Quantum Control of Coherent EUV Radiation: New Methods for Phase Matching at Short Wavelengths

在这个项目中，将探索将非线性光学扩展到软X射线光谱区域的新技术。具体而言，精确控制的光学波形，结构化波导，和准相位匹配在短波长将被用来增加基于激光的相干x射线源的亮度。在过去的两年里。在这一领域已经取得了巨大的进展，展示了将激光有效转换到光谱的极紫外（EUV）区域（波长约为50 eV）的新方法。现在可以产生比同步加速器产生的短1000倍的短波长光脉冲--短到足以直接探测原子运动（10飞秒）。还可以通过使用相位匹配技术来显著提高这些非常高阶的非线性过程的转换效率。例如，通过使激光束传播通过中空纤维，可以使光脉冲的相速度与所产生的x射线束的相速度相匹配，从而提高转换效率。最后，最近表明反馈控制算法可以及时微调驱动高次谐波产生的激光脉冲的形状，使得可以优化该过程并选择性地增强特定的X射线光子能量。这是一种发生在单个原子内的全新类型的相位匹配，其中激光脉冲形状被优化，使得从激光的一个半周期产生的X射线与相邻半周期产生的X射线相长干涉。这与更传统的相位匹配技术相反，在相位匹配技术中，来自大量单个原子的发射被布置为通过匹配驱动波和谐波的相速度来进行相长干涉。新的技术将使我们能够将相位匹配技术应用于更高的光子能量，从50 - 500 eV。简单地扩展以前的工作是不够的，因为更高能量的X射线光子是在更高的电离水平下发射的，在激光和X射线束之间引入了非常大的相速度失配。可能的新技术包括使用结构波导调制系统的非线性响应，以获得准相位匹配，并使用双色激发。这项工作，当结合使用的时间成形脉冲的持续工作，将大大提高在这个高度非线性政权的激光原子相互作用的理解，和我们的能力，以最佳控制的X射线generationprocess.This研究领域提出了一个独特的和具有挑战性的结合前沿基础研究和先进技术。超快、宽带宽的激光脉冲和反馈算法将用于相干控制和设计辐射原子的电子波函数，其非常实用的目标是开发明亮、相干的软X射线光源。这种在亚纳米、亚飞秒、距离和时间尺度上对物质的控制探索了基本原子和分子过程以及光学技术的极限。这项工作将为光学，计算机，电子和EUV技术的学生提供良好的培训-技术上的重要领域，行业的需求远远超过毕业生的可用性。此外，这种新光源在纳米技术、显微镜、计量学、光刻、X射线光学的表征以及使用X射线的超快动态过程的研究中具有潜在的未来应用。我们和其他公司正在积极研究其中的许多应用。

In this project, new techniqucs to extend nonlinear optics into the soft-x-ray region of the spectrum will be explored. Specifically, precisely controlled optical waveforms, structured wave guides, and quasi phase-matching at short wavelengths will be used to increase the brightness of laser-based coherent x-ray sources. In the past 2 years. dramatic progress has been made in this arca, demonstrating new methods for efficient conversion of laser light into the extreme ultraviolet (EUV) region of the spectrum, at wavelengths around 50eV. It is now possible to generate short- wavelength light pulses 1000 times shorter than can be generated by synchrotrons-short enough (10 femtoseconds) to directly probe atomic motion. It is also possible to dramatically improve the conversion efficiency of these very high-order nonlinear processes by using phase matching techniques. For example, by propagating the laser beam through a hollow fiber, the phase velocity of the optical pulse can be made to match that of the generated x-ray beam, thus improving thc conversion efficiency. Finally, very recently it was shown that feedback-control algorithms can fine- tune the shape in time of the laser pulse driving high-harmonic generation, making it possible to optimize the process and selectively enhance a particular x-ray photon energy. This is a fundamentally new type of phase matching that occurs within a single atom, where the laser pulse shape is optimized so that x-rays generated from one half-cycle of the laser interfere constructively with x-rays generated by adjacent half-cycles. This is in contrast to more-conventional phase matching techniques, where emission from a large number of individual atoms is arranged to interfere constructively by matching the phase velocities of the driving and harmonic waves.In the proposed work, several significant remaining challenges for generating coherent light in the EUV will be addressed. New techniques will he developed that will allow us to apply phase- matching techniques to higher photon energies, from 50 - 500eV. Simply extending previous work will not suffice because higher-energy x-ray photons are emitted at higher levels of ionization, introducing a very large phase velocity mismatch between the laser and x-ray beams. Possible new techniques include the use of structured waveguides for modulating the nonlinear response of the system to obtain quasi phase matching, and the use of two-color excitation. This work, when combined with continuing work on the use of temporally-shaped pulses, will greatly enhance the understanding of laser-atom interactions in this highly nonlinear-regime, and our ability to optimally control the x-ray generation process.This area of research presents a unique and challenging combination of forefront basic research and advanced technology. Ultrafast, broad bandwidth laser pulses and feedback algorithms will be used to coherently control and engineer the electron wave function of a radiating atom, with the very practical objective of developing bright, coherent, soft-x-ray light sources. This control of matter on the sub-nanometer, sub-femtosecond, distance- and time-scales explores the limits of fundamental atomic and molecular processes, as well as of optical technology. This work will provide excellent training for students in optical, computer, electronic, and EUV technologies- technologically- significant fields where the needs of industry far outpaee the availability of graduates. Furthermore, this new light-source has potential future applications in nanotechnology, microscopy, metrology, lithography, the characterization of x-ray optics, and in the study of ultrafast dynamic processes using x-rays. We and other are actively pursuing many of these applications.