INDEX Depolarizers

Circular dichroism arises from the same optically active transitions responsible for the Cotton effects observed in ORD curves, but unlike ORD it is an absorption, not a dispersion, phenomenon. Hence, the CD effect is restricted to the region of the transition and can be interpreted more straightforwardly. Both ORD and CD can best be understood if one imagines the incident plane-polarized beam resolved into two in-phase circularly polarized beams whose vectors rotate in opposite directions. A difference in index of refraction between the left and right circularly polarized beams results in rotation of the transmitted plane polarized beam while differential absorption of the two circularly polarized beams results in depolarization of the transmitted beam, so that an incident plane-polarized beam whose frequency is within that of an optically active absorption band becomes both rotated and elliptically polarized upon passage through the sample. This depolarization effect is CD, and the measured parameter is (et — er), the difference in extinction coefficient between the left and right circularly polarized beams. The data is usually recorded as the specific ellipticity, defined as ... 

The relationship between the depolarized time-dependent transmitted intensity l(t) and the refractive index difference in two orthogonal directions x and y, in which one is in the direction of the applied electric field and both are perpendicular to the direction of propagation of the incident light, is given by (2)... 

Although polarization data are normally obtained for liquids and single crystals, it is possible to measure depolarization ratios of Raman lines from solids by suspending them in a material with similar index of refraction (27). 

Appraisals of the optical properties of lipid solutions and dispersions will provide information on concentrations, aggregation and stability, phase transitions, densities, and repeating structuresJ Measurements of refractive index, scattered light intensity (polarized and depolarized), and birefringence are relatively easy laboratory methods on which certain product specifications may be based. Also, fluorescent techniques can readily provide information on lipid movements and transfer of lipid between particles. ... 

Many methods for detennination of molecular order can only measure P2) [Mcbl, Warl]. Among them are measurements of the infrared dichroism and of the refractive index. P2) and P4) can be determined by Raman fluorescence depolarization and from wideline NMR spectra. However, X-ray diffraction and lineshape analysis of resonances from isolated NMR interactions can also provide higher moments. 

An intensity-modulated version of such a sensor was shown to provide a refractive index resolution better than 2x10 RIU. A wavelength-modulated version of this sensor exhibited even better resolution -5 X 10 RIU [64]. However, these SPR sensors suffered from polarization in-stabihty, even when fight depolarization was introduced. To eliminate this source of instability, SPR sensors using polarization-maintaining fibers were introduced [65]. These sensors were demonstrated to provide a refractive index resolution of about 2x10 RIU. 

Figure 9 Polarized Raman measurements are used to characterize grain orientation in polycrystalline films. (A) Principal Raman vibrational modes for the rutile phase of Ti02 (B) Polarized Raman spectra of oriented films (order increases from top to bottom) (C) Refractive index variation with Raman depolarization ratio for the A q mode.

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