Bing-Xiang Li

Electro-optic switching of dielectrically negative nematic through nanosecond electric modification of order parameter

We present experimental studies of nanosecond electric modification of the order parameter (NEMOP) in a variety of nematic materials with negative dielectric anisotropy. The study demonstrates that NEMOP enables a large amplitude of fast (nanoseconds) electro-optic response with the field-induced birefringence on the order of 0.01 and a figure of merit (FoM) on the order of 104 μm2/s; the latter is orders of magnitude higher than the FoM of the Frederiks effect traditionally used in electro-optic nematic devices. The amplitude of the NEMOP response is generally stronger in nematics with larger dielectric anisotropy and with higher natural (field-free) birefringence.

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

We study a nanosecond electro-optic response of a nematic liquid crystal in a geometry where an applied electric field E 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 time scales 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 time scale of nanoseconds. The paper provides a useful guidance in the current search for 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 time scale 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 can be used in applications in which one needs to achieve ultrafast (nanosecond) changes in optical characteristics, such as birefringence.

Kerr effect at high electric field in the isotropic phase of mesogenic materials.

The well-known Kerr effect in isotropic fluids consists in the appearance of uniaxial orientational order and birefringence that grows as the square of the applied electric field. We predict and observe that at a high electric field, the Kerr effect displays features caused by the nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Namely, the field-induced birefringence grows faster than the square of the electric field and the dynamics of birefringence growth slows down as the field increases. As a function of temperature, the field-induced birefringence is inversely proportional to the departure from an asymptotic critical temperature, but this temperature is no longer a constant (corresponding to the lower limit of the supercooled isotropic phase) and increases proportionally to the square of the electric field.

Nanosecond switching of micrometer optical retardance by an electrically controlled nematic liquid crystal cell

Electro-optic response of liquid crystals in mainstream display applications exhibits a millisecond switching of optical retardance on the order of one micrometer. We demonstrate that a similarly large optical retardance can be switched much faster, within 10-100 nanoseconds, by using multiple passes of light through a cell filled with the nematic liquid crystal. The fast response is based on the so-called nanosecond electric modification of order parameters (NEMOP) effect. The described approach can be used to develop ultrafast optical shutters and modulators.

Nanosecond electric modification of order parameter in nematic and isotropic phases of materials with negative and positive dielectric anisotropy

We demonstrate nanosecond electro-optic switching in the nematic and isotropic phases of two nematic materials, one with a negative dielectric anisotropy (HNG715600-100) and another with a positive dielectric anisotropy (8CB). In both cases, the effect is caused by the nanosecond electric modification of the order parameter (NEMOP). In NEMOP effect, the electric field is applied in such a way that the director alignment is not distorted. The NEMOP effects in the nematic phases are compared to the Kerr effects of the isotropic phases of the same two materials. Although the amplitudes of NEMOP and Kerr effects are comparable, we observe differences in temperature dependencies. We also observe that the field-induced Kerr birefringence in the isotropic phases does not follow the expected quadratic dependence on the applied field. Namely, it grows slower in the dielectrically negative material and faster in the dielectrically positive material.

Ultrafast electro-optic switching in liquid crystals

We present the nanosecond electro-optic response of liquid crystals in the nematic and isotropic phases. The results demonstrate that in the isotropic phase (close to the phase transition), both the dielectrically positive and negative liquid crystals have larger field induced birefringence than in the nematic phase. However, a nematic mixture with negative dielectric anisotropy exhibits two advantages, namely, the field induced birefringence is weakly dependent on temperature, and the characteristic time of this nanosecond response does not depend on the electric field. The observed nanosecond response can benefit the ultrafast electro-optic applications.

Electrical tunability of an omnidirectional reflector in liquid-crystal heterostructure composed of periodic and quasi-periodic sturcture

We designed an omnidirectional reflector for both the transverse electric (TE) and the transverse magnetic (TM) modes in one dimensional liquid-crystal-based heterostructure composed of the ThueMorse multilayered structure and periodic medium. The electrical tunabilities of the reflector are theoretically studied by the Berremans matrix method. The omnidirectional reflective bands (ORB) of the TE and TM modes have different behaviours. For the TE mode, the ORB is electric independent, and is wider than its counterpart of the TM mode. The ORB of the TM mode becomes narrow with the increase of the incident angle. The polarity-independent ORB was observed in the proposed structure.

Reconfigurable designs for EIT in solid state plasma metamaterials with multiple transmission windows

A reconfigurable metamaterial analog electromagnettically induced transparency like (EIT-like) effect is theoretically and numerically demonstrated in this paper. The unit cell is composed of a stimulated circular loop element and an unstimulated arc slot element, which are both constructed by semiconductor. The proposed designs can realize a continuously tunable EIT-like effect in a broad frequency range, while the number of EIT-like transmission windows can be configured by increasing the number of arc slots.

Rapid control of liquid crystals for ultrafast communications

Nematic liquid crystals (NLCs) are widely used in electro-optic applications because of their anisotropy. In a typical NLC, the molecules’ long axes are on average aligned parallel to a single direction, called the director, which is also the optical axis of the NLC. Typically, a liquid crystal display is controlled by realigning the director with an external electric field. When the field is off, anisotropic molecular interactions at the bounding plates cause the director to relax back into the uniform state, in which finite birefringence of the NLC results in optical contrast. One of the principal difficulties is that the orientational relaxation of the director in the field-off state is a slow process limited by viscosity; the switch-off time is typically in the range of milliseconds. For applications such as ultrafast electro-optics, quantum computing, and secure communications, however, a nanosecond response time is required. As a result, many research groups have explored ways to reduce the switching time, such as by optimizing the viscoelastic parameters of the NLCs, using overdriving schemes of switching,1 and placing the NLCs into polymer networks.2 These measures show improvements, reducing the response time to the millisecond and sometimes even submillisecond range. For example, a blue-phase polymer template filled with an NLC shows both switching on and off times within hundreds of microseconds2 in a broad (80C) temperature range. Overcoming the submicrosecond and nanosecond barrier remains a challenge caused by the viscous nature of director reorientation. We have proposed a new approach to NLC electro-optics3–5 that eliminates the director reorientation process. For example, in an NLC with a negative anisotropy of dielectric permittivity subject to an electric field that is applied perpendicularly to the director, the NLC molecules do not realign. Although the Figure 1. Dynamics of the field-induced birefringence (ın) of the nematic liquid crystal material known as CCN-47 in response to an applied voltage pulse (U) of amplitude 626V at 49C. The inset shows the geometry of the experiment, in which the nematic cell with planar director (along the z-axis in the geometry BU, and along the y-axis in geometry UF) is probed with an oblique laser beam (red dashed line).