A short optical coating guide

Nowadays, the vast majority of scientific and industrial optics have a thin-layer coating. With the increasing complexity of optical systems, non-coated optics can easily introduce ghost images, back reflections, and can in some instances be an safety hazard or destroy an expensive piece of equipment. On the other hand, thin-film coating introduce critical properties in the optics which use them. Here is an overview of the different most common coatings techniques available.

The simplest of all

The simplest coatings are made of thin layer of metal, such as aluminium, sliver or gold. They are used for broadband mirrors. The best reflectivity comes at higher price: aluminium (Al) is the cheapest (R=88%-92%), followed by silver (Ag, R=95%-99%) and then gold (Au, 98%-99%). But in the blue-violet part of the spectrum, only aluminium is suitable.

Dielectric coatings

These coatings use different thin layers of material (various metal oxides, calcium or magnesium fluorides) to create the desired effect. There are three major techniques used for dielectric coating: electron-beam deposition (E-beam), ion-assisted electron-beam (IAD) and ion beam sputtering (IBS). All of these process are quite similar in their principle. They consist in evaporating some coating material on the substrate. The difference lies in the deposition energy.

Because of the low energies involved when using electron-beam deposition, the thin film material contains bubbles and micropores, like a sponge. These will eventually fill with water, which will change the refractive index of the coating and thus the properties of the optics. (This is known as “environmental shifting”). The presence of water also lowers the damage threshold of the optics: when submitted to an intense light, the water will tend to vaporise and scrap off bits of the coating. Finally, even in the absence of water, the inhomogeneities of the coating layers lower the theoretical damage threshold. The positive points about this technology is that it is cheap, widespread and very versatile. The coating itself is also slightly flexible, which makes the optic more resistant to mechanical stress. Some of the major optics manufacturer only have access to that type of coating at the moment and outsource IBS-coated optics.

Ion-assisted electron-beam is an intermediate technique, between ion-beam sputtering and e-beam. So are its results.

Ion beam sputtering involves energies 100 times higher than e-beams. As a result the molecules of the coating layers form covalent bound when deposited. The result is free from bubbles or pores, more homogenous, more durable, have higher damage threshold and is more repeatable and controllable. They also show lower scattering and absorption properties, and overall higher specifications (more broadband, steeper transitions when needed, better spectral stability…). This is high precision coating, and the surface roughness can be controlled at better than 1 Å RMS (!), that is <λ/5000. Of course, this comes at a higher cost (atom-by-atom removal is very slow), and even worse, it is limited in the types of coatings it can handle: most of the UV coatings for instance involve fluorides which dissociate when sputtered. In this case, e-beam is the only option.

Coating types comparison

We will provide here a brief description of the choices one has to make when choosing its coating type.

Anti-reflective coatings

  • Broadband vs damage threshold: since broadband generally means multi-layer coatings, the more broadband a coating is, the lower its damage threshold and the higher its price. Broadband coatings also show the property to be less sensitive to angle of incidence than other anti-reflection coatings, which makes them valuable when the optics has to be tilted over the course of an experiment for instance. Broadband coating also show lower reflectivity than single layer or V-types coatings.
  • Single-layer vs V-types anti-reflection: Single-layer is more durable than V-types (which are multi-layers), and are enough to lower the reflectivity of BK7 from 4% to about 1.3%. On high refractive index materials (sapphire, Nd:YAG, ruby), single layer coating can go as low as 0.25% reflectivity at normal incidence. V-type coatings, AKA narrowband anti-reflection coatings, are best suited for laser application, since they can show reflectivities lower than 0.25% on common lens substrates.

Mirrors and partial reflectors

  • Broadband vs reflectivity: generally speaking, the more broadband, the lower reflectivity and the lower damage threshold. For very broadband mirrors requirements with more than 1 µm bandwidth, the only solutions available are the metallic coatings. Bare aluminium for instance has >86% reflectivity from 200 nm to well over 20 µm
  • Working in transmission requires a partial reflector. This is used for instance to sample 1% of a beam for diagnostic purposes. Mirrors are not suitable because their back face is generally uncoated with non laser polishing. Beamsplitters can also do that job.
  • Incident polarisation plays an important role in how well a mirror performs. Although mirror are tuned for a specific polarisation and angle of incidence, some configurations are physically limited. Best results are achieved when used at an angle with S polarisation or at normal incidence. At equivalent coatings, a mirror tuned for P polarisation at 45° incidence can hardly achieve less than 2% loss in reflectivity compared to its S equivalent. Sometime the loss is even much higher

Filters

  • Filters for fluorescence: the main issue with fluorescence is to separate the excitation light from the usable signal. Although both are generally far from each other in terms of wavelength and a sensible set-up would use the fluorescent light on a different optical axis than that of the excitation light, fluorescence is generally a low-light measurement. As such it is necessary to have the best achievable transmission at the wavelength of interest. We would generally advise a minimum blocking OD of 3 and a minimum transmission of 80%. Bandwidth and cut-off steepness are of lesser importance provided the excitation wavelength is in the blocking range.
  • Raman spectroscopy can be considered as a demanding application, because of the weakness of the signal and specially because of its spectral closeness to the excitation wavelength (typically ~10-50nm, sometime as far as 100nm). Therefore we would generally advise to go for high quality filters, such as those made using ion-beam sputtering. This can produce filters with transitions of ~3-8nm. There are four basic types of filters to choose from: Long-Wave-Pass (LWP) Edge Filters, Short-Wave-Pass (SWP) Edge Filters, Notch Filters, and a Laser-Line Filters. Laser-Line Filters will transmit only the excitation light (less noise), and Notch Filters are an obvious choice for blocking the excitation line. In systems using these two filter types, both Stokes and Anti-Stokes Raman scattering can be measured simultaneously. However, in many cases Edge Filters provide a superior alternative for both Transmitting and Blocking filters. Edge filters offer better transmission, higher laser-line blocking, and the steepest edge performance to see Raman signals extremely close to the laser line.
  • Broadband attenuation is easily achieved using a ND filter (Neutral Density). Those filter have an uniform spectral response (typically over 400-1200nm) but are limited in the amount of power they can handle (typically less than a few watts). Higher powers require a partial reflector. They are extremely useful for photography and to measure some light with a low damage threshold instrument. They are normally defined by their OD (optical density) value: their transmission on a scale from 0 to 1 is equal to 10-OD. For instance a ND filter with an OD of 3 will transmit 10-3=0.001, which equals 0.1%.

Beamsplitters

    • Wavelength separation / mixing: a beamsplitter can be used to separate or mix beams at different wavelengths. In this case they act as a mirror at one wavelength and as a window at the other wavelength. When building a system that involves wavelength separation, one must pay particular attention to its own design choices: physical laws limit what is achievable with a beamsplitter. When spectral purity at a wavelength is most important, it is better to transmit this wavelength and reflect the others. If getting as much light as possible at a wavelength is the main target rather than cleaning the spectrum, then it is better to reflect that wavelength and transmit the others. Finally, control of the polarisation is important when working at 45°. Best results are achieved when reflecting S polarised light and transmitting P polarised light. Good result are achieved when reflecting and transmitting P polarised light. Other combinations must be avoided if possible, the worst one being reflecting P and transmitting S.

A step towards the future?

Last year, a group of researcher[1] at the Rensselaer Polytechnic Institute in Troy, N.Y., reported achieving broadband virtually reflection-free anti-reflection coating using oblique-angle deposition. This coating results in a thin-film of nanorods, which grow by themselves. Self-organisation of the nanorods is obtained by tilting the substrate during the coating process. This technique is also susceptible to increase highly reflective coating to near 100% reflectivity. If this finds its way to production, the market is likely to be huge.

Reference
1. Xi, J.-Q., Martin F. Schubert, J. K. Kim, E. Fred Schubert, Minfeng Chen, Shawn-Yu Lin, Wayne Liu, and Joe A. Smart “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection” Nature Photonics 1, 176 (March 2007)

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