Silicon is a lightweight rigid material, but it’s thermal conductivity is poor, and it’s natural reflectivity in the visible and infra red is very low. Consequently Silicon mirrors are always used with a vacuum deposited optical coating, such as MaxR for CO2 laser, L/4 reflective phase retarder, or protected silver.
Silicon mirrors tend to be used by lower power CO2 lasers as flat fold mirrors, and in clean conditions where back spatter and dust will not cause catastrophic failure of the mirrors. Typically a Silicon mirror has 1/10th of the laser damage resistance of a copper based mirror.
Chemical polishing of Silicon gives exceptional mirror surface smoothness, and the material is very chemically resistant.
Silicon is brittle, glass like material, and is not machinable by conventional metal working tools such as mills, lathes, etc, As a result Silicon based mirrors are generally circular or have very simple geometries, and cannot have internal water cooling channels. Cooling of silicon mirrors is achieved by water cooled pads in intimate thermal contact with the back of the mirror.
Traditionally Silicon has been used for low cost, high volume mirror applications, but the explosive growth of competing uses of semiconductors in solar energy, photo voltaics etc, has seen it’s price, and availability fluctuate.
Heat transfer compound is often used on the back of Silicon mirrors to sink heat away to a cooling pad. Many brands of heat transfer compound are very strongly absorbing in the infra red, and the slightest residue on optical surfaces of the paste will cause severe absorption and rapid destruction of the mirror. See our technical document about contamination of laser optics.