Metallic coated optical mirrors are coated with various metals to provide reflectivity across the UV, VIS, and IR spectra making them an excellent choice for a wide range of applications. When choosing a metallic coating, one should carefully consider the following:
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Advanced Optics manufactures optical mirrors that offer reflectivity from 190nm &#; 20µm. Please visit our technical bare (unprotected) metallic coatings page to learn more about mirrors without a protective overcoat.
Please visit our optical mirrors page to see a full list of our capabilities and our in-stock optical mirrors.
Protected aluminum mirrors are coated with aluminum and a ½ wave of either silicon monoxide or silicon dioxide overcoat. The choice of overcoat may need to be considered due to the thermal implications of how each is applied and the wavelength of intended use. Either overcoat provides abrasion resistance for the soft aluminum underneath, which should otherwise not be touched or cleaned. The protective overcoat also keeps the aluminum from oxidizing over time.
Protected aluminum is a popular choice in the visible spectrum due to its low cost and good reflectivity with Ravg >85% from 400 &#; 700nm. It continues to provide good reflectivity into the NIR spectrum with Ravg >90% 400 &#; nm making it an excellent choice for many broadband applications.
The disadvantage of this common, general-purpose coating is the dip in reflectivity at 880nm, which is due to the aluminum and enhanced by the overcoat. However, protected aluminum mirrors can be customized by centering the coating at a specific wavelength to maximize reflectivity where it matters most to your application. This makes protected aluminum mirrors not only economical but flexible enough to be used in a wide range of applications.
Advanced Optics fabricates protected aluminum mirrors in almost any configuration from commercial grade down to 1/20 wave. For further information on how we can help you design the proper optical mirror for your application please visit our optical manufacturing capabilities page.
Enhanced aluminum mirrors are overcoated with a thin dielectric layer that provides protection and durability to the fragile aluminum underneath, which otherwise should not be touched or wiped. Enhanced aluminum mirrors can be specifically tailored to improve reflectivity in either the UV or the visible spectrum, thereby extending their application range.
UV and DUV Enhanced Aluminum Mirrors Enhancing the aluminum in the UV spectrum provides Ravg >90% 250 &#; 450nm and Ravg >85% 250 &#; 700nm.
In addition to our standard UV enhanced aluminum coating, Advanced Optics offers a deep UV enhanced aluminum coating for use down to 190nm.
Enhancing the aluminum in the visible spectrum provides Ravg of 93% 400 &#; 700nm, which peaks at 95 &#; 96% at 520nm.
As with our standard protected aluminum coatings, Advanced Optics can design UV enhanced aluminum and visible spectrum enhanced aluminum coatings centered at a specific wavelength to maximize reflectivity in the area of the spectrum that matters most to your application. Advanced Optics fabricates enhanced aluminum mirrors in almost any configuration from commercial grade down to 1/20 wave. Please visit our optical manufacturing capabilities page to see how we can help you meet your requirements.
Protected silver mirrors have a thin dielectric overcoat that provides durability and prevents the silver underneath from rapid oxidation. These mirrors offer higher reflectivity over a broader spectral range than aluminum coated mirrors with Ravg > 97.5% from 450 &#; nm and Ravg > 98% from 2 &#; 20µm. This makes them ideal for applications that require high, fairly flat reflectivity over the visible and NIR spectrum.
The disadvantage of choosing a protected silver mirror is its propensity to oxidize over time. The protective dielectric overcoat is helpful but these mirrors will eventually tarnish, and environmental factors such as humidity will accelerate the process.
Advanced Optics fabricates protected silver mirrors in almost any configuration from commercial grade down to 1/20 wave. Please visit our optical manufacturing capabilities page for additional information
Silver can be enhanced to provide 99% reflectivity at a specific wavelength between 400nm &#; 20µm. However, the overall curve is not as flat as standard protected silver as illustrated in the curve below, which is optimized at 520nm.
The disadvantage of choosing a protected silver mirror is its propensity to oxidize over time. The protective dielectric overcoat is helpful but these mirrors will eventually tarnish, and environmental factors such as humidity will accelerate the process.
Advanced Optics fabricates protected silver mirrors in almost any configuration from commercial grade down to 1/20 wave. For further information on how we can help you design the proper optical mirror for your application please visit our optical manufacturing capabilities page.
Protected gold mirrors have a thin dielectric overcoat that provides durability for the soft gold underneath, which otherwise should not be touched or cleaned. When your application requires high reflectivity in the NIR or IR spectrum, these mirrors excel with Ravg >96% from 700 &#; nm and Ravg >98% from 2 - 10µm.
The disadvantage of a protected gold coating is that it does not reflect well in the visible spectrum.
Advanced Optics can help you customize your protected gold coating to meet your reflectivity requirements. We fabricate protected gold mirrors in almost any configuration from commercial grade down to 1/20 wave. Please visit our optical manufacturing capabilities page to see how we can help you meet your requirements.
Metal mirrors are used when high reflection in a wide spectral range is necessary. The reflectance of metal mirrors, unlike dielectric ones, does not depend very much on polarization the angle of incidence of light. The main disadvantages of metallic mirrors, in comparison with dielectric ones, are a slightly smaller reflection coefficient at given wavelengths and a significantly smaller radiation resistance.
Contact us to discuss your requirements of Precision Grade Optical Mirrors. Our experienced sales team can help you identify the options that best suit your needs.
1. Aluminum Mirrors
2. Silver Mirrors
3. Gold Mirrors
1. Aluminum Mirrors
Aluminum mirrors remain the most commonly used metal mirrors due to its good reflection from UV to far IR and THz range, low cost and high resistance to external influences. Contacting with air, aluminum forms a layer of Al2O3 oxide a few nanometers thick. The layer is optically transparent, very dense and chemically stable. This layer provides the highest chemical resistance of aluminum film, but can not protect it from mechanical influences. If no such effects are expected on the mirror, an aluminum layer without protection can be used. Such mirrors are extremely useful, for example, inside scientific instruments, when the protective layer can be a source of undesirable interference or absorption.
Fig. 1. Reflection of an unprotected aluminum film
But often additional protection of the reflective layer is required.
1.1. Protected Aluminum
1.1.1. Aluminum mirrors with SiO2 and SiO protective coating are among the most commonly used. This coating is mechanically strong enough for most applications, but reduces a bit the reflection in the UV. In addition, it has some absorption by 3 μm (water) and by 9-11 μm (Si-O bond).
Fig. 1.1. Reflection spectrum of an aluminum film with SiO2 protective coating.
1.1.2. As SiO2, and even more SiO, demonstrates high absorption at wavelengths shorter than 250 nm, such mirrors are bad in deep UV. For mirrors operating in the 200-250 nm range, it is preferable to use the MgF2 layer as a protection. It reduces the absorption loss in protection, but its mechanical strength is lower. Another important factor of losses in the GUF is scattering; for reducing it aluminum should be sprayed in a special way.
1.2. Enhanced Aluminum
Instead of a single-layer protection, it is possible to make a multilayer structure on the top of the aluminum film, which will slightly increase the reflection in the visible or near-IR range. Outside the "amplification" range, the reflection will be lower than that of bare aluminum. The damage threshold also remains at the level of unprotected aluminum.
Fig. 1.2.&#;. Reflection of &#;enhanced&#; aluminum
Fig. 1.2.&#;. Comparison of reflection of enhanced, standard and unprotected aluminum films
1.3. Metal-dielectric Mirrors
By increasing the number of dielectric layers on top of the aluminum film, reflection above 99% in the required spectral region can be achieved. A high-grade dielectric mirror is created on top of the aluminum film. The advantage of this design in comparison with the standard dielectric mirror is a high reflection in the entire spectral range and a lower sensitivity of the structure to a change in the angle of incidence and polarization of light. But the damage threshold of these structures remains low, for this reason they are not to be used in power optics.
Fig. 1.3. Reflection of a metal-dielectric mirror.
2. Silver Mirrors
Silver mirrors differ from aluminum ones by a higher reflection in the visible and infrared ranges, but noticeably worse reflection in UV. Unlike Al2O3 , silver oxide does not form a strong and stable film on the surface of silver and can not provide either mechanical or chemical protection of the metal. That is why the use of silver mirrors without a protective layer is almost impossible even when the mirror is protected from mechanical influences.
Wavelength, μm Average reflectance, % Damage threshold,2.1. Silver with Protection
Protecting the silver layer with a dielectric film makes the structure mechanically and chemically resistant.
Fig. 2.1. Reflection of a silver film with a protective coating.
2.2. &#;Sealed&#; Silver
Even the smallest pores in the protective film allow the silver to chemically interact with air, acid gases and water. The lifetime of silver mirrors can be significantly reduced by the heavy loads from the environment, such as high temperature and humidity, high industrial air pollution. The application of an additional protective layer in a special additional process allows to "seal" the pores in the protective film (also on the edges of the mirror) and significantly prolong the service life of the product. This can be especially important for equipment located in hard-to-reach places. Optically, this coating is virtually indistinguishable from standard p-silver.
3. Gold Mirrors
Gold mirrors are most often used in the infrared range. For light with wavelengths shorter than 600 nm, the reflection from gold is very low. Gold is chemically neutral, so even without a protective layer; it practically does not interact with atmospheric gases and water. This allows gold to be used without protection in scientific instruments, when interference or absorption lines in protective layers can be undesirable. But it should be remembered that the gold layer is extremely soft and can easily be damaged mechanically. For most applications, gold should be covered with an oxide protective structure.
Wavelength, μm Average reflectance, % Damage threshold,3.1. Unprotected Gold
Fig. 3.1. Reflection of bare gold
3.2. Protected Gold
Fig. 3.2. Reflection of p-gold
3.3. Gold Mirrors for Carbon Dioxide Lasers
The usual protective layer on the gold mirror has some absorption at 10.6 μm, which reduces the radiation resistance of the product. The special design of the protection allows to reduce the absorption and thereby slightly increase the damage threshold and the reflection coefficient. But it is necessary to keep in mind when working with a CO2 laser in a (quasi) continuous mode, the damage threshold will be determined primarily by the thermal conductivity of the substrate and by other factors.
Fig. 3.3. Reflection of gold mirrors for carbon dioxide lasers
3.4. THz Mirrors
The penetrating power of electromagnetic radiation in a conductor is proportional to the wavelength. Therefore, to ensure high reflection in the THz range, a thicker metal film is required than for working in the visible and near-mid-IR range. Studies confirm that some "standard" metal mirrors show a drop in reflection in the terahertz range. To ensure high reflection, we apply a sufficiently thick layer of metal. For more information about the product, see the article THz mirrors.
The following spectrum helps to compare the optical properties of metal mirrors, discussed above.
Fig. 4. Comparison of optical properties of different types of metal mirrors
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