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

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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.