Nonlinear response of hydrogen atom induced by intense few-cycle laser field and its influence on refractive index

Abstract

Nonlinear response of hydrogen atom in intense few-cycle laser field is investigated through numerical integration of Schrödinger equation. According to the calculation results, in order to investigate the saturation of instantaneous response and higher-order Kerr effect (HOKE), the electron transitions between bound-excited states of hydrogen atom are needed to be taken in account. The pulse-defocusing induced by HOKE is not obvious due to the quick rise of plasma effect when the pulse carrier wavelength is 800nm. If the pulse wavelength is tuned from 800nm to 1800nm, there is an enlarging intensity window in which HOKE can play the defocusing role and can result in ionization-free filamentation, and beyond this window the plasma effect will dominate. With the increasing of pulse duration, this intensity window will become narrower because of the accumulation of plasma density. As a result, short pulses with wavelength longer than 800nm should be used to observe the ionization-free filamentation.