  The exploration of light has traditionally focused on its spatial properties,
particularly its orbital angular momentum (OAM), while its temporal dynamics
have remained an underexplored frontier due to the slow response times of
existing modulation techniques. In this context, we introduce a method to
modulate the OAM of light on a femtosecond scale by engineering a controllable
space-time coupling in ultrashort pulses. By intricately linking azimuthal
position with time, we implement a static, azimuthally varying wavefront
transformation to dynamically alter the spatial distribution of light in a
fixed transverse plane. Our experiments demonstrate self-torqued wave packets
that exhibit spiraling motions and rapid temporal OAM changes down to a few
femtoseconds. We further extend this concept to generate wave packets that
angularly self-accelerate. We reveal that these wave packets dynamically adjust
their OAM by redistributing their energy density across their spectral
bandwidth, all without the influence of external forces. Owing to the unique
properties of self-torque and angular acceleration, these time-varying OAM
beams offer an accessible avenue for exploring light at fundamental time
scales, with far-reaching implications for ultrafast spectroscopy, nano- and
micro-structure manipulation, condensed matter physics, and other related
areas.
