Here at Torr Scientific, we offer many of our products with an optional anti‑reflective coating. This means not only our viewports can be coated, but also our magneto-optical traps and glass cells!
There are various coatings available and the best one for your application will depend on the transmission you are looking for and if you would like to target one, two, or multiple wavelengths.
Both the coating type and the optic you choose will dictate the transmission rates that are possible for the wavelength(s) you want, so choose carefully! It doesn’t matter what coating you put on Kodial – it still won’t be able to transmit very well in the deep infrared.
The different coatings we offer
AR coated ZnSe viewport
AR” stands for “anti-reflective”. It is the simplest coating we offer, made up from a single layer of coating material. This offers an improvement to a range of wavelengths on the electromagnetic spectrum.
“VAR” coatings are more complex than the generalised “AR” coating. These coatings are built up from two layers and result in a high-performance transmission coating. “AR” once again stands for “anti-reflective”, however the “V” in “VAR” isn’t in place of a word. Instead, this represents the shape that the coating gives on our reflectance curves. This coating targets a specific wavelength and minimises the reflection, which in turn maximises the transmission. This appears on curves (reflectance graphs) as a dip towards 0% reflection at that specific wavelength.
For “WAR” coatings, much like our VAR coating, the first section of this name relates to how the curve looks on reflectance graphs, like a “W”. Then the “AR” then stands for “anti-reflective”. The “W” shape comes from the ability of this coating to have two points of minimised reflection, as the WAR coating can target two specific wavelengths. The wavelengths we can target with this coating depends on the material but also, this only works if the wavelengths are a suitable distance from each other. This means sometimes we cannot target two wavelengths that are too close together, or two wavelengths that are too far apart.
“BBAR” coatings – This name is descriptive for its purpose, as it stands for “BroadBand Anti-Reflective”. These are specialised coatings for high-performance transmission across a wide range of wavelengths. BBAR coatings are much more complicated to achieve than AR or VAR coatings, as they are built up from many layers of the coating material.
“SPAR” coatings – standing for “special anti-reflective”. These coatings are part of our non-standard range and these are utilised when none of our other anti-reflective coatings are suitable. They target multiple specific wavelengths – much like how a WAR coating works, but with extra points. If you are interested in a SPAR coating, our technicians can create a graph with the predicted transmission rates at each point.
How do thin‑film coatings work?
One technique of these coatings to improve the transmission, is to increase the efficiency of light transferring between the external atmosphere and the optic – and vice versa. If light changes velocity (commonly due to entering a material with a different refractive index), the transfer between the materials can be inefficient and cause reflections.
The simplest thin film coatings are made from a material that has a refractive index between that of the optic and the external environment. The coatings smooth the jump for light passing from the atmosphere to the optic, like a stepping stone. However, AR coatings at Torr Scientific also go a step further, where we counter-intuitively introduce reflections to increase the transmission rate.
When the coating is tailor-made for the wavelength of light, we can make use of something called “destructive interference” with the help of their reflections. This is a phenomenon that occurs when two waves interact at opposite parts of their cycle — when one is at its peak (highest point of the wave), the other is at its trough (lowest point of the wave). When this happens, they will cancel each other out. This only works if the thickness of the coating is tailored to the wavelength, where the reflected light will be at the exact right point of its cycle to cause destructive interference.
