Using the full Raman depolarisation in the determination of the order parameters in liquid crystal systems

Abstract

The depolarisation ratio for the Raman-active phenyl stretching mode has been measured over the whole of the mesophase range, and the orientational order parameters deduced, in the uniaxial nematic liquid crystal octylcyanobiphenyl (8CB). Linearly polarised light was incident normally on a homogeneously aligned sample and a chi(2) minimisation routine performed on the 360 degrees depolarisation ratio profile. The order parameters (P(200)) and (P(400)), together with the differential polarisability ratio, r , are used as fitting parameters and measured as a function of temperature. Interestingly, we show that the value for r , conventionally measured in the isotropic phase and assumed to remain constant, has a clear temperature dependence, ranging from -0.032 + or - 0.008 in the isotropic phase through to -0.245 + or - 0.015 at the nematic-to-smectic A phase transition. The measured order parameters (P(200)) and (P(400)) varied from 0.35-0.55 + or - 0.02 and 0.180-0.245 + or - 0.02, respectively, across the 8 degrees C wide nematic phase range. The values of both (P(200)) and (P(400)) are in excellent agreement with theory, but it is noteworthy that (P(400)) shows a much better quantitative match than has been reported in previous work. Crucially the temperature dependence of r is shown to be a contributing factor in the low (P(400)) values that have been conventionally reported from Raman scattering measurements. The potential for fitting the entire angular depolarisation ratio distribution in liquid crystalline systems that are described by more order parameters, specifically biaxial materials, is discussed.

Realignment-enhanced coherent anti-Stokes Raman scattering and three-dimensional imaging in anisotropic fluids

“…The intensity is higher for orientation of the linear laser polarization parallel to 0 n and the CN bonds and lower for the orthogonal one, Fig. 1, which is consistent with previous reports [37]. Raman spectra for single-compound LC 5CB (red), nematic mixture E7 (green), and a mixture of nematic AMLC-0010 and chiral additive CB-15 (~1 Wt.…”

“…In general, the threshold power increases with decreasing the LC's optical anisotropy and/or increasing the elastic constants, as well as is much higher in smectic and columnar materials as compared to nematics. In planar cells, the threshold power tends to be larger than in the homeotropic ones because of the LC's effect on the beam polarization [32][33][34][35][36][37]. Moreover, special care has to be taken when monitoring the realignment phenomena in the planar cells as they can be "invisible" in the polarizing microscopy mode because of the so-called Mauguin regime in which the linear polarization of light propagating in the LC medium follows the twist of the LC director [18,38].…”

“…The findings described here are also of general interest for nonlinear microscopy applications in LC research [25][26][27][28][29] as well as for probing molecular/bond orientation patterns in biological, composite thermotropic, and lyotropic LC materials using CARS microscopy [7-16, 23, 30]. Since the laser-induced optical realignment effects [31][32][33][34][35][36][37][38][39] are shown to be significant above the certain threshold powers of CARS excitation beams, this sets some limits in terms of excitation laser powers and how fast CARS imaging can be performed when studying dynamic processes in anisotropic fluids.…”

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

“…However, values of S [54] and K 1 ; K 3 [55] are available. So in this case we make the assumption, experimentally demonstrated for some rod-like LCs, [56] that e 1 and e 3 are linearly proportional to S in the nematic phase.…”

Orientational order of tetra n-hexylesters of perylene and tetrachloroperylene tetracarboxylic acids in low-molar-mass liquid crystal investigated using absorption and fluorescence methods

“…On the other hand, the <P 4 > values for pure liquid crystal 8CB, for instance, determined by using full Raman depolarisation experiment, are positive in agreement with the Maier-Saupe and the Humphries-James-Luckhurst theoretical models. [22,27] The order parameters <P 2 > and <P 4 > are not sufficient to fully describe the distribution of dye probe in liquid crystalline matrices, and the higherorder parameters <P L > (L ≥ 6) are needed to be determined in other experiments. [28] The applied fluorescence method is designed to determine the values of <P 4 > for the dye molecules with cylindrical symmetry (C ∞h ).…”