Attosecond Pulses with Time-Varying Orbital Angular Momentum: The Self-Torque of Light

Propagating laser beams with defined orbital angular momentum (OAM) have found applications that enable new paradigms in optical metrology and communication, quantum information, photomechanical manipulation, and super-resolution imaging. To date, however, the generation and application of OAM beams has been largely based on a static interpretation of optical OAM, where OAM is time-independent. We show for the first time that propagating light waves can possess dynamic, time-varying OAM. We exploit the highly non-linear and non-perturbative process of high-harmonic generation to imprint time-dependent OAM onto extreme ultraviolet (EUV) beams. This new photonic property, self-torque (an analogy to mechanical systems that exhibit a self-induced time variation of their angular momentum), manifests as a few-to-sub femtosecond variation of optical OAM states along each pulse in the underlying attosecond pulse train. We confirm that optical self-torque endows unique properties to coherent EUV waveforms, which enable the delivery of optical torque on the natural time and length scales of charge and spin ordering in materials. Ability to sculpt the OAM wavefront of EUV beams also offers exciting potential in imaging, metrology and sensing methods for semiconductor and quantum materials.