Passivation of nonlinear optical crystals

A method comprising:

• growing a fluorine-doped borate-based nonlinear optical crystal from a melt containing an atomic percent of fluorine of 5-20% of the atomic percentage of boron in the melt to inhibit incorporation of OH into the fluorine-doped borate-based nonlinear optical crystal, wherein the fluorine-doped borate-based nonlinear optical crystal comprises at least one of cesium borate (CBO) or cesium lithium borate (CLBO);
• mechanically preparing the fluorine-doped borate-based nonlinear optical crystal;
• performing an annealing process on the fluorine-doped borate-based nonlinear optical crystal to heat the fluorine-doped borate-based nonlinear optical crystal to drive OH content out of the fluorine-doped borate-based nonlinear optical crystal;
• and
• exposing the fluorine-doped borate-based nonlinear optical crystal to a passivating gas comprising at least one of deuterium mixed with one or more inert gases or a deuterium-containing compound mixed with one or more inert gases, wherein the at least one of deuterium or the deuterium-containing compound of the passivating gas has a selected concentration in a range of 5 to 10%.