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

非线性光学提供了一种独特的方法，通过称为频率转换的过程来创建可访问且具有成本效益的激光源。事实上，一些透明材料的自然反应是将强烈的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指示器到激光点火聚变的应用。激光从一个频率到另一个频率的有效转换需要在这种光-材料相互作用中对材料成分进行仔细且有时复杂的工程设计。这些技术在大范围的应用中非常成功，但受限于可为此目的设计的材料。这个CAREER奖支持的研究不是光-材料相互作用中的工程材料，而是探索我们如何设计光。虽然这项研究的主要应用是开发新光源，但光场工程也可以揭示光转换过程的基本物理学，并为测量材料本身提供非常精确的工具。这项研究将通过指导实验光学科学的本科生，以及通过跨学科的第一年研讨会课程提高科学素养的课程开发，并通过中级光学课程为学生提供早期参与光学在研究中的应用，与研究密集型文理学院的教育相结合。非线性频率转换过程（例如二次谐波产生）的效率的主要挑战是非线性光学材料的色散。最近，一种新的方法，用于校正的色散效应已经开发出来，其中序列的反向传播脉冲被用来干扰周期性的谐波产生过程，实现准相位匹配的全光版本。虽然这种技术的实验演示只显示了高次谐波的产生，该技术应适用于更广泛的非线性过程。在本项目中，将对当前的理论模型进行直接实验测试，以产生二次谐波，从而更好地了解干扰中涉及的物理学。基于这一知识，并与数值模型的发展相一致，相位匹配的效率将使用超快反向传播脉冲的成形来优化。这些结果不仅适用于低阶非线性光学过程，而且适用于高次谐波，这是阿秒科学的主要来源。此外，超快对向传播脉冲的使用将作为复杂非线性材料色散特性的高分辨率原位探测器进行研究，其可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.