Si - Silicon
American Photonics silicon is the most commonly used material for internal cavity fold mirrors and external beam delivery mirrors. The selection is mainly due to its low cost, thermal stability, and good durabilitiy. Silicon is also available from APC as optical grade for infrared transmitting applications.
APC can provide raw or generated blanks to most customer specifications. Circular, square or rectangular geometries are available. See APC's capabilities with optical fabrication and thin film coatings for more details.