Irradiation Effects of Nanosecond Laser on Potassium Dihydrogen Phosphate Crystals and Its Damage Mechanisms

The surface morphology, microstructures, chemical bond vibrations and optical performances of Potassium dihydrogen phosphate(KDP) crystals induced by nanosecond laser with different wavelengths, are studied by means of field emission scanning electron microscopy(FE-SEM) and infrared spectroscopy(IR) microanalysis techniques at different energy densities. SEM results showed that with the increase of laser irradiation energy density, the damage region on KDP surfaces presented three parts evidently. And the damage area increases nonlinearly with the laser energy density. Meanwhile, at the same energy density, the damage area induced by the 355 nm nanosecond laser is much larger than that of the 1 064 nm nanosecond laser. Infrared spectroscopy results revealed that the laser irradiation leads to a break in hydrogen bonds and a dehydration of KDP crystals, in conjunction with a generation of P=O and P-H bonds due to the vibrational mode disorder of the PO₄³⁻ tetrahedron and H₂PO₄⁻ groups after laser irradiation. Simultaneously, after irradiation, the surface reflectivity of KDP crystals decreased and the surface is roughened and deliquesced. Moreover, with the increase of the laser energy density, the central region of the KDP crystal is damaged firstly, and then is annealed and repaired, finally is destroyed again under the thermal effects of the laser energy deposition. In addition, after irradiation with the energy density of 10 J/cm², all vibration modes of central regions of KDP crystals weaken or even disappear, and the sever damage occurs.