# Optics question-Materials for vacuum chamber viewing ports

1. Jul 25, 2012

### ecnerwalc3321

Hi guys,

I am interested in designing a chamber that is either purged with nitrogen gas or vacuum sealed that encloses a thin film sample. The method of analysis is via ellipsometry. (My knowledge of optics is extremely limited but know that ellipsometry measures some shift in polarization of light upon reflection off a surface to determine thickness of film). The most immediate problem for me is determining what material to make the viewing ports for which the incident ray can pass through readily and keep all its properties (good transmittance, no changes in polarization, etc) and what the angle of the ports should be at (we normally do measurements at 65 degrees; so to eliminate refractance due to material, I've figured it should be placed at a 65 degree angle.

Are there other factors that I should consider? Thanks.

2. Jul 25, 2012

### ZapperZ

Staff Emeritus

For example, what range of light are you going to be using? If you are going to perform ellipsometry over visible range, than ordinary glass should be sufficient (you will still need to know the transmission of that glass over that range, which is often provided by the manufacturer). However, if you need to go into the UV range, than you will need quartz or fused-silica window.

No one can answer the "angle" of the port, since we have no idea how you are performing the measurement, and what your vacuum chamber looks like.

Zz.

3. Jul 26, 2012

### ecnerwalc3321

OK, so I have shown a picture of the current setup. The sample is placed in the center. Polarized light comes in from the left barrel, reflects off the sample, and enters the barrel on the right. What I'm looking to build is a cover/chamber this that covers the sample but does not affect measurements to a great extent. Obviously, two windows on the chamber have to be made in order for light to pass through and hit the sample. Visible light is used; normally we analyze data from 400nm upwards. Measurements are taken at a fixed angle (as shown in picture, 65 degrees).

I was hoping to double check my reason with some experts here before having this thing retrofitted. I do not deal with optics at all; after plowing through many textbooks and seeing how fertile the field is, I'm afraid that I'll overlook some physical phenomenon in designing this thing.

I feel that the factors that I need to be concerned about include polarization change as light passes through windows, refractive index, and transmittance. Is there anything else that needs to be considered? Is this problem as simple as building a chamber that has two windows at 65 degrees (normal to incident light to minimize refracted light) with a material that is offers close to 100% transmittance in the wavelength that I want?

Thank you very much for your input.

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