Influence of fatigue processes on the switching currents in lead titanate and lead zirconate-titanate films

The processes of polarization fatigue in PbTiO₃ and Pb(Zr_0.5Ti_0.5)O₃ films on silicon substrates have been studied by the method of switching in rectangular pulsed fields. Dependences of the switchable polarization, the maximum switching current i _max, and the switching time ??s on the number of switching cycles have been obtained from the data on the switching currents. It has been shown that the mobility of domain walls is retained in the fatigue processes and the observed fatigue is connected with switching off a fraction of the switchable volume due to pinning of reverse domain nuclei by charged defects.     

Analysis of polarization switching in ferroelectric crystals in the injection mode

We report on the results of experiments on polarization switching in a ferroelectric TGS crystal during injection of electron beams from a scanning electron microscope under a surface layer. A series of models reflecting the polarization switching dynamics of a ferroelectric crystal under the action of an injected charge is constructed. The implementation of these models is based on the principles of evolution of domain structures taking into account analysis of possible polarization switching mechanisms for ferroelectric samples. A mathematical model developed using these principles demonstrates qualitative similarity of model current pulses and those obtained experimentally in the injection mode.     

1550nm-VCSELs在偏振保持光反馈和正交光注入下的偏振转换特性

基于自旋反转模型,研究了1550 nm垂直腔面发射激光器(1550 nm-VCSELs)在偏振保持光反馈和正交光注入下的偏振转换特性.结果表明:正交光注入下的从激光器会随着注入强度的增加产生偏振转换.在归一化注入电流较小时,改变反馈强度,会使从激光器发生偏振转换的注入强度出现规律不同的变化;改变频率失谐,会使从激光器发生偏振转换的注入强度出现规律相同的变化.     

光电负反馈下垂直腔表面发射激光器偏振开关特性研究

基于扩展的自旋反转模型, 对光电负反馈下垂直腔表面发射激光器的偏振开关特性进行了数值仿真和理论分析. 研究结果表明: 对于不同的自旋反转率, 反馈强度和延迟时间对激光器偏振开关特性产生较大影响.在慢自旋反转率下运行时, 随着反馈强度的增加, 开关点电流呈线性增加, 导致X偏振模被压缩, 这与报道的基于各向同性光反馈的情景相反, 产生这一现象的原因是由于光电负反馈提高了X偏振模的阈值; 延迟时间对开关点电流的影响随反馈强度的变化而不同.在快自旋反转率下运行时, 反馈强度对开关点电流的影响与慢自旋反转率时的情形不同, 开关点电流经历先增加后减小的过程, 开关点电流受反馈强度的影响更加敏感; 而延迟时间的影响规律和慢自旋反转率时相似. 此外, 还发现自发辐射噪声对激光器偏振开关特性有较大影响. 关键词： 垂直腔表面发射激光器 偏振开关 光电负反馈 自发辐射噪声     

Microscopic simulation of the polarization switching at ferroelectric first-order phase transitions

Simulations of the kinetics of polarization switching at first-order ferroelectric phase transitions are carried out within the framework of Ishibashi dipole lattice model. We consider the influence of the dipole–dipole interaction between polar units on the polarization switching taking into account the existence of metastable states. We also examine the size effect on the switching kinetics. We calculate hysteresis loops and switching rates in the systems in which the continuum approximation is not valid. We show that in the case of hundreds of polar units our equations give the known analytical expressions for the switching rate.     

Investigation of the fractal features of the polarization reversal processes in ferroelectric crystals under injection conditions

The features of polarization reversal processes of ferroelectric crystals are analyzed under the condition that electron beams of a scanning electron microscope are injected into the subsurface layer of a sample. The proposed mathematical model describes the dynamics of the polarization switching of a ferroelectric crystal using the injection contact technique. The simulation relies on the basic principles of scale invariance in domain structure evolution. The polarization switching current of ferroelectric crystals is investigated as the dynamic characteristic of a finite medium exhibiting fractal behavior. The application of fractal and multifractal analysis of a time series enables us to calculate the fractal dimension of the polarization reversal process under injection conditions.     

Nanosecond dynamics of ferroelectric/dielectric superlattices

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization. Regions of the superlattice that have been switched into a uniform polarization state by the applied electric field, however, exhibit piezoelectricity during the course of the switching process. We propose a switching model different from previous models of the switching of superlattices, based instead on a spatially heterogeneous transformation between striped and uniform polarization states.     

Wavelength switching based on quantum-dot vertical-cavity surface-emitting laser

This study investigates, for the first time, static and dynamic wavelength switching characteristics of the 1.3 ??m quantum-dot vertical-cavity surface-emitting laser (QD VCSEL). The free-running QD VCSEL with ??1 and ??2 state innately is injected by a laser source with ??1 state. When the injection power exceeds the threshold power, the dominant state of the QD VCSEL changes from ??2 to ??1 state. Results of this study demonstrate that the wavelength switching based on a 1.3 ??m QD VCSEL has a simpler and more cost-effective configuration than those of previous systems.     

Polarization fatigue in poly(vinylidene fluoride (78%)-trifluoroethylene (22%)) ferroelectric thin films: a pulse train study

This paper investigates the influences of polarization fatigue on remanent polarization and switching time by pulse train measurements in ferroelectric poly(vinylidene fluoride (78%) and trifluoroethylene (22%)) thin films. Fatigue was carried out by a series of bipolar switching pulses with constant pulse width (on-time) and various interval times between pulses (off-time). The experimental observations indicated that the off-time period showed no obvious influence on fatigue rate and the switching time increased with the increase of fatigue cycles. The origination of these phenomena was discussed according to the charge injection model.     

Liquid crystal cell for space-borne laser rangefinder to space mission applications

Liquid crystal cell (LCC) for space-borne laser rangefinder to space mission applications was developed, manufactured and tested under cooperation between Military University of Technology (MUT) in Poland and Vavilov State Optical Institute (Vavilov SOI) in Russia. LCC operates in twisted nematic mode, commutating the polarization plane of a laser beam working at 1.064 ??m and the energy density not smaller than 0,15 J/cm² at the pulse duration about 8 ns. The transmission of LCC is not smaller than 95% at the aperture diameter not less than 15 mm. Switching on and switching off times in a 2.5-??m thick LCC driven by voltage of 10 V are not larger than 0.7 ms and 7 ms, respectively, in the operating temperature range from 20°C to 40°C. The LCCs developed in MUT were positively tested under space requirements in Vavilov SOI.     

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.     

Switching time dispersion and retention of bistable states in Langmuir-Blodgett ferroelectric films

Polarization switching and retention of each of the two polarized states in Langmuir—Blodgett (LB) ferroelectric films are studied using nonlinear dielectric spectroscopy. It is found that polarized states can be preserved for a long time, but the polarization dynamics in 10–40-nm-thick LB films is characterized by a considerable switching-time dispersion. In addition, ferroelectric LB films exhibit clearly manifested asymmetry of switching to states with opposite directions of polarization. To explain the experimental results, a polarization-switching model is proposed that takes into account the energy of interaction of a ferroelectric polymer with boundary surfaces. The effect of inhomogeneity of the LB film structure on the ferroelectric switching dynamics is also discussed.     

Instabilities in quantum-dot spin-VCSELs

A modified spin-flip model is used to investigate the stability of optically-pumped quantum-dot spin-vertical cavity surface-emitting lasers. in terms of pump intensity and polarization. The stability maps exhibit pronounced polarization switching in the stable regions. Periodic oscillations in unstable regions are attributed to competition between spin relaxation and birefringence.     

Polarization and transverse-mode dynamics of gain-guided vertical-cavity surface-emitting lasers

Transverse-mode competition and polarization selection in gain-guided vertical-cavity surface-emitting lasers are studied by use of a transverse continuous model that incorporates basic physical mechanisms of polarization dynamics. Polarization stability and polarization switching within the fundamental Gaussian mode are described. The first-order transverse mode always starts lasing orthogonally polarized to the fundamental one. At larger currents polarization coexists with several active transverse modes. These results are shown to be sensitive to the carrier spin-flip relaxation rate.     

Polarization reversal behavior of an asymmetric ferroelectric thin film

Using the Landau–Khalatnikov equation of motion, the polarization reversal behavior in an asymmetric ferroelectric thin film has been studied. Our model first introduces a third power of polarization to describe the asymmetry of a ferroelectric thin film with surface transition layer, which originates from the difference between the surfaces. Interestingly, vertical drift of polarization switching behaviors was found in this system. The properties consisting of hysteresis loop, spontaneous polarization, switching current of an asymmetric ferroelectric thin film with surface transition layer are discussed.     

Thermodynamics and kinetics of the initial stages of polarization switching in ferroelectrics

The thermodynamics and kinetics of polarization switching in ferroelectrics are studied in the specific case of switching in intrinsic ferroelectrics with 180° domains. The initial stage of the switching in the region of weak metastability is analyzed. An expression relating the critical domain size to the switching field is derived. An equation describing the evolution of the size distribution function of the switched domains is obtained. Expressions for calculating the number of polarization switching nuclei as a function of the switching field are derived.     

基于SOA偏振旋转光开关的性能研究

利用SOA中的偏振主态的概念,理论分析了SOA中的偏振旋转的问题,并以此为基础提出了一种基于SOA中偏振旋转的光开关。这种光开关以SOA和偏振分柬器（Polarization Beam Combiner,PBC）为核心元件,利用光在SOA中的偏振旋转实现了光信号的开关,这种光开关具有开关速度快,功率均衡,易集成等优点,实验得到的开关消光比达到24dB以上,开关速度达到20ns。     

Polarization switching properties of spray deposited CsNO3: PVA composite films

The ferroelectric and switching properties of spray deposited cesium nitrate: poly (vinyl alcohol) composite films at different substrate temperatures (T _s ) have been studied. The optimum value of remanent polarization was obtained in the film deposited at T _s =200°C, which may be due to larger structural distortion (c/a ratio) and less porosity as revealed by x-ray diffraction and field emission scanning electron microscope (FESEM) analysis. The switching current transients have been analyzed by nucleation limited switching model (NLS) with the Lorentzian distribution function. This model gives excellent agreement with the experimental polarization current throughout the whole time range. The switching parameters were determined in the composite films deposited at different T _s and found to be optimum at T _s =200°C. The effect of pulse amplitude on the domain switching properties has also been studied and analyzed. The peak value of polarization current exhibits an exponential dependence on the external applied field.