  Twisted light fields carrying orbital angular momentum (OAM) provide powerful
capabilities for applications in optical communications, microscopy, quantum
optics and microparticle rotation. Here we introduce and experimentally
validate a new class of light beams, whose unique property is associated with a
temporal OAM variation along a pulse: the self-torque of light. Self-torque is
a phenomenon that can arise from matter-field interactions in electrodynamics
and general relativity, but to date, there has been no optical analog. In
particular, the self-torque of light is an inherent property, which is
distinguished from the mechanical torque exerted by OAM beams when interacting
with physical systems. We demonstrate that self-torqued beams in the
extreme-ultraviolet (EUV) naturally arise as a necessary consequence of angular
momentum conservation in non-perturbative high-order harmonic generation when
driven by time-delayed pulses with different OAM. In addition, the
time-dependent OAM naturally induces an azimuthal frequency chirp, which
provides a signature for monitoring the self-torque of high-harmonic EUV beams.
Such self-torqued EUV beams can serve as unique tools for imaging magnetic and
topological excitations, for launching selective excitation of quantum matter,
and for manipulating molecules and nanostructures on unprecedented time and
length scales.
