向列相液晶染料可调谐激光器的研究

对向列相液晶染料的可调谐激光器进行了光学特性研究. 以650 nm为中 心波长设计了SiO₂和TiO₂多层膜的一维光子晶体, 以激光染料与向列相液晶的混合物作为增益介质层, 制备了波长可调谐激光器.用Nd: YAG倍频脉冲激光器输出的532 nm激光抽运所制备的激光器样品得出如下光学特性: 激光发射波长随温度调谐范围为605.5---639.8 nm, 达到34.3 nm, 随电压调谐范围为634.5---619.5 nm, 达到15 nm. 发射激光每脉冲的阈值能量为12.3 μJ, 激光线宽小于1 nm.     

Intensity fluctuations of amplified spontaneous emission

The intensity fluctuations of a linearly polarized component of the amplified spontaneous emission at 3.51 μm from a single, variable-length xenon-helium source have been measured. The fluctuations are only slightly reduced despite heavy saturation and the results are found to be in qualitative agreement with theoretical models based on the coupling of the fluctuations in the beams travelling in different directions.     

Theory of travelling-wave amplified spontaneous emission

A rate-equation model for describing the travelling-wave amplified spontaneous emission pulses (TWASE) in a transversally excited travelling wave arrangement is given. 6.35 ps long ASE pulses have been obtained by 12 ps long pump pulses. The effect of pump intensity, pump-pulse duration, molecular parameters of the dyes and pump-sweep velocity on the ASE pulses is studied.     

Creation of tunable absolute bandgaps in a two-dimensional anisotropic photonic crystal modulated by a nematic liquid crystal

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. We have investigated the tunable absolute bandgap in a two-dimensional anisotropic photonic crystal structures modulated by a nematic liquid crystal. The PC structure composed of an anisotropic-dielectric cylinder in the liquid crystal medium is studied by solving Maxwell's equations using the plane wave expansion method. The photonic band structures are found to exhibit absolute bandgaps for the square and triangular lattices. Numerical simulations show that the absolute bandgaps can be continuously tuned in the square and triangular lattices consisting of anisotropic-dielectric cylinders by infiltrating nematic liquid crystals. Such a mechanism of bandgap adjustment should open up a new application for designing components in photonic integrated circuits.     

Widely tunable dual-pump parametric amplifiers with photonic crystal fibres

This paper theoretically studies the double-pumped fibre-optical parametric amplifiers （FOPAs） in photonic crystal fibres. Two distinct working regimes of FOPAs are researched, which depend on the dispersion at the central wavelength of the two pumps. Extremely broad tuning range can be obtained when the central pump wavelength is in the normal dispersion regime and is insensitive to the wavelength separation between the two pumps, while the tuning range is narrow in the anomalous dispersion regime and can be significantly enhanced by increasing the wavelength separation. Impacts of higher-order dispersions and temporal walk-off on the gain spectra are also discussed.     

Widely tunable dual-pump parametric amplifiers with photonic crystal fibres

This paper theoretically studies the double-pumped fibre-optical parametric amplifiers (FOPAs) in photonic crystal fibres. Two distinct working regimes of FOPAs are researched, which depend on the dispersion at the central wavelength of the two pumps. Extremely broad tuning range can be obtained when the central pump wavelength is in the normal dispersion regime and is insensitive to the wavelength separation between the two pumps, while the tuning range is narrow in the anomalous dispersion regime and can be significantly enhanced by increasing the wavelength separation. Impacts of higher-order dispersions and temporal walk-off on the gain spectra are also discussed.     

Widely tunable photonic crystal fiber Fabry-Perot optical parametric oscillator

We present a widely tunable low-threshold chi(3) optical parametric oscillator. The oscillator cavity is formed by butt coupling dichroic mirrors to either end of a highly nonlinear index-guiding photonic crystal fiber. This yields a singly resonant Fabry-Perot oscillator with a high feedback fraction for the resonant parametric sideband. The tuning range of the output parametric sideband stretches from 23 to 164 THz above the pump frequency. The threshold power of the oscillator is only 15 W.     

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.     

A biaxial nematic liquid crystal

We report a biaxial nematic liquid crystal in a simple thermotropic system.     

The output of amplified spontaneous emission lasers

The output of amplified spontaneous emission (ASE) lasers such as X-ray lasers operated without mirrors is calculated exactly for Gaussian and Lorentzian small signal gain profiles by a simple Taylor series expansion. The accuracy of the ‘Linford’ formula commonly used as an approximation for the output of ASE lasers is evaluated by comparison to our exact solutions. The Linford formula is accurate to better than 10% for intensities produced by a Gaussian gain profile, but requires multiplication by a correction factor of at gain length product greater than 5 for Lorentzian gain profiles.     

Widely tunable optical parametric generation in a photonic crystal fiber

We report on the observation of widely tunable optical parametric generation in a photonic crystal fiber. The frequency shift of the generated sidebands that arise from modulational instability is strongly dependent on the detuning of the pump from the fiber's zero-dispersion wavelength. We are able to demonstrate experimentally more than 450 nm of sideband tunability as we tune the pump wavelength over 10 nm. Excellent agreement has been found between the experimentally measured and theoretically predicted shifts.     

Nanostructures in a nematic liquid crystal

The effect of nanoobjects, molecular objects, and α particles introduced into a liquid-crystal matrix that is placed in a dc electric field is studied theoretically and experimentally. The introduced particles form a charged defect, the radius of which is much larger than the particle sizes, in the orientational structure of the liquid crystal. This defect is easily observed under an optical microscope, and its image agrees with calculated data. It is suggested that this effect be used for detection and diagnostics of small objects and for design of ionizing radiation detectors. The phenomena revealed in this work can help in producing new charged-defect-saturated liquid-crystal structures formed by introducing nanoparticles in a medium or by irradiating a medium by ionizing radiation. A nonmechanical principle of transport of nanoparticles and their regular incorporation into the lattice is implemented. The essence of this principle is control of molecular orientations in a liquid crystal by quasi-stationary electric fields.     

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.     

Numerical study of femtosecond travelling-wave amplified spontaneous emission

The generation of pulses from amplified spontaneous emission in dye solutions is studied in the case of femtosecond pump pulses. The effects of ultrafast vibrational relaxation, self-phase modulation and optimum synchronization are discussed.     

扭曲向列相液晶电光效应的研究

分析了扭曲向列相液晶的电光效应.采用溴钨灯作为光源,运用光栅单色仪选择380~1 100 nm波段的光进行照射,测量在0~8 V电压条件下扭曲向列相液晶对于不同波长入射光的透射率.     

向列型液晶盒的光电响应特性

测量向列型液晶盒的光电效应,得出了P-V曲线的变化趋势,根据P-V曲线可以确定阈值电压,也能说明液晶分子在缓慢变化的电场作用下分子取向变化的情况.     

Electro-optics of bipolar nematic liquid crystal droplets

We directly visualize the response and relaxation dynamics of bipolar nematic liquid crystal droplets to an applied electric field E. Despite strong planar anchoring, there is no critical field for switching. Instead, upon application of E, the surface region first reorients, followed by movement of the disclinations and the bipolar axis. After removing E, elastic forces restore the drop to its original state. The collective electro-optic properties of ordered hexagonal-close-packed monolayers of drops are probed by diffraction experiments confirming the proposed switching mechanism.