Polar in-plane surface orientation of a ferroelectric nematic liquid crystal: Polar monodomains and twisted state electro-optics

Abstract

We show that surface interactions can vectorially structure the three-dimensional polarization field of a ferroelectric fluid. The contact between a ferroelectric nematic liquid crystal and a surface with in-plane polarity generates a preferred in-plane orientation of the polarization field at that interface. This is a route to the formation of fluid or glassy monodomains of high polarization without the need for electric field poling. For example, unidirectional buffing of polyimide films on planar surfaces to give quadrupolar in-plane anisotropy also induces macroscopic in-plane polar order at the surfaces, enabling the formation of a variety of azimuthal polar director structures in the cell interior, including uniform and twisted states. In a π-twist cell, obtained with antiparallel, unidirectional buffing on opposing surfaces, we demonstrate three distinct modes of ferroelectric nematic electro-optic response: intrinsic, viscosity-limited, field-induced molecular reorientation; field-induced motion of domain walls separating twisted states of opposite chirality; and propagation of polarization reorientation solitons from the cell plates to the cell center upon field reversal. Chirally doped ferroelectric nematics in antiparallel-rubbed cells produce Grandjean textures of helical twist that can be unwound via field-induced polar surface reorientation transitions. Fields required are in the 3-V/mm range, indicating an in-plane polar anchoring energy of wP ∼3 × 10−3 J/m2.

Soliton walls paired by polar surface interactions in a ferroelectric nematic liquid crystal

“…Recent synthesis and evaluation [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] of new mesogens with large molecular dipoles led to a demonstration of a fluid ferroelectric nematic liquid crystal (NF) with a uniaxial polar ordering of the molecules 13,14 . The ferroelectric nature of NF has been established by polarizing optical microscopy observations of domains with opposite orientations of the polarization density vector P and their response to a direct current (dc) electric field 𝐄 13,14 . The surface orientation of 𝐏 is set by buffed polymer layers at glass substrates that sandwich the liquid crystal 13,14 .…”

“…The ferroelectric nature of NF has been established by polarizing optical microscopy observations of domains with opposite orientations of the polarization density vector P and their response to a direct current (dc) electric field 𝐄 13,14 . The surface orientation of 𝐏 is set by buffed polymer layers at glass substrates that sandwich the liquid crystal 13,14 . This sensitivity to the field polarity and in-plane surface polarity makes NF clearly different from its dielectrically anisotropic but apolar paraelectric nematic counterpart N.…”

How Far Can We Push the Rigid Oligomers/Polymers toward Ferroelectric Nematic Liquid Crystals?

“…The ferroelectric nematic ( N F ) liquid crystal (LC) is a novel type of emerging polar soft matter discovered in 2017, − where the head-to-tail symmetry of two permissible director orientations of LC mesogen, i.e., n and - n , is broken. The molecular dipoles nearly parallel to the director axis spontaneously orient to a preferred direction, producing a macroscopic electric polarization P through a ferroelectric-like packing. − Although the nomenclature of the “ N F ” phase is still in debate, ,− the unique integration of giant dielectricity, ,− strong nonlinear optical signal, high response sensitivity to electric field, , and fluidity in N F materials endows rich opportunities for the development of advanced optical or electrical LC devices. − …”

Revealing the polar nature of a ferroelectric nematic by means of circular alignment

“…In the N F -phase ( b ) the polarization (red) aligns with the rubbing direction. In the case of anti-parallel rubbing the structure has to make a right-handed (R) or left-handed (L) twist of 180° to match the in-plane polar boundary conditions 9 , 10 . …”

“…For the polar N F phase the situation is fundamentally different: on a rubbed surface the N F phase aligns with the spontaneous polarization parallel to the rubbing direction 9 . The striking consequence is that when the rubbing directions are anti-parallel, we get a ± 180° director twist between the surfaces, cf.…”