  We study the spontaneous formation of exciton polariton vortices in an
all-optical non-resonantly excited annular trap, which is formed by ring-shaped
subpicosecond laser pulses. Since the light emitted by these vortices carries
orbital angular momentum (OAM) corresponding to their topological charge, we
apply a dedicated OAM spectroscopy technique to detect the OAM of the
measurement signal with picosecond time resolution. This allows us to identify
the formation of OAM modes and investigate the dynamics of the vortex formation
process. We also study the power dependence of this process and how the ring
diameter influences the formation of OAM modes.
