  'Space-time' (ST) wave packets are propagation-invariant pulsed optical beams
that travel rigidly in linear media without diffraction or dispersion at a
potentially arbitrary group velocity. These unique characteristics are a result
of spatio-temporal spectral correlations introduced into the field;
specifically, each spatial frequency is associated with a single temporal
frequency (or wavelength). Consequently, the spatial and temporal bandwidths of
ST wave packets are correlated, so that exploiting an optical source with a
large temporal bandwidth or achieving an ultralow group velocity necessitate an
exorbitantly large numerical aperture. Here we show that `spectral recycling'
can help overcome these challenges. 'Recycling' or `reusing' each spatial
frequency by associating it with multiple distinct but widely separated
temporal frequencies allows one to circumvent the proportionality between the
spatial and temporal bandwidths of ST wave packets, which has been one of their
permanent characteristics since their inception. We demonstrate experimentally
that the propagation invariance, maximum propagation distance, and group
velocity of ST wave packets are unaffected by spectral recycling. We also
synthesize a ST wave packet with group velocity c/14.3 (c is the speed of light
in vacuum) with a reasonable numerical aperture made possible by spectral
recycling.
