Dielectric mirror

By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified reflectivity at different wavelengths of light.

This is the same principle used in multi-layer anti-reflection coatings, which are dielectric stacks which have been designed to minimize rather than maximize reflectivity.

Simple dielectric mirrors function like one-dimensional photonic crystals, consisting of a stack of layers with a high refractive index interleaved with layers of a low refractive index (see diagram).

The reflections from the low-index layers have exactly half a wavelength in path length difference, but there is a 180-degree difference in phase shift at a low-to-high index boundary, compared to a high-to-low index boundary, which means that these reflections are also in phase.

A well-designed multilayer dielectric coating can provide a reflectivity of over 99% across the visible light spectrum.

[2] As shown in the GIF, the transmitted color shifts towards the blue with increasing angle of incidence.

is the transmitted wavelength under perpendicular angle of incidence and The manufacturing techniques for dielectric mirrors are based on thin-film deposition methods.

Polymeric dielectric mirrors are fabricated industrially via co-extrusion of melt polymers,[5] and by spin-coating[6] or dip-coating[7] on smaller scale.

An infrared dielectric mirror in a mirror mount
Diagram of a dielectric mirror. Thin layers with a high refractive index n 1 are interleaved with thicker layers with a lower refractive index n 2 . The path lengths l A and l B differ by exactly one wavelength, which leads to constructive interference.
The color transmitted by the dielectric filters shifts when the angle of incident light changes.
An electron microscope image of an approximately 13 micrometre piece of dielectric mirror being cut from a larger substrate. Alternating layers of Ta 2 O 5 and SiO 2 are visible on the bottom edge.