Figure 4: Quantitative characterization of group matching.
From: Giant narrowband twin-beam generation along the pump-energy propagation direction
Frequency spectra of PDC are recorded at external angles 0.142, 0.139, 0.137, 0.133, 0.125 and 0.117 rad for one (dashed line) and four (solid line) BBO crystals of length 5 mm oriented at 37.5°. In 20 mm of BBO crystal, the emission at the group-matched wavelength 533.5 nm (shown by green arrow) is enhanced 250 times compared with the wavelength 637.5 nm, where there is no group-velocity matching. In a 5-mm crystal, the enhancement is not so dramatic.
