  There is significant interest in tailoring wave packets that transversely
accelerate during propagation. The first realisation was in the optical domain,
the Airy beam. Valid in the paraxial approximation, such beams were shown to
transversely accelerate while following a parabolic caustic, even if the
centroid itself travelled along a rectilinear path. Later, non-paraxial optical
fields in the form of Weber and Mathieu beams were demonstrated, as well as
non-optical wave packets with electrons. Such fields have found a plethora of
applications from particle manipulation, spatial-temporal beam control, plasma
control to non-linear optics. Here we demonstrate, for the first time, the
angular acceleration of light, achieved by non-canonical superpositions of
Bessel beams carrying orbital angular momentum. We show that by adjustment of a
single parameter the acceleration and deceleration may be tuned continuously.
We observe an unexpected energy transfer mechanism between spatial regions of
the field which we investigate experimentally and theoretically. Our findings
offer a new class of optical field that will enable studies in matter waves and
opto-fluidics.
