCPB) is presented based on the modified complex-source model (sink-source model). The fields are exact solutions of Maxwell’s equations and applicable to a focused pulsed beam with a pulse duration down to and below one cycle of the carrier wave and with arbitrary polarization state and arbitrary focusing. Depending on the pulse duration, the pulse is blueshifted, and its wings are chirped. This effect, which we refer to as intrinsic chirp goes beyond the carrier-envelope description. The corresponding phase is a temporal analog of the Gouy phase.To verify the existence of the intrinsic chirp in sub-cycle pulses, we obtained the sub-single-cycle terahertz (THz) waveforms with a standard THz time-domain spectroscopy system. The time-frequency analysis results of the obtained waveforms show clearly the existence of the intrinsic chirp. Before this work, the Fourier transform-limited pulses have been considered to be free of chirp, which is somehow proved to be misleading.Further more, for the description of plane-wave sub-cycle pulses, the inconsistency of the popular vector potential definition is noticed and we proposed a modified version with analytic envelopes. This new model makes the study of intense laser-matter interaction process, such as above threshold ionization and high harmonic generation, possible. In addition, we proposed the time domain representation of the pulse train synthesized in the frequency domain with analytic signal structure. By using such a technique, the meaningful concepts like the carrier frequency, CEP and chirps of the synthesized pulse train could be easily derived.We studied the above threshold ionization of atoms illuminated by intense linear polarized sub-cycle pulses with numerical solutions of the time-dependent Schrodinger equation for a hydrogenlike atom. In this regime, the presumption for the adiabatic ionization theories that atoms are ionized directly from an initial bound state to the continuum state will fail. The nonadiabatic ionization channels turn out to play important roles:the bound electrons can climb up the energy ladder and get ionized from a certain bound state other than the original one. This process leaves significant fingerprints in carrier-envelope-phase-sensitive phenomena like total ionization yield and momentum asymmetry of the photoelectrons.In the sub-cycle regime, the high harmonic generation is also different from the cases of long pulses. A reduction of the cutoff energy occurs as the pulse width goes down to sub-cycle. The amount of the reduction is envelope dependent. The blueshift of the center frequency is considered to be the cause of the reduction. The generated high harmonic spectrum is found to be closely related to the intrinsic chirp of the pulse.Moreover, we studied the population inversion of atoms interacting with sub-cycle pulses. By numerically solve the density matrix equations, we studied the population inversion between two initially coherent bound states driven by sub-cycle pulses which are far from resonance. We found that the population inversion is a function of CEP with a period of2π which is in impressive contrast to the case of initially incoherent states in which a period of π has been found. Furthermore, the initially coherent states are much more sensitive to the driving pulses and the degree of the population inversion can be enhanced by several orders. The multiphoton nature of this process is demonstrated. These characters make the population transfer between coherent states a promising way to detect the CEPs of sub-cycle pulses within broad intensity and carrier frequency range. In turns, it also provides one way to manipulate the population of the coherent states and to detect the initial phase difference between the coherent states.