Scientists Realize Rapid Growth of a Large KDP Crystal with a Long-seed
KDP (KH2PO4) and its isomorphs, DKDP (KDxH2-xPO4) are the only available nonlinear crystals used as electro-optical switches and frequency converters for inertial confinement fusion (ICF) systems on account of their particular properties.
For the 'point-seed' rapidly grown KDP-type crystals, all prismatic faces and pyramidal faces grow under high supersaturated solutions, forming the so-called pyramid-prism (PY-PR) boundary which is detrimental to efficient frequency tripling and phase matching.
More recently, a cuboid DKDP crystal was rapidly grown. Without a pyramidal sector, the cuboid DKDP crystal had high cutting efficiency to obtain third harmonic generation (THG)-oriented samples for ICF. However, it is not always easy to get a long seed to grow a large cuboid KDP-type crystal. Therefore, it is of practical significance to reduce the length of the seed while maintaining the advantages of the cuboid KDP-type crystal.
The research led by the team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (CAS), had successfully grown large long-seed KDP crystal with the size of 471 mm × 480 mm × 400 mm. Their study was published in High Power Laser Science and Engineering.
They found that, with almost the same high cutting efficiency to obtain THG-oriented samples, this long-seed KDP-type crystal could grow from a seed that is shorter than that of the cuboid KDP-type crystal. For example, to get 10 THG-oriented samples with size 460 mm × 430 mm × 10 mm, the size of the seed should be at least 486 mm for the cuboid KDP-type crystal, while it was just 348 mm for the long-seed KDP-type crystal.
The experiment showed that the long-seed KDP crystal also exhibited good crystalline quality since the FWHM of HRXRD was the same as that of the cuboid DKDP crystal. As to the transmission and the LID performance, the long-seed KDP crystal was even better than the cuboid DKDP crystal.
Furthermore, new seeds with much longer size in the  crystallography direction than the original seed could be taken from the pyramidal portion down through the prismatic portion. In addition, because the carrier had a height greater than that of the long seed, there was no need to replace the carrier for a different seed with that of different height.
For the cuboid KDP-type crystal, once the height of the carrier is set, the length of the seed must be equal to the height of the carrier. Therefore, the long-seed KDP-type crystal is more promising than the cuboid KDP-type crystal in engineering application.
Long-seed KDP crystal and HRXRD of the (200) crystalline face (Image by SIOM)