高吸收ZnSe多晶无镜光学双稳的实验观测

本文对具高吸收的ZnSe多晶材料的增加吸收型光学双稳性进行了实验观测,发现在介质的吸收远高于过去提出的理论判据时,仍能获得双稳输出,并测定了阈值开关功率与光斑尺寸及样品厚度的关系.采用分层模型分析实验结果,定性地解释了实验现象.     

Influence of nonlinear absorption effects on optical bistability in semiconductor ring resonators

We present a theoretical investigation of optical bistability in a nonlinear semiconductor ring resonator, by taking into account the two-photon absorption and the free-carrier absorption. By solving the intensity equation within the ring resonator, a parametric formulation is obtained for describing the bistability relation between the input and the output intensities. Numerical results show that the nonlinear absorption effects can affect the critical points and domain of the optical bistability.     

增强吸收型光双稳的耦合动力学方程及动态分析

本文从唯象的角度首次提出了增强吸收到光双稳的耦合动力学方程，利用计算机模拟，描述了增强吸收型光双稳的静态和动态特性。     

Magnetooptical bistability of a layered semiconductor in the region of exciton absorption

The effect of weak external magnetic fields on the conditions for implementing optical bistability in the region of exciton absorption of layered semiconductors has been theoretically investigated. By the example of the 2H polytype of lead diiodide, it is shown that, changing the strength of an external magnetic field, one can form conditions for implementing bistability and effectively control the position and sizes of the hysteresis loop of exciton absorption.     

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.     

Microcharacterization of the surface oxidation of Py/Cu multilayers by scanning X-ray absorption spectromicroscopy

The local surface oxidation of the permalloy surface layer in Py/Cu GMR multilayers on a micron lateral scale has been analyzed by means of a microspot-X-ray absorption spectromicroscope utilizing synchrotron radiation from the Advanced Light Source bending magnet beamline 6.3.2. Additionally, the GMR multilayer samples prepared by dc magnetron sputtering have been analyzed by cross-sectional transmission electron microscopy, hard X-ray reflection and magnetoresistance measurements. The formation of a passivating iron-oxide layer on the sample surface was identified by X-ray absorption near edge structure spectroscopy (XANES) near the Fe-2p edge while no indication for nickel-oxide formation could be found. Small micron-size pits of reduced iron-oxide concentration could be identified by XANES microscopy while the corresponding nickel distribution appeared to be homogeneous. The results are explained in terms of a local breakdown of the passivating oxide layer. Received:16 October 2000 / Accepted: 4 December 2000 / Published online: 21 March 2001     

Optical bistability induced by mirror absorption: measurement of absorption coefficients at the sub-ppm level

We have observed optical bistability caused by absorption-induced thermal expansion of mirrors forming a Fabry-Perot interferometer. From the resulting anomalous transmission line shapes, absorption coefficients of the mirror coatings as low as 0.2parts in 10(6) (ppm) have been successfully measured.     

有三重态的四能级系统的光学双稳态

对一种有着单重态和三重态的典型的有机物的四能级系统的光学双稳进行了研究。由密度矩阵的速率方程近拟推导出了光学双稳的状态方程。对单重态和三重态吸收或色散情况下的双稳情况用数值解的方法进行了讨论。文中分析了单重态和三重态的合作参数、原子失谐参数和腔失谐参数对双稳的影响。     

Intrinsic absorption threshold of stishovite and coesite

The optical absorption spectra of the small mono-crystals samples of stishovite and coesite were studied at first. The intrinsic absorption threshold of stishovite is determined at 8.75 eV, being probably, highest in the family of different crystalline polymorph modifications of silicon dioxide. The absorption spectrum of stishovite is independent of temperature (studied in the range 290-450 K). The intrinsic absorption threshold of coesite mono-crystal situated near 8.6 eV at 293 K, coincides within experimental errors with that of α-quartz crystal, and depends on temperature, as used to be for the tetrahedron structured silicon dioxide crystalline modifications. A broad absorption band with a first spread maximum near 7.6 eV sides with intrinsic absorption threshold was found in the stishovite mono-crystal sample. Its low intensity (about 10 cm⁻¹) in an as-received sample shows on a defective nature of this band. No analogous band was detected in the sample of coesite.     

Manufacturing and modelling of sintered micro-porous absorption material for low frequency applications

A novel sintering based method to produce thin ultrahigh molecular weight polyethylene, UHMWPE, absorption material layer to increase absorption at low frequencies is introduced. The experimental impedance tube measurement results show that a 4 mm thick sintered sample layer increases absorption at a low frequency range below 1000 Hz compared with commercial melamine and polyester absorption foam samples. To cover a wider frequency range, multilayer structures composed of a sintered micro-porous material layer and commercial melamine and polyester foam layers are created and examined. The sintered sample layer also increases absorption in multilayer structures at low frequencies. Absorption coefficient values above 0.5 are reached starting from 200 Hz with multilayer structures. Software exploiting Biot’s theory of porous materials has been adopted to fit the experimental absorption data for sintered samples, commercial foams and multilayers. Software based on Biot’s theory was found to deliver quite good correlation with measured absorption coefficient values, with disagreements below 10% between the measured and estimated values.     

Sensitivity enhanced all-optical switching using prism-grating coupled surface plasmon modes

A sensitivity enhanced all-optical-switch design using prism-sinusoidal grating coupled surface plasmon modes is demonstrated numerically. Owing to the simultaneous excitation of surface plasmons for the pump and signal at the interface between a nonlinear waveguide layer and a metal layer, a sensitivity enhanced bistability effect in the structure can be obtained. The nonlinear refractive index of the waveguide layer experiences an abrupt change of the local pump field owing to this bistability effect. Such a characteristic greatly improves the performance of optical switch by reducing the threshold pump intensity. An all-optical-switch design with a pump threshold as low as 0.9 GW/cm² is presented.     

Analytic solutions for two-photon absorption in counterpropagating beams in the cubic approximation

Exact stationary-state solutions of the cubic (unsaturated) nonlinearity model of degenerate two-photon absorption by counterpropagating beams are found, and are used to describe power limiting and multiple optical bistability within a Fabry-Perot etalon.     

Optical absorption bistability in quasi-one-dimensional peierls semiconductors

The optical absorption coefficient γ of a thin film of a Peierls semiconductor has been calculated in the dipole approximation for direct interband transitions as a function of intensity I and central frequency ω₀ of a quasi-monochromatic frequency spectrum near the lower optical transition edge. It is shown that the γ(I, ω₀) dependence is strongly nonlinear, and within a certain region of I and ω₀ variation, not single-valued, which indicates the onset of resonator-free optical bistability with increasing absorption.     

Anomalous absorption of bulk shear sagittal acoustic waves in a layered structure with viscous fluid

It is demonstrated theoretically that the absorptivity of bulk shear sagittal waves by an ultra-thin layer of viscous fluid between two different elastic media has a strong maximum (in some cases as good as 100%) at an optimal layer thickness. This thickness is usually much smaller than the penetration depths and lengths of transverse and longitudinal waves in the fluid. The angular dependencies of the absorptivity are demonstrated to have significant and unusual structure near critical angles of incidence. The effect of non-Newtonian properties and non-uniformities of the fluid layer on the absorptivity is also investigated. In particular, it is shown that the absorption in a thin layer of viscous fluid is much more sensitive to non-zero relaxation time(s) in the fluid layer than the absorption at an isolated solid-fluid interface.     

非相干泵浦过程对光学双稳的影响

研究了低能级偶极子的非相干泵浦过程环形腔中双稳行为的影响，结果表明，耦合系统的双稳机制为非相干泵浦过程的干涉性，双稳迟滞环与非相干泵浦场有严格的对应关系，在一定条件下，甚至出现多稳态，可以利用非相干泵浦的干步性能控制光学双稳。     

Optimization of two-photon absorption enhancement in one-dimensional photonic crystals with defect states

One-dimensional photonic crystals with a defect layer of CdS were fabricated. The observed enhancement of two-photon absorption (TPA) in the CdS layer can be attributed to the intensified optical field confined within the defect layer of the photonic crystal. The results show that the enhancement of TPA coefficient depends basically on the number of periods of the photonic crystal and the defect mode position in the photonic band gap. The observation agrees qualitatively with the expectations of a computation by matrix transfer formulation.     

Optical bistability in one-dimensional photonic band gap structures with coupled nonlinear defects

Using the transfer matrix method, we study the transmission property of one-dimensional photonic band gap structure with two coupled nonlinear defects separated by a linear middle layer and find that such composite structure exhibits a bistability that’s strongly dependent of the linear middle layer. The switching threshold values can be greatly reduced just by choosing the proper parameters of the linear middle layer.     

Influence of substrate in photothermal measurements of thin film absorption

In photothermal measurements of low optical absorption coefficients, as in the case of thin films of amorphous semiconductors, the substrate absorption can play a non-negligible role. In this paper, the substrate influence is discussed and generalized on the basis of a theoretical model of a two-layer absorbing sample, and the interaction of the thermal and optical parameters which control the photothermal process in the sample is shown. Some experimental results obtained on thin films of amorphous silicon carbide deposited on different substrates are presented and discussed.     

多次漫反射法测量粉末中的杂质吸收光谱及其计算机模拟

本文提出了用多次漫反射法来测定粉末样品的杂质吸收光谱,其结果与同材料的透明片状样品的杂质吸收光谱基本一致,为了验证更普遍的对不同折射率粉末情况,文中采用假设模型,以计算机模拟方法,其所得的结果说明,当颗粒足够小,色散又不太大时,所得的粉末样品与透明片状样品的吸收光谱一致,只差一个未知的吸收光程,如再与另一特性参数(寿命或量子效率)测量相结合,即可用来确定稀土杂质在材料中的辐射跃迁几率等物理参数。     

Collisionless absorption in an overdense plasma with anisotropic electron distribution function

Collisionless absorption of linearly polarized electromagnetic wave in a plasma with anisotropic bi-Maxwellian electron velocity distribution is investigated. Due to the wave magnetic field influence on the electron kinetics in the skin layer, the wave absorption is found to significantly depend on the degree of the electron temperature anisotropy. Depending on the value of the skin layer anomaly parameter, and on the electron temperature anisotropy degree, the conditions are found when a significant decrease or increase of the collisionless absorption is expected. Received 25 January 2002