Dielectric Coating
Dielectric coatings (also called thin-film coatings, interference coatings, or multilayer coatings) are stacks of very thin layers of transparent, non-conducting dielectric materials deposited on an optical substrate. In lasers and photonics they control reflectance, transmittance, and polarization by thin-film interference rather than by metallic reflection.
Unlike metal coatings, they can have extremely low absorption and can be engineered for nearly 0% to >99.99% reflectance in a chosen wavelength band.
How they work:
Light reflects at every interface between layers of different refractive index. Layer thicknesses (typically a fraction of a wavelength, often λ/4) are chosen so the reflected waves interfere constructively or destructively.
High-reflector (HR) / laser mirrors: Alternating high- and low-index layers (quarter-wave or Bragg stack) produce constructive interference and very high reflectance, often >99.9%.
Anti-reflection (AR) coatings: Thicknesses and indices are chosen so reflections cancel (destructive interference), reducing Fresnel loss from ~4% per uncoated glass surface to well below 0.5% (sometimes <0.1%).
Performance depends on wavelength, angle of incidence, and polarization. More complex designs give dichroic mirrors, beam splitters, filters, polarizers, and chirped/dispersive mirrors.
Common materials are metal oxides such as SiO2 (low index) paired with TiO2, Ta2O5, HfO2, or similar high-index oxides. Deposition methods include electron-beam evaporation (often ion-assisted) and ion-beam sputtering (IBS), which produces denser, more durable, lower-scatter films with higher laser-induced damage thresholds (LIDT).
Why they matter for lasers:
Metallic mirrors absorb some light and are easily damaged by high intensity. Dielectric coatings can be nearly lossless and designed for high LIDT, so they are the standard choice for laser resonator mirrors, output couplers, and beam-steering optics. Limited reflection bandwidth is often useful: a folding mirror can reflect the laser wavelength while transmitting pump light (dichroic design).
Main applications:
Laser resonator high reflectors and output couplers.
Dichroic mirrors (separate or combine pump and laser wavelengths).
AR coatings on laser crystals, nonlinear crystals, lenses, windows, and fiber ends.
Beam splitters and thin-film polarizers.
Optical filters (bandpass, notch, edge).
Dispersive / chirped mirrors for ultrafast lasers.
High-power and ultrafast systems (petawatt beamlines, fs/ps lasers) where LIDT, low absorption, and controlled group-delay dispersion are critical.
Coatings on semiconductor lasers (edge emitters, VCSELs) and photodiodes.
They are also used outside lasers in astronomy, space optics, imaging, and precision interferometry. Limitations include angle and wavelength sensitivity, possible coating stress, and the need to match LIDT specifications (pulse duration, wavelength, fluence) to the actual laser.