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Optical Coatings

Metallic Coatings

Melles Griot offers eight forms of standard metallic high-reflection coatings formed by vacuum deposition. These coatings, which can be used at any angle of incidence, can be applied to most optical components. Simply append the coating suffix number to the component product number.

Metallic reflective coatings are delicate and require care during cleaning. Dielectric overcoats substantially improve abrasion resistance, but they are not impervious to abrasive cleaning techniques. Clean, dry pressurized gas can be used to blow off loose particles, then clean, deionized water, a mild detergent, and alcohol can be used. Gentle cleaning with a swab is recommended.
Aluminum (/016)
Aluminum coating
Aluminum coating /016

 
  • The most widely used metallic mirror coating
  • Provides consistently high reflectance throughout the near-ultraviolet, visible, and near-infrared regions
  • Ravg > 90% from 400 to 1200 nm
  • Damage threshold: 0.2 J/cm2 ± 10%, 10-nsec pulse (12 MW/cm2) at 532 nm;
    0.3 J/cm2 ± 10%, 20-nsec pulse (14 MW/cm2) at 1064 nm
Aluminum, the most widely used metal for reflecting films, offers consistently high reflectance throughout the visible, near-infrared, and near-ultraviolet regions of the spectrum. While silver exhibits slightly higher reflectance than aluminum through most of the visible spectrum, the advantage is temporary because of oxidation tarnishing. Aluminum also oxidizes, though more slowly, and its oxide is tough and corrosion resistant. Oxidation significantly reduces aluminum reflectance in the ultraviolet and causes slight scattering throughout the spectrum.
Protected Aluminum (/011)
Protected aluminum
Protected aluminum coating /011

 
  • The best general-purpose metallic reflector for visible to near-infrared
  • Protective overcoat extends life of mirror and protects surface
  • Ravg > 87% from 400 to 800 nm
  • Damage threshold: 0.3 J/cm2 ± 10%, 10-nsec pulse (21 MW/cm2) at 532 nm;
    0.5 J/cm2 ± 10%, 20-nsec pulse (22 MW/cm2) at 1064 nm
Protected aluminum is the very best general-purpose, metallic coating for use as an external reflector in the visible and near-infrared spectra. Unless we specify otherwise or you specifically request a different coating, our mirrors are coated with protected aluminum. Protected aluminum is coated with a dielectric film of disilicon trioxide (Si2O3) of half-wavelength optical thickness at 550 nm. The protective film arrests oxidation and helps maintain high reflectance. It is durable enough to protect the aluminum coating from minor abrasions.
Enhanced Aluminum (/023)
Enhanced aluminum
Enhanced aluminum coating

 
  • Enhanced performance in the mid-visible region
  • Durability of protected aluminum
  • Ravg > 93% from 450 to 750 nm)
  • Damage threshold: 0.4 J/cm2 ± 10%, 10-nsec pulse (33 MW/cm2) at 532 nm,
    0.3 J/cm2 ± 10%, 20-nsec pulse (12 MW/cm2) at 1064 nm
By coating the aluminum with a multilayer dielectric film, reflectance is increased over a wide range of wavelengths. The durable enhancing multilayer produces a peak reflectance of 95% with an average across the visible spectrum of 93%. This coating is well suited for applications requiring the durability and reliability of protected aluminum, but with higher reflectance in the mid-visible regions. The reflectance of enhanced aluminum peaks between 530 nm and 580 nm and is high from 400 nm to 800 nm.
Ultraviolet-Enhanced Aluminum (/028)
UV-enhanced aluminum
Ultraviolet-enhanced aluminum coating /028

 
  • Maintains reflectance in the ultraviolet region
  • Dielectric overcoat prevents oxidation and increases abrasion resistance
  • Ravg > 86% from 250 to 400 nm
  • Ravg > 85% from 400 to 700 nm
  • Damage threshold: 0.07 J/cm2 ± 10%, 10-nsec pulse (5.7 MW/cm2) at 355 nm
By applying a film of an ultraviolet-transmitting dielectric (usually MgF2), the reflectance of pure, bare aluminum can be preserved in)the ultraviolet. The dielectric layer prevents oxidation of the aluminum surface and provides abrasion resistance. While the resulting surface is not as abrasion resistant as our protected aluminum, this coating may be cleaned with care. Reflectance averages over 86% from 250 to 400 nm and over 86% throughout the visible spectrum. This coating can be applied to all Melles Griot mirrors.
Internal Silver (/036)
Internal silver
Internal silver coating

 
  • For internal reflection (second-surface) mirrors and prisms only
  • Preferred for visible to near-infrared region
  • Less polarization effects than aluminum
  • Ravg > 98% from 400 nm to 1200 nm
  • Damage threshold: 0.9 J/cm2 ± 10%, 10-nsec pulse (75 MW/cm2) at 532 nm,
    1.6 J/cm2 ± 10%, 20-nsec pulse (73 MW/cm2) at 1064 nm
Through most of the visible and near-infrared spectra, silver has higher reflectance than aluminum, at least for a short time following deposition. Rapid oxidation quickly causes unprotected silver coatings to deteriorate. In the internal silver coating, oxidation and tarnish are prevented by coating the external surface with an additional layer of either Inconel® or copper. The Inconel or copper layers are subsequently painted to increase abrasion resistance. In this way, the high initial reflectance of silver is indefinitely preserved.

Silver is frequently used in the near-infrared (the interval containing neodymium and gallium arsenide laser lines) because it avoids the small dip in reflectance exhibited by aluminum in this interval. In the near ultraviolet, silver has very low reflectance, and aluminum is a preferable choice. From the visible into the middle-infrared, silver offers the highest internal reflectance available from a metallic coating. Silver has less effect than aluminum on the polarization state in these regions of the spectrum.
Protected Silver (/38)
Protected silver
Protected silver coating /038

 
  • Extremely versatile mirror coating
  • Excellent performance for the visible to infrared region
  • Ravg > 95% from 400 nm to 20 µm
  • Can be used for ultrafast Ti:Sapphire laser applications
  • Damage threshold: 0.9 J/cm2 ± 10%, 10-nsec pulse (75 MW/cm2) at 532 nm,
    1.6 J/cm2 ± 10%, 20-nsec pulse (73 MW/cm2) at 1064 nm
Melles Griot uses a proprietary coating and edge-sealing technology to offer a first-surface external protected silver coating. In recent tests, the protected silver coating has shown no broadening effect on a 52-femtosecond pulse. This information is presented as a guideline for femtosecond applications and no warranty is implied. Protected silver offers extremely broad performance, from 400 nm to well into the infrared, with excellent durability.

Due to the specialized tooling required to produce the protected silver coating, it is offered only on a limited range of substrates.
Bare Gold (/45)
Bare Gold
Bare gold coating /045

 
Because it combines good tarnish resistance with consistently high reflectance throughout the near, middle, and far infrared, gold is widely used in these regions. While it is possible to construct multilayer films that may surpass the reflectance of gold at specific wavelengths, the useful range of gold is unequaled. Gold is especially effective in controlling thermal radiation. Because bare gold is soft and scratches easily, bare-gold mirrors should be cleaned only by flow-washing with solvents and clean water or by blowing the surface clean with a low-pressure stream of dry air.
Protected Gold (/055)
Protected gold


 
  • The performance of the durability of dielectrics
  • Protective overcoat extends coating life
  • Ravg > 98% from 650 nm to 16 µm
  • Damage threshold: 0.4 J/cm2 ± 10%, 20-nsec pulse (17 MW/cm2) at 1064 nm
The Melles Griot proprietary protected gold mirror coating combines the natural spectral performance of gold with the durability of hard dielectrics. Protected gold provides over 96% average reflectance from 650 to 1700 nm, and over 98% average reflectance from 2 to 16 µm. As well as the damage threshold listed above, the /055 coating was tested for laser-induced damage and was found to withstand up to 18 ± 2 J/cm2 with a 260-µs pulse (0.4 MW/cm2) at a wavelength of 3 µm. These mirrors can be cleaned regularly using standard organic solvents, such as alcohol or acetone.

Metallic High-Reflection Coatings
Coating Type Wavelength Range
(nm) 		Average Reflectance
(%) 		Damage Threshold COATING SUFFIX
Aluminum 400-1200 90 0.2 J/cm2 ± 10%, 10-nsec pulse (12 MW/cm2) at 532 nm;
0.3 J/cm2 ± 10%, 20-nsec pulse (14 MW/cm2) at 1064 nm		 /016
Protected Aluminum 400-800 87 0.3 J/cm2 ± 10%, 10-nsec pulse (21 MW/cm2) at 532 nm;
0.5 J/cm2 ± 10%, 20-nsec pulse (22 MW/cm2) at 1064 nm		 /011
Enhanced Aluminum 450-750 93 0.4 J/cm2 ± 10%, 10-nsec pulse (33 MW/cm2) at 532 nm,
0.3 J/cm2 ± 10%, 20-nsec pulse (12 MW/cm2) at 1064 nm 		/023
UV-Enhanced Aluminum 250-400 86 0.9 J/cm2 ± 10%, 10-nsec pulse (75 MW/cm2) at 532 nm,
1.6 J/cm2 ± 10%, 20-nsec pulse (73 MW/cm2) at 1064 nm		 /028
Internal Silver 400-1200 98 0.9 J/cm2 ± 10%, 10-nsec pulse (75 MW/cm2) at 532 nm,
1.6 J/cm2 ± 10%, 20-nsec pulse (73 MW/cm2) at 1064 nm 		/036
Protected Silver 400-20,000 95 0.9 J/cm2 ± 10%, 10-nsec pulse (75 MW/cm2) at 532 nm,
1.6 J/cm2 ± 10%, 20-nsec pulse (73 MW/cm2) at 1064 nm 		/038
Bare Gold 700-20,000 99 1.1 J/cm2 ± 10%, 20-nsec pulse (48 MW/cm2) at 1064 nm /045
Protected Gold 650-16,000 98 0.4 J/cm2 ± 10%, 20-nsec pulse (17 MW/cm2) at 1064 nm /055

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Optics Guide Copyright 2002 Melles Griot Inc.