Why is dielectric material used for anti-reflective coating?

[says]

Why is dielectric material used for anti-reflective coating? / Why not use a material this is NOT a dielectric material?

Is it because dielectric materials are highly transparent?

 

[Simon Bridge]

The material is used because it has the desired properties.

 

[the student]

Wave reflection is directly proportional to the thickness of the dielectric material.The interference effects in the coating cause the wave reflected from the anti-reflection coating top surface to be out of phase with the wave reflected from the semiconductor surfaces.

Most insulators (or dielectric materials) are held together by ionic bonds. Thus, these materials do not have free conduction electrons, and the bonding electrons reflect only a small fraction of the incident wave. The remaining frequencies (or wavelengths) are free to propagate (or be transmitted). This class of materials includes all ceramics and glasses.

If a dielectric material does not include light-absorbent additive molecules (pigments, dyes, colorants), it is usually transparent to the spectrum of visible light. Color centers (or dye molecules) in a dielectric absorb a portion of the incoming light wave. The remaining frequencies (or wavelengths) are free to be reflected or transmitted. Any other materials cannot be used because they are light absorber and will not reflect light.

 

[Baluncore]

Why is dielectric material used for anti-reflective coating?

When the refractive index of the dielectric is the geometric mean of for example, glass and air, a layer of dielectric one quarter of a wavelength thick will make what is called a quarter wave transformer that perfectly matches the air to the glass. That eliminates reflection at the impedance mismatch of glass and air because the phase of reflections from the two surfaces of the dielectric then cancel.

https://en.wikipedia.org/wiki/Quarter-wave_impedance_transformer
https://en.wikipedia.org/wiki/Anti-reflective_coating

We live in a universe of incompatible material interfaces. When an interface is compatible it becomes invisible to us.