Liquid Crystal Q-plate Research Articles

A liquid crystal device-based apparatus for observing optical vortex and manipulating micro-particles

Abstract In this paper, we disclose a self-developed liquid crystal device-based optical apparatus for the teaching experiment of college students. Three relevant experiments have been presented. Through the teaching experiment class, the students have learnt the knowledge on the light-matter interaction, the photo-patterning of a liquid crystal q-plate and the generation of optical vortex, and the manipulation of micro-particle via angular momentum. Most students held positive remarks on the optical apparatus, and were satisfied with the assigned tasks in the experiment. Indeed, our proposed optical apparatus is not only applicable to the explorative research work, but also suitable for the college students’ teaching laboratories.

Fractional Talbot Lithography for Predesigned Large-Area Liquid-Crystal Alignment

To address the increasing demands for cost-effective, large-area, and precisely patterned alignment of liquid crystals, a fractional Talbot lithography alignment technique was proposed. A light intensity distribution with a double spatial frequency of a photomask could be achieved based on the fractional Talbot effect, which not only enhanced the resolution of lithography but also slashed system costs with remarkable efficiency. To verify the feasibility of the alignment method, we prepared a one-dimensional polymer grating as an alignment layer. A uniform alignment over a large area was achieved thanks to the perfect periodicity and groove depth of several hundred nanometers. The anchoring energy of the alignment layer was 1.82 × 10−4 J/m2, measured using the twist balance method, which surpassed that of conventional rubbing alignment. Furthermore, to demonstrate its ability for non-uniform alignment, we prepared polymer concentric rings as an alignment layer, resulting in a liquid-crystal q-plate with q = 1 and α0 = π/2. This device, with a wide tuning range (phase retardation of ~6π @ 633 nm for 0 to 5 V), was used to generate special optical fields. The results demonstrate that this approach allows for the uniform large-area orientation of liquid-crystal molecules with superior anchoring energy and customizable patterned alignment, which has extensive application value in liquid-crystal displays, generating special optical fields and intricate liquid-crystal topological defects over a large area.

Controlled Generation of Orbital Angular Momentum Beams With Coherent Beam Combining Digital Laser and Liquid-Crystal q-Plate

Controlled Generation of Orbital Angular Momentum Beams With Coherent Beam Combining Digital Laser and Liquid-Crystal q-Plate

Color-selective optical edge detection enabled by thermally stimulated cholesteric liquid crystals

Optical edge detection can significantly compress the data volume and is highly pursued in imaging processing. The commonly used methods extract the optical edge information but lose the capability to distinguish colors, which is another key information for imaging. Here, a design for color-selective optical edge detection is proposed and demonstrated with a cholesteric liquid crystal q-plate. The corresponding optical edge detection exhibits a narrow reflection band characteristic due to the one-dimensional photonic crystal configuration of the cholesteric liquid crystal. The monochromatic band is thermally and reversibly tuned ∼300 nm within 5.3 °C. Color-selective optical edge detection is verified by a series of chromatic images. This work introduces a thermally responsive liquid crystal device to release the color dimension of optical edge detection, which may upgrade existing imaging processing techniques.

Robust optical edge detection enabled by a twisted reflective q-plate.

Optical edge detection significantly reduces the image information load and is highly sought after in instant image processing. Robustness to the wavelength and polarization of light as well as mechanical vibration is a key requirement for practical applications. Here, a robust optical edge detector is proposed and demonstrated based on a reflective twisted liquid crystal q-plate. The device is composed of a mirror and a 1.46-μm-thick liquid crystal layer with a twist angle of 69.2°. The backtracking of the light inside the twisted medium forms a mirror symmetric twisted design and thus leads to a broadband self-compensated spiral phase modulation. By this means, an optical edge detector with excellent wavelength and polarization independence is presented for both coherent and partially coherent light sources. Additionally, the reflective design makes the system more compact and stable. This work supplies a practical design for robust optical edge detection, which may upgrade existing image processing techniques.

All-Optical Cross-Connection of Cylindrical Vector Beam Multiplexing Channels

The achievement of cylindrical vector beam (CVB) multiplexing has shown advantages in enlarging communication capacities and all-optical interconnections. While various mode transformation technologies have been developed to process CVB multiplexing signals, previous approaches have failed to cross-connect CVB channels, which could play a pivotal role in all-optical interconnection. Since CVBs can be decomposed into two orthogonal circularly polarized vortex beams with opposite topological charges, it is possible to cross-connect CVB channels via spin-dependent orbital angular momentum mode cross-transformation. Exploiting the spin-orbit interaction of photons in anisotropic liquid crystal q-plates, we performed conjugate helical phase modulations to these two spin components using the local polarization conversion of the birefringent effect, where the conjugate relationship of the two spin components was maintained during the transformation. Constrained by the spin-dependent phase modulation, the spatially separated target CVB mode was independently transformed with other higher-order modes via the partitioned modulation of the liquid crystal q-plate for CVB mode cross-transformation. As a proof of concept, a three-channel CVB multiplexing communication system was constructed. We show that the data signals between two CVB channels were successfully cross-connected, and 150 Gbit/s quadrature phase-shift keying signals were transmitted with a bit-error-rate approaching 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> .

Liquid-crystal q-plates with a phase core to generation vortex beams with controllable number of singularities

Based on the method of azo dye photoalignment, electrically controlled liquid crystal (LC) q-plates for the generation of a given number of polarization and phase optical singularities have been designed. The results of experimental studies of polarization and phase characteristics of the generated singular beams are in good agreement with the proposed theoretical model. The proposed devices can be adapted for a variety of problems of modern photonics, including implementation of structured illumination microscopy, information coding, optical security technology, microrheology etc.

Liquid Crystal Q-Plates with a Phase Core for Generation of the Controlled Number of Singularities Over the Radiation Profile

Liquid Crystal Q-Plates with a Phase Core for Generation of the Controlled Number of Singularities Over the Radiation Profile

Mueller Matrix Polarimetric Imaging Analysis of Optical Components for the Generation of Cylindrical Vector Beams

In this work, we performed a Mueller matrix imaging analysis of two commercial optical components usually employed to generate and manipulate vector beams—a radial polarizer and a liquid-crystal q-plate. These two elements generate vector beams by different polarization mechanisms—polarizance and retardance, respectively. The quality of the vector beams relies on the quality of the device that generates them. Therefore, it is of interest to apply the well-established polarimetric imaging techniques to evaluate these optical components by identifying their spatial homogeneity in diattenuation, polarizance, depolarization, and retardance, as well as the spatial variation of the angles of polarizance and retardance vectors. For this purpose, we applied a customized imaging Mueller matrix polarimeter based on liquid-crystal retarders and a polarization camera. Experimental results were compared to the numerical simulations, considering the theoretical Mueller matrix. This kind of polarimetric characterization could be very helpful to the manufacturers and users of these devices.

Photoaligning and Photopatterning: New LC Technology

We demonstrate a physical model of photoalignment and photopatterning based on rotational diffusion in solid azo-dye nanolayers. We also highlight the new applications of photoalignment and photopatterning in display and photonics such as: (i) liquid crystal (LC) E-paper devices, including optically rewritable LC E-paper on flexible substrates as 3D E-paper, as well as optically rewritable technology for photonics devices; (ii) photonics LC devices, such as LC Switches, polarization controllers and polarization rotators, variable optical attenuators, LC filled photonic crystal fiber, switchable diffraction grating; (iii) patterned micro-polarizer array using photo-alignment technology for image sensor; (iv) electrically tunable liquid crystal q-plates; (v) electrically switchable liquid crystal Fresnel lens; (vi) liquid crystal optical elements with integrated Pancharatnam-Berry phases. We are sure, that in the field of (LC), the main point is no longer display research, but new photonic applications of LC are emerging in telecommunication, fiber optical communication systems, sensors, switchable lenses, LC light converters and other LC photonics devices.

Taming the swirl of self-structured liquid crystal q-plates

Spontaneously formed liquid crystal topological defects under external fields offer a nature-assisted route to the creation of geometric phase optical vortex generators (q-plates). Here we report on the consequences of the unavoidable swirled transverse spatial distribution of the optical axis of such optical elements on the beam shaping and we propose a swirl-compensation scheme based on the arithmetic of geometric phase optical elements.

Comparison of backward flow values in the sharp focus of light fields with polarization and phase singularity

Using Jones matrices and vectors, we show that an optical metasurface composed of a set of subwavelength binary diffraction gratings and characterized by an anisotropic transmittance described by a polarization rotation matrix by the angle mφ, where φ is the polar angle, forms an m-th order azimuthally or radially polarized beam when illuminated by linearly polarized light, generating an optical vortex with the topological charge m upon illumination by circularly polarized light. Such a polarization-phase converter (PPC) performs a spin-orbit transformation, similar to that performed by liquid-crystal q-plates. Using a FDTD method, it is numerically shown that when illuminating the PPC by a uniformly (linearly or circularly) polarized field with topological charge m = 2 and then focusing the output beam with a binary zone plate, a reverse on-axis light flow is formed, being comparable in magnitude with the direct optical flow. Moreover, the reverse flows obtained when focusing the circularly polarized optical vortex with the topological charge m = 2 and the second-order polarization vortex are shown to be the same in magnitude.

Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere based on liquid crystal device.

We propose theoretically and verify experimentally a method of using electrically tunable liquid crystal q-plate and wave plate for generating arbitrary vector vortex beams on a hybrid-order Poincaré sphere (HyOPS). The generated vector vortex beam is verified and shows decent agreement with the prediction. This method brings many advantages, such as high conversion efficiency, good electrical controllability, and integration. This system can provide fundamental optical system support for various structured beam applications.

Self-induced liquid crystal q-plate by photoelectric interface activation

Here, we report on the experimental demonstration that highly efficient self-induced spin-orbit optical vortex generation can be achieved by using standard liquid crystal materials and surface treatment agents. This is done by revisiting the recent attempt by Zolot'ko and coworkers to produce self-induced liquid crystal vortex plates using the dc electric field [I. A. Budagovsky, S. A. Shvetsov, and A. S. Zolot'ko, Mol. Cryst. Liq. Cryst. 637, 47 (2016)] that remains, so far, limited to moderate efficiencies. The phenomenon is the result of the self-back-action of light arising from the spontaneous creation of a liquid crystal topological defect. These results demonstrate photo-electric interface activation as a candidate towards the development of a self-adapted spin-orbit photonic toolbox, thus enabling agile management of the orbital angular momentum of light.

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.

Performance of a q-plate tunable retarder in reflection for the switchable generation of both first- and second-order vector beams

We examine the performance of a tunable liquid crystal q-plate in a reflective geometry. When the device is tuned to a half-wave retardance, it operates as a q-plate with twice the value (2q) by adding a quarter-wave retarder between the mirror and the q-plate. However, when the device is tuned to a quarter-wave retardance, it acts as the original q-plate without the retarder. Experimental results are shown. Using an input tunable polarization state generator, the system allows the switchable production of all states on both the first- and second-order Poincaré spheres.

Using the nonseparability of vector beams to encode information for optical communication.

In this work, it is experimentally demonstrated that the nonseparability of vector beams (e.g., radial and azimuthal polarization) can be used to encode information for optical communication. By exploiting the nonseparability of a vector beam's space and polarization degrees of freedom using conventional wave plates, it is shown that 2 bits of information can be encoded when applying the identity and three Pauli operators to its polarization degree of freedom. It is also shown that vector beams can be efficiently decoded with as low as 2.7% cross talk using a Mach-Zehnder interferometer that exploits a higher-order Pancharatnam-Berry phase and liquid crystal q-plates.

4 × 20 Gbit/s mode division multiplexing over free space using vector modes and a q-plate mode (de)multiplexer.

Vector modes are spatial modes that have spatially inhomogeneous states of polarization, such as, radial and azimuthal polarization. In this work, the spatially inhomogeneous states of polarization of vector modes are used to increase the transmission data rate of free-space optical communication via mode division multiplexing. A mode (de)multiplexer for vector modes based on a liquid crystal q-plate is introduced. As a proof of principle, four vector modes each carrying a 20-Gbit/s quadrature phase shift keying signal (aggregate 80 Gbit/s) on a single wavelength channel (λ∼1550 nm) were transmitted ∼1 m over the lab table with <-16.4 dB mode crosstalk. Bit error rates for all vector modes were measured at the 7% forward error correction threshold with power penalties <3.41 dB.

Measuring the self-healing of the spatially inhomogeneous states of polarization of vector Bessel beams

We experimentally measured the self-healing of the spatially inhomogeneous states of polarization of vector Bessel beams. Radially and azimuthally polarized vector Bessel beams were experimentally generated via a digital version of Durnin's method, using a spatial light modulator in concert with a liquid crystal q-plate. As a proof of principle, their intensities and spatially inhomogeneous states of polarization were experimentally measured using Stokes polarimetry as they propagated through two disparate obstructions. It was found, similar to their intensities, that their spatially inhomogeneous states of polarization self-healed. The self-healing can be understood via geometric optics, i.e., the interference of the unobstructed conical rays in the shadow region of the obstruction, and may have applications in, for example, optical trapping.

Tunable supercontinuum light vector vortex beam generator using a q-plate

Spatially coherent multicolored optical vector vortex beams were created using a tunable liquid crystal q-plate and a supercontinuum light source. The feasibility of the q-plate as a tunable spectral filter (switch) was demonstrated, and the polarization topology of the resulting vector vortex beam was mapped. Potential applications include multiplexing for broadband high-speed optical communication, ultradense data networking, and super-resolution microscopy.