# optical homogeneity of organic thin films

Prior to further analysis (like variable angle ellipsometric spectroscopy) I check thin films of organic materials vacuum deposited on glass substrates for their optical homogeneity under lineraly polarized (white) light. This is done as precaution, as areas without the materials deposited or uneven deposition of the material may lead to erronous determination of the refractive indices to be determined.

Years ago I once attended classes dealing with polarization microscopy. Provided by an institute of geology, obviously their samples were thin polished cuts of minerals, typically of about 0.03 mm layer thickness. Observation of the optical parameters, like birefringence, was taught as indication of the crystal class; to discriminate between optically isometric (glasses and cubic system), mono- (tetragonal and hexagonal system) and biaxial (orthorhombic, monoclinic, or triclinic class).

Now I ponder about the application of this method to the thin organic films under investigation. Because even (dependent on the wavelength of investigation) some of the refractive indices rise up to $n \approx 1.8$ (there are minerals with similar refractive indices, too), if a much lesser layer thickness of, for example, 120 nm deposited is

• (still) significant enough to observe and characterize the optical indicatrix

or

• I do not observe the expected optical biaxial properties under crossed Nicol's because the deposition yielded an amorphous glass -- what initially was one of the aims of the very deposition experiment.

I both appreaciate that it may not that easy to cleave organic crystalline material to such little layer thickness to cross check this. Without embedding into a matrix, perhaps this were difficult to achieve with a microtome, too. (And I do not have access to this technique, either).
On the other side, if crystalline, I do know that monoclinic spacegroups (like $P2_1/c$, $P\bar{1}$, $P2_1$, $C2/c$) and the orthorhombic $P2_1 2_1 2_1$, are prevalent in the structure models of organic structures deposited at the Cambridge Crystal Database CSD.

• Hard to tell what your question is. Minerals cut 0.03 mm thick are birefringent if they have the right crystal structure. You don't see this in organic films to a thickness of 120 nm. Is this because they are too thin or because the film is amorphous? Is that the essence of your question? – mmesser314 May 7 '15 at 13:33
• @mmesser314 Yes, my question boils down to "are birefringent properties absent because the film is amorphous" or "is this below the detection limit as the film is too thin". – Buttonwood May 7 '15 at 14:04
• Not necessarily too thin. A 1/4 wavelength of material makes an anti-reflection coating. Light that reflects off the bottom surface travels 1/2 wavelength farther than light that reflects of the top. The two are out of phase and cancel. Amorphous sounds more likely. As you say, birefringence depends on crystal structure. – mmesser314 May 8 '15 at 2:33