Highreflectivity

High reflectivity refers to the property of a surface or optical coating that reflects a large fraction of incident light over a specified wavelength range. It is quantified by the reflectivity R, the ratio of reflected to incident power, often expressed as a percentage. In optical design, high reflectivity is achieved with mirrors and coatings designed to minimize transmission and absorption losses.

Dielectric multilayer mirrors use alternating layers of materials with high and low refractive indices, engineered as quarter-wave stacks to produce constructive interference of reflected waves. These dielectric high-reflectivity coatings can reach reflectivities exceeding 99.9% at a target wavelength, and can be tailored to narrow or broaden spectral bands. Metallic coatings such as silver or aluminum provide relatively high reflectivity across broad spectral ranges but suffer higher absorption losses at some wavelengths and can degrade over time.

High reflectivity is sensitive to wavelength and angle of incidence. Dielectric stacks are typically designed for a specific wavelength or band; reflectivity falls off outside the design region. At non-normal incidence, polarization effects cause different reflectance for s- and p-polarized light, and substrate properties can influence performance.

Applications include laser resonators, optical cavities, telescope mirrors, interferometers, and photonics. In laser engineering, high-reflectivity mirrors maximize intracavity power and reduce losses. In astronomy, high-reflectivity mirrors improve throughput. In precision metrology and gravitational wave detectors, ultra-high reflectivity reduces phase noise.

Measurement and quality control involve assessing reflectivity with spectrophotometers or ellipsometers. Typical values for specialized mirrors exceed 99.9% per surface in the visible to near-infrared, with some dielectric stacks reaching 99.999% in narrow bands. Production requires precise deposition, low surface roughness, and environmental stability. Limitations include absorption, scattering from roughness, long-term exposure effects, and angular sensitivity.