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A dielectric mirror is designed from multiple layers of transparent optical materials, like thin-film coatings, dielectric coatings, and interference coatings. The Fresnel reflection coefficient from an only interface between two different materials is not big. However, many interfaces’ reflections interfere and result in a high reflectivity of the device.
The Bragg mirror is the most common design and it leads to the biggest possible reflectivity for a number of materials and layer pairs. Dichroic mirrors can also be designed by using controlled properties for distinct wavelengths.
How are they designed?
To properly design a dielectric mirror, there have to be certain criteria, like:
These designs are only found by using numerical optimization algorithms. It is very challenging, as the mirrors need certain parameter space and high dimensionality.
Designing a mirror is not an easy task. Sometimes, there has to be a compromise between the required number of layers, growth precision and obtained optical properties.
How are the mirror properties calculated?
A mirror’s reflecting properties are calculated with modeling software based on a matrix method. Every layer is associated with a 2-by-2 complex matrix. Moreover, each matrix is multiplied so it results in a matrix of the whole layer structure. From here, the amplitudes of waves can be calculated. The properties of chromatic dispersion are the result of the frequency dependence of the transmission/reflection coefficients. This can be calculated through Fresnel equations.