Tuning curve of type-0 spontaneous parametric down-conversion
  We study the tuning curve of entangled photons generated by type-0 spontaneous parametric down-conversion in a periodically poled KTP crystal. We demonstrate the X-shaped spatiotemporal structure of the spectrum by means of measurements and numerical simulations. Experiments for different pump waists, crystal temperatures, and crystal lengths are in good agreement with numerical simulations. 
Stefan Lerch, B\"anz Bessire, Christof Bernhard, Thomas Feurer, and   Andr\'e Stefanov
quant-ph physics.optics