Bing-Yan Wei

Generating switchable and reconfigurable optical vortices via photopatterning of liquid crystals

Liquid-crystal fork gratings are demonstrated through photopatterning realized on a DMD-based microlithography system. This supplies a new strategy for generating fast switchable, reconfigurable, wavelength-tolerant and polarization-insensitive optical vortices. The technique has great potential in broad fields such as OAM-based quantum computations, optical communications, and micromanipulation.

Generation of arbitrary vector beams with liquid crystal polarization converters and vector-photoaligned q-plates

Arbitrary vector beams (VBs) are realized by the designed polarization converters and corresponding vector-photoaligned q-plates. The polarization converter is a specific twisted nematic cell with one substrate homogeneously aligned and the other space-variantly aligned. By combining a polarization-sensitive alignment agent with a dynamic micro-lithography system, various categories of liquid crystal polarization converters are demonstrated. Besides, traditional radially/azimuthally polarized light, high-order and multi-ringed VBs, and a VB array with different orders are generated. The obtained converters are further utilized as polarization masks to implement vector-photoaligning. The technique facilitates both the volume duplication of these converters and the generation of another promising optical element, the q-plate, which is suitable for the generation of VBs for coherent lasers. The combination of proposed polarization converters and correspondingly fabricated q-plates would drastically enhance the capability of polarization control and may bring more possibilities for the design of photonic devices.

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.

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.

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.

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.

Fork gratings based on ferroelectric liquid crystals.

In this article, we disclose a fork grating (FG) based on the photo-aligned ferroelectric liquid crystal (FLC). The Digital Micro-mirror Device based system is used as a dynamic photomask to generated different holograms. Because of controlled anchoring energy, the photo alignment process offers optimal conditions for the multi-domain FLC alignment. Two different electro-optical modes namely DIFF/TRANS and DIFF/OFF switchable modes have been proposed where the diffraction can be switched either to no diffraction or to a completely black state, respectively. The FLC FG shows high diffraction efficiency and fast response time of 50µs that is relatively faster than existing technologies. Thus, the FLC FG may pave a good foundation toward optical vertices generation and manipulation that could find applications in a variety of devices.

Generation of self-healing and transverse accelerating optical vortices

Self-healing and transverse accelerating optical vortices are generated via modulating Gaussian beams through subsequent liquid crystal q-plate and polarization Airy mask. We analyze the propagation dynamics of these vortex Airy beams, and find that they possess the features of both optical vortices and Airy beams. Topological charges and characteristics of nondiffraction, self-healing, and transverse acceleration are experimentally verified. In addition, vortex Airy beams with both topological charge and radial index are demonstrated and mode switch among Gaussian, vortex, vector, Airy beams and their combinations can be acquired easily. Our design provides a flexible and highly efficient way to generate unique optical vortices with self-healing and transverse acceleration properties, and facilitates prospective applications in optics and photonics.