The characteristics of "hair" inclusions in large aperture KDP/DKDP crystals and their impact on laser damage

The "hair" inclusion defect is typical in large aperture KDP/DKDP crystals, severely limiting the optical lifetime in high-power laser devices. However, the characterization of this defect and its impact on the crystal laserinduced damage performance are still unclear. In this work, the macroscopic and microscopic characteristics of the "hair" inclusions inside KDP/DKDP crystals are investigated in detail by laser scattering technique and optical microscopy. It is found that the chains of inclusions extend inside the crystals at an angle of 88 degrees similar to 123 degrees to the {101} crystal faces. The chains consist of hundreds or thousands of independent inclusions, which are mostly pill-shaped, spherical, and elongated. Besides, the Raman spectroscopy demonstrates that the inclusions are mainly mother solutions and may contain gaseous phases inside. Thereafter, the local light intensity enhancement model of the inclusions is established to analyze the modulation effect by inclusion size and orientation angle. The average size of the inclusions and the typical inclination angle are located in the field strength enhancement range, causing a sharp amplification of the laser energy. In addition, the laser damage resistance at 355 nm of the crystals is measured. The intrusion of inclusions, which act as a "lens", produces the light intensity enhancement and would cause the crystal substrate to be the first to damage, lowering the laser-induced damage thresholds (LIDTs) of the KDP/DKDP crystals. This study offers guidance for controlling "hair" inclusion defects in KDP/DKDP crystals and provides insights into the mechanism of laser and inclusion-induced damage of crystals.