  The concept of angular momentum is ubiquitous to many areas of physics. In
classical mechanics, a system may possess an angular momentum which can be
either transverse (e.g., in a spinning wheel) or longitudinal (e.g., for a
fluidic vortex) to the direction of motion. Photons, however, are well-known to
exhibit intrinsic angular momentum which is longitudinal only: the spin angular
momentum defining the beam polarization and the orbital angular momentum
associated with a spiraling phase front. Here we show that it is possible to
generate a novel state of light that contains purely transverse angular
momentum, the analogue of a spinning mechanical wheel. We use an optical
nano-probing technique to experimentally demonstrate its occurrence in our
setup. Such a state of light can provide additional rotational degree of
freedom in optical tweezers and optical manipulation.
