Shi-Jun Ge

Arbitrary and reconfigurable optical vortex generation: a high-efficiency technique using director-varying liquid crystal fork gratings

A high-efficiency technique for optical vortex (OV) generation is proposed and demonstrated. The technique is based on liquid crystal fork gratings with space-variant azimuthal orientations, which are locally controlled via polarization-sensitive alignment layers. Thanks to the optical rewritability of the alignment agent and the dynamic image generation of the digital micro-mirror device, fork gratings can be instantly and arbitrarily reconfigured. Corresponding optical vortices carrying arbitrary azimuthal and radial indices are demonstrated with a conversion efficiency of 98.5%, exhibiting features of polarization control and electrical switching. The technique may pave a bright road toward OV generation, manipulation, and detection.

Fast switchable optical vortex generator based on blue phase liquid crystal fork grating

Optical vortices have great potentials in optical communications, quantum computations, micro-manipulations and so on. At present, fast switching and reconfiguring of these beam vortices are still challenges. We proposed a blue phase liquid crystal fork grating by applying a vertical electric field with a forked electrode to the polymer stabilized blue phase liquid crystal cell. A fork shaped phase profile with alternation of isotropic and ordinary refractive indices in the lateral direction is thus obtained. Both fork gratings and fork grating array with different topological charges are demonstrated. They permit rapid optical vortex switching and topological charge tuning, and also exhibit excellent polarization independency and high efficiency.

Polarization-controllable Airy beams generated via a photoaligned director-variant liquid crystal mask.

Researches on Airy beams have grown explosively since the first demonstration in 2007 due to the distinguishing properties of nondiffraction, transverse acceleration and self-healing. To date, a simple and compact approach for generating Airy beams in high quality and efficiency has remained challenging. Here, we propose and demonstrate a liquid crystal (LC) polarization Airy mask (PAM) featured by spatially variant LC azimuthal director. The PAM is fabricated through photoaligning LC via a polarization-sensitive alignment agent suophonic azo dye SD1. Thanks to the special design, a novel feature of polarization-controllable switch between dual Airy beams of orthogonal circular polarization is presented. The molecular-level continuity of LC director significantly improves the quality and efficiency of resultant Airy beams. Besides, the PAM can handle intense light due to the absence of absorptive electrodes. Additional merits of compact size, low cost and broad wavelength tolerance are also exhibited. This work settles a fundamental requirement for Airy beam applications of optical manipulations, biology science and even some uncharted territories.

Fast-response and high-efficiency optical switch based on dual-frequency liquid crystal polarization grating

A dual-frequency liquid crystal polarization grating is fabricated by photoalignment and demonstrated as an optical switch. A high diffraction efficiency up to 95% is obtained for a single first order with circular incident polarization. Via merely alternating the frequency of applied electric field, the switch On and Off time reach 350 μs and 550 μs, respectively. This work supplies a new design for fast-response and high-efficiency optical switch with the merits of easy fabrication and low power consumption.

Optical array generator based on blue phase liquid crystal Dammann grating

Blue phase liquid crystal Dammann grating is demonstrated as an optical array generator. The phase profile is formed by the alternation of isotropic refractive index and vertical field induced ordinary index. Periodical Dammann grating can generate equal energy distributed optical array. When forked dislocations are introduced, multiple optical vortex beams with different topological charges are generated. This approach supplies a novel design for fast-response optical array generator, which has great potentials in array illumination, beam shaping and optical communications.

Digitalizing Self‐Assembled Chiral Superstructures for Optical Vortex Processing

Cholesteric liquid crystal (CLC) chiral superstructures exhibit unique features; that is, polychromatic and spin-determined phase modulation. Here, a concept for digitalized chiral superstructures is proposed, which further enables the arbitrary manipulation of reflective geometric phase and may significantly upgrade existing optical apparatus. By encoding a specifically designed binary pattern, an innovative CLC optical vortex (OV) processor is demonstrated. Up to 25 different OVs are extracted with equal efficiency over a wavelength range of 116 nm. The multiplexed OVs can be detected simultaneously without mode crosstalk or distortion, permitting a polychromatic, large-capacity, and in situ method for parallel OV processing. Such complex but easily fabricated self-assembled chiral superstructures exhibit versatile functionalities, and provide a satisfactory platform for OV manipulation and other cutting-edge territories. This work is a vital step towards extending the fundamental understanding and fantastic applications of ordered soft matter.

Simulation and optimization of liquid crystal gratings with alternate twisted nematic and planar aligned regions.

Electro-optical properties of liquid crystal (LC) gratings with alternate twisted nematic (TN) and planar aligned (PA) regions are simulated. Three typical steps are introduced: first, the LC director distributions of the two different regions are simulated. Then, the phase and amplitude of the emergent light in each region are calculated through Jones matrix. Based on this information, the voltage-dependent diffraction efficiency is achieved by Fourier transformation, finally. It gives an exact explanation for the mechanism of this kind of gratings. Experiments with optimized parameters are carried out through photopatterning. The trend of the measured voltage-dependent efficiency fits the simulation result very well. This method can be used to optimize the performance of LC gratings with alternate TN and PA regions, and exhibits great potential in the simulation of corresponding photonics and display applications.

Terahertz vortex beam generator based on a photopatterned large birefringence liquid crystal

A terahertz (THz) q-plate is proposed and demonstrated to generate THz vortex beams. It is composed of a large birefringence liquid crystal (LC) with spatially-varying director distribution sandwiched by two pieces of fused silica glass. A polarization-sensitive alignment agent is photopatterned to carry out the specific LC director distribution. THz vortex beams with different topological charges are characterized with a THz digital holographic imaging system. The intensity and phase distributions consistent with theoretical analyses are obtained. Besides, an eight-lobed intensity distribution is observed corresponding to the vertical polarization component of a cylindrical vector beam. This work may inspire novel THz applications.

Bridging the terahertz near-field and far-field observations of liquid crystal based metamaterial absorbers*

Metamaterial-based absorbers play a significant role in applications ranging from energy harvesting and thermal emitters to sensors and imaging devices. The middle dielectric layer of conventional metamaterial absorbers has always been solid. Researchers could not detect the near field distribution in this layer or utilize it effectively. Here, we use anisotropic liquid crystal as the dielectric layer to realize electrically fast tunable terahertz metamaterial absorbers. We demonstrate strong, position-dependent terahertz near-field enhancement with sub-wavelength resolution inside the metamaterial absorber. We measure the terahertz far-field absorption as the driving voltage increases. By combining experimental results with liquid crystal simulations, we verify the near-field distribution in the middle layer indirectly and bridge the near-field and far-field observations. Our work opens new opportunities for creating high-performance, fast, tunable, terahertz metamaterial devices that can be applied in biological imaging and sensing.

Graphene-assisted high-efficiency liquid crystal tunable terahertz metamaterial absorber

In this paper, few-layer porous graphene is integrated onto the surface of a metasurface layer to provide a uniform static electric field to efficiently control liquid crystal, thereby enabling flexible metamaterial designs. We demonstrate a tunable cross-shaped metamaterial absorber with different arm lengths driven by this combined metasurface and graphene electrode. The resulting absorber supports a resonant frequency tunable from 0.75 to 1 THz with a high-quality factor, and amplitude modulation of ~80% at these frequencies with an applied voltage of 10 V. Furthermore, the near-field intensity and hot spot distribution can be manipulated over a broad range.