Designs and Materials for General and High Pulse Rate Laser Reflectors
Reflecting coatings are used to control optical paths integral to a variety of optical functions and instruments, including household mirrors, high performance reflectors used in space telescopes, short-pulse high-frequency high-energy laser applications with high damage resistance, and more. In this technical paper, we briefly discuss standard and advanced reflector designs, materials, and their applications.
General Reflector Designs
High reflector designs fall into two categories that are application dependent: protected metal and multilayer dielectric. Metallic reflectors cover the widest span of wavelengths and have low polarization properties. Highly reflecting metals are mechanically vulnerable to scratching and chemically corrosive environments and therefore, require abrasion and environmental protection. Commercial applications use Aluminum (Al), either as a second surface reflector or with a protective overcoating layer of Titanium Dioxide (TiO2) or Silicon Monoxide (SiO).
SiO forms hard dense amorphous films that have low permeability to moisture. Because Silicon Dioxide (SiO2) is the favored composition state of Si + O2, careful consideration must be taken to manufacture SiO starting material and to deposit SiO as SiO. Thermal evaporation is typically used since the composition is easier to control than it is when using e-beam and sputtering deposition. SiO absorbs at wavelengths <450 nm and has a limited laser energy damage threshold. However, durability enhancement is a benefit for general purpose first-surface mirrors, including scientific and medical instruments. Similar coatings from the fully oxidized composition can be porous or require more complex stacks to achieve the same high performance and environmental durability as obtained with the monoxide.
Bare Aluminum reaches its maximum reflectance 88-90% and has the highest average Reflectance of any metal for wavelengths as short as ~200 nm at a thickness ~100 nm. To enhance the reflection to values ~95%, two stacks of high/low index dielectric layers can be added. Applications for enhanced reflection mirrors include scientific and medical instruments.
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