Zhi-gang Zheng

Three-dimensional control of the helical axis of a chiral nematic liquid crystal by light

Chiral nematic liquid crystals—otherwise referred to as cholesteric liquid crystals (CLCs)—are self-organized helical superstructures that find practical application in, for example, thermography, reflective displays, tuneable colour filters and mirrorless lasing. Dynamic, remote and three-dimensional control over the helical axis of CLCs is desirable, but challenging. For example, the orientation of the helical axis relative to the substrate can be changed from perpendicular to parallel by applying an alternating-current electric field, by changing the anchoring conditions of the substrate, or by altering the topography of the substrate’s surface; separately, in-plane rotation of the helical axis parallel to the substrate can be driven by a direct-current field. Here we report three-dimensional manipulation of the helical axis of a CLC, together with inversion of its handedness, achieved solely with a light stimulus. We use this technique to carry out light-activated, wide-area, reversible two-dimensional beam steering—previously accomplished using complex integrated systems and optical phased arrays. During the three-dimensional manipulation by light, the helical axis undergoes, in sequence, a reversible transition from perpendicular to parallel, followed by in-plane rotation on the substrate surface. Such reversible manipulation depends on experimental parameters such as cell thickness, surface anchoring condition, and pitch length. Because there is no thermal relaxation, the system can be driven either forwards or backwards from any light-activated intermediate state. We also describe reversible photocontrol between a two-dimensional diffraction state, a one-dimensional diffraction state and a diffraction ‘off’ state in a bilayer cell.

Self-polarizing terahertz liquid crystal phase shifter

Using sub-wavelength metallic gratings as both transparent electrodes and broadband high-efficiency polarizers, a highly-compact self-polarizing phase shifter is demonstrated by electrically tuning the effective birefringence of a nematic liquid crystal cell. The metal grating polarizers ensure a good polarizing efficiency in the range of 0.2 to 2 THz. Phase shift of more than π/3 is achieved in a 256 μm-thick cell with a saturation root mean square voltage of around 130 V in this integrated device.

Blue phase liquid crystals induced by bent-shaped molecules based on 1,3,4-oxadiazole derivatives

Five bent-shaped molecules derived from the 1,3,4-oxadiazole core are synthesised and doped into chiral nematic liquid crystals to induce the blue phase. The effects of their terminal chain length on the blue phase range are compared and analysed experimentally. The results show that the blue phase range is significantly influenced by terminal chain length, and the widest range could be obtained in the case that the chain length is neither too long nor too short. In addition, it is also found that the mechanism of a blue phase induced by bent-shaped molecules is mainly related with its geometry, no matter whether a liquid crystal phase exists or not.

Large birefringence liquid crystal material in terahertz range

We develop a fluorinated phenyl-tolane based nematic mixture NJU-LDn-4 and evaluate its frequency-dependent birefringence utilizing terahertz time domain spectroscopy (THz-TDS). A large mean birefringence of 0.306 is obtained in a broad range from 0.4 to 1.6 THz, with a maximum of 0.314 at 1.6 THz. Furthermore, relation between molecular structures and birefringence property is discussed. This work reveals new insights for tailing liquid crystal molecules with desirable large birefringence in THz range, which is extremely meaningful for the design and fabrication of fast, compact and tunable terahertz devices.

Polarization‐independent blue‐phase liquid‐crystal gratings driven by vertical electric field

— A blue-phase liquid-crystal grating is proposed by applying a vertical electric field with lateral periodic distribution. Simulation on electric-field distribution was also carried out, the results of which suggest the alternation of isotropic and ordinary refractive indices in the lateral direction. Through the electrode configuration design, both 1 D and 2D gratings were demonstrated with high transmittance of ca. 85%. The diffraction efficiency of the first order reached up to 38.7% and 1 7.8% for the 1D and 2D cases, respectively. The field-induced fast phase modulation permits a rapid switching of diffraction orders down to the submillisecond scale.

Frequency‐Driven Self‐Organized Helical Superstructures Loaded with Mesogen‐Grafted Silica Nanoparticles

Adding colloidal nanoparticles into liquid-crystal media has become a promising pathway either to enhance or to introduce novel properties for improved device performance. Here we designed and synthesized new colloidal hybrid silica nanoparticles passivated with a mesogenic monolayer on the surface to facilitate their organo-solubility and compatibility in a liquid-crystal host. The resulting nanoparticles were identified by 1 H NMR spectroscopy, TEM, TGA, and UV/Vis techniques, and the hybrid nanoparticles were doped into a dual-frequency cholesteric liquid-crystal host to appraise both their compatibility with the host and the effect of the doping concentration on their electro-optical properties. Interestingly, the silica-nanoparticle-doped liquid-crystalline nanocomposites were found to be able to dynamically self-organize into a helical configuration and exhibit multi-stability, that is, homeotropic (transparent), focal conic (opaque), and planar states (partially transparent), depending on the frequency applied at sustained low voltage. Significantly, a higher contrast ratio between the transparent state and scattering state was accomplished in the nanoparticle-embedded liquid-crystal systems.

Thermally reversible full color selective reflection in a self-organized helical superstructure enabled by a bent-core oligomesogen exhibiting a twist-bend nematic phase

A self-organized helical superstructure was found to exhibit thermally tunable, reversible selective reflection of light across the whole visible region upon doping with an achiral bent-core hybrid trimer having a twist-bend nematic phase.

Wide tunable lasing in photoresponsive chiral liquid crystal emulsion

A new tunable laser based on a photoresponsive chiral liquid crystal emulsion is reported. Such a laser can be reversibly photo-tuned in a wide spectral range of 112 nm (566–678 nm), and it simultaneously possesses stable emission performance and quasi-continuous tunability. Typically, unlike the conventional liquid crystal laser, the proposed laser can be fabricated by simply coating the material on a single substrate, which will simplify the production process and broaden the application of liquid crystal laser. In addition, the mechanisms of tunability are explored on a molecular scale. The studies reveal that the molecular conformation transition of a photosensitive chiral switch during isomerization leads to changes in not only the molecular geometry and the direction of dipole moment, but also the molecular interactions and the miscibility of the materials. These changes cause the rearrangement of liquid crystals, thus rendering the variation of helical twisted power and achieving tuning on the photonic band-edge of the chiral liquid crystal, consequently forming a photo-tunable laser.

Photoinduced phase transition behaviours of the liquid crystal blue phase doped with azobenzene bent-shaped molecules

A kind of azobenzene-based bent-shaped molecule is doped into a chiral nematic liquid crystal, so as to induce a 25°C blue phase range. As the sample is irradiated by 365 nm ultraviolet radiation, phase transitions from the blue phase to the chiral nematic, and then to the isotropic state are observed, because of the trans–cis isomerisation of the azobenzene moiety; on the other hand, the cis–trans isomerisation of azobenzene leads to the recovery from the isotropic to the blue phase. The transition time from the blue phase to the isotropic is about ∼1–2 minutes, and the time for the recovery is about 2 hours; these times are significantly faster than that of conventional materials. It is also found that the transition time from the blue phase to the isotropic is extended with increase of the material thickness and the weakening of ultraviolet intensity; however, no distinct effects on the time of the reverse process are seen. In addition, a stripe pattern is fabricated by photomask exposure, and such results indicate prospects for application of this material in light-driven displays and information storage.

Controllable Dynamic Zigzag Pattern Formation in a Soft Helical Superstructure

Zigzag pattern formation is a common and important phenomenon in nature serving a multitude of purposes. For example, the zigzag-shaped edge of green leaves boosts the transportation and absorption of nutrients. However, the elucidation of this complicated shape formation is challenging in fluid mechanics and soft condensed matter systems. Herein, a dynamically reconfigurable zigzag pattern deformation of a soft helical superstructure is demonstrated in a photoresponsive self-organized cholesteric liquid crystal superstructure under the simultaneous influence of an applied electric field and light irradiation. The zigzag-shaped pattern can not only be generated and terminated repeatedly on demand, but can also be easily manipulated by alternating irradiation of ultraviolet and visible light while under the influence of a sustained electric field. This unique behavior results from a delicate balance among the variable experimental parameters. The evolution of the zigzag-shaped pattern is successfully modeled by numerical simulations and has been monitored through diffraction of a probe laser. Interestingly, this fascinating zigzag-shaped pattern yields crescent-shaped diffraction pattern. The reversibly controllable dynamic zigzag pattern could enable the fabrication of novel photonic devices and architectures, besides greatly advancing the fundamental understanding of temporal behavior of ordered soft materials under combined stimuli.