Ge Zhu

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.

Arbitrary photo-patterning in liquid crystal alignments using DMD based lithography system

We propose and implement a technique for arbitrary pattern fabrication in liquid crystal (LC) alignments and local polarization control for light wavefront. A micro-lithography system with a digital micro-mirror device as dynamic mask forms arbitrary micro-images on photoalignment layers and further guides the LC molecule orientations. Besides normal phase gratings, more complex 2D patterns such as quasicrystal and checkerboard structures are demonstrated. To characterize the optical performances of the fabricated structures, the electro-optically tunable diffraction patterns and efficiencies are demonstrated in several 1D/2D phase gratings. Compared to other techniques, our method enables the arbitrary and instant manipulation of LC alignments and light polarization states, facilitating wide applications in display and photonic fields.

Polarization independent liquid crystal gratings based on orthogonal photoalignments

The polarization independence of liquid crystal gratings with alternate orthogonal aligned regions is theoritically studied and demonstrated by means of photoalignment technique. The different alignments in adjecent regions are achieved by two-step photo exposure to guide orientations of sulfonic azo dye layers and further align the liquid crystal molecules. Both one-dimensional and two-dimensional switchable phase gratings have been demonstrated. Such polarizer-free gratings show very high transmittance (∼92%), diffraction efficiency (over 31%), and optical contrast (over 150) including low power consumption.

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.

Fast response dual-frequency liquid crystal switch with photo-patterned alignments

A dual-frequency liquid crystal based optical switch with orthogonally photo-patterned alignments is designed and fabricated. The cell gap is theoretically optimized for high switching performance. The measured extinction ratio of first diffraction order is over 20 dB with a low electric field of 3  V/μm. The switch On-Off time are measured to be 350 μs and 600 μs, respectively, both of which have reached submillisecond scale. Moreover, the switch is polarization independent, which has been predicted theoretically and further proved experimentally. This design is suitable in wide applications requiring fast response and polarizer free properties.

A fast response variable optical attenuator based on blue phase liquid crystal

Blue phase liquid crystals (BPLCs) are promising candidates for next generation display thanks to their fast response and quasi-isotropic optical properties. By taking these advantages, we propose to introduce the material into fiber-optic applications. As an example, a BPLC based variable optical attenuator (VOA) is demonstrated with a polarization independent design. The device shows normally-off feature when no field is applied. Response time down to submillisecond scale is achieved in switching between two arbitrary attenuation states. The attenuation range is also measured from 1480 to 1550 nm, which cover the whole telecomm S-band and part of the C-band. The overall performances reach the requirements for practical use; while still have room for further improvement. Through this example, the applicability of BPLC in fiber-optic devices is presented, which may impel the development of many other photonic applications from infrared to even microwave regions.

Liquid crystal blue phase induced by bent-shaped molecules with allylic end groups

A blue phase is induced by bent-shaped monomer with allylic end bonds when doped into chiral nematic liquid crystal. The mechanism of blue phase induction and stabilization is investigated. Its temperature range is further widened to 10.2 K by UV irradiation with a slowly cooling process. The widening principle is distinguishable from traditional polymer stabilization mechanism. The study provides some useful insights into the molecular design of suitable bent-shaped dopants towards wide range blue phase liquid crystals.

Brief review of recent research on blue phase liquid crystal materials and devices

Both polymer stabilization and bent-shaped molecule doping strategies are utilized to widen the blue phase range of liquid crystals. The molecular structures of compositions are optimized for high thermooptical and electro-optical performances. A low temperature applicable blue phase liquid crystal with suppressed hysteresis is achieved. The bistability of one blue phase liquid crystal is investigated. Based on these materials, fast tunable devices such as gratings with polarization insensitivity are designed and fabricated. The materials and device designs demonstrated here are suitable in wide applications requiring fast response time.

Low‐temperature‐applicable polymer‐stabilized blue‐phase liquid crystal and its Kerr effect

— A type of polymer-stabilized blue-phase liquid crystal, which can be used in a low-temperature environment, is proposed. The blue-phase range after polymerization was widened to more than 73°C, and the blue-phase texture is very stable even at a temperature as low as −35°C. The electro-optical performances dependence on polymer concentration was investigated. The results indicate that the saturation voltage increases and the hysteresis enhances as the polymer concentration increases. The rise and decay times could reach as low as 391 and 789 μsec, respectively. Such material also shows good electro-optical behavior at a temperature of −35°C. In addition, the Kerr constant was tested under a uniformly distributed electric field to be 2.195 nm/V2 at room temperature and 2.077 nm/V2 at −35°C. The Kerr constant tested under white-light illumination was 1.975 nm/V2, which shows a small dispersion.

Bistable state in polymer stabilized blue phase liquid crystal

The bistable phenomenon is found in polymer stabilized blue phase liquid crystal. Two stable phases, blue phase and chiral nematic state, are obtained and we succeeded in the switch between these two states through different operation procedures. The mechanism of the bistable phenomenon is discussed, according to which we consider the bistability may widely exist in various polymer stabilized blue phase liquid crystal systems. This work provides some useful insights into the application of polymer stabilized blue phase liquid crystal in bistable devices.