Pushing the boundaries of nonlinearities using structured light

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Buldt, Finn, Ontje
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Spatial light modulators give us the ability to manipulate the fundamental constituents of light (amplitude and phase). This ability has ushered in a new way to approach light matter interaction (to control light matter interactions) by modulating the wavefront incident on a material. Light can carry two types of angular momentum, spin angular momentum and orbital angular momentum. The former is linked to the rotation of the polarization vector whereas the latter stems from a helical phase structure. Some structured light beams are specially designed to have this rotating phase structured resulting in beams which carry and have the ability to transfer orbital angular momentum. In this work we use structured light fields shaped by spatial light modulators in conjunction with nonlinear phenomena to create new light beams. The nonlinear processes of concern are second harmonic generation and spontaneous parametric down-conversion . Both have been extensively studied using Gaussian beam profiles, but not structured light fields. The research presented here focuses on developing new, interesting beams and more efficient, brighter sources of entangled photons. To create structured light fields such as a Laguerre-Gaussian or a Bessel-Gaussian, physical optical elements such as a spatial phase plate or an axicon had to be used to produce the respective beam. As these physical elements can only produce a single mode, this limits what can be studied. However, by encoding holographic phase masks onto spatial light modulators one can use a single tool to create structured light fields. We use the versatility afforded to us by the SLM to study the second harmonic signal pumped by three different types of structured light fields: the Bessel-Gaussian, the Laguerre-Gaussian, and the Hermite-Gaussian. With the Hermite-Gaussian beam, we present the difference in mode-conservation of the SH depending on the focusing of the pump. For Laguerre-Gaussian beams, we examine the conservation of total angular momentum in the second harmonic process. Then, as Hermite-Gaussian and Laguerre-Gaussian modes can be inter-converted using astigmatic transform, we study the effect of such a transformation of the pump on the resultant second harmonic signal. Lastly, we present the novel propagation behavior of a frequency doubled Bessel-Gaussian which can be likened to the scattering of solitons. Spontaneous parametric down-conversion is used to generate entangled photons. Commonly, the surface area of the crystal is larger than the incident laser on it. Therefore, we devised a method to sub-divide the beam before incident on the crystal such that each sub-divided beam drives its own down-conversion process. Instead of a single source of entangled photons per crystal, we have multiple sources from the same crystal. Thereby, when multiple sources or a more complex entanglement product is required, our method is more efficient. In order to generate a brighter source of entangled photons or single photons, we combine wavefront shaping with spontaneous parametric down-conversion. Through a simple feedback loop formed by the spatial light modulator and a camera capturing the down-conversion. We optimize the wavefront incident on the nonlinear medium to increase the intensity of the down-conversion at a desired location (and its conjugate).
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Rensselaer Polytechnic Institute, Troy, NY
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