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Chidley
Posted on Thursday, June 02, 2005 - 5:36 am:   

Can anyone tell me of materials that absorb between wavelengths 1000nm and 3000 nm ? Any class of material, e.g., lquids, polymers, inorganics. They should preferably, but not necessarily, be "transparent" at wavelengths shorter than 1000nm. They do not need to absorbing over whole band (mixtures are allowed). I haven't found anything searching the internet: I am aware of many compound semiconductors, but they are opaque in the visible.
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Gabi Levin
Posted on Thursday, June 02, 2005 - 6:01 am:   

It all depends on the money you can spend. If you contact companies like II-VI, Coherent, they specialize in interference filters, etc. They can tell you if such wide band filters are possible. The problem you are looking at is one material, or two, that will absorb in the NIR, including part of the near near NIR (1000 to 1200nm) but be transpaernt in the visible and the other part of the near near NIR, 700 to 1000, which will be very difficult to find.

The filters I am talking baout are made from several layers of oxides that have different refraction coefficients, and thus can be built to transmit "almost" any band, and block other bands of light. They can probably use ZnSe which is transparent (not 100%) in the visible and the NIR and deposit few layers (thickness of such layers is usually submicron) that will block the NIR region, and leave the shorter wavelength to go through.
The companies I mentioned are just an example, others exist.

I hope this will help you,

Gabi Levin
Brimrose Corp. of America
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hlmark
Posted on Thursday, June 02, 2005 - 8:46 am:   

Chidley - a lot depends on what characteristics you want to have at wavelengths longer than 3000 nm (3 microns). To some degree or other, almost everything absorbs in the 1 - 3 micron range, with increasing absorbance as you go toward longer wavelengths. In fact, you'd have a much more difficult job finding materials that don't absorb in that region, there are only about 4 or 5 solids that are reasonable candidates for that and 3 liquids, that I know of.

If you can also have the filter absorb at wavelengths longer than 3 microns, then it's a relatively easy task, but you will probably need a combination of filters. One of them could be something as simple as a cell containing water, which will absorb most radiation from almost any desired wavelength on, depending on the thickness. Then a broadband interference filter can be used to give a sharper edge at the short-wavelength end than a purely absorbing filter will do. Broadband interference filters in the NIR are fairly readily available, as Gabi said. You might even find suitable filters off-the-shelf from some optical suppliers, such as Edmund Industrial Optics.

Howard

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hlmark
Posted on Thursday, June 02, 2005 - 8:54 am:   

Come to think of it, something else you should look at is Schott filter glasses - those are available off-the-shelf with all sorts of absorption characteristics. If you can't find a single one that is suitable, there might a combination that will be

Howard

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Edward Chidley
Posted on Friday, July 22, 2005 - 6:11 am:   

Ladies and Gents,

(Gabi, Howard, having had time to do a few searches, I wish to re-state my problem, to see if you or anyone else can help, more precisely and probably to a slightly diferent specification ! Thanks for you replies: the suggestion about interference filters - these can be designed not to transmit light over the required waveband, for one angle of incidence; the requirement is for a truly absorbing material, that blocks/transmits over all angles. Sorry, I should have said)

As I said before, the substance is required to highly absorbing between approximately 1000 - 3000 nm (Howard, thanks for the suggestion about Schott filter glasses, Schott provide an Excel spreadsheet with "data" out to 5200 nm, for KG1-5. I looked at KG5 and it is indeed largely absorbing between 1000 nm and 3000 nm - but read on). And it needs to have a high degree of transparency between about 3000 nm and 12000 nm (KG5 is very absorbing beyond ~4000 nm, and glass is very absorbing beyond abound 5000 nm; but for example, sapphire is highly transmitting from UV to beyond 15000 nm), and to be moderately transparent in the visible part of the EM spectrum - it's allowed to have absorbtion bands there, or be less transparent though. Of course this will all depend on thickness, but I mean by 'largely absorbing' and 'high degree of transparency' the layer will transmit less than 10% and transmit 80%, i.e., the requirement is about relative transparency.

However, a combination of materials which absorb anywhere between 1000 nm and 3000 nm is allowed, as long as they have low absorbtion beyond 3000 nm. Or the best approximation the specification.

Edward Chidley
22.7.05
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hlmark
Posted on Friday, July 22, 2005 - 9:00 am:   

Edward - I dunno, but you may be out of luck, unless you can afford the time, resources and MONEY to do the research that can possibly allow you to come up with your own material formulation. F'rinstance, if you could work with a sapphire manufacturer to make doped sapphire with characteristics similar to what Schott does with glass, that might be an approach. Otherwise, I'm fresh out of ideas.

But maybe you shouldn't discount interference filters yet. When light enters an IF at an angle, it still undergoes the interference phenomenon, but the effective wavelength changes. If you stacked several narrow-to-medium bandwidth filters, different ones would take out different wavelengths, and when light came in at an angle, there would simply be a different one taking out any given wavelength.

Howard

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edward chidley
Posted on Friday, July 29, 2005 - 5:23 am:   

Howard,

Thanks for your suggestions. I posted the same question again yesterday as I couldn't find this discussion at all !
As regards IR transmitting materials, have a look at the Oxford Instruments website, page http://www.oxinst.com/SCNPDP140.htm, download "windows for cyogenic environments guide" - this includes some materials suitable for use in the IR, and spectra too (materials are mostly aslo only suitable for cryogenic work). But you probably know all that.

Edward
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hlmark
Posted on Friday, July 29, 2005 - 8:40 am:   

Edward - I posted a slightly different answer yesterday; I suppose you've seen it. I looked at the Oxford page you mentioned, and noted that they also list two of the materials the I also mentioned: sapphire and CaF2.

But those are only the materials that they provide, for their own equipment. The possible range materials that are clear in the mid-IR is much broader than that, starting with most of the "ordinary" binary salts (NaCl, KBr, etc.) up to CsI. Mostly, the heavier the atoms in the salt, the longer the long-wave limit is. CsI, for example, is clear and transparent to 50 microns (50,000 nm). The problem with all of those is that they are soluble in water. Even if you don't plan on using aqueous media, they tend to become fogged by atmospheric humidity, and are a real pain in the bazooka to work with, for that reason - just ask any mid-IR spectroscopist.

But there are lots of other salt-like materials that are available, too, that are not water-sensitive. CaF2 and BaF2 (that I mentioned previously) are among them, as is ZnSe, TlBr, salt mixtures such as KRS-5 (a proprietary designation for a ternary salt mixture) various silver salts, etc. With such a wide range of possibilities, it becomes mainly a matter of choosing one with the desired mechanical properties as well as the optical range you want. But then there is still the issue of making them absorb in the range where you want them to not be transparent.

If you want to contact me off the discussion group, we can discuss this further.

Howard

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