椭圆极化激光场驱动下氢原子发射高次谐波的特点

High-order harmonic generation of H atom in elliptically polarized laser field

通过数值求解两维含时薛定谔方程,研究了在不同波长的椭圆极化激光驱动下氢原子发射高次谐波的特点,得到了氢原子所产生的高次谐波谱截止位置处发射效率随椭偏率的依赖关系.研究结果表明,在激光光强保持不变的情况下,激光波长越大,截止位置处谐波发射效率对椭偏率的依赖越强;而在相同的激光波长驱动下,随着激光光强的增大,氢原子高次谐波谱截止位置处的效率随着驱动场椭偏率的增大下降得更快.最后,利用不同波长下高次谐波发射效率随椭偏率的依赖关系,通过数值计算和拟合研究了电子波包在隧穿电离后的横向扩散速率,研究发现在谐波截止位置附近由ADK模型得到的横向扩散速率比数值求解两维含时薛定谔方程得到的结果约小17%,正是这种横向扩散速率偏小的原因导致由ADK模型计算的谐波发射效率随椭偏率的增大相对于实验结果来说下降的较慢,而数值求解两维模型原子得到的结果与实验结果一致.

The ellipticity of a laser field provides an additional control parameter for laser-atom interactions and introduces some special features in nonlinear photo-processes. For example, high-order harmonic generation （HHG） in gases is the most important method for generating extreme ultraviolet （XUV） attosecond pulses from intense infrared laser field. Some alternative methods to generate isolated attosecond pules are known as polarized gating （PG）, double optical gating （DOG）, and generalized double optical gating （GDOG）, each of them making use of the strong dependence of the HHG on the ellipticity of the driving pulse. The ellipticity dependence of the yield in HHG as a function of the driving laser wavelength, intensity, target atom, and harmonic order, should be very useful for designing polarization gating-based schemes for isolated attosecond pulses. As was shown in numerical and experimental studies, HHG is very sensitive to the driving field ellipticity. In particular, the harmonic yield rapidly decreases with increasing ellipticity. According to the semiclassical three-step model, after tunneling ionization, the electron oscillates quasifreely driven by the laser field and acquires additional kinetic energy, finally it can recombine with the parent ion and emit a harmonic photon, whereas for an elliptically polarized laser field, a small ellipticity ε=E0y/E0x （here E0x is the electric field component along the major polarization axis and Eoy is the minor axis） can displace the electron along the minor polarization axis, which leads to a rapid decrease of the recombination efficiency with the increasing ellipticity. The decrease of the HHG efficiency can be attributed to the fact that the electron wave packet misses the parent ion due to lateral shift driven by the transverse component of the elliptically polarized field. However, the high-order harmonic generation could still occur owing to spreading of the electron wave packet after the tunnel ionization which can compensate for the lateral shift. By solving the two dimensional Time-dependent Schrodinger equation numerically using the Crank-Nicolson method, we study high-order harmonic generation of model H atom in an elliptically polarized laser field with different wavelengths. We use laser pulses with wavelengths of 800, 1000, 1300, 1500, and 1800 nm, respectively. The peak intensities are 1.5×10^14 and 2.5×10^14 W/cm^2. We show that the numerical calculated wavelength scaling of elliptical-polarization dependence are in good agreement with a function （ε1/2 ∝ λ^-1）, where G1/2 is the ellipticity required to suppress the harmonic yield by a factor 2. For the same wavelength, the ellipticity dependence of the HHG becomes stronger with increasing laser peak intensity. Furthermore, the wave packet spreading rates obtained from ADK model are 17% smaller than our numerical result, that is in good agreement with experimental measurement. So, it is two dimensional model of H atom that in the elliptical polarized laser field is an effective method to study the HHG spectrum.