Metallic Mirror

A metallic mirror is a type of mirror where the reflection is obtained from a thin metal coating, such as aluminum, silver or gold, on a substrate of glass or metal. Metallic mirrors have a broad spectral range of reflection (from visible to infrared), but a lower reflectivity (around 90% to 98%) than dielectric mirrors. They are also insensitive to polarization, angle of incidence and phase dispersion. Metallic mirrors are often used for general-purpose optics and ultrashort-pulse lasers

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PROS & CONS of Metallic Mirrors

Advantages:

  • They have a broad spectral range of reflection (from visible to infrared), which makes them suitable for applications that require broad bandwidth and low dispersion.
  • They are insensitive to polarization, angle of incidence and phase dispersion, which makes them suitable for applications that require constant phase shift and low loss.
  • They are relatively easy and cheap to fabricate compared to dielectric mirrors.

Disadvantages:

  • They have lower reflectivity (around 90% to 98%) than dielectric mirrors, which means they transmit or absorb more light.
  • They have softer coatings that are more susceptible to damage and oxidation, which requires special care when cleaning.
  • They can cause ghost images due to secondary reflections from the substrate material.

Types of Metallic Mirrors Coating

  • Aluminum: This is a widely used metal coating for mirrors, as it has high reflectivity over a broad spectral range from UV to visible and near-infrared. Aluminum mirrors are often protected or enhanced with a dielectric layer (such as silica) to increase their durability and resistance to oxidation.
  • Silver: This is another common metal coating for mirrors, as it has higher reflectivity than aluminum in the visible and infrared regions. Silver mirrors are also protected or enhanced with a dielectric layer (such as silica) to prevent tarnishing and improve their damage threshold.
  • Gold: This is a metal coating for mirrors that offers high reflectivity for near-infrared and infrared wavelengths. Gold mirrors are also protected with a dielectric layer (such as silica) to increase their durability and resistance to oxidation.

Comparison of Metallic Coatings

Aluminum Coating:

Aluminum is a common metal coating that offers high reflectance over a wide spectral range, from UV to near-IR. It is also relatively inexpensive and durable. However, aluminum has lower reflectance than silver or gold in the visible and IR regions, and it can oxidize over time. To prevent oxidation and increase durability, aluminum mirrors are often protected or enhanced by a dielectric overcoat, such as silica or silicon nitride.

  

Silver Coating:

Silver is another metal coating that provides high reflectance from 500 nm to 20 μm. It has higher reflectance than aluminum in the visible and IR regions, and it also has higher laser damage threshold than aluminum mirrors. However, silver is more prone to tarnishing when exposed to humidity or corrosive gases, which can reduce its performance over time. To protect silver mirrors from tarnishing, they are often coated with a dielectric layer, such as silica or silicon nitride.

  

Gold Coating:

Gold is a metal coating that offers high reflectance for near-IR and IR wavelengths, from 700 nm to 20 μm. It has higher reflectance than aluminum or silver in this spectral range, and it is also resistant to oxidation and tarnishing. However, gold has lower reflectance than aluminum or silver in the visible region, and it is also more expensive than other metal coatings.