To analyze the mechanism of damage threshold enhancement after laser conditioning, the fluorescent and stimulated Raman scattering properties of unconditioned and laser-conditioned KD_2PO_4 crystals are compared in detail. It is revealed that the intensity of fluorescence decreases significantly, especially fluorescence <400 nm, after nanosecond laser conditioning, and the change of the stimulated Raman scattering peak at 921 cm~(-1) is very weak. Moreover, the intensity of fluorescence further decreases after subnanosecond laser conditioning. A sharp decrease in the fluorescence intensity <400 nm reflects a change in energy levels of the electron defects in a crystal. The Raman scattering proves very weak change of the PO_4 vibrational modes. Furthermore, a laser-conditioning mechanism is discussed in combination with electronic transition and thermal processes.
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