周期极化铌酸锂晶体中高质量拉盖尔-高斯光束的非线性产生及调控

Laguerre-Gaussian (LG) beams are characterized by a set of unique features: they have helical phase front, they carry orbital angular momentum and they have phase singularity at the core. These features were shown to be useful for a variety of applications, such as rotating optical tweezers for micro-particles, classical and quantum optical communications, super-resolution microscopy, astronomic observations, etc. However, it is still challenging to efficiently generate bright, pure and short-wavelength LG modes in compact devices. This proposal focuses on new methods for generation and manipulation of LG beams by nonlinear three wave mixing processes in a periodically poled LiNbO3 (PPLN) crystal. It aims to address a wide range of topics, starting from fundamental questions related to the mode purity in wave mixing processes, to the development of practical and efficient methods for nonlinear generation of LG beams and up to the generation of these beams in the ultra-violet spectral range by compact and efficient cascaded nonlinear processes. Whereas the first topic will enable to understand the limitations of generating LG beams by nonlinear processes, the latter studies will enable to generate LG beams at wavelengths in which both lasers and linear beams shaping elements (such as spatial light modulators or Q-plates) are either inefficient or completely unavailable. Specifically, the research program includes 3 main topics: the effects of (non)-collinearity and the phase matching scheme - Bragg, Raman-Nath or Cerenkov - on the mode purity of the generated LG beam in nonlinear mixing process; third harmonic generation in the blue and UV of LG beams; and finally intra-cavity second- and third-harmonic generation of high-purity LG beams. The group from Nanjing University is active in this field for many years and have made some of the pioneering experiments on nonlinear generation and all-optical manipulation of LG beams. In this project, we expect to understand the fundamental physics in the nonlinear manipulation of LG beams in PPLN crystals, and to realize high-efficiency generation of high-purity, high power and short wavelength LG beams.

拉盖尔-高斯（LG）光束具有连续涡旋状相位波前、光子轨道角动量、中心相位奇点等特点，由于在光学操控、经典/量子通讯、超分辨成像等领域有重要应用，在近年来成为光学领域的研究热点之一。随着应用的需求不断发展，对LG光束质量（包括高纯度、高功率、短波长等）的要求也在日益提高，达到了用传统方法难以实现的高标准。本项目主要研究周期极化铌酸锂晶体这一人工微结构材料体系中高质量LG光束的非线性产生及操控，探索LG光束非线性混频过程中的新型相位匹配机制，发展LG光束高效非线性光学转换的方法，并利用空间模式匹配、光学腔等方法实现高纯度LG光束。申请人的课题组在该领域有多年的工作积累，在不同类型周期极化铌酸锂晶体的设计和制备方面有较好的工作基础。通过本项目的实施，将从基本物理层面理解周期极化铌酸锂晶体中操控LG光束的机制，并面向应用需求实现具有高纯度、高强度和短波长LG光束的高效产生。

在本项目中，我们主要研究周期极化铌酸锂晶体中的新型制备技术、相位匹配机制和非线性混频原理，结合高阶模式光学腔设计，在周期极化铌酸锂晶体中实现携带轨道角动量的拉盖尔-高斯光束的高质量产生和有效操控，取得了若干创新成果，具体包括：（1）基于周期极化铌酸锂晶体建立了产生和操控拉盖尔-高斯光束的理论模型，深入理解了光子轨道角动量的守恒规律及其调控方法；（2）利用新发展的激光畴工程技术，发展了三维相位匹配机制，设计并制备了特定构型的三维铌酸锂非线性光子晶体，实现了二次谐波拉盖尔-高斯光束的高效输出；（3）开发了腔内模式转换技术，实现了高纯度拉盖尔-高斯光束和矢量偏振光束的激光输出，模式可控、效率高、纯度高；（4）还基于高质量拉盖尔-高斯光束实现了超长二次谐波光针制备、旋转物体直接成像、以及大容量非线性全息复用等应用演示。

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