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.     

Determining the influence of ferroelectric polarization on electrical characteristics in organic ferroelectric field-effect transistors

Organic ferroelectric field-effect transistors(OFeFETs) are regarded as a promising technology for low-cost flexible memories. However, the electrical instability is still a critical obstacle, which limits the commercialization process. Based on already established models for polarization in ferroelectrics and charge transport in OFeFETs, simulation work is performed to determine the influence of polarization fatigue and ferroelectric switching transient on electrical characteristics in OFeFETs. The polarization fatigue results in the decrease of the on-state drain current and the memory window width and thus degrades the memory performance. The output measurements during the ferroelectric switching process show a hysteresis due to the instable polarization. In the on/off measurements, a large writing/erasing pulse frequency weakens the polarization modulation and thus results in a small separation between on- and off-state drain currents. According to the electrical properties of the ferroelectric layer, suggestions are given to obtain optimal electrical characterization for OFeFETs.     

Analysis of ferroelectric switching in sodium nitrite:poly(vinyl alcohol) nanocomposite films

The Fourier transform infrared (FTIR) spectra and switching current response in sodium nitrite:poly(vinyl alcohol) nanocomposite films have been studied as a function of composition of NaNO₂. The switching current data fitted well to infinite-grain model (IGM) in the region t<t _s and to finite-grain model (FGM) in the region t≥t _s . The microscopic parameters like the dimensionality, the domain wall velocity, and the nucleation rate have been evaluated which provide more physical insight of the switching phenomena in the composite films. The polarization current and nucleation rate are optimum in 50 wt.% composite film and have been discussed in terms of grain size and strain variations with the composition. The effect of applied field and pulse width variation on the switching behavior of 50 wt.% composition has also been studied. The exponential field dependence of the domain wall velocity and the nucleation rate indicate that nucleation mechanism is responsible for switching phenomena in the composite films. The writing pulse width affects significantly on the switching behavior of the composite films.     

Ultra-short laser ablation of metals and semiconductors: evidence of ultra-fast Coulomb explosion

We have studied ultra-fast laser ablation of Si and a metal via the neutral and ion yield, the energy distribution of emitted neutrals and the charge distribution as a function of the laser pulse width. Two processes, one leading to the ejection of fast (3–7 eV), the other to slow thermal particles, can be identified. The origin of the first process can be correlated with laser pulse widths (or pump–probe delays) and processes on a time scale below 100 fs. Results for Si confirm recent findings for Coulomb explosion (CE) and we show for the first time that CE exists as a mechanism of material removal from metals for ultra-short laser pulses. PACS 06.60.Jn; 79.20.Ds; 79.60.Bm; 42.62.Fi; 71.20.-b     

Charge storage characteristics in Al/AlN/Si metal–insulator–semiconductor structure based on deep traps in AlN layer

Charge storage characteristics in an Al/AlN/p-Si metal–insulator–semiconductor (MIS) structure have been investigated by capacitance–voltage and long-term capacitance measurements. Good program/erase behavior is observed in the AlN/Si structure, which is attributed to the trapping and detrapping of charges in deep traps of the AlN layer. In the long-term retention mode, a clear memory window is found 2000 s after removing a program/erase voltage of ±3 V, indicating good charge retention capability of the MIS structure. Further investigation shows that for a program pulse width of 500 ms, the charge storage does not occur when the pulse amplitude is smaller than a threshold value of ∼1 V. The trapped charge density increases linearly with increase of the pulse amplitude (>1 V) and tends to saturate at 2.5 V. With increasing program pulse width, the trapped charged density increases a little more than logarithmically. PACS 73.40.Kp; 72.20.Jv; 71.55.Eq     

Dynamics of twist effect in a dual-frequency nematic liquid crystal

The dynamics of the electrooptical 90° twist effect in a dual-frequency nematic liquid crystal is investigated for wavelengths of 0.65 and 1.55μ m. It is shown that the boundary conditions of the interaction between the phases affect the optical threshold of the twist effect, the contrast, and the working voltage range. The switching time of the twist effect from the off to the on state upon a variation of the amplitude of a rectangular dc voltage pulse from 15 to 50 V changes from 1.5 to 0.3 ms for a thickness of the nematic crystal layer of about 7 μ m. The minimal time of switching from the “on” to the “off” state was 3 ms in the case when relaxation of molecules in a cell with asymmetric boundary conditions was controlled electrically. The dynamic range of transmittance variation at a wavelength of 1.55 μm extended to 30 dB.     

Fine control of optical pulse width via doublet gain lines

We demonstrate a general technique for realizing fine control of optical pulse width via doublet gain or loss lines. The optimized doublet gain or loss lines for fine control of optical pulse width can be designed by using an inverse design method. The preliminary experimental demonstration of fine control of optical pulse width is performed by using doublet Brillouin gain lines in an optical fiber. Tunable pulse compression or stretching ratio of 1.47–0.43 accompanying a time delay is achieved by controlling the separation between two gain lines, for an input pulse train with a 40 ns pulse width and a repetition rate of 5 MHz, in a 4 km silica fiber with a fixed pump power of 88.1 mW.     

Vector boson decays of the Higgs boson

We derive the width of the Higgs boson into vector bosons. General formulas are derived both for the on–shell decay as well for the off–shell decays, and , where . For the off-shell decays the width of the decaying vector boson is properly included. The formulas are valid both for the Standard Model as well as for arbitrary extensions. As an example we study in detail the gauge-invariant effective Lagrangian models where we can have sizable enhancements over the Standard Model that could be observed at LEP. Received: 31 July 1998 / Revised version: 23 September 1998 / Published online: 6 November 1998     

Ferroelectric Properties of Polycrystalline Ceramics with Dipolar Defect Simulated from the Potts--Ising Model

Physical properties of polycrystailine ferroelectrics including the contributions of the fixed dipolar defects and the average grain size in the Potts-Ising model are simulated by using the Monte Carlo method. Domain pattern, hysteresis loop and switching current of the polarization reversal process are obtained. Two processes are considered in our simulation. In the first one, the grain texture of ferroelectric ceramics are produced from the Ports model, and then the Ising model is implemented in the obtained polycrystailine texture to produce the domain pattern, hysteresis loop and switching current. It is concluded that the defect has the ability to decrease the remnant polarization P~ as well as the coercive field E~. The back switching is obviously observed after the electric field is off, and it shows some variation after introducing the fixed dipolar defect. Meanwhile, the spike of the switching current is found to lower with the increasing defect concentration and the decreasing average grain size.     

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.     

Surface polaritons in a dielectric at a boundary with a metal in a static electric field

The existence of surface polaritons in a dielectric at a boundary with an ideal metal or superconductor in a static electric field is predicted. The frequency regions in which polaritons exist are substantially different for opposite orientations of the electric field, so that a change in the direction of the field signifies “switching on” or “switching off” of surface polaritons with a fixed frequency. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 876–880 (25 December 1998)     

Polarization Switching Bistability and Dynamics in Vertical-Cavity Surface-Emitting Laser under Orthogonal Optical Injection

Bistability as well as dynamics associated to polarization switching in a vertical-cavity surface-emitting laser (VCSEL) submitted to orthogonal optical injection from an external master laser are investigated. Intensity-polarization switching bistability is studied by increasing and decreasing the intensity of the injected beam and for a fixed frequency detuning, i.e. the frequency offset between master and the free running VCSEL. Depending on the frequency detuning, bistable switching is experimentally found to occur with or without injection locking of the fundamental mode to the master laser frequency. For large positive detuning, injection locking of the first-order transverse mode with a strong depression of the fundamental mode has been observed. The case of frequency – polarization switching bistability, i.e. when the frequency detuning is first decreased and then increased for a fixed injected power, has also been theoretically analyzed. Qualitative comparison with previous experimental work is presented for the dependence of the width of the bistable switching region on the injection strength level. Finally we show an experimental result of complicated dynamics including period doubling route to chaos that accompany polarization switching and which motivates future theoretical investigations.     

Local charge formation in LiNbO3 using a mobile needle-shaped electrode

It is shown that local formation of electric charge in a region near the surface of lithium niobate crystals can take place both by local polarization switching of the sample and by local injection of electric charge from a needle-shaped electrode. A local change, due to the electrooptic effect, in the birefringence of the sample near the charged region is used to investigate the nature and the formation mechanism of the charge relief. Fiz. Tverd. Tela (St. Petersburg) 41, 510–512 (March 1999)     

Theoretical studies of ultrafast ablation of metal targets dominated by phase explosion

Ultrashort pulse laser ablation of metallic targets is investigated theoretically through establishing a modified two-temperature model that takes into account both the temperature dependent electron–lattice coupling and the electron–electron-collision dominated electron diffusion processes for higher electron temperature regime. The electron–lattice energy coupling rate is found to reduce only slowly with increasing pulse duration, but grow rapidly with laser fluence, implying that the melting time of metallic materials decreases as the laser intensity increases. By taking phase explosion as the primary ablation mechanism, the predicted dependences of ablation rates on laser energy fluences for different laser pulse widths match very well with the experimental data. It is also found that during phase explosion the ablation rate is almost independent of the pulse width, whereas the ablation threshold fluence increases with the pulse duration even for femtosecond pulses. These theoretical results should be useful in having proper understanding of the ablation physics of ultrafast micromachining of metal targets. PACS 52.50.Jm; 61.80.Az; 72.15.Cz; 79.20.Ap; 79.20.Ds     

电流型脉冲序列控制Buck变换器工作在电感电流连续导电模式时的多周期行为

脉冲序列控制(PT)是一种离散的非线性开关变换器控制方法,具有瞬态响应快、无需补偿网络、控制电路实现简单等优点.根据控制脉冲的产生方式不同,脉冲序列控制可分为电压型脉冲序列控制(voltage-mode PT,VM-PT)和电流型脉冲序列控制(current-mode PT,CM-PT).研究表明,工作于电感电流连续导电模式(continuous conduction mode,CCM)与工作于电感电流断续导电模式(discontinuous conduction mode)的VM-PT控制开关变换器的工作特性存在明显差别,VM-PT控制CCM开关变换器存在的低频振荡现象严重影响了其稳态及瞬态性能.目前,对CM-PT控制CCM开关变换器的工作特性还未见相关报道.研究了CM-PT控制CCM开关变换器的工作特性,对其控制参数以及稳定工作域进行了分析.分析结果表明,通过参数的合理设计,虽然可以避免低频振荡现象的发生以及开关管不能正常关断的问题,但存在变换器功率范围窄的缺点.最后针对这一缺点提出了一种改进的控制方法.     

Q-switched mode-locking femtosecond all-fiber laser based on Er3+/Yb3+ codoped phosphate fiber

A Q-switched mode-locking femtosecond all-fiber laser based on a 2 cm long homemade Er³⁺/Yb³⁺ codoped phosphate glass fiber has been reported. By using the nonlinear polarization evolution technique, a nearly 100% modulation depth of mode-locking pulse train is achieved. At a pump power of 410 mW, the energy of each Q-switched envelope, whose width is about 220 μs, is 10 μJ, while the duration of mode-locking pulse within the Q-switched envelope is 318-fs.     

基于光子晶体光纤中脉冲俘获的超高速光开关

提出一种基于光子晶体光纤中脉冲俘获现象的超高速全光开关.信号脉冲处于正常色散区,它们彼此的时域间隔为1 ps.通过数值求解光子晶体光纤中脉冲传播满足的耦合非线性薛定谔方程,发现用孤子脉冲可俘获信号脉冲串中的任何一个脉冲,被俘获的信号脉冲的中心波长明显蓝移,在频域上和其他信号脉冲分离开来,于是让信号脉冲串在输出端通过布喇格光纤光栅,被俘获的信号脉冲将被过滤掉.数值模拟表明,用脉冲俘获实现的光开关响应速率可以达到1 THz.     

The von Neumann representation as a joint time–frequency parameterization for polarization-shaped femtosecond laser pulses

We generalize the joint time–frequency von Neumann representation of femtosecond laser pulses for usage with time-dependent polarization states. The electric field is expanded in terms of Gaussian-shaped transform-limited subpulses located on a discrete time–frequency lattice, each with a specific polarization state. This formalism provides an intuitive picture for the time- and frequency-dependent polarization state. It can also serve as a basis for polarization pulse shaping. As an illustration, we define pulses for which polarization parameters (ellipticity and orientation) are given directly in time–frequency phase space. This approach has applications in quantum control and other areas for which time- and frequency-dependent light polarization is relevant.     

An investigation on magnetism, spin–phonon coupling, and?ferroelectricity in multiferroic GdMn2O5

The magnetization, Raman spectroscopy, and ferroelectricity of multiferroic GdMn₂O₅ as a function of temperature and magnetic field are investigated. The complicated magnetic transitions at low temperatures are featured with anomalous Raman mode shifts, dielectric response, and ferroelectricity generation, indicating the significant spin–phonon coupling. It is argued that this coupling is possibly responsible for the electrical polarization generation associated with the incommensurate–commensurate transition.      

Centrifugal acceleration of plasma in pulsar magnetosphere

We present a relativistic model for the centrifugal acceleration of plasma bunches and the coherent radio emission in pulsar magnetosphere. We find that rotation broadens the width of leading component compared to the width of trailing component. We explain this difference in the component widths using the nested cone emission geometry. We estimate the effect of pulsar spin on the Stokes parameters, and find that the inclination between the rotation and magnetic axes can introduce an asymmetry in the circular polarization of the conal components. We analyse the single pulse polarization data of PSR B032954 at 606 MHz, and find that in its conal components, one sense of circular polarization dominates in the leading component while the other sense dominates in the trailing component. Our simulation shows that changing the sign of the impact parameter changes the sense of circular polarization as well as the swing of polarization angle Article presented at the International Conference on the Frontiers of Plasma Physics and Technology, 9–14 December 2002, Bangalore, India.