Investigation of the pyramid–prism boundary of a rapidly grown KDP crystal

To investigate the pyramid-prism (PY-PR) boundary of a rapidly grown KDP crystal, the exact position of the PY-PR boundary of an X-cut rapidly grown KDP sample is determined by measuring the refractive-index non-uniformity. The peak-valley value of Delta(n(e)-n(o)) near the PY-PR boundary is approximately 3 x 10(-6), and the width of the distortion area near the PY-PR boundary is approximately 0.6 mm. The FWHM values of the high-resolution X-ray diffraction peaks in the pyramidal sector, the prismatic sector, and the region containing the PY-PR boundary are all 0.008 degrees, indicating the fine crystalline quality of the crystal. Moreover, the existence of the PY-PR boundary does not affect the fine crystalline quality. From the pyramidal sector to the prismatic sector, the transmittance in the ultraviolet region decreases gradually when crossing the PY-PR boundary. The fluences corresponding to 50% laser-induced damage (LID) probability are 11.9, 12.8, and 17.0 J cm(-2) (3 ns, 355 nm) for the PY-PR boundary, the pyramidal sector, and the prismatic sector, respectively. By identifying different types of defects in each sector, it is found that the PY-PR boundary exhibited the worst LID performance. This is because there are more bulk microdefects in the PY-PR boundary. These defects are more likely to be inclusions induced by the instability of the steps near the PY-PR boundary.