Optical measurements of orientational order in uniaxial and biaxial nematic liquid crystals

• Published in: Liquid crystals Vol. 37; no. 6-7; pp. 949 - 959
• Main Authors: Gleeson, H.F., Southern, C.D., Brimicombe, P.D., Goodby, J.W., Görtz, V.
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis 01.06.2010; Taylor & Francis Ltd
• Subjects: Anisotropy; biaxial order; Crystallography; Crystals; Liquid crystals; Materials science; Methodology; Nematic; Order parameters; orientational order parameters; polarised Raman spectroscopy; Raman spectroscopy; Refractive index; Refractivity; Spectrum analysis
• ISSN: 0267-8292; 1366-5855
• DOI: 10.1080/02678292.2010.488818

The use of optical methods to study the orientational order in nematic liquid crystals is both convenient and powerful. Some of the earliest approaches made use of measurements of the refractive index anisotropy, following the methodology of Halle [ 1 ] to measure the order parameter, . This method is reviewed briefly and some typical areas of application are presented. Raman spectroscopy is a second approach discussed in this paper. Although technically a more complex methodology, Raman spectroscopy has the significant advantage that in addition to being able to measure it is also possible to determine . A review of current approaches is presented. Very recently, it has been shown that polarised Raman spectroscopy can be employed to determine biaxial order parameters in liquid crystals. The methodology is described, and data are presented for such measurements on a series of bent-core nematic materials that could exhibit the elusive biaxial nematic phase.