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 |
1 |
|
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
|
| UV enhanced |
250-600 |
>85 |
1 |
| VIS enhanced |
400-700 |
>92 |
1 |
|
Fig. 2-1 UV enhanced Al coating.
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 |
2-3 |
| 0.532 & 0.633 and 10.6 |
>97 at 0.4-0.7
>= 99.0 at 10.6 |
2-3 |
|
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 |
2-3 |
| NIR enhanced |
700-900 |
>97 |
2-3 |
|
Fig. 4-1 Enhanced Ag coating at 400-700 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 |
2-3 |
|
6. Protected Gold coating
| Wavelength range, µm |
Average reflection, % |
Damage threshold |
| 0.6-10 |
>98 |
2-3 J/cm2, 50 ns pulse |
|
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 |
|
Fig. 7-1 Enhanced Gold coating at 532 nm.
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.
Fig. 7-3 Comparative figure for metallic coating.
