Polar structure of disclination loops in nematic liquid crystals probed by second-harmonic-light scattering

Abstract

Angle-resolved, second harmonic light scattering (SHLS) measurements are reported for three different classes of thermotropic nematic liquid crystals (NLCs): polar and nonpolar rodlike compounds and a bent-core compound. Results revealing well-defined scattering peaks are interpreted in terms of the electric polarization induced by distortions of the nematic orientational field ("flexopolarity") associated with inversion wall defects -non-singular disclinations, analogous to Neel walls in ferromagnets, that often exhibit a closed loop morphology in NLCs. Analysis of the SHLS patterns based on this model provides a "proof-of-concept" for a new approach to probe the flexopolar properties of NLCs.

The stabilisation of the Nxphase in mixtures

“…The correlation with theoretical models is not fully clear and multilevel ordering has been considered and discussed. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Indeed, a considerable theoretical and simulation effort is currently targeted at understanding the liquid crystal phase behaviour in systems with conformational chirality. Due to these questions, in this contribution the phase is termed N x .…”

Optical second harmonic generation in a ferromagnetic liquid crystal

“…=[ * sin (( )) + ( ( ) )cos (( ))] , (4) where * is the local low frequency electric field generated by an external voltage U applied along the z-axis (Fig. 1a), (.n)n is the flexoelectric poling field associated with the splay-type distortion of n(r), 9,10,14 and CE and CF are constants. In a distorted LC structure, such as the one depicted in Fig.…”

“…11,12 The flexoelectric effect is responsible for occurrence of SHG in the vicinity of topological defects and other imperfections in the LC structure. 13,14 It also causes the appearance of scattered second harmonic (SH) radiation provoked by thermally-driven orientational fluctuations of the LC molecules (SH scattering). 15,16 Another source of SHG are contact regions of the LC medium with surrounding surfaces, where censtrosymmetry is intrinsically absent.…”