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Conference Paper Birefringence and Grain Size Effects on Optical Transmittanc...

Dear Hadi Jabbar Alagealy , thank you for suggesting the paper dealing with birefringence and grain size effect on transmittances (total forward transmittance and real in-line transmittance). Unfortunately, this paper does not discuss the determination of the refractive index on polycrystalline material. The refractive indices in the table for MgF2 were determined on single crystals and no ellipsometer was used. (Dodge, M. J. (1984). Refractive properties of magnesium fluoride. Applied Optics, 23 (12), 1980. doi: 10.1364 / ao.23.001980). Thank you anyway.

Dear Jaroslav Kastyl ,

to my opinion due to the polycrystalline state of your sample the birefringence is only restricted to the crystallites. An overall birefringence is destroyed due to the distribution of crystallite orientations. There is no chance for ellipsometry to get any reliable birefringence results. In additon you will propably get a lot of light scatter rather than a well defined beam.

The paper above mentioned by Hadi Jabbar Alagealy deals with the overall sample transmission as a function of the size and the birefringence of the crystallites, but not with the overall birefringence.

So far my comments.

Best regards

G.M.

Dear Gerhard Martens ,

I agree with you, you are 100% right.

Do you think it would be possible to determine some "effective" refractive index? Without distinguishing between ordinary and extraordinary refractive index..

I already tried some measurement and I got quite convincing results for effective refractive index, i.e. refractive index of polycrystal with tetragonal lattice.

Please check the plots below.

And thank you for your comments.

Best wishes

JK

First of all, as Maxwell's waveequation shows, the refractive index is a wave property. The overarching question is therefore if the material is homogeneous relative to the wavelength of light. Roughly, it will be if the diameter of the crystallites is smaller than a tenth of the wavelength. If not, then the light will "see" the anisotropy of every crystallite (even if macroscopically randomly oriented, there will be therefore non-zero cross polarization terms for such samples) and the spectra will look the more different the larger the anisotropy. In this case you can determine a pseudorefractive index function, but it will be meaningless. I have added an example from the infrared spectral range, where scattering does not play a role.

More details you can find in my lecture notes, pp. 225-237: Preprint Wave optics in Infrared Spectroscopy

Hello Thomas Mayerhöfer,

I secretly hoped you would notice my question :) and thank you for answer.

Let say, I will determine some "pseudorefractive" index and I will do it for different grain sizes (e.g. in range 100-1000 nm with 100 nm step). It could be helpful for some scientist, or not?

I would definitely love to see such a study and please extend it also to the infrared spectral range! Concerning the helpfulness, I consider such investigations fundamental and very important for spectroscopists to see and know about such effects. As Lord Rayleigh once stated, we all have a tendency to extend simple concepts beyond the limits where they are applicable. It is therefore helpful to be reminded once in a while that nature is often more complex than we think it is... ;-)

Dear Jaroslav Kastyl ,

thanks for feed back.

Indeed there is some 'effective' refractive index or "pseudorefractive" index as Thomas Mayerhöfer calls it.

If you see any benefit having such a number, you should measure it and its dependence on the grain size etc...

Good luck and

best regards

G.M.

As Gerhard Martens noted "..having such a number, you should measure it..".

Maybe you should know why I am interest in such number.

In my science field is important to know refractive indices for some simulations, also to express theoretical transmittance etc. but you'll maybe surprised because some refractive indices just aren't determined yet. And therefore it could help to have at least some "effective" refractive index from polycrystalline structure. It is quite different to do some calculations with refractive index 2.12 or 2.22, there you will probably agree.

I have studied some great refractive indices' databases (e.g. E.Palik Handbook of Optical Constants of Solids) and even in some recent papers authored by well respected researchers is stated that these and those refractive indices are not know. And scientist are thus forced to use some estimations, like n = 2.12 vs 2.22.

I really appreciate comments from both of you and thank you Gerhard Martens and Thomas Mayerhöfer very much.

Jaroslav Kastyl Which angle(s) of incidence do you use for ellipsometry? I just remembered that there is one problem with large domain materials, which is that for large angles of incidence the p-polarized reflectance can become larger than the s-polarized reflectance, which can never happen for scalar materials (materials which can be described by a scalar dielectric function). This may pose a problem when you invert the ellipsometry equations to determine the optical constants. There is a chapter about this problem in my habilitation thesis (Thesis Optics and IR-Spectroscopy of Polydomain Materials

Thomas Mayerhöfer always at three varying angles in one measuring set: 65-70-75° or 55-60-65° or 60-65-70°, etc..., it depends on type of my samples (e.g. lower for alumina, higher for zirconia). I always check the shapes and deviations of Psi, Delta curves - if they look reliable, then I measure more precisely at chosen angles, but if deviations are high, than I change the focused area or select different angles.

Thanks for suggesting your thesis for more detailed info. To be honest, I have read your papers, mainly dealing with (Boguer)-Beer-Lambert law and beyond its limits, really nice works!

Perhaps you noticed, I am not so skilled (relatively young scientist but more ceramicts than spectroscopists), so I am just a beginner in spectroscopy and at optical properties, but I enjoy this field.. Maybe, I have some idea, some offer to you. But now I am at home office due to covid illness. So, firstly I need to discuss the idea with my friend - a project mate and then, I will send you personal message.

Thank you, this reminds me of my misspelling concerning Pierre Bouguer's name in the first paper... ;-)

Anyway, I actually started spectroscopy on ceramic samples... it is definitely a nice field, and mainly difficult, because some spectroscopists, in particular those that write textbooks, love to believe that light has no wave properties... ;-)

I hope you get well soon and I am looking forward to future discussions!

i agree with Gerhard Martens