  Today, it is well known that light possesses a linear momentum which is along
the propagation direction. Besides, scientists also discovered that light can
possess an angular momentum (AM), a spin angular momentum (SAM) associated with
circular polarization and an orbital angular momentum (OAM) owing to the
azimuthally dependent phase. Even though such angular momenta are longitudinal
in general, a SAM transverse to the propagation has opened up a variety of key
applications [1]. In contrast, investigations of the transverse OAM are quite
rare due to its complex nature. Here we demonstrate a simple method to generate
a three dimensional (3D) optical wave packet with a controllable purely
transverse OAM. Such a wave packet is a spatiotemporal (ST) vortex, which
resembles an advancing cyclone, with optical energy flowing in the spatial and
temporal dimension. Contrary to the transverse SAM, the magnitude of the
transverse OAM carried by the photonic cyclone is scalable to a larger value by
simple adjustments. Since the ST vortex carries a controllable OAM in the
unique transverse dimension, it has a strong potential for novel applications
that may not be possible otherwise. The scheme reported here can be readily
adapted for the other spectra regime and different wave fields, opening
tremendous opportunities for the study and applications of ST vortex in much
broader scopes.
