Dielectric Mirror
Dielectric mirror (also called a Bragg mirror or distributed Bragg reflector, DBR) is a high-performance reflector made from a stack of thin, transparent, non-conducting layers rather than a metal coating. In lasers and photonics it is the standard choice for resonator mirrors, beam steering, and wavelength-selective optics because it can reach very high reflectance with very low absorption.
How it works:
Light reflects weakly at every interface between two materials of different refractive index (Fresnel reflection). A dielectric mirror stacks many such interfaces so those weak reflections add in phase (constructive interference) over a designed wavelength band. The result is overall reflectance that can exceed 99.9% and, in special “supermirrors,” 99.999% or more.
The simplest and most common design is a quarter-wave (Bragg) stack: alternating high-index and low-index layers, each with optical thickness λ/4 at the design wavelength. Path-length differences and phase shifts then line up so every interface contribution interferes constructively.
Common material pairs include TiO₂ / SiO₂ or Ta₂O₅ / SiO₂. More layers and higher index contrast give higher peak reflectance and a wider stop-band. Complex (non-periodic) designs produce broadband mirrors, dichroic mirrors, chirped/dispersive mirrors, or polarizing coatings.
Properties depend on angle of incidence and polarization (s-pol usually reflects more strongly than p-pol at oblique incidence). Absorption and scatter can be kept extremely low (ppm level), which is why damage thresholds are typically much higher than for metallic mirrors.
Why lasers use them instead of metal mirrors:
Metallic coatings (Al, Ag, Au) are broadband and simple, but they absorb a few percent of the light and convert it to heat. That limits peak reflectance (~90–98%) and laser-induced damage threshold. Dielectric stacks reflect by interference, not free-electron absorption, so they are preferred wherever loss, heat, or high intensity matters: laser cavities, high-power beam delivery, and ultrafast systems. Trade-off: the high-reflectance band is narrower unless the design is deliberately chirped or multi-stack.
Applications:
Laser resonators: high-reflector (HR) end mirrors and output couplers. One mirror often transmits the pump wavelength while reflecting the laser line (dichroic design).
Beam steering and folding in high-power or precision systems (cutting, welding, lidar, interferometry).
Dichroic / harmonic separators: reflect one wavelength (e.g. 1064 nm) and transmit another (532 nm or a pump beam).
Ultrafast lasers: low group-delay-dispersion (GDD) or chirped mirrors that compensate pulse broadening.
Thin-film polarizers and polarizing beam splitters.
Semiconductor lasers and VCSELs: epitaxial DBR stacks grown as the cavity mirrors.
Filters, hot/cold mirrors, and high-finesse cavities (optical clocks, gravitational-wave detectors use supermirrors).
They are fabricated by electron-beam evaporation, ion-assisted deposition, or ion-beam sputtering (IBS) onto substrates such as fused silica, often as first-surface coatings so the beam never travels through glass before reflection.