CAREER: Understanding and Controlling Nonlinear Frequency Conversion with Counter Propagating Light

非线性光学通过称为频率转换的过程，为创建可访问且具有成本效益的激光源提供了独特的手段。事实上，一些透明材料的自然反应是将强激光从一种颜色或频率转换为另一种颜色或频率，从而实现从绿色激光笔到激光点燃聚变的各种应用。激光从一种频率到另一种频率的有效转换需要在这种光-物质相互作用中对材料成分进行仔细的，有时是复杂的工程设计。这些技术在大范围的应用中已经非常成功，但受限于可以为此目的设计的材料。与光-材料相互作用中的工程材料不同，这项由CAREER奖支持的研究探索了我们如何设计光。虽然这项研究的主要应用是开发新的光源，但光场工程也可以揭示光转换过程的基本物理原理，并为材料本身的测量提供极其精确的工具。本研究将与一所研究密集型文理学院的教育相结合，通过对实验光学本科生的指导，以及通过跨学科的第一年研讨会课程来提高科学素养的课程开发，并通过中级光学课程让学生尽早参与光学在研究中的应用。非线性光学材料的色散是影响二次谐波产生等非线性频率转换过程效率的主要问题。最近，一种校正色散效应的新方法被开发出来，该方法使用反传播脉冲序列周期性地干涉谐波产生过程，实现准相位匹配的全光学版本。虽然该技术的实验证明仅用于高次谐波产生，但该技术应适用于更广泛的非线性过程。在这个项目中，将对当前的理论模型进行二次谐波产生的直接实验测试，从而更好地理解干涉所涉及的物理。在此基础上，结合数值模型的发展，将利用超快反传播脉冲的整形来优化相位匹配的效率。这些研究结果不仅适用于低阶非线性光学过程，也适用于高次谐波，这是阿秒科学的主要来源。此外，将研究使用超快反传播脉冲作为复杂非线性材料色散特性的高分辨率原位探针，这可能用于周期性极化介质的表征或生物材料的成像。

Nonlinear optics provides a unique means for creating accessible and cost-effective laser sources, through a process called frequency conversion. The natural response of some transparent materials, in fact, is to convert intense laser light from one color, or frequency, to another, enabling applications ranging from green laser pointers to laser-ignited fusion. Efficient conversion of laser light from one frequency to another requires careful and sometimes complex engineering of the material component in this light-material interaction. These techniques have been very successful for a large range of applications, but are limited by what materials can be engineered for this purpose. Instead of engineering materials in the light-material interaction, the research supported by this CAREER award explores how we can engineer the light. While the main application of this research is the development of new light sources, engineering of light fields could also reveal fundamental physics of the light conversion process, as well as provide an extremely precise tool for performing measurements of the materials themselves. This research will be integrated with education at a research-intensive liberal arts college through mentoring of undergraduates in experimental optical science, as well as the development of curricula for improved scientific literacy through an interdisciplinary first-year seminar course, and providing students early engagement with applications of optics in research through an intermediate-level Optics course. The main challenge for the efficiency of nonlinear frequency conversion processes such as second harmonic generation is the chromatic dispersion of the nonlinear optical material. Recently, a novel method for correcting the dispersion effects has been developed, in which sequences of counterpropagating pulses are used to interfere periodically with the harmonic generation process, achieving an all-optical version of quasi-phase matching. While experimental demonstrations of this technique have been shown only for high-order harmonic generation, the technique should be applicable to a much wider range of nonlinear processes. In this project, direct experimental testing of current theoretical models will be performed for second harmonic generation, providing a better understanding of the physics involved in the interference. Building on this knowledge and in concert with development of numerical models, the efficiency of phase matching will be optimized using shaping of the ultrafast counterpropagating pulses. The results from these studies are applicable not only to low-order nonlinear optical processes, but also high harmonics, the major source for attosecond science. Additionally, the use of ultrafast counterpropagating pulses will be investigated as a high-resolution, in-situ probe of the dispersion properties of complex nonlinear materials, which may be used in the characterization of periodically-poled media or imaging of biological materials.