title: The role of spatial and temporal modes in pulsed parametric down-conversion.
abstract: We explore spatial correlations created by stimulated pair emission in frequency degenerate parametric down-conversion from a periodically poled KTP crystal pumped by ∼2 ps duration laser pulses. The ratio of stimulated pairs over spontaneous pairs reaches as high 0.8 in the experiment. This ratio is a direct measure of the total number of modes relevant to the down-conversion process. We identify a universal curve for this ratio that accounts for the effect of the focused pump, introducing a coherence diameter r(0) related to the diffraction limited size of the pump beam in the far-field. Measurements of the spatial correlations of the PDC light for longer crystals and tight focusing conditions show that the description given in terms of a universal curve is surprisingly robust and breaks down only for a laser beam focussed to a waist smaller than 40 μm in a 2 mm long PPKTP crystal.
authors.forename: S C
authors.initials: SC
authors.lastname: Yorulmaz
authors.forename: M P
authors.initials: MP
authors.lastname: van Exter
authors.forename: M J A
authors.initials: MJ
authors.lastname: de Dood