Institute for Lasers, Life and Biophotonics Amsterdam, VU University, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands.
Phys Rev Lett. 2010 Aug 6;105(6):063001. doi: 10.1103/PhysRevLett.105.063001. Epub 2010 Aug 2.
The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over P states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a (4)He ionization energy at h×5 945 204 212(6) MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system.
频率梳（FC）激光器的出色精度被转移到极紫外（XUV，波长短于 100nm），这是这些设备以前无法到达的频率区域。通过放大和相干上转换源自近红外飞秒 FC 激光器的一对脉冲，在接近 51nm 的 XUV 波长处产生了一个频率梳。使用氦原子作为标尺和相位探测器，证明了在 XUV 中源的相位相干性。当 FC 激光器在氦的 P 态上扫描时，观察到具有高对比度的稳定类似拉姆齐条纹的信号，从中可以提取出 XUV 中的绝对跃迁频率。该程序产生了 h×5945204212(6)MHz 的（4）He 电离能，精度提高了近一个数量级，从而对该双电子系统的 QED 计算提出了挑战。
