Thick-crystal regime in photon pair sources

Abstract

The workhorse technique for generating correlated pairs of photons is based on spontaneous parametric downconversion in nonlinear crystals. These photon pair sources are usually designed with relatively short crystal lengths, in the belief that this is necessary to attain good performance. We show, contrary to common practice, that concurrent high brightness and efficiency are also available to longer crystals. We present comprehensive measurement data on the pump and collection beam parameters necessary to achieve high collection efficiency (89.0%±1.7% and 81.9%±3.7% for signal and idler) together with high brightness when a single thick β-barium borate crystal (15.76 mm) is pumped with a narrow linewidth laser. Spectral analysis of the collinear, nondegenerate photons suggests that the effective interaction length within the crystal is dominated by the collection beam mode, and the use of longer crystals with increased spatial walk-off does not necessarily lead to reduced collection efficiency. This surprising result is an important consideration for optical designers who seek to develop practical photon pair sources.