Orbital angular momentum (OAM) light, combined with the nonlinear process to expand the frequency range, has drawn increasing research interest in recent years. Here, we implement the first, to the best of our knowledge, experimental fourth-harmonic generation of OAM light with two cascaded quasi-phase-matching crystals. A Laguerre–Gaussian beam was transmitted through a duplet crystals system and frequency-doubled twice by two separate second-harmonic generation processes, which transduced the frequency of the OAM beam from telecom band to visible band and then to ultraviolet (UV) band. The topological charge of the OAM beam was increased substantially in the cascaded frequency conversion processes. In this experiment, we verify the OAM conservation by utilizing a specially designed interferometer, and the results correspond well with the numerical simulation. This work provides an effective method for the generation of UV OAM beams with high topological charges.

中文翻译：

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级联准相位匹配晶体的四次谐波产生轨道角动量光

轨道角动量（OAM）光与非线性过程相结合以扩展频率范围，近年来引起了越来越多的研究兴趣。在此，我们以我们所知，实现了具有两个级联准相位匹配晶体的OAM光的实验性第四谐波生成。Laguerre-Gaussian光束通过二重晶体系统传输，并通过两个单独的二次谐波生成过程将其倍频两次，这将OAM光束的频率从电信频段转换为可见频段，然后转换为紫外线（UV）频段。在级联频率转换过程中，OAM束的拓扑电荷显着增加。在本实验中，我们通过使用专门设计的干涉仪来验证OAM守恒，结果与数值模拟吻合良好。这项工作为生成具有高拓扑电荷的UV OAM光束提供了一种有效的方法。