1：Experimental Design and Method Selection:

The study compares HPHT and LPHT annealing of type Ib synthetic diamonds at 1870 °C. HPHT annealing was performed at

2：2 GPa for 4 hours, while LPHT annealing was conducted in hydrogen atmosphere at normal pressure for 5 hours. Sample Selection and Data Sources:

Two yellow single crystal type Ib synthetic diamonds produced by AdamasInvest, Ltd were used. Samples were cut and polished to thicknesses varying from

3：32 mm to 8 mm. List of Experimental Equipment and Materials:

High pressure apparatus BARS for HPHT annealing and HTT-G10 graphite furnace for LPHT annealing. Spectroscopic measurements were performed using Agilent Cary 300 UV-VIS spectrophotometer, Thermo Scientific Nicolet iS50 spectrometer, Bruker Vertex 70 spectrometer, and Renishaw inVia Raman confocal spectrometers.

4：Experimental Procedures and Operational Workflow:

After annealing, graphitized surface layers were removed, and samples were chemically cleaned. Optical absorption and photoluminescence measurements were conducted before and after annealing.

5：Data Analysis Methods:

Absorption coefficients and concentrations of nitrogen defects were calculated from IR spectra. Photoluminescence intensities were adjusted to equal intensity of Raman line for semi-quantitative comparison.