Feedback-free programmable optical torques enabled by spin-orbit-engineered metasurfaces

Programmable optical torque is essential for advanced optical manipulation, yet existing approaches rely on feedback-driven, time-varying optical fields, introducing complexity and limiting scalability. Here, we introduce a new concept of freeform optical torques enabled by metasurfaces, and experimentally demonstrate their feedback-free, programmable control with flexibly shaped rotational-angle dependence, overcoming the long-standing limitation of torque functions confined to constant or sinusoidal forms. This capability arises from converting the temporal degree of control into a spatial one, realized through the interaction between temporally static vector optical fields and spin-orbit-engineered metasurfaces. To characterize these optical torque functions, we developed a torsion-balance system operating near the thermal noise limit. The system achieves high sensitivity of 0.22 fNm·Hz^-0.5 for macroscopic samples and enables direct measurement of fNm-level optical angle-torque functions that were previously unresolvable. This strategy provides unprecedented freedom in optical torque control, enabling passive, all-optical attitude and rotational motion control with broad implications from microscale robotics to macroscopic lightsails.