Electrotunable Non‐reciprocal Laser Emission from a Liquid‐Crystal Photonic Device

A liquid crystal (LC) photonic device with an anisotropic optical heterojunction structure has been fabricated. The device has a phase‐retarding nematic LC (NLC) layer sandwiched between two polymer cholesteric LC films with right‐handed helices of different pitches. Electrotunable non‐reciprocal light transmittance and unidirectional circularly polarized (CP) lasing emission have been successfully demonstrated for this device structure. Two left CP (LCP) lasing emission peaks are observed at the edges of the overlapping region between the two photonic bands in the structure and are shifted upon the application of a voltage. In contrast, a non‐reciprocal right CP (RCP) lasing emission peak emerges at one of the band edges and diminishes upon the application of a voltage. These phenomena are interpreted based on the selective reflection of RCP light and the reorientation of the NLC molecules by the application of a voltage.     

级联光子晶体Mach-Zehnder干涉仪的可调谐滤波特性

基于液晶的电控双折射特性和光子晶体自准直特性,在二维光子晶体引入复合缺陷提出一种可调谐Mach-Zehnder 干涉(MZI)滤波器结构,应用液晶系统的自由能理论,推导了外加电压与液晶有效折射率的关系,并结合MZI 相位调制原理和光子晶体等效折射率理论,得到液晶的电控双折射特性与MZI 滤波器的输出光谱间的数学关系,运用时域有限差分法(FDTD)对滤波器的输出进行仿真模拟,并根据仿真结果对结构进行一级级联的优化设计。结果表明:通过控制外加电压的大小可以改变输出端的透射波长,达到可调谐滤波的效果,并且一级级联后的滤波效果比级联前有很大的改善,滤波的半波带宽从20 nm 减小为7 nm,可调谐范围从15 nm 增大为40 nm,因此可以通过进一步级联的方式使滤波器结构更加的优化,以便运用到光波分复用系统中。     

Polarization sensitive performance of one dimensional photonic crystal filters with a liquid crystal layer

A tunable photonic crystal (PC) filter is proposed with the twisted nematic (TN) liquid crystal (LC) as a defect layer. The polarization sensitive performance and the eigenmodes are analyzed. The results show that there are two kinds of transmission modes, namely o and e, when the voltage is low. The different modes can be selected by rotating the filter axially. The e mode shifts toward the shorter wavelength side as the applied voltage increases. Then the peaks of two modes merge into each other and the polarization sensitive performance disappears. The photonic band gap is insensitive to the voltage. The tunable range of the filter is 53 nm.     

填充液晶的光子晶体的光控可调光子带隙

可调光子晶体由于其潜在的应用价值成为现今光子晶体研究中的一个热点。文章首次提出了通过光诱导液晶取向技术来调制光子晶体光子禁带的方法,进而采用平面波展开法分析了填充液晶的二维三角形光子晶体禁带结构的可调节性。数值模拟结果表明:通过偏振光控制空气孔中所填充的相列液晶的方向可以对光子晶体的禁带结构进行调节。与电场调制方法相比,该光控液晶取向技术具有响应速度快、结构简单的优点。这种可调光子晶体可用于制作场敏偏光片、全光光开关和可调光衰减器。     

Switch energies of electronic and optical switches: a simplecomparative study

A comparison of the switch energies of electrically controlled optical absorption switches and electronic switches is performed. It is shown that in the investigated case with quantum box based absorption switches, the switch energies of the optical switches are generally orders of magnitude larger than those of the electronic switches for very high frequencies. Consequences on integration of optical devices as well as on the potential for photonic switching and processing are briefly discussed     

采用双向取向表面的永久双稳态扭曲向列相显示器

开发了一种新的双向取向表面。所述取向表面可以用于控制液晶引流（backflow）方向。基于所述新型取向层,成功制造了一种双稳态扭曲向列相LCD,该LCD在U和T态之间转换,不伴随表面锚泊能的破坏（surface anchoring energy breaking）。     

Inside Front Cover: Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters (Adv. Funct. Mater. 2/2008)

On p. 212, Torsten Hegmann and co‐workers describe nematic liquid crystals (N‐LCs) confined in planar liquid crystal cells after doping with small quantities of gold nanoclusters. These give rise to a dual alignment mode and electro‐optic response (Freedericksz transition). By fine‐tuning of experimental conditions, N‐LCs doped with gold nanoclusters can be electrically reoriented and aligned either like N‐LCs with a positive dielectric anisotropy (used in twisted nematic displays) in a planar cell or alternatively as N‐LCs with a negative dielectric anisotropy (used in large LCD TVs based on the vertical alignment mode). We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.     

Tunable Frequency Selective Surfaces and Negative-Zero-Positive Index Metamaterials Based on Liquid Crystals

We utilize the properties of aligned nematic liquid crystal (LC) cells in the design of: (i) a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values and (ii) micron-scale metallodielectric and all-dielectric tunable frequency selective surfaces (FSSs). The metamaterial is constructed by randomly doping a liquid crystal substrate with coated dielectric (non-magnetic) spheres and can be utilized over a large spectral range. FSSs with a liquid crystal superstrate are synthesized using conventional and genetic algorithm methods to exhibit broadband tunable filter characteristics at mid-infrared (mid-IR) wavelengths. These LC tunable FSS structures can be used to develop a new class of infrared/optical switches for terahertz applications.     

Electronic tunability of ferroelectric liquid crystal infiltrated photonic crystal fibre

The electronic tunability of ferroelectric liquid crystal filled photonic crystal fibres is demonstrated for the first time. The tunability is obtained in the wavelength range of 1500-1600-nm, with a free spectral range of-20-nm. The electronic tunability is achieved by applying an electric field onto the infiltrated photonic crystal fibre which facilitates the photonic bandgap tuning owing to the microsecond-order switching speeds of ferroelectric liquid crystals. The polarised transmission spectrum shows the behaviour of a tunable notch filter with a change in applied voltage. The results obtained could pave the way for the fabrication of fast tunable all-in-fibre devices in the telecom wavelength range.     

用于自适应红外焦平面探测器的液晶微透镜阵列

主要讨论了面阵电控液晶微透镜在设计、制作和测试等方面的若干关键性问题,主要包括:(一)进行了液晶结构电极间的电场分布仿真;(二)通过标准微电子工艺制作了原理性器件;(三)采用高斯激光束照射液晶结构,在改变驱动电压的情形下获得了所需要的电控光学干涉性能.所制作的阵列液晶微透镜具有制作工艺简单、器件紧凑、灵巧、功耗低、操作方便、易于匹配耦合与集成等特点.分析了将所发展的液晶微透镜技术用于改善和增强红外焦平面探测结构的光电性能等方面的问题.     

电可调谐的聚合物液晶光子晶体

设计了一个无顶的（top-cut）六角棱镜,棱镜高h=3.73cm ,侧面与底面夹角alfa=66 。利用此棱镜和光全息刻蚀的方法,在光抗材料上制作了对称六角结构、蜂窝结构和椭圆柱非对称等结构的光子晶体;并且在聚合物分散液晶材料上制作了大面积电可调谐的光子晶体结构。该光路系统结构简单、稳定、重复性好。光子晶体的理论干涉模式和实验制作结构相比较,结果吻合的非常好。另外还研究了光子晶体的远场衍射模式和电转换特性,对称的六角衍射模式更好的揭示了光子晶体的六角结构。晶体结构中液晶微滴（droplet）的尺寸最小可到10nm,直流转换电压最小达到13.3V/?m。这对研究电调谐的光子晶体激光器和电光转换装置有非常深远的意义。     

Symmetric self-electrooptic effect device: optical set-reset latch,differential logic gate, and differential modulator/detector

The symmetric self-electrooptic-effect device (S-SEED), a structure consisting of two p-i-n diodes electrically connected in series and acting as an optically bistable set-reset latch, is discussed. Applications and extensions of this device are also discussed. The devices do not require the critical biasing that is common to most optically bistable devices and thus is more useful for system applications. They have been optically cascaded in a photonic ring counter and have been used to perform different NOR, OR, NAND, and AND logic functions. Using the same device, a differential modulator that generates a set of complementary output beams with a single voltage control lead and a differential detector that gives an output voltage dependent on the ratio of the two optical input powers have been demonstrated     

Compact Design of an Electrically Tunable and Rotatable Polarizer Based on a Liquid Crystal Photonic Bandgap Fiber

In this letter, a compact electrically controlled broadband liquid crystal (LC) photonic bandgap fiber polarizer is designed and fabricated. A good fiber coupling quality between two single-mode fibers and one 10-mm-long LC-filled photonic crystal fiber is obtained and protected by using SU-8 fiber fixing structures during the device assembly. The total insertion loss of this all-in-fiber device is 2.7 dB. An electrically tunable polarization extinction ratio of 21.3 dB is achieved with 45deg rotatable transmission axis as well as switched on and off in the wavelength range of 1300-1600 nm.     

基于铁电液晶电控双折射的调谐滤波器的研究

将铁电液晶作为缺陷层引入一维光子晶体中,基于液晶的电控双折射,形成快速窄带调谐滤波器.用传输矩阵法研究了铁电液晶缺陷光子晶体的可调谐滤波特性,计算了电场对滤波器透射峰的位置、强度、个数和带宽的影响,结果表明该滤波器呈现良好的滤波功能.     

Characterization of a ferroelectric liquid crystal-based time delayunit for phased array antenna applications

An experimental demonstration of a photonic time delay unit (PTDU) based on ferroelectric liquid crystal devices and Thompson polarization beamsplitters (TBSs) is presented. High signal-to-noise ratios (SNRs) (>96 dB) and optical polarization extinction ratios (ER,~40 dB) are demonstrated for the PTDU using an active noise filter technique. Using a 5 V peak square wave switch drive signal, a 70 μs switching speed is demonstrated for this PTDU that can be used in fast (e.g., 5000 beam/s) beam scanning phased array systems. A shorter switching speed of 35 μs is also observed using a high 15 V peak transitional voltage driving signal     

取向层厚度及介电常数对TN-LCD影响的研究

在具有高驱动电压的液晶显示器中,为了对液晶器件起到保护作用,以采用增加取向层厚度的方法来解决某些显示器件由于驱动电压过高导致的问题。当取向层厚度不可忽略时,随着取向层厚度增加会导致器件驱动电压升高。本文利用扭曲向列相液晶显示器结构,通过模拟和实验分析了取向层厚度对LCD的影响以及不同介电常数的取向层对LCD的影响。结果表明:当取向层的介电常数大于20时,能够有效降低TN-LCD驱动电压。当其介电常数大于500时,其厚度变化对驱动电压的影响变得很小。本文结果对降低高驱动电压液晶器件的驱动电压有重要的指导性意义。     

Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters

We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.     

Design and Integration in Electro‐Optic Devices of Highly Efficient and Robust Red‐NIR Phosphorescent Nematic Hybrid Liquid Crystals Containing [Mo6I8(OCOCnF2n+1)6]2− (n = 1, 2, 3) Nanoclusters

By combining [Mo₆I₈(C _n F_2n+1COO)₆]^2‐ (n = 1, 2, 3) nanocluster units with liquid crystalline ammonium cations, a new series of hybrid materials is developed that show a nematic liquid crystal phase, the most fluid of all LC phase, on a large range of temperatures including room temperature. The photophysical studies performed in the LC state show that these self‐assembled hybrid materials emit in the red‐NIR with absolute quantum yields up to 0.7 and show a very good photostability under continuous irradiation. They are further integrated up to 20 wt% in E7, a well‐known nematic commercial LC mixture. Mixtures are investigated in terms of homogeneity and stability to select the best suitable candidate for the design of electro‐controlled devices. Studies of optical switching, contrast, viscosity, and behavior toward an electrical stimulus demonstrate the high potential of these hybrid materials in the fields of photonic or optoelectronic.     

Tunable Polarization Properties of Hybrid-Guiding Liquid-Crystal Photonic Crystal Fibers

Tunable polarization properties in hybrid-guiding photonic crystal fibers infiltrated with nematic liquid-crystalline materials are theoretically investigated, both in the case of uniform switching of the liquid crystal molecules and for realistic nematic director patterns. It is demonstrated that the bandgap formation mechanism and the modal transmission properties in such fibers offer the possibility for electrically switchable, wavelength-selective single-polarization and/or zero to extremely high-birefringence guidance.     

液晶的介电各向异性分布对液晶微带线特性的影响

基于向列液晶的微带传输线模型,研究了其模场解和1～100 GHz频段内传输特性,采用有限元法较为精确和全面地分析了各向异性非均匀分布的液晶介质对微带传输线模场解和S参量的影响,并且分析了微带电极上施加偏置电压对该传输线S参数的影响。仿真结果表明:该微波传输线模型的回波损耗在考虑液晶的介电各向异性非均匀分布时比传统仿真方法所得结果更低,随着偏置电压在一定范围内连续增大其S参数连续变化,同时谐振频率连续移动,并且这种精确的分析方法计算得到的谐振频率点的偏移量相比传统分析方法修正误差2.4 GHz,这为液晶微波可调器件的进一步研究奠定了理论基础。