Polarization Switching Phenomena Research Articles

The Interplay Between Ferroelectricity and Electrochemical Reactivity on the Surface of Binary Ferroelectric AlxB1‐xN

AbstractPolarization dynamics and domain structure evolution in ferroelectric Al0.93B0.07N are studied using piezoresponse force microscopy and spectroscopies in ambient and controlled atmosphere environments. The application of negative unipolar and bipolar first‐order reverse curve (FORC) waveforms leads to a protrusion‐like feature on the Al0.93B0.07N surface and a reduction of electromechanical response due to electrochemical reactivity. A surface change is also observed on the application of fast alternating current bias. At the same time, the application of positive biases does not lead to surface changes. Comparatively in a controlled glove box atmosphere, stable polarization patterns can be observed, with minuscule changes in surface morphology. This surface morphology change is not isolated to applying biases to free surface, a similar topographical change is also observed at the electrode edges when cycling a capacitor in an ambient environment. The study suggests that surface electrochemical reactivity may have a significant impact on the functionality of this material in the ambient environment. However, even in the controlled atmosphere, the participation of the surface ions in polarization switching phenomena and ionic compensation is possible.

Broad tunable photonic microwave signal generation using optically-injected 1310 nm spin-VCSELs

We theoretically propose and demonstrate photonic microwave signal generation with a very broad tunability range from below 2.5 GHz to around 100 GHz. The proposed scheme is based on sustained oscillation dynamics produced in spin-VCSELs under right circularly polarized optical injection. The generated microwave frequencies can be tuned by detuning the frequency of the optical injection signal. The system also takes the advantage of polarization switching phenomena to generate broader microwave frequencies where the generated frequencies are equal to the sum of the absolute detuning frequency and the birefringence split between the spin-VCSEL modes which is 47.7 GHz. Sustained oscillation dynamics provokes tunable periodical oscillations through the adjustment of the frequency detuning and injection strength which can be used as a microwave signal generator. Enhancement of the RF power of the oscillations can be achieved by increasing the injection strength towards the locking regions. The microwave linewidth is reduced from 58 MHz to a medium linewidth of about 14 MHz under optical injection technique. This simple configuration paves the way for potential uses of spin-VCSELs in Radio-over-Fiber (RoF) applications and future technologies.

Ferroelectric properties of HfO2 nanodots with a diameter smaller than 10 nm deposited on an ITO bottom electrode

HfO2 nanodots and epitaxial HfO2 thin films with ferroelectric properties were grown on yttria-stabilized zirconia substrates with indium tin oxide bottom electrodes via a pulsed laser deposition (PLD) method. The crystallinity and ferroelectric properties of the epitaxial HfO2 thin films were analyzed to compare ferroelectric properties resulting from crystallinity and the strained structure of HfO2 nanodots. Using the total number of PLD pulses as a control, it was possible to grow HfO2 nanodots with diameters of 7, 12, and 20 nm and heights of 4.9, 7.8, and 14.8 nm. Based on the d33 piezoelectric hysteresis loops and polarization switching phenomena, it was confirmed that the HfO2 nanodots exhibited good ferroelectric properties even for scaling of less than 7 nm in diameter. The observation of the piezoelectric d33 hysteresis loop of the HfO2 nanodots and epitaxial HfO2 thin films revealed that HfO2 nanodots had improved ferroelectric properties due to the size effect.

Enhanced Electric Polarization and Polar Switching of Dipolar Aromatic Liquids Confined in Supramolecular Gel Networks.

Dipolar aromatic liquids confined in the interstitial domains of chiral organogels show significantly enhanced electric polarization, as compared with those of pure liquids alone or organogels formed with nonpolar liquids. Intriguingly, nitrobenzene gels showed a supramolecular polar switching phenomenon; i.e., hysteresis in the polarization (P)-electric field (E) curves was observed for the gel above the melting point of the solvent. This indicates that the nitrobenzene molecules confined in the chiral nanofibrous gel networks exert macroscopic polarization whose direction is inversed depending on the direction of the external electric field. The anomalously enhanced electric polarization and polar switching phenomenon of supramolecular gels in varied solvents are scrutinized by the positive-up-negative-down (PUND) measurements, and the interactions between the gel nanofibers and the polar solvent molecules play crucial roles for the emergence of the polar switching phenomenon. This work presents for the first time that dipolar liquid molecules filling the interstitial space of supramolecular gels exhibit a significant confinement effect. It provides a new perspective to design electric-field-responsive soft materials based on the functional liquid domains confined in their porous networks.

Polarization dynamics in a resonant optical medium with initial coherence between degenerate states

<p style='text-indent:20px;'>We investigate the polarization switching phenomenon for a one-soliton solution to the <inline-formula><tex-math id="M1">\begin{document}$ \Lambda $\end{document}</tex-math></inline-formula>-configuration Maxwell-Bloch equations in the case of initial coherence in the material, corresponding to the forbidden transition between the two lower energy levels. We find two polarization states that are stationary. They are not the purely right- and left-circularly polarized solitons, as in the case of zero initial coherence, but rather two mixed, elliptically polarized, states. These polarization states, of which only one is asymptotically stable, depend on both the initial population levels of the lower states and the coherence value. We also find the existence of superluminal soliton propagation, but through numerical simulations show this solution to be unstable, and therefore likely not realizable physically.

Dynamic characteristics of 1550 nm vertical-cavity surface-emitting laser subject to polarization-rotated optical feedback：the short cavity regime

Based on the extended spin-flip model (SFM), we investigate theoretically the dynamic characteristics of 1550nm vertical-cavity surface-emitting laser subject to polarization-rotated optical feedback: the short cavity regime. Results show that increasing the feedback strength will result in multiple polarization switching (PS) phenomena, and there will appear rich dynamics under the condition of medium feedback intensity, such as single period, period-doubling, quasi-periodic and chaotic states. At the same time, the increase of injection current will result in the reduction of working area of Y direction polarization mode. As the feedback delay time increases, under the condition of weak optical feedback polarization mode, the hopping phenomenon will take place at a particular frequency; the frequency of mode hopping will increase with the increase of moderate feedback strength, and the laser shows a variety of new dynamic characteristics, such as single period, pulse envelope, quasi-periodic and chaotic states, by taking a beat frequency signal. These dynamic characteristics are very sensitive to the phase change so the beat frequency effect between external cavity modes plays a key role. In addition, the hopping phenomenon between various dynamic states can also be found along with the mode hopping.

Molecular modeling and molecular dynamics simulation of the polarization switching phenomena in the ferroelectric polymers PVDF at the nanoscale

The molecular modeling and molecular dynamics of polarization switching for the ferroelectric films model of polyvinylidene fluoride (PVDF) are investigated at the nanoscale. We consider a molecular model of PVDF film, consisting of two and four a chains [–CH2–CF2–]n limited by n=6 elementary units. The first-principle approach is applied to the switching and kinetics of these models. Two types of behavior were established for PVDF chains: simultaneous and sequential rotation in high and low electric fields. Kinetics of sequential polarization switching shows a homogeneous critical behavior in the low electric field with a critical point at Landau–Ginzburg–Devonshire (LGD) coercive field E=EC. This type of kinetics demonstrates a kink-like behavior for polarization solitary wave propagation. The simultaneous type of kinetics demonstrates the total domain-like polarization switching, corresponding to exponential behavior of switching time in high electric field as for bulk samples. Corresponding LGD intrinsic coercive field for a two-chain and four-chains model is EC~2.0GV/m with revealing size effect. Obtained results show common quantum nature of PVDF chains switching phenomena—the quantum interaction of the PVDF molecular orbitals under applied electric field at the nanoscale level. The results obtained are compared with experimental data.

Gain Anisotropy Analysis in Green Semipolar InGaN Quantum Wells with Inhomogeneous Broadening

Polarization switching phenomena in semipolar (1122)-oriented InGaN quantum wells (QWs) were theoretically investigated by a newly formulated model considering the effect of In compositional fluctuation. The theoretical model successfully reproduced the reported polarization switching phenomena for both the emission wavelength and the excitation density in blue-green (1122) QWs, and this showed the importance of inhomogeneous broadening effects to understand polarization properties of semipolar quantum wells. Then, the model was applied for pure-green (1122) QWs, and we predicted that optical polarization was kept in the [1123] direction up to the carrier density high enough to create population inversion in such long-wavelength QWs. These results support the possibility for semipolar-oriented pure green InGaN laser diodes with cleaved facet cavity mirrors.

Electronic model for VCSELs: Switching mode, control of threshold current and saturation

An electronic model of VCSELs based on the mathematical rate equations of Danckaert et al. [J. Danckaert, B. Nagler, J. Albert, K.K. Panajotov, I. Veretennicoff, T. Erneux, Opt. Commun. 201 (2002) 129] is built. The polarization switching (PS) phenomenon is generated. The PS phenomenon here is analyzed using two methods. In the first method, the behavior of the polarization modes is investigated in terms of the injection voltage parameter. A threshold voltage value and a switching value are obtained as prescribed by the theoretical model. In the second method, the ratio of the voltages (modes) outputs is plotted in terms of the voltage parameter. It is found that the ratio reaches a maximum at a critical value of the voltage parameter. Controlling the threshold current, it is found that it can be reduced voluntarily by the use of an external voltage supplier (denoted E 2). When E 2 increases, the threshold current decreases and the maximum reached by the polarization mode increases. When the control voltage is high and below a second critical value, the polarization modes saturate to the same value. But above this critical value, they saturate to opposite values.

Optical anisotropy in [0001]-orientedAlxGa1−xN/AlNquantum wells(x&gt;0.69)

The optical polarization in [0001]-oriented ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}/\text{AlN}$ multiple quantum wells (QWs) in the deep-ultraviolet region $(xg0.69)$ was studied. Photoluminescence spectroscopy performed at 8.5 K revealed that the predominant polarization direction in QWs with a well width of $\ensuremath{\sim}1.5\text{ }\text{nm}$ switched from GaN-like $\mathbit{E}\ensuremath{\perp}[0001]$ to AlN-like $\mathbit{E}\ensuremath{\parallel}[0001]$ at an Al composition $x$ of $\ensuremath{\sim}0.83$, where $\mathbit{E}$ is the electric field vector of emitted light. This Al composition is much higher than the previously reported critical compositions for polarization switching phenomena. Furthermore, decreasing the well width from more than 10 to 1.5 nm promoted $\mathbit{E}\ensuremath{\perp}[0001]$ polarization. These results can be explained by the effect of strain and quantum confinement on the valence-band structures.

Modeling the fracture behavior of piezoelectric materials using a gradual polarization switching model

Polarization switching experiments conducted on polycrystal piezoelectric materials indicate that the polarization switching is gradual (occurs over a range of applied electrical and mechanical loads). An empirical model that considers the gradual change in the polarization direction is proposed to capture the polarization switching phenomena and apply it to modeling the fracture behavior of piezoelectric materials. Changes in the dielectric constants (measured from a 90° polarization switching experiment) and the polarization direction are used to estimate the amount of domain switching for an applied electric field. To model polarization switching in a generalized state of stresses and electric fields, the applied electric field (during the experiment) is converted to the corresponding value of internal energy density causing polarization switching. Internal energy density is used as the parameter to estimate the amount of domain switching and the resulting gradual change in the polarization direction. The gradual polarization switching model computes changes in the material properties and spontaneous strain contributions (arising from crystal shape change in the switching domains) based on the current polarization direction. A finite element solution procedure that incorporates the gradual polarization switching model is developed and interfaced with ABAQUS. Stress–strain variations obtained from the compression experiments and FEA are compared for PZT-4 (Zr/Ti = 52/48) and PZT-5H (Zr/Ti = 53/47). The C(T) specimen that was used in Mode-I fracture experiments is analyzed with a slit crack assumption. Microscopic observations of the region near the crack tip of a C(T) fracture specimen is used to capture the actual crack geometry generated by the machining process. A finite element model with the actual crack geometry is used to investigate the influence of finite dimensions of the crack cavity on the size of the polarization switching zones in the vicinity of the crack tip and its effects on the opening stress and the stress intensity ahead of the crack tip.

The Effects of Stress, Temperature, and Spin Flips on Polarization Switching in Vertical-Cavity Surface-Emitting Lasers

We discuss the effect of uniaxial planar stress on polarization switching in vertical-cavity surface-emitting lasers (VCSELs). The approach is based on an explicit form of a frequency-dependent complex susceptibility of the uniaxially stressed quantum-well semiconductor material. In this mesoscopic framework, we have taken cavity anisotropies, spin carrier dynamics, and thermal shift of the gain curve into account. In this way, we present a model that provides a global overview of the polarization switching phenomenon. The results are compared with experiments on an air-post VCSEL operating at 980 nm

Switching spectroscopy piezoresponse force microscopy of ferroelectric materials

The application of ferroelectric materials for electronic devices necessitates the quantitative study of local switching behavior, including imprint, coercive bias, remanent and saturation responses, and work of switching. Here we introduce switching spectroscopy piezoresponse force microscopy as a tool for real-space imaging of switching properties on the nanoscale. The hysteresis curves, acquired at each point in the image, are analyzed in the thermodynamic and kinetic limits. We expect that this approach will further understanding of the relationships between material microstructure and polarization switching phenomena on the nanoscale, and provide a quantitative tool for ferroelectric-based device characterization.

All-optical circular polarization switching phenomenon observed in photo chemically etched silicon

We demonstrate an all-optical circular polarization switching (AOCPS) phenomenon by using the storing polarization effect in a photo chemically etched silicon (PCE Si). The luminescence, structural and, chemical characteristics of the PCE Si are also observed, and are compared with the porous silicon (PS). In addition, the mechanism of an AOCPS phenomenon is discussed in terms of interaction between spinning electrons and circularly polarized photons.

On the local and average energy release in polarization switching phenomena

We derive an expression for the energy release during polarization switching in ferroelectric/ferroelastic materials with piezoelectric coupling. The energy release density is proportional to the local changes of the remanent and material properties due to crystal reorientation, and depends on the local electromechanical fields before and after switching. Its equivalent in a representative volume element is applied to constitutive modeling of repolarization and analysis of switching stability in a ferroelectric material.

Polarization phenomena in a laser coherently pumped by a linearly polarized field

The field intensity and polarization behaviour of an optically pumped laser is investigated in different operating conditions. For a linearly polarized pump field, a strong gain anisotropy is induced which favours generation of light with a polarization parallel to that of the pump field. Thus gain anisotropy can be counterbalanced by cavity-loss anisotropy only at low pumping field intensities, and the interplay between both types of anisotropy leads to polarization switching phenomena. In contrast to the case of the incoherently pumped laser, the decay rate for the magnetic dipole induced on the J = 1 level plays a minor role in determining the polarization dynamics. The influence of a longitudinal magnetic field or of modulation of the pump-field polarization orientation leads to new features and complex dynamic behaviour which includes full polarization (or vector) chaos.

Polarization switching in vertical-cavity surface emitting lasers observed at constant active region temperature

Polarization switching in gain-guided, vertical-cavity, surface-emitting lasers was studied as a function of the active region temperature. We show that polarization switching occurs even when the active region temperature is kept constant during fast pulse low duty cycle operation. This temperature independent polarization switching phenomenon is explained in terms of a recently developed model.

Polarization reversal and hysteresis loops of anti-ferroelectric liquid crystal, MHPOBC

Abstract Temperature dependence and field impressing rate, dE/dt, dependence of polarization switching phenomena and field-induced phase transition in an antiferroelectric liquid crystal (MHPOBC) were investigated by simultaneous measurement of polarization reversal current and optical transmittance. The dE/dt dependence of hysteresis loops suggests that two phases spatially coexist; such as, ferroelectric and antiferroelectric phases or ferroelectric and ferrielectric. With decreasing dE/dt, the coercive field of hysteresis loop in the ferroelectric phase continues to decrease from a positive value to zero and then becomes negative. Ionic current and accumulation of free charges near the electrodes are possible to be the origin of the phenomenon.

Temperature Effect on Ferroelectric Polarization Switching in Poly(vinylidene fluoride)

Ferroelectric polarization switching experiments were carried out on Form I crystal of poly(vinylidene fluoride) over a temperature range from −76°C to 125°C and at applied electric fields E up to 200 MV m1. At 25°C, notable polarization switching currents which decayed within a few ms were observed at E≥75 MV m−1. These currents became more intensive with increasing applied electric field. The maximum switching current density Jmax and the maximum switching time τmax could be expressed approximately by Jmax (Am−2)=0.72 (E−75) and τmax−1 (s−1)=37(E−75), respectively, as also in the case with common ferroelectric crystals such as BaTiO3 and Rochelle salts. The polarization switching behavior changed remarkably with temperature and could be separated into three parts at about −44°C and 60°C. These temperatures coincide well with the reported β (glass transition) and α (crystalline) relaxation temperatures, respectively. This fact indicates that the polarization switching phenomenon in poly(vinylidene fluoride) has close correlation with molecular chain motions. Finally, molecular motions responsible for the polarization switching in the three temperature regions are discussed.