Space-time coupling in the up-conversion of broadband down-converted   light
  We investigate the up-conversion process of broadband light from parametric down-conversion (PDC), focusing on the spatio-temporal spectral properties of the sum-frequency generated (SFG) radiation. We demonstrate that the incoherent component of the SFG spectrum is characterized by a skewed geometry in space-time, which originates from a compensation between the group-velocity mismatch and the spatial walk-off of the fundamental and the SFG fields. The results are illustrated both by a theoretical modeling of the optical system and by experimental measurements. 
Enrico Brambilla, Ottavia Jedrkiewicz, Paolo di Trapani and Alessandra   Gatti
physics.optics