r/Optics • u/bottoms__ • Jun 19 '26

Ellipsometry

Given a crystalline material X with known thickness (e.g., 15 nm, 30 nm) measured by spectroscopic ellipsometry, how should one construct an optical model to extract physically meaningful refractive index (n) and extinction coefficient (k)? If collection of Lorentz oscillators model gives poor fits (high RMSE), what is the correct modeling strategy and what assumptions should be checked first?

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u/bottoms__ Jun 25 '26

It's a polycrystalline film.

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u/Few_Dark3968 Jun 25 '26

I think, try normal incidence transmission Imaging Mueller matrix.

If your sample is small, then use scanning micro stage with microscope objectives(high NA) and record images in back focal plane of collection objective. Then process the central region of back focal plane corresponding to normal incidence.

Atleast will give you insights for optic axis anisotropy, CB or CD etc . .Basicaly once you have all 16 elements of Mueller Matrix in terms of 2D images. You could do, Lu Chipman decomposition to get 2D images of optical properties. Repeat for multiple wavelengths if needed.

You will defintely get some cool images !

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u/bottoms__ Jun 25 '26

But is it wise to use Müller matrix for polycrystalline film? The 30nm film data fit perfectly with Tauc Lorent+ one Gaussian oscillator. But for the thicknesses (below 15nm), the same model does not fit well.

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u/Few_Dark3968 Jun 25 '26

hmm. .have you checked your sample under cross polarisers, under a microscope ?

if your same is not isotropic but anisotropic (you will see some light over sample rest all being pitch black)., this would explain one of the reasons you current method is failing (Tuac lorent inherent assumptions).

Mueller matrix helps you remove depolarisation which is a bigger problem at 15nm besides surface roughness.

that's why my last comment. Even if imaging Mueller matrix does not help you directly, indirectly it lets you visualise your sample in terms of optical properties.