Qidong Wang

Electrooptical properties of new type fluorinated phenyl-tolane isothiocyanate liquid crystal compounds

ABSTRACT A series of 4ʹ-(4-alkyl-phenyl)-2ʹ,6ʹ-difluorotolane isothiocyanate liquid crystal (LC) compounds were synthesised, and their phase transitions and electrooptical properties were investigated. These compounds exhibit high birefringence, about 0.47–0.52. As the number of carbon atoms in the alkyl chains increases, these LC compounds can exhibit smectic phases. When these LCs were mixed into the commercial LCs, the birefringence values of LC mixtures become higher than pure commercial LCs, and the visco-elastic coefficients of five LC mixtures are very close to each other at every test temperature. The results of response properties indicate that the compounds with a tri-fluorine substitution and n-propyl end group possess better response performance than the others. These LC compounds are particularly attractive for achieving fast response times in LC optic devices. Graphical Abstract

Tunable surface-emitting dual-wavelength laser from a blended gain layer with an external holographic grating feedback structure

A tunable surface-emitting dual-wavelength laser emitted from the blended organic gain layer based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating feedback structure was reported. The organic blended gain layer was formed from Poly (2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and Poly (2-methoxy-5-(3′, 7’-dimethyloctyloxy)-1, 4-phenylenevinylene) (MDMO-PPV) with a weight ratio of 2:1. The dual-wavelength laser located at 629.9 nm and 640 nm was obtained in a single beam. The optical characteristics of these two organic semiconducting materials and the dual-wavelength laser performance under an electric field are investigated and illustrated. This simple tunable dual-wavelength laser shows the potential to extend the development of organic lasers.

Organic Solid-State Tri-Wavelength Lasing from Holographic Polymer-Dispersed Liquid Crystal and a Distributed Feedback Laser with a Doped Laser Dye and a Semiconducting Polymer Film

Organic solid-state tri-wavelength lasing was demonstrated from dye-doped holographic polymer-dispersed liquid crystal (HPDLC) distributed feedback (DFB) laser with semiconducting polymer poly[-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and laser dye [4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran] (DCM) by a one-step holography technique, which centered at 605.5 nm, 611.9 nm, and 671.1 nm. The temperature-dependence tuning range for the tri-wavelength dye-doped HPDLC DFB laser was as high as 8 nm. The lasing emission from the 9th order HPDLC DFB laser with MEH-PPV as active medium was also investigated, which showed excellent s-polarization characterization. The diffraction order is 9th and 8th for the dual-wavelength lasing with DCM as the active medium. The results of this work provide a method for constructing the compact and cost-effective all solid-state smart laser systems, which may find application in scientific and applied research where multi-wavelength radiation is required.

The optimal cell gap determination of a liquid crystal wavefront corrector from a single photoelectric measurement

It has been a crucial technique to improve the dynamic response characteristics of a liquid crystal wavefront corrector (LCWFC) with optimal cell gap since the LCWFC needs at least 2π (or π) phase modulation in adaptive optics systems (AOSs). We have given a complete process for obtaining the optimal cell gap accurately from a single photoelectric measurement, which can be conducted with a liquid crystal (LC) cell of any known thickness. This method has been analysed theoretically and confirmed experimentally by using a wedge-shaped cell; the experimental results match very well with the theoretical analysis. The response time of an optimal gap cell can be a novel evaluation method of response performance of LC materials.

Evaluation of phase state and rotational viscosity of fast response liquid crystals using a fully atomistic molecular dynamics

ABSTRACT Rational design of liquid crystals (LCs) with excellent phase state and rotational viscosity has been a crucial technique for response speed improvement of LC wavefront corrector. A complete process for theoretically evaluating the phase state and rotational viscosity of fast response LCs using a fully atomistic molecular dynamics is reported. Predicted trends in molecular order, phase-transition temperature between metastable states and rotational viscosity show excellent agreement with experimental results. We also demonstrate that overestimation of the attraction both between and within molecules in the general Amber force field mainly leads to a systematic shift in the phase-transition temperature, rotational viscosity and figure-of-merit for fast response LCs. With further optimisations of intermolecular potential, simulation procedure and data processing, this fully atomistic simulation will be a useful evaluation method of response performance of LC materials. GRAPHICAL ABSTRACT

Electrically tunable photo-aligned hybrid double-frequency liquid crystal polarisation grating

ABSTRACT A polarisation grating (PG) based on the hybrid photo-aligned double-frequency liquid crystal has been demonstrated and fabricated in this paper. The single-order diffraction efficiency is above 95% for circular polarisation incident light. The PG can be driven by 2.5 V/µm electric field with alternating frequency and has no Freedericksz transition threshold. In response time, the rise time and decay time are 650 and 950 , respectively; the sub-millisecond can be achieved under a low driving voltage. Graphical Abstract

High pump efficiency of a second-order distributed feedback laser based on holographic polymer dispersed liquid crystals with preferred liquid crystal molecular orientation

We report on the fabrication and characterization of a surface-emitting distributed feedback (DFB) organic semiconductor laser based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating. Specifically, by adopting an acrylate-based monomer with low functionality and a narrower grating, the phase separated liquid crystal (LC) molecules were aligned along the direction of the grating groove, which can exhibit better lasing feedback performance in the HPDLC layer. The device shows single-mode laser emission at 630.4 nm with a threshold of 0.18 μJ, and the conversion efficiency is 6.4%. The curing intensity, light loss and diffraction efficiency were also investigated for HPDLC structures to identify the effects of the grating period and monomer functionality.