Background Seed orientation is a key parameter for oriented crystal growth, yet its systematic influence on growth kinetics, micro-defect formation, and macroscopic properties remains unclear. Although oriented growth has been explored as a potential method to improve crystal utilization efficiency, the fundamental mechanisms linking seed orientation to defect formation, optical quality, and morphological evolution have not been systematically established.

Purpose This study aims to systematically investigate how seed orientation modulates the morphological evolution, optical properties, and micro-defect characteristics of KDP/DKDP crystals, and to establish quantitative correlations between orientation-dependent defect formation mechanisms and macroscopic crystal performance.

Methods The traditional temperature reduction method was employed to grow KDP/DKDP crystals with seed orientations at angles of 0°, 30°, 45°, and 60° relative to the Z-plane. Positron Annihilation Lifetime Spectroscopy (PALS) was used to characterize vacancy-type defects and calculate defect trapping rates (κ_d). UV transmittance measurements at 355 nm and refractive index uniformity assessments (Δn_RMS) were performed to evaluate optical quality. Three-dimensional morphological reconstruction was conducted to quantify recovery region geometry, including volume calculations and symmetry analysis of 101 pyramidal faces.

Results PALS analysis reveals distinct orientation-dependent defect formation mechanisms: the 30°-cut exhibits the highest defect trapping rate (κ_d=3.68) and lowest UV transmittance (～89%) due to highly asymmetric development of 101 pyramidal faces and large stress field gradients in the recovery region; the 0°-cut demonstrates the lowest defect trapping rate (κ_d = 2.93) and optimal refractive index uniformity (Δn_RMS = 1.43×10⁻⁶) owing to the highest recovery region symmetry and most uniform stress field; the 45°-cut produces the smallest recovery region volume (0.02×10³ cm³), while the 60°-cut yields the largest (11.49×10³ cm³) with complicated recovery paths involving passive exposure of 100 faces. A significant negative correlation between defect trapping rate κd and transmittance at 355 nm was established (R²=0.87), demonstrating the direct impact of orientation-induced defects on optical performance.

Conclusions Seed orientation systematically modulates crystal quality by altering the geometry, symmetry, and stress distribution of the recovery region through control of 101 pyramidal face exposure and growth kinetics. While oriented grown crystals cannot perfectly replace conventional Z-cut seeds, strategic selection of seed orientation (particularly 0° and 45°-cuts) can reduce recovery region volume, improve crystal utilization efficiency, and maintain high optical quality for large-aperture KDP/DKDP crystal applications in high-power laser systems.