Schlaepfer F, Ludwig A, Lucchini M, Kasmi L, Volkov M, Gallmann L, Keller U
Opt Express. 2017 Feb 20;25(4):3646-3655. doi: 10.1364/OE.25.003646.
Attosecond pump-probe measurements are typically performed by combining attosecond pulses with more intense femtosecond, phase-locked infrared (IR) pulses because of the low average photon flux of attosecond light sources based on high-harmonic generation (HHG). Furthermore, the strong absorption of materials at the extreme ultraviolet (XUV) wavelengths of the attosecond pulses typically prevents the use of transmissive optics. As a result, pump and probe beams are typically recombined geometrically with a center-hole mirror that reflects the larger IR beam and transmits the smaller XUV, which leads to an annular beam profile of the IR. This modification of the IR beam can affect the pump-probe measurements because the propagation that follows the reflection on the center-hole mirror can strongly deviate from that of an ideal Gaussian beam. Here we present a detailed experimental study of the Gouy phase of an annular IR beam across the focus using a two-foci attosecond beamline and the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) technique. Our measurements show a Gouy phase shift of the truncated beam as large as 2π and a corresponding rate of 50 as/mm time delay change across the focus in a RABBITT measurement. These results are essential for attosecond pump-probe experiments that compare measurements of spatially separated targets.
由于基于高次谐波产生（HHG）的阿秒光源平均光子通量较低，阿秒泵浦-探测测量通常是通过将阿秒脉冲与更强的飞秒锁相红外（IR）脉冲相结合来进行的。此外，材料在阿秒脉冲的极紫外（XUV）波长处有强烈吸收，这通常会妨碍使用透射光学元件。因此，泵浦光束和探测光束通常通过中心孔镜进行几何重组，该中心孔镜反射较大的红外光束并透射较小的XUV光束，这导致红外光束呈环形光束轮廓。红外光束的这种改变会影响泵浦-探测测量，因为在中心孔镜上反射后的传播可能会与理想高斯光束的传播有很大偏差。在此，我们使用双焦点阿秒光束线和RABBITT（通过双光子跃迁干涉重建阿秒拍频）技术，对环形红外光束在焦点处的古依相位进行了详细的实验研究。我们的测量结果表明，在RABBITT测量中，截断光束的古依相移高达2π，并且在焦点处每毫米时间延迟变化对应的速率为50。这些结果对于比较空间分离目标测量的阿秒泵浦-探测实验至关重要。
