Fig. 4: Spectral measurement of the PDC signal in GaAs.

Spectra for a the (2 0 0) atomic planes, b the (4 0 0) atomic planes, c the (1 1 1) atomic planes, and d the (3 1 1) atomic planes. The features in a and b can be attributed to atomic resonances or to inter-band transitions in the crystal. The vertical error bars indicate the counting statistics. See further details in the text.