Enhanced Optical Properties of CVD Diamond through HPHT Annealing

Diamond, with its outstanding physicochemical properties, stands out as an ideal material for nonlinear optical applications. However, the subpar optical performance of commercial diamonds, particularly their insufficient optical homogeneity, significantly hindered their broader application in high-power nonlinear lasers. In this study, annealing experiments were successfully conducted on CVD diamond at high temperatures of up to 2400 °C under a pressure of 6 GPa. Post annealing, the optical homogeneity of the diamond exhibited a significant improvement, the extent of which varied based on the annealing temperature. We hypothesize that the enhancement could be linked to the coalescence of dislocations within the crystal. As the annealing temperature exceeded 2000 °C, a significant reduction in C–H bonds, NV centers, and SiV centers in diamonds was noted, accompanied by a substantial decrease in internal stress, leading to a marked improvement in diamond transmittance. Additionally, we investigated the impact of cooling time, emphasizing that excessively short cooling times could introduce additional thermal stress, leading to degradation of the optical properties of diamond. Our experiment provides the first confirmation that high-temperature/high-pressure (HPHT) annealing could enhance the optical homogeneity of diamond, offering a convenient method to enhance the optical properties of commercial CVD diamonds grown rapidly. The impact of HPHT annealing on the optical properties of CVD diamonds is studied. Results demonstrate that HPHT annealing could enhance the optical uniformity of diamonds, with the extent of enhancement being temperature-dependent. Furthermore, a notable decrease in the content of C–H bonds, NV centers, and SiV centers in diamonds is observed post annealing.