High-harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, thus far, the shortest isolated attosecond pulses have only been produced with linear polarization, which limits the range of physics that can be explored. Here, we demonstrate robust polarization control of isolated extreme-ultraviolet pulses by exploiting non-collinear high-harmonic generation driven by two counter-rotating few-cycle laser beams. The circularly polarized supercontinuum is produced at a central photon energy of 33 eV with a transform limit of 190 as and a predicted linear chirp of 330 as. By adjusting the ellipticity of the two counter-rotating driving pulses simultaneously, we control the polarization state of isolated extreme-ultraviolet pulses—from circular through elliptical to linear polarization—without sacrificing conversion efficiency. Access to the purely circularly polarized supercontinuum, combined with full helicity and ellipticity control, paves the way towards attosecond metrology of circular dichroism.

飞秒激光驱动的高谐波产生，使得捕获分子和材料中最快的动力学成为可能。但是，到目前为止，仅产生了具有线性极化的最短隔离阿秒脉冲，这限制了可以探索的物理范围。在这里，我们通过利用由两个反向旋转的少周期激光束驱动的非共线高谐波产生，展示了对孤立的极端紫外脉冲的鲁棒偏振控制。圆偏振超连续谱是在33 eV的中心光子能量下产生的，其转换极限为190 as，并且预测线性chi为330 as。通过同时调整两个反向旋转的驱动脉冲的椭圆率，我们控制隔离的极紫外脉冲的极化状态（从圆形极化到椭圆极化到线性极化），而不会牺牲转换效率。进入纯圆极化超连续谱并结合完全螺旋和椭圆度控制，为圆二色性的亚秒计量学铺平了道路。