The interaction between a quantum particle’s spin angular momentum¹ and its orbital angular momentum² is ubiquitous in nature. In optics, the spin–orbit optical phenomenon is closely related with the light–matter interaction³ and has been of great interest^4,5. With the development of laser technology⁶, the high-power and ultrafast light sources now serve as a crucial tool in revealing the behaviour of matter under extreme conditions. A comprehensive knowledge of the spin–orbit interaction for intense light is of utmost importance. Here, we report the in situ modulation and visualization of the optical orbital-to-spin conversion in the strong-field regime. We show that, through manipulating the morphology of femtosecond cylindrical vector vortex pulses⁷ by a slit, the photon’s orbital angular momentum can be controllably transformed into spin after focusing. By employing a strong-field ionization experiment, the orbital-to-spin conversion can be imaged and measured through the photoelectron momentum distributions. Such detection and consequent control of the spin–orbit dynamics of intense laser fields has implications for controlling photoelectron holography and coherent extreme-ultraviolet radiation⁸.

本质上，量子粒子的自旋角动量¹和其轨道角动量²之间的相互作用是普遍存在的。在光学中，自旋轨道光学现象与光-质相互作用³密切相关，并且引起了人们的极大关注^4,5。随着激光技术的发展⁶，高功率和超快光源现在成为揭示物质在极端条件下的行为的关键工具。对强光的自旋-轨道相互作用的全面了解至关重要。在这里，我们报告在强场体制中的光学轨道到自旋转换的原位调制和可视化。我们表明，通过操纵飞秒圆柱矢量涡旋脉冲的形态⁷通过狭缝，聚焦后，光子的轨道角动量可控制地转变为自旋。通过采用强场电离实验，可以通过光电子动量分布来成像和测量轨道到自旋的转换。这种检测和对强激光场自旋轨道动力学的控制，对于控制光电子全息术和相干极紫外辐射具有重要意义⁸。