Laser pulses generate electron-hole pairs in a ZnSe crystal and selective trapping alters the carrier concentration. A result is modulated optical absorption Δα, probed by a second monochromatic beam. The effect is extrinsic and may be associated with ionized Cu in substitution for Zn. Phase-sensitive kinetics measurements reveal a thermal component and indicate that at least two impurity levels are involved, the latter with photon-capture cross sections q in the range 0.3 × 10 −16 to 0.9 × 10 −16cm 2, corresponding to non-equilibrium carrier concentrations Δn≅10 13 cm −3.