MgO (Magnesium oxide) substrates are materials used in various fields such as electronics and optics. They have high thermal stability and are known for their insulating properties. In electronics, MgO substrates are used as insulators in electronic devices such as capacitors and transistors. In optics, MgO substrates are used as substrates for thin film coatings in a variety of applications including anti-reflection coatings, beam splitters, and polarizers. The high thermal stability and high resistance to thermal shock make MgO an attractive material for these applications. Additionally, MgO has a low refractive index, making it an ideal substrate for anti-reflection coatings.