A method is proposed to determine the temporal width of high-brightness radio-frequency compressed electron pulses based on cross-correlation technique involving electron bunches and laser-induced plasma. The temporal evolution of 2-dimensional transverse profile of ultrafast electron bunches repelled by the formed transient electric field of laser-induced plasma on a silver needle is investigated, and the pulse-width can be obtained by analyzing these time-dependent images.This approach can characterize radio-frequency compressed ultrafast electron bunches with picosecond or sub-picosecond timescale and up to 105 electron numbers.
By applying a nonperturbative quantum electrodynamic theory, we study ponderomotive resonances when an electron beam is scattered by a standing photon wave. Our study shows that the pondero- motive parameter Up, the ponderomotive energy per laser-photon energy, for each of the two traveling laser modes possesses a minimum valuehω/（mec2）. Ponderomotive resonances occur only when the ratio of the laser photon energy to the electron rest-mass energy is a fraction, where the denomina- tor is twice the square of a positive integer and the numerator is the total ponderomotive number, which is also a positive integer.
We theoretically and experimentally study the polarization and phase control of two-photon absorption in an isotropic molecular system. We theoretically show that the two-photon transition probability decreases when the laser polarization changes from linear through elliptical to circular, and the laser polarization does not affect the control efficiency of two-photon transition probability by shaping the spectral phase. These theoretical results are experimentally confirmed in coumarin 480. Furthermore, we propose that the combination of the laser polarization with the spectral phase modulation can further increase the control efficiency of the two-photon absorption.
将Guo-Aberg-Crasemann形式散射理论推广到高次谐波产生过程,获得了高次谐波产生概率公式.利用这一公式,计算了不同惰性气体原子的高次谐波谱.理论分析和数值计算显示高次谐波有新的截断定律qchω=(9-4^(1/2))U_p+(2~/1/2)-1)I_p≈3.34U_p+1.83I_p,其中,U_p为电子的有质动能,I_p为原子电离能,hω为激光光子能量,q_c为高次谐波的截断阶数.这一截断定律与近期Popmintchev等(Popmintchev et al.2012Science 336 1287)的实验观测符合得很好.
