高频飞秒激光对硅材料烧蚀的热积累效应

对硅材料飞秒激光损伤进行了数值模拟。首先,在飞秒激光与硅相互作用的双温模型中引入能带电子激发、双光子吸收,俄歇复合等一系列非线性项,通过每一个光场半周期中的麦克斯韦电磁场公式,得出材料中不同时间的光场强度,作为描述材料对光强的响应物质方程的输入项。通过麦克斯韦方程与物质方程间的迭代可以得到烧蚀的硅的一系列参数,如电子浓度、晶格温度等。在此基础上分析了皮秒、亚皮秒量级双脉冲和三脉冲飞秒激光作用下,激光能量密度和脉冲延迟时间对热积累效应的影响。     

基于光弹调制的原子磁强计中光信号检测方法

在无自旋交换弛豫原子磁强计中,需要检测极小的旋光角度。基于光弹调制器的偏振调制技术由于其较低的噪声和长时间的稳定性在各种检测方法中是优选的。但光弹调制器的输出信号里包含有大量噪声和高次谐波,严重影响了原子磁强计的性能。针对以上问题分析了基于光弹调制器的偏振调制技术的原理和待检测信号的特性,并提出一种基于双通道数字锁相放大器的原子磁强计微弱信号检测方法。该方法简化锁相放大算法,减小电路复杂度,并能准确地同时检测一次谐波和二次谐波的幅值。理论分析和仿真结果表明,该检测系统工作良好,可以准确地检测微弱信号,误差在0.1%以内。     

A gigahertz surface magneto-plasmon optical modulator

We propose a novel high-speed magnetooptic (MO) modulator based on the optical excitation of surface magneto-plasmon (SMP) waves in bismuth-substituted yttrium iron garnet (Bi-YIG). A model describing magnetization dynamics of the Bi-YIG film is used in conjunction with an SMP reflectivity model to evaluate the performance of the device. In developing the reflectivity model, the dispersion relation for the SMP waves at a Bi-YIG-metal interface is derived. The optical response of the device is modeled for various driving pulses, and the performance of the device is discussed in terms of response time, efficiency, and bandwidth. Using practical material parameters, it is found that the modulator has an improved efficiency over MO modulation devices relying on bulk propagation for comparable driving current pulse durations, as well as the added benefits of a miniature design, multigigahertz operation, and tunable bandwidth. The analysis presented here provides a useful framework for the design and development of magneto-plasmon photonic devices.     

惰性气体超极化的光抽运方法及比较

某些无放射性的具有自旋特性的惰性气体同位素（如^3He和^129Xe）,可通过激光抽运超极化,得到很高的非热平衡极化度和长驰豫时间的惰性气体。极化后惰性气体同位素核磁共振的灵敏度提高10^3～10^5倍。介绍了目前极化惰性气体常用的两种方法——自旋交换光抽运和亚稳态能级交换。     

全固态皮秒径向偏振激光器及其加工特性

在自主研制的全固态皮秒激光器基础上,腔外加入偏振转换元件输出皮秒径向偏振激光,并对其进行侧泵Nd:YAG晶体放大,最终得到中心波长1 064 nm、平均功率1.95 W、重复频率1 kHz、峰值功率1.77108 W、光束质量2.95以及纯度92%的皮秒径向偏振激光器。用该激光器对0.5 mm厚不锈钢材料进行钻孔和刻槽实验,并与同等加工工艺参数条件下皮秒线偏振激光器钻孔圆度和刻槽深度进行对比,分析两种激光器对加工效果的影响。实验结果表明,相比皮秒线偏振激光,利用皮秒径向偏振激光进行加工,钻孔圆度更好、刻槽深度更深且槽侧壁更为平坦。该结果为皮秒径向偏振激光器在材料加工领域的应用提供参考。     

Spin-dependent tunneling recombination in heterostructures with a magnetic layer

We propose a mechanism for the generation of spin polarization in semiconductor heterostructures with a quantum well and a magnetic impurity layer spatially separated from it. The spin polarization of carriers in a quantum well originates from spin-dependent tunneling recombination at impurity states in the magnetic layer, which is accompanied by a fast linear increase in the degree of circular polarization of photoluminescence from the quantum well. Two situations are theoretically considered. In the first case, resonant tunneling to the spin-split sublevels of the impurity center occurs and spin polarization is caused by different populations of resonance levels in the quantum well for opposite spin projections. In the second, nonresonant case, the spin-split impurity level lies above the occupied states of electrons in the quantum well and plays the role of an intermediate state in the two-stage coherent spin-dependent recombination of an electron from the quantum well and a hole in the impurity layer. The developed theory allows us to explain both qualitatively and quantitatively the kinetics of photoexcited electrons in experiments with photoluminescence with time resolution in Mn-doped InGaAs heterostructures.     

Spin filtration of unpolarized electrons by impurity centers in semiconductors

It is shown that unpolarized paramagnetic centers can implement the spin filtration of unpolarized conduction electrons in semiconductors. This ability of paramagnetic centers is caused by the difference in the spin evolution of the states of electron-paramagnetic-center pairs and by the spin selectivity of electron capture exclusively from singlet pairs. The electron spin polarization should be opposite to the paramagneticcenter polarization. To implement spin filtration, an external magnetic field is necessary. The polarization can attain the largest values (∼10%) if the probability of spin-selective electron capture from singlet pairs exceeds the pair-decay rate by a factor of 5–7.     

GaAs (110)量子阱的电子自旋弛豫

采用固态源分子束外延的方法在GaAs(110)取向衬底上生长了GaAs/AlGaAs多量子阱结构.对样品进行了低温光致发光谱和时间分辨光致发光谱的测量,结果表明激发功率和激发波长对室温下量子阱内电子的自旋弛豫时间有强烈的影响.对于常见的GaAs(100)量子阱起支配作用的D'yakonov-Perel' (DP)自旋弛豫机制,在GaAs (110)量子阱材料里被充分地抑制了.对于缺失了DP相互作用的GaAs (110)多量子阱,电子-空穴相互作用对自旋弛豫时间随激发功率变化有重要的影响.     

Nonlinear response to picosecond pulse excitations inmagneto-dielectric media

Nonlinear responses of magneto-dielectric thin layers to picosecond excitation have been calculated utilizing direct time-domain integration. Relaxations, hysteresis, and magnetic saturation phenomena, associated with the response are discussed in depth. The purpose of the calculation is to provide an analytical method by which physical properties of materials can be identified under picosecond-pulse excitation as well as to explore special cases of excitations in which picosecond pulses are efficiently absorbed. The calculations indicate that by examining the reflected waveform of the incident rectangular picosecond pulse, the amount of magnetic hysteresis and saturation of the material may be estimated. It is shown that magnetic hysteresis will affect the shape of the trailing edge of the reflected signal, whereas the magnetic saturation effect can be identified from the slope of the reflected pulse step. Examples of designs for effective picosecond-pulse screening structures are given     

Optical and electrical characterization of (Ga,Mn)N/InGaN multiquantum well light-emitting diodes

(Ga,Mn)/N/InGaN multiquantum well (MQW) diodes were grown by molecular beam epitaxy (MBE). The current-voltage characteristics of the diodes show the presence of a parasitic junction between the (Ga,Mn)N and the n-GaN in the top contact layer due to the low conductivity of the former layer. Both the (Ga,Mn)N/InGaN diodes and control samples without Mn doping show no or very low (up to 10% at the lowest temperatures) optical (spin) polarization at zero field or 5 T, respectively. The observed polarization is shown to correspond to the intrinsic optical polarization of the InGaN MQW, due to population distribution between spin sublevels at low temperature, as separately studied by resonant optical excitation with a photon energy lower than the bandgap of both the GaN and (Ga,Mn)N. This indicates efficient losses in the studied structures of any spin polarization generated by optical spin orientation or electrical spin injection. The observed vanishing spin injection efficiency of the spin light-emitting diode (LED) is tentatively attributed to spin losses during the energy relaxation process to the ground state of the excitons giving rise to the light emission.     

Specific features of optical orientation and relaxation of electron spins in quantum wells with a large spin splitting

The processes of optical spin orientation and spin relaxation of electrons are treated theoretically for semiconductor quantum wells, in which the spin splitting of the energy spectrum is comparable with the characteristic energy of charge carriers. The density matrix of photoexcited electrons at the instant of optical excitation is obtained in explicit form. A system of kinetic equations describing the behavior of the spin density matrix at an arbitrary relation between the average energy of charge carriers and the spin splitting is derived. It is demonstrated that, upon photoexcitation, a noticeable degree of orientation can be attained only in the pulse mode of operation, when the photoexcitation pulse duration is comparable with the period of spin precession in the field of spin splitting. It is shown that the total spin of the ensemble of electrons exhibits oscillations damping with time; the shape and damping time of the oscillations are sensitive to the parameters of photoexcitation and the spin splitting.     

ZnO Doped With Transition Metal Ions

Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches     

Picosecond time-resolved laser mass spectroscopy

The extension of laser mass spectroscopy to the picosecond time domain and the application of this method to the study of fast surface processes is described. The technique is based on a pump-probe scheme in which a first pulse stimulates the desorption of surface material, and a second delayed pulse is used to photoionize the desorbed particles to allow detection with a mass spectrometer. Measurements on picosecond-laser-heated GaAs surfaces demonstrate the feasibility of the method. For example, it was possible to measure the thermal desorption of Ga and of Ga₂O with picosecond time resolution     

啁啾光脉冲在高阶色散控制光纤链中的稳定传输

基于一种二阶和三阶色散补偿的光纤级联系统模型 ,用数值法对啁啾皮秒光脉冲作了传输模拟。结果表明 ,完全补偿的高阶色散控制系统消除了三阶色散所引起的脉冲边沿部的振荡 ,减弱了脉冲峰的时间移动 ;另外 ,在确定的配置下 ,给输入脉冲附加一最佳的频率啁啾 ,可使得色散控制孤子稳定传输。脉冲宽度和啁啾以及光强度都围绕在初始值附近波动 ,在每个补偿周期末端 ,基本恢复到初始值 ;最佳啁啾的选取与二阶色散的配置有关 ,与三阶色散的配置无关 ;文中画出了 10 0Gbit/s码率的 64位随机高斯光脉冲序列在完全补偿系统中传输 10 0 0 0km后的眼图。由清晰的眼图可知 ,这种完全补偿系统减弱了脉冲之间的相互作用     

Encoding Information on the Excited State of a Molecular Spin Chain

The quantum states of nano-objects can drive electrical transport properties across lateral and local-probe junctions. This raises the prospect, in a solid-state device, of electrically encoding information at the quantum level using spin-flip excitations between electron spins. However, this electronic state has no defined magnetic orientation and is short-lived. Using a novel vertical nanojunction process, these limitations are overcome and this steady-state capability is experimentally demonstrated in solid-state spintronic devices. The excited quantum state of a spin chain formed by Co phthalocyanine molecules coupled to a ferromagnetic electrode constitutes a distinct magnetic unit endowed with a coercive field. This generates a specific steady-state magnetoresistance trace that is tied to the spin-flip conductance channel, and is opposite in sign to the ground state magnetoresistance term, as expected from spin excitation transition rules. The experimental 5.9 meV thermal energy barrier between the ground and excited spin states is confirmed by density functional theory, in line with macrospin phenomenological modeling of magnetotransport results. This low-voltage control over a spin chain's quantum state and spintronic contribution lay a path for transmitting spin wave-encoded information across molecular layers in devices. It should also stimulate quantum prospects for the antiferromagnetic spintronics and oxides electronics communities.     

Optical pulse generation using guided-wave electrooptic modulator with resonant electrodes and polarization reversal

This paper reports on optical pulse generation using a new guided-wave electrooptic (EO) phase modulator with long resonant electrodes and polarization reversal. Adopting polarization-reversal structure to a guided-wave EO modulator with long standing-wave resonant electrodes, high-efficiency optical modulation is obtained by the compensation of the transit-time effect. By operating a fabricated EO phase modulator with a large amplitude modulation signal, optical sidebands over 100 GHz were obtained. Furthermore, optical pulse trains of an /spl sim/ 25-ps pulsewidth and a 15.28-GHz repetition frequency were successfully obtained by controlling the generated optical sidebands by use of an optical synthesizer.     

High-density photogenerated free-carrier spin relaxation processesin Wurtzite semiconductors: CdSe and semimagnetic semiconductorCd1-xMnxSe

Picosecond time-resolves spin relaxation kinetics of high-density free carriers is investigated at low temperatures on CdSe (x=0) and in the dilute semimagnetic semiconductor Cd_1-xMn_x Se for x=0.05 and 0.10. The fast spins relaxation observed in CdSe results from a mechanism associated with the noncentrosymmetric character of the band structure for this material. This process is similar to the one proposed by M.I. D'yakonov and V.I. Perel' (1971) for the zinc blende crystal structures. The spin relaxation times in CdSe are on the order of 30 ps. The spin relaxation time are <20 ps in semimagnetic semiconductor Cd_1-xMnSe and are consistent with spin-flip Raman scattering measurements. The increase in spin relaxation rate relative to CdSe is explained in terms of the carrier spin exchange between the carriers and the magnetic spin sites. A probable cause for the reduction in the observed spin polarization factor for carriers in Cd_1-xMn_xSe ( x≠0) is presented     

Investigation of GaAs/AlGaAs quantum cascade structures by optical methods based on hot luminescence in the near-infrared range

The time-resolved photoluminescence of GaAs/AlGaAs quantum-cascade structures under intense pulse excitation is studied. Aside from optical transitions between the ground electron and hole states of a system of two tunnel-coupled quantum wells, the photoluminescence spectrum at short times after the excitation pulse exhibits features corresponding to transitions between the excited states of these wells, which are not observed in time-integrated photoluminescence spectra. It is shown that, due to a high pump level, the electron gas is initially strongly heated, which makes it possible to observe band-to-band transitions between both the ground and excited states. Nonequilibrium carriers cool down with a characteristic relaxation time of ～125 ps.     

Chiral Photodetector Based on GaAsN

The detection of light helicity is key for various applications, from drug production to optical communications. However, the light helicity direct measurement is inherently impossible with conventional photodetectors based on III–V or IV–VI non-chiral semiconductors. The prior polarization analysis by often moving optical elements is necessary before light is sent to the detector. A method is here presented to effectively give the conventional dilute nitride GaAs-based semiconductor epilayer a chiral photoconductivity. The detection scheme relies on the giant spin-dependent recombination of conduction electrons and the accompanying spin polarization of the engineered defects to control the conduction band. As the conduction electron spin polarization is, in turn, intimately linked to the excitation light polarization, the light polarization state and intensity can be determined by a simple conductivity measurement. This approach, removing the need for any optical elements in front of a non-chiral detector, could offer easier integration and miniaturization. This new chiral photodetector could potentially operate in a spectral range from the visible to the infra-red using (InGaAl)AsN alloys or ion-implanted nitrogen-free III–V compounds.     

Picosecond all-optical polarization switching in InGaAsP MQW's at1.52 μm

An all-optical switch operating at a wavelength of 1.52 μm is demonstrated. The switch makes use of electron spin polarization in a multiple-quantum-well semiconductor structure. The polarization change of a signal beam was observed to recover with a time constant of 20 ps determined by the spin relaxation time