Nanosecond electro-optics of a nematic liquid crystal with negative dielectric anisotropy

Abstract

Abstract. We study a nanosecond electro-optic response of a nematic liquid crystal in a geometry where an applied electric field Ε modifies the tensor order parameter but does not change the orientation of the optic axis (director N ). We use a nematic with negative dielectric anisotropy with the electric field applied perpendicularly to N . The field changes the dielectric tensor at optical frequencies (optic tensor) due to the following mechanisms: (a) nanosecond creation of the biaxial orientational order; (b) uniaxial modification of the orientational order that occurs over timescales of tens of nanoseconds, and (c) the quenching of director fluctuations with a wide range of characteristic times up to milliseconds. We develop a model to describe the dynamics of all three mechanisms. We design the experimental conditions to selectively suppress the contributions from the quenching of director fluctuations (c) and from the biaxial order effect (a) and thus, separate the contributions of the three mechanisms in the electro-optic response. As a result, the experimental data can be well fitted with the model parameters. The analysis provides a rather detailed physical picture of how the liquid crystal responds to a strong electric field on a timescale of nanoseconds. This work provides a useful guidance in the current search of the biaxial nematic phase. Namely, the temperature dependence of the biaxial susceptibility allows one to estimate the temperature of the potential uniaxial-to-biaxial phase transition. An analysis of the quenching of director fluctuations indicates that on a timescale of nanoseconds, the classic model with constant viscoelastic material parameters might reach its limit of validity. The effect of nanosecond electric modification of the order parameter (NEMOP) can be used in applications in which one needs to achieve ultrafast (nanosecond) changes of optical characteristics, such as birefringence.2

Microsecond Electro-Optic Switching of Nematic Liquid Crystals with Giant Dielectric Anisotropy

“…The field-induced birefringence n  in the MEMOP effect in GPDA200 is significant and is achieved at electric fields that are by an order of magnitude lower than those reported previously for NEMOP in NLCs with a negative dielectric anisotropy [10][11][12]. For example, in 7 the previously studied HNG715600-100 ( 12   = − , 23 C T =  ) 0.013 n  = was achieved under the applied field 170 V/μm E = [12], while the same n  in GPDA200 is achieved already at E= (11-33) V/μm, depending on the temperature, Fig.…”

“…The field-induced birefringence in materials with positive dielectric anisotropy is caused by the quenching of the director fluctuations [14] and modification of the uniaxial order parameter [11]. The modification of the uniaxial order parameter is proportional to the square of the field [11]. Quenching of the director fluctuations by the electric field is a well-known phenomenon associated with the anisotropy of the liquid crystals.…”

“…The described MEMOP effect yields the FoM one or two orders of magnitude higher than the conventional Frederiks effect. It is also of interest to compare the described MEMOP effect to the NEMOP effect observed in nematics with 0   [10][11][12]. As compared to NEMOP, the MEMOP effect requires electric fields that are one order of magnitude lower to achieve a similar field-induced birefringence [12].…”

Search for microscopic and macroscopic biaxiality in the cybotactic nematic phase of new oxadiazole bent-core mesogens

“…Given the unequivocal local biaxiality in this unique class of BCNs, possibly promoted by enhanced orientational correlations in the transverse molecular packing (compared to the other BCNs), synthetic efforts are underway to develop variants of these molecules that may be able to extend the local biaxial correlations to the macroscopic scale. Another potentially interesting direction of research might be the measurements of the biaxial susceptibility of the electric field-induced modification of the optical tensor; that would indicate how close the u N might be to forming b N in the absence of an electric field [58].…”

Polymerization enabled reduction of the electrically induced birefringence change in nematic liquid crystals

“…Recently, the birefringence of NLCs with a negative dielectric anisotropy has been electrically changed on the time scale of nanoseconds for both switching-on and switching-off processes by the so-called nanosecond electrically modified order parameter (NEMOP) effect [29][30][31] . In the NEMOP effect, the applied electric field changes the optical property of NLCs by the enhanced uniaxial order parameters and the induced biaxial order parameters [29][30][31] . However, the NEMOP effect has not been explored in PSLCs.…”