Lithium (Li) is a soft, silvery-white alkali metal that is the lightest metal and the lightest solid element. It is highly reactive and flammable, requiring special handling and storage procedures. In the context of optics and photonics, lithium is used as a component in various optical materials like lithium niobate (LiNbO3) and lithium tantalate (LiTaO3), which are commonly used for nonlinear optics, modulators, and other optical devices due to their high electro-optic coefficients. Additionally, lithium is used in the manufacturing of specialty glasses and ceramics, contributing to their desirable optical properties. Although lithium itself is not used in optical applications, its compounds are highly valuable in the field.