Metallic Coatings

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Metallic coatings are used in applications requiring a high degree of reflection over a very wide wavelength range. The reflectivity of metallic coatings varies less as a function of the beam's polarization and angle of incidence than that does that of dielectric coatings, but metallic coatings tend to be more lossy.

Aluminum coatings are widely used because of that metal's excellent reflectance over practically the entire optical spectrum, including from UV out to infrared. In addition, Aluminum is the only metal used in UV mirror coatings.

1. Protected Aluminum coating

The Aluminum is overcoated with a single additional dielectric layer that protects the delicate metal layer from oxidation and scratches.

Wavelength range, µm Average
reflection, %
Damage threshold,
J/cm2, 50 ns pulse
0.2-10.0 >90 0.25-0.3

Protected Al coating at 0.2-10.0 µm
Fig. 1 Protected Al coating at 0.2-5 µm.

 2. Enhanced Aluminum coating

The Aluminum is overcoated with multiple dielectric layers, boosting the reflectivity of the metallic coating within a desired wavelength sub-range.

Type Wavelength range, nm Average
reflection, %
Damage threshold,
J/cm2, 50 ns pulse
VIS enhanced 400-700 >92 0.25-0.3

Enhanced Al coating at 250-600 nm
Fig. 2-1 UV enhanced Al coating.

Enhanced Al coating at 400-700 nm
Fig. 2-2 VIS enhanced Al coating.

 Silver is preferred for visible to near-infrared wavelengths. Silver coatings show a higher damage threshold than Aluminum coatings. Silver is also routinely protected and/or enhanced by the addition of dielectric layers.

3. Protected Silver coating

Wavelength(s) range, µm Average
reflection, %
Damage threshold,
J/cm2, 50 ns pulse
0.4-10.0 >95 0.25-0.3

Protected Ag coating at 400-10000 nm
Fig. 3 Protected Ag coating at 0.4-10 µm.

 4. Enhanced Silver coating

Type Wavelength range, nm Average
reflection, %
Damage threshold,
J/cm2, 50 ns pulse
VIS enhanced 400-700 >97 0.25-0.3
NIR enhanced 700-900 >97 0.25-0.3

Enhanced Ag coating at 400-700 nm
Fig. 4-1 Enhanced Ag coating at 400-700 nm.

Enhanced Ag coating at 700-900 nm
Fig. 4-2 Enhanced Ag coating at 700-900 nm.

 Gold coatings are used primarily for near-, mid- and far-infrared wavelengths. Gold starts to have attractive reflective properties at around 600 nm and above.

5. Bare Gold coating

Bare gold is soft and very easily scratched, but the absence of protective layers eliminates it unwanted interference effects which can be detrimental to spectroscopic instruments operating over a very wide wavelength range.

Like Aluminum and Silver, Gold is also routinely protected and/or enhanced by the addition of dielectric layers. Note however that a bare gold coating has a slightly better average reflectivity than a protected gold coating.

Wavelength range, µm Average reflection, % Damage threshold,
J/cm2, 50 ns pulse
0.6-10 >98.5 0.25-0.3

6. Protected Gold coating

Wavelength range, µm Average reflection, % Damage threshold
0.6-10 >98 0.25-0.3 J/cm2, 50 ns pulse

Protected Gold coating at 600-10000 nm
Fig. 6 Protected Gold coating.

7. Enhanced Gold coating

Wavelength(s) range, µm Average reflection, % Damage threshold
0.532 & 0.633 and 10.6 >70 at 0.532
>85 at 0.633
>=99.2 at 10.6
1 J/cm2, 10 ns pulse &
1 J/cm2, 10 ns pulse &
2 kW/cm2, CW mode

Enhanced Gold coating at 532 nm
Fig. 7-1 Enhanced Gold coating at 532 nm.

Enhanced Gold coating at 10.6 mm
Fig. 7-2 Enhanced Gold coating at 10.6 μm.

The following plot summarizes the performance of the different types of metallic coatings that have been discussed above.

 Comparative figure for metallic coating
Fig. 7-3 Comparative figure for metallic coating.

 

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