Photoinduced phase transitions in a system with a transformable electron spectrum

An electron theory of photoinduced phase transitions and cavityless optical bistability in a light field with a finite optical-spectrum width is constructed. The Liouville equation for the density matrix is used to derive the existence criteria and calculate the main characteristics of the given phenomena. Broadening of the optical spectrum is shown to reduce the possibility of observing critical features (the existence criterion becomes more stringent and the hysteresis-loop area becomes smaller). Finally, results are compared with experimental data for CdS and amorphous GeS₂. Zh. éksp. Teor. Fiz. 111, 2147–2157 (June 1997)     

Size effects upon phase transitions in vanadium oxide nanocomposites

A phenomenological model has been proposed for the formation of the major thermal hysteresis loop of optical parameters upon the semiconductor-metal phase transition in vanadium oxide nanocomposites. It has been demonstrated that the effects associated with the influence of nanocrystallite sizes on the phase transition temperature in an individual nanocrystallite, which is determined by the width of the elementary hysteresis loop and the position of the phase equilibrium temperature on the temperature scale, manifest themselves in the form of pronounced features in the loop shape. In particular, the size effects for VO₂ polycrystalline films are observed in the formation of a trapezoidal shape of the optical thermal hysteresis loop with a wide lower base, whereas these effects for nanocomposites based on porous glasses with VO₂ nanocrystals are revealed in the form of the optical hysteresis loop with a narrow lower base (wide upper base). The proposed model also explains the symmetric shape of the major hysteresis loop for vanadium oxide nanocomposites based on opals. The size effects in opal nanocomposites with strictly fixed sizes of pores and, therefore, nanocrystallite sizes manifest themselves in the form of well-defined steps in the heating and cooling branches of the major hysteresis loop.     

Photoinduced phase transition in an organic radical crystal with room-temperature optical and magnetic bistability

A phase control by photoirradiation is successfully achieved in a spin-Peierls system of the organic radical crystal, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which exhibits optical and magnetic bistability around room temperature with a large hysteresis loop. A nanosecond laser pulse is found to induce a transition from a diamagnetic low-temperature phase to a paramagnetic high-temperature phase both inside (296 K) and outside (11 K) the hysteresis loop. Comparison of the excitation energy dependence between transition efficiency and photoconductivity suggests that the photoinduced transition is driven by suppression of the spin-Peierls instability by the accumulation of photocarriers.     

Ultrafast semiconductor-metal phase transition in vanadium dioxide induced by a femtosecond laser pulse

A model is proposed for a photoinduced Peierls-type semiconductor-metal phase transition that makes it possible to determine the time dependence of the bandgap width in the electronic spectrum of vanadium dioxide subjected to a light field and the dependence of the time at which a photoinduced semiconductor-metal phase transition occurs on the laser pulse duration. The theoretical results obtained are consistent with experimental data on the illumination of a VO₂ film with an intense laser pulse.     

Quasi-static loops of the dielectric hysteresis for a photosensitive relaxor ferroelectric crystal in the temperature range of the diffuse phase transition

This paper reports on the results of an investigation into the dielectric hysteresis loops of La-and Ce-doped barium strontium niobate, a photosensitive relaxor ferroelectric, both in the dark and under illumination with an intensity of 0.22 mW/cm². Measurements are performed in a quasi-static electric field at a frequency of 5 × 10^?4 Hz for different temperatures in the range of the diffuse phase transition. Illumination increases the hysteresis loop amplitude. The amplitude and the coercive field decrease under heating, and the difference between the loops measured for illuminated and unilluminated crystals disappears gradually as the temperature of the maximum in the permittivity is approached. The experimental results are not in conflict with the assumption that random internal relaxor fields are screened by photoinduced carriers.     

Effect of grain sizes on the metal-semiconductor phase transition in vanadium dioxide polycrystalline thin films

Experimental data on the effect of grain sizes on the shape and width of the hysteresis loop characterizing a metal-semiconductor phase transition in vanadium dioxide films are analyzed in terms of the classical theory of nucleation. It is shown that the factors responsible for the changes in the shape and width of the hysteresis loop with variations in the size of the grains making up a film are associated with the heterogeneous character of nucleation of a new phase, on the one hand, and with the elastic stresses arising in the phase transition, on the other.     

玻璃与液晶非线性光学界面反射特性的研究

本文系统地研究了由玻璃与液晶构成的非线性光学界面。用一台调Q的红宝石激光器研究了由玻璃与处于各向同性液相的液晶所构成的非线性界面。在不同入射角时非线性界面由内部全反射跃变到部分透射的阈值光强与Kaplen的平面波理论计算结果相一致。在T-T_c=2.5℃条件下测量了时间分辨的非线性界面的反射率。观察到了反射率的滞后迴线。非线性界面的这种反射率的滞后迴线可以归之于液晶分子退取向的弛豫过程。用Ar⁺激光器研究了玻璃与向列相液晶所构成的非线性光学界面,观察到了由相变而产 关键词：     

Photoinduced light absorption by C60 films in the 0.08–4.0-eV spectral range

Spectra of photoinduced light absorption in C₆₀ films at high and low excitations in the temperature range between 15 and 300 K have been measured. In addition to the well-known explanation of photoinduced absorption in terms of optical transitions in the system of photogenerated singlet excitons, triplet excitons, and polarons, changes in the absorption spectrum of the fullerite ground state must be considered. We suggest taking into account the effect of crystal field in explaining the features of the photoinduced absorption spectrum. A feature similar to the inverted luminescence spectrum and ascribed to optical excitation of singlet excitons, which is partially allowed owing to intermolecular interaction, has been detected in spectrum of photoinduced absorption. Zh. éksp. Teor. Fiz. 112, 246–256 (July 1997)     

Electro-optical phase shift in polymer dispersed liquid crystals

An anisotropic version of the Maxwell Garnett approximation is applied for studying the electro-optical phase modulation by polymer dispersed liquid crystals (PDLC). The PDLC contain bipolar liquid crystal droplets that can be reoriented by an external field causing a change in the optical birefringence. This approach provides an explicit link between the droplet orientation distribution and the electro-optical phase shift. For aligned droplets we find that the sharpness of the change in the birefringence may be controlled by selecting the initial orientation. For a planar distribution we find sharp transitions with a hysteresis loop whose width depends on the droplet concentration. For a random distribution, the droplet orientation and the optical phase shift change more gradually with the applied field. These results demonstrate that PDLC may be suitable for a wide range of electro-optic applications based on their field-induced phase modulation properties. In addition, it is apparent that the optical phase shift is quite sensitive to changes in droplet orientation. It should therefore be useful for studying reorientation phenomena in PDLC, overcoming the problems due to light scattering in these materials. Received 25 November 1999 and Received in final form 20 January 2000     

Optical data recording with vanadium dioxide-based film reversible media

Thermally induced semiconductor-metal phase transitions taking place in Al/VO₂/insulator and VO₂/insulator film systems upon recording optical data are investigated. Experiments show that the rate of the processes depends on thermophysical parameters of the films, as well as on the energy exposure. The dynamic range of the energy exposure sensitivity is defined by the hysteresis loop width. The shape and width of the hysteresis loop depends on the VO₂ film thickness and the nonstoichiometric ratio profile across the film.     

Analytical solution of 1D Ising-like systems modified by weak long range interaction

It is well-known that 1D systems with only nearest neighbour interaction exhibit no phase transition. It is shown that the presence of a small long range interaction treated by the mean field approximation in addition to strong nearest neighbour interaction gives rise to hysteresis curves of large width. This situation is believed to exist in spin crossover systems where by the deformation of the spin changing molecules, an elastic coupling leads to a long range interaction, and strong bonding between the molecules in a chain compound leads to large values for nearest neighbour interaction constants. For this interaction scheme an analytical solution has been derived and the interplay between these two types of interaction is discussed on the basis of experimental data of the chain compound which exhibits a very large hysteresis of 50 K above RT at 370 K. The width and shape of the hysteresis loop depend on the balance between long and short range interaction. For short range interaction energies much larger than the transition temperature the hysteresis width is determined by the long range interaction alone. Received 26 November 1998     

一种基于角谱理论的改进型相位恢复迭代算法

进行星间激光通信的光学发射天线光束整形器设计时, 首要解决的问题是根据输入光场及理想的输出光场, 确定整形器的相位分布, 其核心就是相位恢复. 基于角谱传播理论, 在传统 Gerchberg-Saxton (G-S)迭代算法的基础上, 提出了一种幅度梯度加成迭代算法, 给出了算法的详细流程与分析. 与G-S相比, 新算法利用迭代过程, 构建光场幅度反馈回路, 利用梯度搜索最佳迭代路径, 两者的联合作用加速其迭代收敛进程. 数值仿真表明, 新算法的单位迭代次数所引起迭代误差下降的速度是G-S算法的1.7倍, 其收敛速度明显优于G-S算法; 对不同的随机初始相位, 新算法都能进行有效迭代, 表现出适应性强, 且收敛一致性好的优点. 幅度梯度加成迭代算法为复杂光场的高效相位恢复提供了一种新思路, 为设计各种衍射光学元件提供了技术支持. 关键词： 相位恢复 迭代算法 角谱理论 光通信     

Thermochromism and Magnetic Phase Transition in an Organic Radical 1,3,5-Trithia-2,4,6-Triazapentalenyl

Thermochromic and magnetic properties of an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal were investigated through polarized reflection and EPR measurements. TTTA shows a first-order magnetic phase transition with a wide thermal hysteresis loop near room temperature. The high temperature (HT) phase is paramagnetic due to a uniform one-dimensional stacking of the radical molecules, while the low temperature (LT) phase is diamagnetic due to strong dimerization along the stacking direction. The colors of the HT and LT phases are purple and green, respectively, and both phases show a strong dichroism. We also found that part of the LT phase changes to the HT phase by laser light irradiation. The experimental results suggest the possibility of optical control of magnetic bistabitity in TTTA crystal.     

Photoinduced spin-Peierls phase transition

An equation is obtained for the order parameter ξ of the spin-Peierls phase transition in a light field. This equation is used to derive the expressions for the frequency ν of the photogenerated longitudinal optical phonon mode and the time τ of the photoinduced phase transition. Numerical values of the quantities ν and τ calculated in terms of the proposed model agree well with the experimental data for the quasi-one-dimensional compound potassium tetracyanoquinodimethane (K TCNQ) irradiated by a laser pulse. Original Russian Text ? A.L. Semenov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 164–167.     

Optical feedback effect on bistable laser diodes

The optical feedback effect on multi-section bistable laser diode characteristics, especially on light/current hysteresis, has been studied. A decrease in the bistable laser diode hysteresis width under optical feedback was observed for the first time. A theoretical analysis has been carried out by solving carrier density rate equations, combined with the light-field equation. The analysis shows that the observed hysteresis width decrease is due to the phase detuning, which is governed by the difference between the values of the α-parameters (line-width enhancement factors) in the gain section and the loss section.     

Optical bistability of a layered semiconductor in the exciton absorption region

The effect of specific features of the exciton and phonon spectra of layered semiconductors on the conditions of realization of optical bistability in the exciton absorption region has been theoretically investigated by the Green’s function method. By the example of the 2H polytype of PbI₂, it is shown that effective exciton scattering from low-energy bending-wave vibrations leads to a blue shift of the frequency range of bistability realization and its narrowing; expansion of the temperature range of bistability observation; and a shift of the hysteresis loop to higher intensities, accompanied by a decrease in its height and width. The possibility of observing polarization optical bistability, which is related to the dependence of the exciton energy spectrum on the propagation direction of a light wave in a crystal, is also substantiated.     

Dynamic triggering of a spin-transition by a pulsed magnetic field

We report the first study of the effect of a high pulsed magnetic field on a spin transition complex in the solid state. The high spin fraction was determined by optical reflectivity. Sizeable effects are observed for the well-known spin transition solid Fe(Phen)₂(NCS)₂. In the hysteresis loop temperature range, an increase in the HS fraction is obtained, with an irreversible (reversible) character in the ascending (descending) branch of the loop. The time dependence of the HS fraction provides information on the kinetics of the spin-crossover process at the spin transition. Received 23 February 1999 and Received in final form 8 June 1999     

10.6 μm Infrared light photoinduced insulator-to-metal transition in vanadium dioxide

Vanadium dioxide has excellent phase transition characteristic. Before or after phase transition, its optical, electrical, magnetic characteristic hangs hugely. It has a wide application prospect in many areas. Now, the light which can make vanadium dioxide come to pass photoinduced phase transition range from soft X-ray to medium infrared light (6.9 μm, 180 meV). However, whether 10.6 μm (117 meV) long wave infrared light can make vanadium dioxide generate photoinduced phase transition has been not studied. In this paper, we researched the response characteristic of vanadium dioxide excited by 10.6 μm infrared light. We prepared the vanadium dioxide and test the changes of vanadium dioxide thin film’s transmittance to 632.8 nm infrared light when the thin film is irradiate by CO₂ laser. We also test the resistivity of vanadium dioxide. Excluding the effect of thermal induced phase transition, we find that the transmittance of vanadium dioxide thin film to 632.8 nm light and resistivity both changes when irradiating by 10.6 μm laser. This indicates that 10.6 μm infrared light can make the vanadium dioxide come to pass photoinduced phase transition. The finding makes vanadium has a potential application in recording the long-wave infrared hologram and making infrared detector with high resolution.     

Oxygen stoichiometry controlled sharp insulator-metal transition in highly oriented VO2/TiO2 thin films

The insulator-metal transition (IMT) in vanadium dioxide (VO₂) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO₂ epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO₂ thin films on (001)-oriented TiO₂ substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO₂. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO₂/TiO₂ films.     

Phonon-Assisted Photoluminescence in a Spherical Nanocrystal

Using the matrix density in the representation of path integrals for an electron, the multiphoton nonlinear absorption light coefficient in the second order of interaction energy with polar optical phonons is derived. This coefficient describes any electron interaction mechanism with phonons. From the interaction mechanism, the main role is played by dimensional resonance when the electron continuously absorbs energy from the field as a result of synchronizing its oscillation with the field. This dimensional resonance is possible when the frequency characterizing the laser field is a multiple of the phonon frequency. Whether a photon is absorbed or emitted, the initial level from where the transition occurs defines the temperature dependence. The absorption spectrum has the form of stripes whose intensity depends on the resonance character. The most pronounced absorption is at the triple resonance, where values of radiation and oscillatory and optical phonon frequencies are equal. __________ Published in Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 654–659, September–October, 2005.