  Spatio-temporal optical vortices (STOVs) are a new type of optical orbital
angular momentum (OAM) with optical phase circulation in space-time. In prior
work [N. Jhajj et al., Phys. Rev X 6, 031037 (2016)], we demonstrated that a
STOV is a universal structure emerging from the arrest of self-focusing
collapse leading to nonlinear self-guiding in material media. Here, we
demonstrate linear generation and propagation in free space of STOV-carrying
pulses. Our measurements and simulations demonstrate STOV mediation of
space-time energy flow within the pulse and conservation of OAM in space-time.
Single-shot amplitude and phase images of STOVs are taken using a new
diagnostic, transient grating single-shot supercontinuum spectral
interferometry (TG-SSSI).
