Observation of ultra-broadband, beamlike parametric downconversion

We report spontaneous parametric downconversion having an unusually wide spectral bandwidth. A collinear type-1 phase-matching configuration is employed with degeneracy near the zero group velocity dispersion frequency. With a spectral width of 1080 nm and degenerate wavelength 1885 nm, the source also emits a high flux of photon pairs constrained to a cone of only 2 degree half-angle. A rigorous theoretical approach is developed that confirms the experimental observations. The source properties are consistent with an ultra-short photon-pair correlation time and, for a narrowband pump, extremely high-dimensional spectral entanglement.