Tracing the phase of focused broadband laser pulses
Tracing the phase of focused broadband laser pulses
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DOI：
10.1038/nphys4185
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发表时间：
2017-10-01
期刊：
影响因子：
19.6
通讯作者：
Hommelhoff, Peter
中科院分区：
文献类型：
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作者：
Hoff, Dominik;Krueger, Michael;Hommelhoff, Peter
精确了解聚焦光束中光的相位行为对于理解和控制激光驱动过程至关重要。一百多年前，人们发现了聚焦单色光束的轴向相位异常，现在通常称为古伊相位（1-4）。最近的理论工作已经对将这种单色相位公式应用于今天变得无处不在的宽带脉冲的有效性提出了质疑（5，6）。基于电子在尖锐的纳米尺度金属尖端上的背散射，一种方法可用于测量具有亚波长空间分辨率和亚光周期时间分辨率的光场（7-9）。在这里，我们报告这样一个直接的，三维测量的光学相位的空间依赖性的聚焦，4-fs，近红外脉冲激光束。所观察到的光学相位基本上偏离单色古伊相位，表现出更复杂的空间依赖性，无论是沿着传播轴和径向。在我们的测量中，这些显著的偏差是规律，而不是聚焦宽带激光脉冲的例外。因此，我们预计所有宽带激光-物质相互作用的广泛分支，例如高谐波和阿秒脉冲产生，飞秒化学（10），眼科光学相干断层扫描（11，12）和光波电子学（13）。
Precise knowledge of the behaviour of the phase of light in a focused beam is fundamental to understanding and controlling laser-driven processes. More than a hundred years ago, an axial phase anomaly for focused monochromatic light beams was discovered and is now commonly known as the Gouy phase(1-4). Recent theoretical work has brought into question the validity of applying this monochromatic phase formulation to the broadband pulses becoming ubiquitous today(5,6). Based on electron backscattering at sharp nanometre-scale metal tips, a method is available to measure light fields with sub-wavelength spatial resolution and sub-optical-cycle time resolution(7-9). Here we report such a direct, three-dimensional measurement of the spatial dependence of the optical phase of a focused, 4-fs, near-infrared pulsed laser beam. The observed optical phase deviates substantially from the monochromatic Gouy phase-exhibiting a much more complex spatial dependence, both along the propagation axis and in the radial direction. In our measurements, these significant deviations are the rule and not the exception for focused, broadband laser pulses. Therefore, we expect wide ramifications for all broadband laser-matter interactions, such as in high-harmonic and attosecond pulse generation, femtochemistry(10), ophthalmological optical coherence tomography(11,12) and light-wave electronics(13).