Shapeoptics Optical Mirror Selection Guide

Optical Mirrors are probably the most commonly used optical elements in your lab, and their quality, performance, and reliability are key to the success of your experiment. That’s why we provide a variety of mirrors so you can be assured to find what you need. When choosing an Optical Mirror, keep in mind the reflectivity, laser damage resistance, and coating durability.

Metallic Mirror

Broadband metallic coated mirrors are good general-purpose mirrors because they can be used over a very broad spectral range from 200 nm to 12 µm. They are also insensitive to polarization and angle of incidence, and provide a constant phase shift, making them appropriate for ultrashort-pulse applications. Their softer coating, however, makes them more susceptible to damage, and special care must be taken when cleaning.

Metallic Coatings

In the following we give some hints for the use of these metals and about the role of protective coatings:

Silver coated Optical Mirrors
  • Highest reflectivity in the VIS and NIR
  • Shapeoptics produces protective layers by magnetron sputtering. These layers with very high packing density make silver mirrors as stable as mirrors of other metals (e.g. aluminum). Lifetimes of 10 years in normal atmosphere were demonstrated.
  • The use of protective layers is mandatory, because unprotected silver is chemically unstable and soft
  • See separate data sheets in our S-shop
Gold coated Optical Mirrors
  • Similar reflectance as silver in the NIR
  • Chemically stable, but soft
  • Protective layers are necessary to make gold mirrors cleanable
  • We recommend to use protected silver mirrors instead of protected gold, because the sputtered protective layers overcome the insufficiency of silver and make it a better choice because of the broader wavelength range, the slightly higher reflectivity and the more favourable price.
  • See separate data sheet in our S-shop
Aluminum coated Optical Mirrors
  • Relatively high and constant reflectance in the VIS and NIR
  • Highest reflectance in the UV
  • Surface oxide layer absorbs in the deep UV
  • A protective layer is recommended, because aluminum is soft
  • See separate data sheet in our S-shop

Dielectric Mirror

Dielectric mirrors offer higher reflectivity over a broad spectral range of a few 100 nm. Their coating is more durable, making them easier to clean, and more resistant to laser damage. We offer broadband dielectric mirrors that are ideal for general laboratory use as well as mirrors especially for high-power Nd:YAG applications at 1.064 µm and 532 nm and DUV and UV applications.

Reflectivity for a different number of layers

The most common mirror design is the so called quarterwave stack, i.e. a stack of alternating high and low index layers with an equal optical thickness of n·t = λ/4 for the desired wavelength. This results in constructive interference of the reflected beams arising at each interface between the layers. The spectral width of the reflection band and the achievable reflectivity for a given number of layer pairs depends on the ratio of the refractive indices of the layer materials. A large refractive index ratio results in a broad reflection band while a narrow reflection band can be produced using materials with a low refractive index ratio.

Assuming ideal coatings with zero absorption and scattering losses the theoretical reflectivity will approach R=100% with increasing number of layer pairs. Also partial reflectors with several discrete reflectivity values between R=0% and R=100% can be manufactured using only a small number of layer pairs (see the figure above). Adding some non-quarterwave layers to such a stack allows to optimize the reflectivity to any desired value.

Metal-Dielectric Mirror

In general, all layer systems consisting of metals and dielectric layers can be called “metal dielectric Mirror”. The most familiar ones are metal-dielectric filters consisting of transparent metal layers which are separated by a dielectric layer. These filters are characterized by extremely broad blocking ranges which result from the reflectivity and absorption of the metallic layers. The spectral position of the transmission band is determined by the optical thickness of the dielectric spacer layer.

Comparison of Reflectance Spectra (silver vs metal-dielectric silver)

Reflectance spectra of a protected silver mirror and a metal dielectric silver mirror, both optimized for high reflectivity in the visible spectral range for use in astronomical telescopes.

Here we want to draw the attention of the reader, however, to metal-dielectric reflectors. Metals and metallic Mirror show an extremely broadband natural reflectivity which is, however, restricted to about 90% in the UV spectral range (aluminum), 96% in the VIS (silver) and 99% in the NIR (gold and silver). Moreover, most of the metals must be protected by dielectric Mirror to overcome limitations of chemical (silver) or mechanical stability (aluminum, silver, gold).

More strictly speaking, almost all metallic mirrors are metal-dielectric Mirror. The protective Mirror always influence the reflectivity of the metals. Single dielectric layers of any thickness lower the reflectivity in most parts of the spectrum. However, multilayer coatings on metals can increase the reflectivity of the metallic coating.

Disclaimer:

    The material used in this knowledge sharing, is only for research, academic, non-profit educational or personal use, the blog owner has strived to credit the original sources, but cannot warrant the accuracy of copyrights or completeness of the information sources.

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