Waveform Synthesis of Attosecond Optical Pulses: A Common Route to Attosecond Pump-Probe Spectroscopy and Nanoscopy in Aqueous Solution

当一个分子吸收光时，它的电子云几乎立即发生反应，随后是在飞秒（fs=10-15秒）到皮秒（ps=10-12秒）时间尺度上的旋转核动力学。例如，在光合作用中，叶绿素分子对紫外光的吸收引起电子偶极子振荡，其周期在1fs范围内，随后是其较慢的衰变和能量重新分配到核自由度。在过去的几十年里，时间分辨光谱学显示了如何使用短周期飞秒激光脉冲来引导核波包和控制基本反应的结果。然而，由于缺乏更短的脉冲，对亚振荡周期时间尺度上的电子波包的控制仍未得到探索。是否有可能跟踪并最终更有效地控制从电子偶极力引发核运动的最早时刻开始的反应动力学？****新兴的阿秒物理学领域为这些问题提供了一个新的视角。事实上，隔离极紫外（XUV, 10-100 eV）和宽带光学阿秒（as=10-18 s）脉冲可以同时产生，并与普通桌面飞秒光学技术同步。最近的激光发展甚至将XUV的阿秒发射扩展到“水窗”软x射线（280-500eV）甚至更高。这一壮举与迄今为止仅能从大型（0.1-10公里）加速器设施中获得的最短皮秒脉冲完全不同。水窗指的是水对辐射基本上是透明的，而碳和氮原子对辐射有高度吸收的光谱范围；在水中实现新颖的高对比度，元素特异性，时间分辨光谱和纳米显微镜。结合我在时间分辨分子光谱方面的经验（博士，渥太华大学，主任：Paul Corkum）和在加拿大独特的阿秒技术和光波形合成方面的专业知识(2013-2014年在慕尼黑马克斯普朗克量子光学研究所班廷博士后奖学金，主任：Ferenc Krausz)，我的研究小组有一个长期的愿景：***通过实现提供光学和水窗软x射线阿秒脉冲的光学波形合成，使水溶液中生物分子的光谱具有未开发的时间和空间分辨率成为可能。***到第5年年底，我们将有：***1)演示光学波形合成产生相干软x射线水窗辐射。***2)在未探测的阿秒状态下，将该辐射用于水相分子的光谱学。***3)利用相干软x射线的超短波长（2-10 nm）对纳米结构进行空间分辨。这项跨学科研究将培养高素质的人才，并为卫生科学领域的新技术应用开辟道路。*** *** *** *

When a molecule absorbs light, its electronic cloud reacts almost instantaneously followed by rovibrational nuclear dynamics on the femtosecond (fs=10-15s.) to picosecond (ps=10-12s.) timescale. In photosynthesis, for example, the absorption of ultraviolet light by chlorophyll molecules induces an electronic dipole oscillation, which has a period in the range of 1 fs, followed by its slower decay and energy redistribution to the nuclear degrees of freedom. Over the last decades, time-resolved spectroscopy showed how few-cycle femtosecond laser pulses can be used to steer nuclear wave packets and control the outcome of elementary reactions. The lack of shorter pulses, however, has left control of the electron wave packets on the sub-oscillation period timescale unexplored. Would it be possible to follow and, ultimately, more efficiently control reaction dynamics right from the earliest moments when electron dipole forces initiate nuclear motion?****The emerging field of Attosecond Physics offers a new take on these questions. In fact, both isolated extreme ultraviolet (XUV, 10-100 eV) and broadband optical attosecond (as=10-18 s.) pulses can be simultaneously produced and synchronized with common tabletop femtosecond optical technology. Recent laser developments even extend the attosecond emission from the XUV to "water window" soft X-rays (280-500eV) and beyond. This feat radically departs from the shortest picosecond pulses so far only available from large scale (0.1-10 km) accelerator facilities. The water window designates the spectral range where water is essentially transparent to the radiation while carbon and nitrogen atoms are highly absorptive; enabling novel high-contrast, element-specific, time-resolved spectroscopy and nanoscopy in water. Combining my experience in time-resolved molecular spectroscopy (Ph.D, University of Ottawa, director: Paul Corkum) with a unique -in Canada- expertise in attosecond technology and optical waveform synthesis (Banting postdoctoral fellowship at the Max-Planck-Institut for Quantum Optics in Munich, 2013-2014, director: Ferenc Krausz), my research group has the long term vision:***To make possible the spectroscopy of biomolecules in aqueous solution with unexplored time and spatial resolutions through the implementation of optical waveform synthesis providing both optical and water window soft X-ray attosecond pulses.***By the end of year 5, we will have:***1) Demonstrated optical waveform synthesis to generate coherent soft X-ray water window radiation. ***2) Used this radiation for the spectroscopy of molecules in aqueous phase in unexplored attosecond regime.***3) Used the ultrashort wavelength of coherent soft X-rays (2-10 nm) to spatially resolve nanometric structures.***This interdisciplinary research will train highly qualified personnel and open avenues for new technological applications in health sciences.*** *** *** **