Spin and orbital angular momentum of an optical beam are two independent parameters that exhibit distinct effects on mechanical objects. However, when laser beams with angular momentum interact with plasmas, one can observe the interplay between the spin and the orbital angular momentum. Here, by measuring the helical phase of the second harmonic 2ω radiation generated in an underdense plasma using a known spin and orbital angular momentum pump beam, we verify that the total angular momentum of photons is conserved and observe the conversion of spin to orbital angular momentum. We further determine the source of the 2ω photons by analyzing near field intensity distributions of the 2ω light. The 2ω images are consistent with these photons being generated near the largest intensity gradients of the pump beam in the plasma as predicted by the combined effect of spin and orbital angular momentum when Laguerre-Gaussian beams are used.

光束的自旋和轨道角动量是两个独立的参数，对机械物体表现出明显的影响。然而，当具有角动量的激光束与等离子体相互作用时，人们可以观察到自旋和轨道角动量之间的相互作用。在这里，通过使用已知的自旋和轨道角动量泵浦光束测量在低密度等离子体中产生的二次谐波2ω辐射的螺旋相位，我们验证了光子的总角动量是守恒的，并观察了自旋向轨道角的转换动量。我们通过分析2ω光的近场强度分布进一步确定2ω光子的来源。2 ω 图像与这些光子一致，这些光子是在等离子体中泵浦光束的最大强度梯度附近生成的，这是由使用拉盖尔-高斯光束时自旋和轨道角动量的组合效应所预测的。