Spectroscopic birefringence technique

K. Hongladarom, W. R. Burghardt, S. G. Baek, S. Cementwala, and J. J. Magda, Molecular alignment of polymer liquid crystals in shear flows. I. Spectroscopic birefringence technique, steady-state orientation, and normal stress behavior in poly(benzyl glutamate) solutions, Macromolecules, 26, 772 (1993). 

Little is known about infrared refractive indices of organic compounds, and only very few such studies related to liquid crystals are reported. To some extend this is due to the fact that special techniques and even dedicated equipment are required. On the other hand birefringence can be derived from the polarization pattern produced by the phase difference between the ordinary and the extraordinary beam. This experiment had been outlined by Born and Wolf (1980) and was applied to liquid crystals by Wu et al. (1984). The procedure is primarily suitable in transparent regions, for a more comprehensive optical characterization it should be extended to complete ellipsometry (Reins et al., 1993). Results obtained by infrared-spectroscopic ellipsometry are shown in Figs. 4.6-5 and 4.6-6. 

Spectroscopic ellipsometry is an elaborate technique, which has been used as a robust and extremely sensitive tool for material characterization [107]. During the last decade, its domain of application has been extended to the study of nanostruc-tured samples, although it is mostly appropriate for the determination of order along the film normal. Small refractive index contrast multilayer structures and birefringence can be experimentally accessed [108] in some conditions, but may require sophisticated models. 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

Since 1875, thanks to Kerr s discovery [ 1], it is known that a static electric field can induce a modification of the optical properties of a liquid. Many years later researchers found out that also an optical electromagnetic field was capable of producing a measurable modification of the dielectric properties, inducing a hirefiringence effect the first experimental observation of the optical Ken-effect (OKE) was reported in 1963 [2]. After few years, with the introduction of the first pulsed lasers, spectroscopists discovered the chance to induce in a material a transient birefringence and to measure its relaxation toward the equiUhrium [3]. They also realized that this could be a relevant new spectroscopic tool able to collect new information on the dynamical processes present in the material. The spectroscopic research, worked out in the following years, confirmed this forecast beyond the expectations. Two important experimental improvements of this spectroscopic technique have been made. On one hand, the pulsed laser sources have become able to produce very short pulses of high... 

Jordanov, B., and D. Tsankov. 1984. A spectroscopic method for measuring linear birefringence in the IR region—Another application of the common beam technique. J. Mol. Struct. 115 457 60. 

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