Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals

We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.     

Chiral luminescent compounds as a perspective for cholesteric liquid crystal lasers

We present the synthesis and characterization of fluorene based materials for mirror less laser applications. Mirror less lasing is obtained from a cholesteric liquid crystals mixture, that acts as a resonator, doped with a photoluminescent dye, that acts as an active medium. We propose to use fluorene based compounds to combine two necessary properties to achieve laser emission: chirality and luminescence. Two different compounds, a trimer and an oligomer are synthesized. The trimer and the oligomer are both prepared with chiral moieties and, in addition, they are liquid crystalline compatible. More, oligofluorenes are good blue emitters and the emission quantum yield is around 0.8 in the violet–blue range. These fluorene based compounds are used in cholesteric liquid crystals mixtures as luminescent chiral dopants and a fine tuning of the laser emission is obtained. The dependence of lasing efficiency on concentration and the handedness of chiral luminescent dopants are investigated.     

Light polarization states of a cholesteric liquid crystal probed with optical ellipsometry

Herein a useful methodology to study optical properties of cholesteric liquid crystals (Ch-LC) is proposed by using the Fourier decomposition ellipsometry technique to calculate the Stokes parameters of transmitted and reflected light in the UV–Vis spectral range. Combining Bragg reflection and optical activity we were able to obtain ∼100% of linear or circular light polarization from the Ch-LC sample using achromatic and non-polarized light source. The photonic bandgap and the polarization components can be controlled with the temperature as a result of alterations in the helix pitch of the cholesteric phase. Finally, it is demonstrated the correlation between the dissymmetry factor (g) calculated via the Stokes parameter S₃ and the reflection spectrum. The data revealed that the maximum value of S₃ is not coincident with the peak of maximum reflection. The reflected or transmitted light analysis via Stokes parameters obtained by ellipsometry showed an alternative and low cost method for optical characterization in Ch-LC.     

复合光导层液晶光阀的研究

介绍了一种具有柱状结构的a-Si：H／nc—Si复合光导层的液晶光阀，复合光导层是通过热蒸发和等离子体辉光放电沉积方法，采用Al诱导a-Si：H制成的。经过测试，这种柱状结构薄膜具有电导各向异性，其横向电导率小于纵向电导率。用这种薄膜制成的液晶光阀其分辨率达到5001p／inch。     

Advances in the display quality of dye-doped polymer-dispersed liquid crystal films by addition of disc-shaped multidirectional light-control film

Typically, the addition of a dye is considered to increase the contrast ratio (CR) of a polymer-dispersed liquid crystal (PDLC) display; however, with the addition of a small amount of dye, the contrast ratio (CR) unexpectedly decreased as compare to the CR of a normal white PDLC display, attributed to various reasons. In this study, an additional multidirectional light scattering polymer film is developed over a conventional dye-doped polymer-dispersed liquid crystal (DPDLC) film to enhance the CR. For this, a disc-shape (DS) pattern on PN393 film is coated. This structure is believed to enhance the scattering properties of DPDLC displays via multiple-directional scattering nature of DS pattern film. Fabrications of such devices are simple and have advantageous of its low-cost production. This optimal designed disc-shaped light-scattering (DSS) film resulted in the enhancement of the CR of the DPDLCs.     

Distributed feedback leaky laser emission from dye doped gel-glass dispersed liquid crystal thin film patterned by soft lithography

Gel-glass dispersed liquid crystal (GDLC) thin films doped with organic laser dye Rhodamine 6G (R6G) were prepared via a sol-gel procedure of tetraethyl orthosilicate (TEOS) and ethyl triethoxisilane (ETES). As characterized by scanning electronic microscope (SEM), surface-relief structures were successfully patterned on lower refractive index GDLC thin films by soft lithographic technology, which support distributed feedback (DFB) laser emission based on leaky mode propagation. The performance of the DFB laser emission was investigated and the spectral narrowing of the emitted radiation and the fine structure pattern were found to be controlled by the doping concentration of liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB). We also showed the synchronous excitation of a DFB lasing with random lasing mediated by light scattering inside the same GDLC leaky waveguide.     

A random laser made of nematic liquid crystal doped with a laser dye

We report on random laser emission obtained in 5CB and E7 nematic liquid crystal (LC) mixtures doped with 1% weight to weight ratio of DCM laser dye. The LC cell was used as asymmetric planar waveguide were emission was collected from the edge of the sample. Variable stripe length method was utilized to estimate the gain and the losses coefficients. Both systems have shown the threshold energy fluence in order of several mJ/cm². In both cases above Fredericks potential, significant increase of emission intensity was observed due to the increase of light scattering on liquid crystalline domains. Moreover the use of fifth order of diffraction grating covered with thin alumina film resulted in strong multimode and directional laser emission.     

Absolute measurement of the generated aberrations from liquid crystal spatial light modulator using a common-path interferometry

A compact common-path interferometry is proposed to measure the wavefront aberration generated from liquid crystal spatial light modulator (LC-SLM). The LC-SLM is encoded with an aberration pattern and illuminated with a linearly polarized light oriented at ±45° with respect to the fast axis of liquid crystal, which is vertically oriented. The horizontal polarization component of the incident beam is not affected by the driving signal, while the vertical polarization component is modulated to the aberration loaded to the LC-SLM. By imposing a quarter-wave plate and a rotating analyzer, these two waves create four frames of phase-stepped interferograms. The aberration to be measured can be retrieved, and the result does not include any systematic error such as the substrate error of LC-SLM. Therefore, this method can implement absolute measurement, and help us to evaluate perfectly the fitting accuracy of the LC-SLM.     

Temperature control of the ultra-short laser pulse compression in a one-dimensional photonic band gap structure with nematic liquid crystal as a defect layer

We investigate numerically the controllable chirped pulse compression in a one-dimensional photonic structure containing a nematic liquid crystal defect layer using the temperature dependent refractive index of the liquid crystal. We consider the structure under irradiation by near-infrared ultra-short laser pulses polarized parallel to the liquid crystal director at a normal angle of incidence. It is found that the dispersion behaviour and consequently the compression ability of the system can be changed in a controlled manner due to the variation in the defect temperature. When the temperature increased from 290 to 305 K, the transmitted pulse duration decreased from 75 to 42 fs in the middle of the structure, correspondingly. As a result, a novel low-loss tunable pulse compressor with a really compact size and high compression factor is achieved. The so-called transfer matrix method is utilized for numerical simulations of the band structure and reflection/transmission spectra of the structure under investigation.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

Abstract

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA?) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA? LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (～73%), response time and grayscale switching performance of the device.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA∗) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA∗ LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (∼73%), response time and grayscale switching performance of the device.     

Optical properties of organo-soluble polyimide thin films and their applications in liquid crystal displays

Three organo-soluble polyimide powders have been synthesized. Their imidization was verified by Fourier transform infrared (FTIR) and thermal gravimetric analysis (TGA) techniques. The amorphous morphology of their thin films were confirmed by X-ray diffraction. Polyimide thin films were prepared by solution casting or spin coating. UV–visible transmission spectra of thin films revealed that they are almost transparent in the range of visible light. With in-plane orientation, revealed by FTIR spectra, negative birefringence (Δn) of thin films were observed, and refractive indices of the thin films along the film plane (n_TE) and normal to the plane (n_TM) were measured by a prism coupler. Because of negative birefringence of the thin films, they can be substituted for the compensation films for twisted nematic liquid crystal displays (TN-LCDs) to extend their viewing angles. In this paper, a 90°C TN-LCD and 120°C TN-LCD were taken as examples to show the compensation effect of thin films of a qualified polyimide.