Optical coatings

The absolute value of residual reflection R does not exceed 0.25% per surface at normal incidence for any wavelength from 220 nm to 2.5 μm.

Example: Antireflective coating at 650 nm.

It is possible to optimize the parameters of antireflective coatings for two or three wavelengths.

Examples:

• R < 0.25% at 355 nm and R < 0.25% at 532 nm
• R < 0.25% at 532 nm and R < 0.25% at 1064 nm
• R < 0.5% at 355 nm, R < 0.5% at 532 nm, and R < 0.5% at 1064 nm

Example: Antireflective coating at 532 nm and 1061 nm.

The average value of residual reflection R does not exceed 0.5% per surface at normal incidence for standard ranges:

• 300 – 400 nm
• 400 – 700 nm
• 600 – 900 nm
• 700 – 1100 nm

Example: Antireflective coating for the 400–700 nm range.

The functioning of such mirrors is based on the reflective properties of the metal from which they are made.

• Metals with high reflectivity are used, such as silver, gold, aluminum, copper, etc.
• They provide broadband reflection.
• The reflectivity of silver in the visible range exceeds 90%.
• Metals have significant absorption, which limits efficiency in some applications.
• Unlike dielectric coatings, metallic coatings are less dependent on the angle of incidence and polarization, while still being suitable for broadband use.

Unlike dielectric coatings, metallic coatings are only slightly dependent on the angle of incidence and polarization, while being suitable for wide wavelength ranges. To protect against mechanical damage and oxidation, metallic coatings are often overlaid with a protective dielectric layer or coated with protective varnish in the case of internal reflection.

1.1 Aluminum coatings

Aluminum is the most accessible and simple metallic coating, preferred for the UV and visible parts of the spectrum.

Examples of aluminum coating (at 0° incidence):

• Standard protected aluminum 400 – 10,000 nm: R_avg > 90%
• UV-enhanced aluminum 220 – 600 nm: R_avg > 85%
• Enhanced aluminum for the visible range 400 – 700 nm: R_avg > 92%

Example: Aluminum reflection

1.2 Silver coatings

Silver provides the best reflection in the visible part of the spectrum.

Examples of silver coating (at 0° incidence):

• Standard protected silver 400 – 10,000 nm: R_avg > 95%
• Enhanced silver for the visible range 400 – 700 nm: R_avg > 97%

Example: Standard silver coating

1.3 Gold coatings

Gold provides the best reflection in the infrared part of the spectrum.

Examples of gold coating (at 0° incidence):

• Standard protected gold 600 – 10,000 nm: R_avg > 98%
• Unprotected gold 600 – 10,000 nm: R_avg > 98.4%

Example: Standard gold coating

These are coatings whose reflectivity is achieved through a multilayer stack of thin dielectric layers with different refractive indices. The thickness and materials of the layers are selected so that the light waves reflected from each interface interfere constructively, providing high reflection in the specified spectral range.

Dielectric coatings consist of multilayer structures made from dielectric materials (e.g., SiO₂, MgF₂, Al₂O₃, etc.).

• Provide extremely high reflectivity (up to 99.9% and above) in a narrow or broad wavelength range.
• Reflection is achieved through multilayer interference rather than absorption, making such mirrors suitable for use in high-power laser systems.

2.1 Single-wavelength reflective coatings

For any wavelength from 200 nm to 2100 nm:

• R > 99.5% at 0° incidence and at 45° incidence for S-polarization
• R > 99% at 45° incidence for P-polarization and for unpolarized radiation

2.2 Broadband reflective coatings

Examples of average residual reflection (R) depending on the wavelength range (values given for normal incidence):

• 350 – 450 nm: R_avg > 99%
• 400 – 700 nm: R_avg > 97%
• 700 – 900 nm: R_avg > 99%

Example: Reflective coating for 750 – 1100 nm at an incidence angle of 45°.

Unlike metallic mirrors, dielectric mirrors have a strong dependence of their spectral reflection characteristics on the angle of incidence, which usually does not exceed 5° from normal.

This type of coating combines a metallic layer with dielectric layers, which improves reflectivity and protects the metal from oxidation and damage.

Features and properties

• High reflectivity:
  Depending on the design, reflectance can reach up to 99.999%, which is especially important for laser systems and precision optics.
• Broadband or narrowband reflection:
  Dielectric layers can be optimized for a narrow spectrum (e.g., a laser wavelength) or broadband reflection (e.g., the visible spectrum).
• Layer thickness and count:
  Achieving maximum reflectivity and the desired bandwidth requires dozens of layers with carefully calculated thickness. Increasing the number of layers boosts reflectivity within the band but does not significantly expand it.
• Stability and durability:
  Dielectric coatings are resistant to wear, chemical effects, and temperature variations, which is crucial for operation under harsh conditions.

Applications

• Fiber-optic communication. For light control and increased transmission efficiency.
• Laser systems. In laser resonators and optical components to ensure high efficiency and minimize losses.
• Optical instruments. In spectrometers, interferometers, telescopes, and other precision devices to improve measurement quality and accuracy.
• Aerospace and scientific equipment. For creating mirrors and reflectors with high reflectivity and stability.

Specialized beamsplitters with a specified ratio of reflection and transmission coefficients are available, for example 30/70, 10/90, and others, depending on the requirements of the optical system.

Examples of standard coatings (incidence angle 0°, λ = 532 nm):

• R = 10% ± 2%
• R = 50% ± 5%
• R = 70% ± 4%
• R = 80% ± 3%
• R = 98% ± 1%

An antireflective coating is usually applied on the opposite side at the same wavelength to reduce losses.рь.

Semi-transparent mirrors provide light splitting with a specified R/T ratio (e.g., 50/50, 30/70, 60/40) over a wide wavelength range (greater than 150 nm). They are widely used in interferometers, laser systems, and optical sensors.

Example of a beamsplitter coating for wavelength ranges at 0° incidence:

• 400 – 700 nm: R = 50% ± 8%
• 700 – 900 nm: R = 50% ± 6%

Example: Broadband beamsplitter coating for the 400–700 nm range

Dichroic beamsplitters allow light to be separated spectrally — reflecting certain wavelengths (e.g., in the shortwave region 300–450 nm) and transmitting others (e.g., in the longwave region 550–900 nm). They are used in spectroscopy and color separation systems.

Examples of dichroic coating at 0° incidence:

• R > 99.5% at 1064 nm, T > 92% at 532 nm
• R > 99.5% at 1064 nm, T > 90% at 808 nm

Example: Long-pass dichroic beamsplitter coating with a cut-off wavelength of 550 nm for 45° incidence.

Narrowband interference filters can be formed by combining thin films made of materials with different refractive indices. Such filters are used when it is necessary to isolate a single wavelength or a narrow spectral range of about 10 nm. Narrowband interference filters provide transmission of the main wavelength above 90% in the passband, and less than 1% in the blocking regions due to reflection.

These coatings are often applied to colored glass to extend the blocking region through material absorption.

Example: Narrowband filtering coating at 550 nm.