双环场高次谐波螺旋度的控制
Control of the helicity of high-order harmonics generated by bicircular field
摘要：
双环激光场产生的高次谐波具有在$+1$和$-1$之间交替的螺旋度。为了产生圆偏振的高次谐波脉冲，这对于应用是重要的，有必要实现具有相反螺旋度的谐波的发射的不对称性。我们从理论上研究了一个广泛的双环场分量强度，并发现谐波强度高，螺旋度不对称性大的地区。我们研究了$ω$ -- $2ω$和$ω$ -- $3ω$双环场以及具有$s$和$p$基态的原子（分别以He和Ne原子为例）的情况。我们已经证明，对于He原子，使用具有更强的第二场分量的$ω$- $3ω$双环场可以产生具有正螺旋度的强高次谐波。对于Ne原子的螺旋度不对称性可以是大的，在一个更宽的范围内的驱动场分量的强度和更高的谐波阶数。二次场分量越强，谐波强度越高，高次谐波的螺旋度不对称参数越大。用椭圆偏振阿秒高次谐波场的参量图说明了Ne原子的结果。
英文摘要：
High-order harmonics generated by bicircular laser field have helicities which alternate between $+1$ and $-1$. In order to generate circularly polarized high-harmonic pulses, which are important for applications, it is necessary to achieve asymmetry in emission of harmonics having opposite helicities. We theoretically investigated a wide range of bicircular field component intensities and found areas where both the harmonic intensity is high and the helicity asymmetry is large. We investigated the cases of $ω$--$2ω$ and $ω$--$3ω$ bicircular fields and atoms having the $s$ and $p$ ground states, exemplified by He and Ne atoms, respectively. We have shown that for He atoms strong high harmonics having positive helicity can be generated using $ω$--$3ω$ bicircular field with a much stronger second field component. For Ne atoms the helicity asymmetry can be large in a wider range of the driving field component intensities and for higher harmonic orders. For the stronger second field component the harmonic intensity is higher and the helicity asymmetry parameter is larger for higher harmonic orders. The results for Ne atoms are illustrated with the parametric plots of elliptically polarized attosecond high-harmonic field.
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