A simple way of classifying the potential use of a mirror is to decide if the mirror is to be used for beam delivery (i.e in the atmosphere) or in the laser cavity under vacuum.

Substrates

Copper – Has excellent heat conductivity, is readily available, and can be machined into various shapes. Generally used coated as oxidation occurs in the atmosphere. Use in the highest powered lasers, both in beam delivery, and for cavity optics.

Molybdenum - Hard, durable metal, but expensive and difficult to process. Always used uncoated. Natural reflectivity is poor (97%), limited to beam delivery use.

Silicon – Lightweight semiconductor that is cheap and easily polished. Never used uncoated. Difficult to machine and profile. Used in beam delivery and cavity applications. Limited to low and medium powered lasers.

Aluminium- Lightweight metal, readily available, easily machined. Always used with coating such as gold. Often limited to scanning or where lightweight mirrors are needed. Can be used with low and medium powered lasers.

Beryllium – Lightweight stiff metal. Dust is severe health hazard and strictly controlled. Use limited to military and aerospace applications.

Coatings

Gold electroplate – Coating of choice for beam delivery, has zero phase shift to eliminate polarisation effects. Will work at the very highest of powers and resists damage even when burnt or scratched. Not quite reflective enough for cavity use. Broadband reflectance is good for multi wavelength applications both pulsed and CW. Cannot be made on silicon

Max R Dielectric – Favoured for cavity use where the high reflectance of 99.9% is needed to give maximum power output. Can be used for beam delivery as well. Generally more fragile than gold coatings. UV resistance is an important feature for cavity optics. The reflectance is generally optimised for a specific wavelength and angle of incidence which must be known at time or ordering. Can be made on copper, or silicon

Phase retarders- A vacuum deposited dielectric coating that introduces a ¼ wave phase shift. This circularly polarises a linearly polarised beam. Very sensistive to incident angle (45 degrees) and wavelength (10.6 microns). Can be made on copper or silicon.