Effect of polarization on femtosecond pulsed laser ablation of surface relief gratings using a novel interferometer

In this paper, we report the effects of polarization state of individual beams in ablation of surface relief gratings using a two-beam interferometric technique. We have carried out ablation experiments on (1 1 1) silicon to form surface relief gratings by interfering two femtosecond laser beams under different polarization combinations. Four combinations of polarization were studied, i.e., s-: s-polarization, s-: p-polarization, p-: p-polarization and circular-: circular-polarization (c-: c-polarization). A novel interferometer was used for the investigation. The grating depths, surface roughness and ablation thresholds have been shown to depend on the polarization state of the interfering beams.     

Temperature and orientation dependence of surface relief gratings based on dye-doped polymer film with the interface of nematic liquid crystals

The formation of surface relief grating on dye-doped polymer film with the interface of nematic liquid crystals has been investigated by means of the holographic technique. The first-order diffraction efficiency of surface relief grating depends on the temperature and the orientation of molecular director in the interface of nematic liquid crystals. The diffraction efficiency is roughly independent of thermal fluctuations of molecular director in the most part of nematic temperature range and apparently drops near the transition temperature. The morphology of surface relief grating demonstrates that the surface modulation is larger for molecular director parallel to the groove direction. The experimental result also shows that the first-order diffraction efficiency is dependent on the surface modulation of surface relief grating.     

波长尺寸表面浮雕光栅的严格理论计算和分析

本文采用麦克斯韦(Maxwell)方程微分积分方程对波长尺寸的梯形反射光栅在TE模式(横向电场)偏振状态入射下,进行严格理论的计算和分析.表面电场计算表明,光栅刻槽斜边边界对于金属材料金和铝光栅表面电场分布具有明显效应,对于入射波长为λ=1.0μm附近的红外区域,金材料具有强烈的金属特性,此时表面电场的斜边边界效应更加明显.表面电场及其远场衍射效率伴随刻槽深度变化,而振荡特性进一步阐明了此效应对近,远场的作用.该方程适用于任意形状光栅或单一散射体对任意入射方向,任意偏振态的衍射和散射问题.     

Electrically controlled switching of light beams in the plane of liquid crystal layer

A simple experimental method and a set-up to control light beams in the plane of liquid crystal (LC) cell is developed. The main idea of the method is the reflection and the refraction of light beams propagating in a plane of LC layer from the boundary, which separates the regions of a liquid crystal cell with different director orientations and refractive indices. This boundary can be produced by electric field applied to the LC layer with a planar homogeneous orientation. The disclination lines that separate the extremely sharp changes of LC orientation can be also used for this purpose. The possibility of discrete deflection of light beam from the initial direction is shown. A computer simulation confirmed that the proposed experimental geometry was most suitable to control intensity of extraordinary rays propagating in the plane of liquid crystal layer.     

各向异性介质浮雕光栅体内光波场的传输

分析了各向异性介质光栅体内光波场的空间分布,并将RTRA递推算法推向了各向异性介质内光波场的传输.数值计算表明,光波场光强的空间分布随透射深度的变化而变化,各向异性介质光栅体内光波场的空间分布相对各向同性介质要均匀,光栅材料的性质参数和光束的入射参数对光波场光强的空间分布有较大的影响等.     

Electrically tunable long-period fiber gratings with low-birefringence liquid crystal near the turn-around point

In this work, an electrically tunable long-period fiber grating (LPFG) coated with liquid crystal layer (LC) is presented. As a LC layer, a prototype low-birefringence 1550A LC mixture was chosen. As a LPFG host, two types of gratings were studied: the LPFGs based on a standard telecommunication fiber, produced by an electric arc technique with a period of 222 μm, and the LPFGs based on a boron co-doped fiber written by a UV technique with a period of 226.8 μm. The relatively short period of these gratings allowed exploiting unique sensing properties of the attenuation bands associated with modes close to the turn-around point. Experiments carried out showed that for the UV-induced LPFG with a LC layer, on the powered state the attenuation band could be offset from the attenuation band measured in the unpowered state by almost 130 nm. When the arc-induced LPFG was coated with the LC, the depth of the attenuation band could be efficiently controlled by applying an external E-field. Additionally, all experimental results obtained in this work were supported by the theoretical analysis based on a model developed with Optigrating v.4.2 software.     

Electro-optical Kerr effect in the isotropic phase of the two antiferroelectric liquid crystal mixtures

We have experimentally studied for the first time the electro-optical Kerr effect and the pre-transitional behavior in the isotropic phase of two antiferroelectric liquid crystal mixtures, W-232 and W-204D, composed of rod-like ester molecules exhibiting the direct smectic-A to isotropic (SmA-I) phase transition. The Kerr law has been confirmed for the two compounds and the variation of inverse Kerr constant with temperature above the smectic–isotropic transition temperatures were determined. Both the mixtures with very broad antiferroelectric phase around room temperature have similar sequence of the phases (i.e., Cr-SmC*A-SmC*-SmA-I). Although, the pre-transitional behavior is usually complex in the isotropic phase of the chiral smectic liquid crystal compounds, the investigated compounds showed a similar behavior compared to that of nematic–isotropic behavior.     

Tunable microlens arrays using polymer network liquid crystal

A tunable-focus microlens array based on polymer network liquid crystal (PNLC) is demonstrated. The PNLC was prepared using an ultraviolet (UV) light exposure through a patterned photomask. The photocurable monomer in each of the UV exposed spot forms an inhomogeneous centro-symmetrical polymer network which acts as a lens when a homogeneous electric field is applied to the cell. The focal length of the microlens arrays is tunable with the applied voltage.     

Observation of dislocation line motion in a stressed cholesteric liquid crystal

Summary By means of an optical microscope we observed the instantaneous displacement and motion of dislocation lines in a cholesteric liquid crystal, when the sample was subjected to a mechanical normal strain. It was found that in the linear response regime the equilibrium is reached in a quasi-periodic way. We explain our results just in a phenomenological way. A preliminary account of this work was presented at the IX International Conference on Liquid Crystals, Bangalore (1982).     

Time-dependent behaviour of mechanical shear deformations in a cholesteric liquid crystal

Summary In this paper we present the experimental results of time-resolved variations of the apparent pitch of a cholesteric mixture after a steplike shearing deformation is applied to it. We show that the relative variation of the wave-length of the maximum reflection ΔP/P _o varies from initial positive values (P>P ₀) to negative ones (P<P ₀ after a few seconds and finally it tends to a saturation value with ΔP/P ₀<0 in a long time. A qualitative explanation based on the mechanism of the unwinding of the twisted helix structure and a possible tilt of the molecular is also given.

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Riassunto In questo lavoro sono presentati i risultati sperimentali della variazione risolta nel tempo del passo apparente di un CLC sottoposto ad una deformazione di taglio a gradino. Si mostra che la variazione relativa della lunghezza d’onda del massimo di riflessione ΔP/P ₀ varia da un valore iniziale positivo (P>P ₀) ad un valore negativo (P<P ₀) dopo pochi secondi, e tende ad un valore di stturazione con ΔP/P ₀<0 all’infinito. Noi diamo una spiegazione qualitativa basata sul meccanismo dello srotolamento della struttura elicoidale e sul possibile tilt degli strati molecolari.

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Резюме В этой статье приводятся экспериментальные результаты для разрешенных во времени изменений видимого шага холестеричиой смеси после приложения ступенчатой деформации сдвига. Мы показываем, что относительное изменение длины волны максимального отражения ΔP/P ₀ изменяется от начальных ппложительных знаьений (P<P ₀) до отрицательных значениь (P<P ₀ после нескольких секунд и окончательно стремится к величине насьшения с ΔP/P ₀<0 в течение длительного времени. Предлагается качественное объяснение, основанное на механизме структуры закрученной спирали и возможного изменния наклона молекул.

     

Phase transition detection by surface photo charge effect in liquid crystals

The surface photo charge effect (SPCE) was applied for the first time at structure and phase transitions study of hydrogen bonded in dimer liquid crystals (HBDLCs). Due to the high sensitivity of this method, besides first-order phase transitions, characteristic for the p,n-octyloxibenzoic acids (8OBA), an order transition was definitely detected within the nematic range. We state that the SPCE, arising at the solid–HBDLCs interface due to the double electrical layer, is invariably concomitant with solid surface–liquid interfaces, and indicates that the changes of the characteristics of this layer, under incident optical irradiation, induce surface charge rearrangement and alternating potential difference. A mechanism of induction of the SPCE at the interface of solid surface–anisotropic liquids is proposed. We also indicate that this mechanism can be adapted for solid surface–isotropic liquid interface, including colloids (milk) and fog (aerosols)–condensed medium.     

Tunable optical vortex arrays using spontaneous periodic pattern formation in nematic liquid crystal cells

Controllable liquid crystal (LC) defects can provide an effective approach to creating tunable optical vortices. We develop a method to create tunable matter vortex arrays in an LC cell, in which +1 and −1 defects are periodically arranged in a square grid lattice. Spontaneous formation of the periodic defect array is achieved using a spontaneous standing pressure wave without using any patterned electrode or patterned alignment layer. The +1 and −1 defects in the array can induce optical vortices with opposite handedness, and the matter vortex array produces a periodic optical vortex array with orbital angular momenta of −2ℏ and +2ℏ in the same grid lattice. Because the pitch of the grid can be controlled, the method can provide a useful pathway to producing tunable optical vortex arrays for various applications such as advanced optical communication and quantum computation.     

Electro-optical Kerr effect of azo-dye-doped liquid crystals using nulled intensity method

ABSTRACT

The electro-optical Kerr effect and pre-transition behavior have been investigated for different concentration of dimethyl-aminoazobenzene dye-doped nematic liquid crystal by using the nulled intensity method. Measurement results have showed that the molecular reorientation of liquid crystals induced by photoisomerization of azo dyes come about to increase an amount of Kerr constant at low percentages of dye concentration. Additionally, the results of pre-transition temperature which were obtained by extrapolating the invers Kerr constant as a function of temperature have been found to be in good agreement with predictions of the Landau-De Gennes model.     

A reflectometric method to measure the azimuthal anchoring energy of a nematic liquid crystal

Summary A new reflectometric method to measure the azimuthal anchoring energy at the interface between a nematic liquid crystal and an isotropic method is proposed. The spurious contribution to the reflectivity coefficients due to the presence of a director-field distortion near the interface is calculated by using the numerical Berreman algorithm and is shown to be negligible for magnetic fields small enough (H<10 kG). Preliminary measurements of the azimuthal anchoring energy at the interface between the NLC 5CB and a rubbed glass plate are given. Research supported in part by MPI and in part by CNR.     

Optimal period for diffraction gratings recorded in polymer dispersed liquid crystals

New diffusion model of recording diffraction gratings in the media of PDLC is described in which besides diffusion of monomer molecules also diffusion of polymer molecules and non-locality of diffusion coefficient are taken into account. It lets us to explain why diffraction efficiency is low for low and high values of intensities of grating recording beam. With the considered model, we have theoretically got optimal period for grating recording.     

Efficient tunable negative refraction photonic crystal achieved by an elliptic rod lattice with a nematic liquid crystal

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated the electromagnetic wave propagation inside an elliptic rod PC slab by means of finite-difference time-domain simulations. The band structure of the PC composed of elliptic rod in the square and triangular lattices is studied by solving Maxwell's equations using the plane wave expansion method. Numerical simulations show that the refractive angle can be tuned greatly by rotating the directors of elliptic rod in the PC slab. Furthermore, an optical switch based on elliptic rod PC structures with nematic liquid crystals was proposed. In the on/off switching system, the partial band gap can be controlled when the normalized operation frequency is 0.28. The modulation induced by liquid crystals created a sharp switching in the photonic devices. Such a mechanism of negative refraction PCs should open up a new application for designing components in photonic integrated circuits.     

Tunable local surface plasmon resonance in liquid-crystal-coated Ag nanoparticles

The far-field and near-field properties of a spherical nematic liquid crystal (NLC) coated metal nanoparticle (NPs) have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of liquid crystals (LCs). The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the surface plasmon resonance (SPR) shows an obvious red shift and field enhancement near outer surface of the shell always is stronger.     

线偏振高斯光束通过条形浮雕光栅的传输

使用角谱表示和严格的模式理论研究了线偏振高斯光束通过特征尺寸与波长可比拟的条形浮雕光栅的传输，系统参数误差对浮雕层中光强最大值的影响.研究表明，光束在条形浮雕光栅各层中有不同的光强分布，浮雕层的介质填充部分中的光强明显高于展空部分的光强.光栅周期误差对浮雕层中最大光强的影响起主要作用.并利用模拟退火优化算法优化系统参数以控制出射光束的光强分布.     

液晶衍射现象的实验研究

研究了液晶光学器件的衍射现象,实验表明,在一定的电压区间,出射光会出现周期性结构的衍射图样,探讨了这种衍射与液晶结构之间的关系以及各衍射光斑的偏振特性.     

利用泽尼克系数求取衍射光栅的分辨本领

光栅分辨本领检测设备的焦距通常达几米甚至十几米,采用直接测量法难度大、成本高,利用衍射波前间接求取光栅分辨本领是解决该问题的有效途径之一。在光栅光谱成像傅里叶变换理论基础上,建立了利用泽尼克多项式拟合系数求解衍射光栅分辨本领的归一化模型,揭示了光栅衍射波前与分辨本领的求取关系,提出了依据泽尼克多项式拟合系数求取衍射光栅分辨本领的新方法。根据该方法实测了一块衍射光栅的分辨本领,并与直接测量法进行对比测试。结果表明该方法误差小于4.42%,降低了分辨本领的测试难度,是衍射光栅分辨本领求取的有效手段,应用于ZYGO干涉仪等仪器中,通过简单的波前测试即可得到定量的衍射光栅分辨本领指标。