  Space-time (ST) wave packets are a class of pulsed optical beams whose
spatio-temporal spectral structure results in propagation invariance, tunable
group velocity, and fascinating refractive phenomena. Here, we investigate the
refraction of ST wave packets at a planar interface between two dispersive,
homogeneous, isotropic media. We formulate a new refractive invariant for ST
wave packets in this configuration, from which we obtain a law of refraction
that determines the change in their group velocity across the interface. We
verify this new refraction law in ZnSe and CdSe, both of which manifest large
chromatic dispersion at near-infrared frequencies in the vicinity of their band
edges. ST wave packets can thus be a tool in nonlinear optics for bridging
large group-velocity mismatches in highly dispersive scenarios.
