异质结中桥分子电子转移的飞秒激光控制研究

以染料分子通过桥分子吸附在半导体表面所组成的异质结为研究对象, 用一维约化振动模型研究了异质结中桥分子电子转移的飞秒激光优化控制过程,分别对单个桥分子和两个并列桥分子连接的异质结系统的电子传输及飞秒激光控制做了仔细的计算和讨论,理论上分析了桥分子的存在对异质结中超快电子转移路径的影响. 关键词： 飞秒激光 优化控制 异质结电子转移     

异质结线性吸收谱中的等离激元效应

将染料分子与半导体表面所组成的异质结与金属纳米粒子的耦合系统作为研究对象,考虑分子的振动态,在偶极-偶极近似下研究其线性吸收谱及相关的电荷转移动力学过程. 对金属纳米粒子作用下吸收谱变宽及相关的电子转移增强过程作了仔细的分析和讨论,理论分析了金属钠米粒子的表面等离激元对异质结中超快电子转移的影响. 关键词： 金属纳米粒子 等离激元 异质结电荷转移     

双色场控制与测量原子分子超快电子动力学过程的研究进展

超短超快激光脉冲的出现为人们探索原子分子中超快电子动力学过程提供了强有力的工具.阿秒脉冲和强激光脉冲电场整形技术能够获得电子波包在亚飞秒时间尺度的动力学信息,发展出了一系列阿秒超快光谱学技术,如阿秒条纹相机、阿秒瞬态吸收谱等,成功地应用于原子、分子和固体中电子运动的探测.其中双色激光场就是通过电场整形技术实现电子相干运动控制和探测的一种重要手段,本文综述了中科院上海光机所强场激光物理国家重点实验室近几年来在双色场控制与测量原子分子超快过程方面的研究工作.     

半导体纳米颗粒电子的超快弛豫过程

为了研究在飞秒激光作用下半导体纳米颗粒的超快动力学过程,建立了一个带表面态的三能级结构的载流子弛豫简化模型,得出各能级的电子速率方程.利用数值模拟方法模拟出各能级电子密度和差分吸收率随时间的变化情况,得知由于吸收截面的变化,差分吸收谱会有一个超快的变化过程.并将模拟结果与Fanxin Wu等人的实验结果相比较,其曲线特征基本一致.说明该模型有一定的合理性.     

半导体纳米颗粒电子的超快弛豫过程

为了研究在飞秒激光作用下半导体纳米颗粒的超快动力学过程,建立了一个带表面态的三能级结构的载流子弛豫简化模型,得出各能级的电子速率方程.利用数值模拟方法模拟出各能级电子密度和差分吸收率随时间的变化情况,得知由于吸收截面的变化,差分吸收谱会有一个超快的变化过程.并将模拟结果与FanxinWu等人的实验结果相比较,其曲线特征基本一致.说明该模型有一定的合理性.     

飞秒激光诱导自组织纳米周期结构及其光学特性的研究进展

飞秒激光具有超快、超强的特点.飞秒激光微纳加工发展非常迅速. 本文综述了近十年来利用飞秒激光在金属、半导体、介质等各类材料中制备的纳米周期结构, 阐述了若干关于飞秒激光诱导纳米周期结构的物理机理的观点.讨论了基于偏振调制的多光束 干涉在半导体表面制备纳米周期结构,简要叙述了周期结构对材料光学特性的影响. 关键词： 飞秒激光 纳米周期结构 多光束干涉 光学特性     

飞秒激光低压N2等离子体特性的实验研究

在低于一个标准大气压的条件下对飞秒激光产生的N₂等离子体光谱进行了实验研究.结果表明, 各种样品气压下的激光N₂等离子体光谱均表现为连续谱和线状谱的叠加.随着样品气压的降低, 连续谱和原子谱线的强度经历了由缓慢增强发展为缓慢降低再到迅速降低的过程, 而正一价离子谱线强度呈现逐步增强的特征.在气压低于0.3 atm (1 atm=101325 Pa)时, 出现了正二价态的离子谱线. 给出了低压N₂等离子体对于飞秒激光传输和能量吸收的物理特性, 并初步讨论了低压等离子体通道特性.这些结果有助于加深了解飞秒激光等离子体的特性和机理, 特别是给出了在实验上测量不同价态离子光谱的条件, 为今后的研究提供了有益的实验依据. 关键词： 飞秒激光 气压 等离子体光谱 激光传输     

中红外飞秒激光场中氮分子高次谐波的多轨道干涉特性研究

通过系统研究氮分子高次谐波产生过程中的电子超快动力学过程,实验上发现在中红外飞秒强激光场驱动下高次谐波谱的截止区附近存在清晰的谐波谱极小值.进一步研究表明谐波谱极小值对应的光子能量强烈依赖于驱动激光脉冲的光强和波长,而与分子取向角无关,由此推断该极小值来源于氮分子最高占据分子轨道和次最高占据分子轨道产生的高次谐波之间的相消干涉.本研究结果将对极端强场条件下多轨道电子超快动力学研究起到积极推动作用.     

飞秒激光作用下全向高反膜破坏的激发过程

设计和制备了全向高反膜SiO₂/TiO₂，研究了它在不同脉冲宽度、不同脉冲能量的飞秒激光作用下的破坏阈值和烧蚀深度.利用发展的抽运-探针方法，研究了抽运脉冲作用下材料中导带电子的超快激发和能量沉积过程，建立并求解了飞秒激光激发材料和材料的激发对抽运光自身反作用的耦合动力学模型.模型较好地揭示了材料破坏的激发过程. 关键词： 飞秒激光 全向高反膜 激发过程 破坏机制     

超快非均质电子转移的优化控制研究(Ⅰ)

本文用三能级的单振动模型,模拟了从二萘嵌苯到TiO2超快电子转移的动力学过程,发现在弱电子转移耦合下,电子在激光场的作用下激发至分子激发态转移然后到半导体导带,在强电子转移耦合下,电子直接由基态转移到半导体导带。在优化控制的理论模拟中以电子激发态的振动基态为目标态,考虑了不同注入位置条件下的电子转移的动力学过程,研究了优化激光场在给定时间内实现目标态的过程,由于从分子激发态到半导体的超快电子转移,只有当分子激发态能级与半导体导带底能级简并时,才能实现较高的目标态产生率。     

First principles analysis of the MgAl2O4:Ni absorption spectrum

Analysis of the energy-level scheme and absorption spectrum of the Ni²⁺ ion in MgAl₂O₄ was performed. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron (DV-ME) method [K. Ogasawara, et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, complete energy-level scheme of Ni²⁺ and its absorption spectra were calculated, assigned and compared with experimental data on the ground and excited state absorption spectra. Numerical contributions of all possible electron configurations into the calculated energy states were determined. By performing analysis of the molecular orbitals population, numerical contributions of the oxygen 2p- and 2s-orbitals into the 3d molecular orbitals were determined.     

Crystal-field induced mixing of electron states in C60 crystals at high pressure

Optical absorption spectra of thin fullerene (C₆₀) crystals in the range 1.7 to 3.8 eV have been measured at T=300 K and at pressures up to 2.5 GPa. The spectrum shifts toward the red with pressure, and the electron absorption intensity is redistributed among its bands. The intensity of the band associated with the lowest direct electron interband transition monotonically increases with pressure, whereas the intensity of the upper interband feature decreases. Bands related to weak edge absorption in the range between 1.7 and 2.2 eV gradually merge with the band associated with the lowest interband transition, whose intensity rises with pressure. A similar redistribution of intensity among electron transition bands has been observed when comparing the spectrum of an isolated C₆₀ molecule and that of a C₆₀ crystal. The results indicate that the crystal-field induced mixing of electron states is present in solid C₆₀, and they can be discussed in terms of the Craig-McClure model, which was suggested to describe crystal-field induced mixing of electron states in anthracene and naphthalene molecular crystals. Zh. éksp. Teor. Fiz. 113, 313–322 (January 1998)     

Excited states and absorption spectra of β-diketonate complexes of boron difluoride with aromatic substituents

In the approximation of the time-dependent electron density functional theory, we have studied using the quantum-chemical method the nature of excited states of boron difluoride acetylacetonate F₂BAA and its substituted derivatives that contain aromatic groups with one or two benzene cycles in the β-position. Optimization of the geometry of complexes show coplanar positions of cycles for all compounds, except for that with the substituent C₆H₃(CH₃)₂. Based on the calculated transition energies and oscillator strengths, we have simulated the absorption spectra in the prevacuum range. The calculated absorption spectra have been compared with the experimental spectra in the gas phase or in solutions. We show that, in the absorption spectra of complexes that contain substituents with one benzene cycle, the first three bands are caused by the transition of π electrons of the substituent to the LUMO of the chelate cycle. In complexes with two cycles in the substituent, the number of these transitions increases to five. As the π system becomes more extended, a bathochromic shift of the first absorption band and an increase in the transition probability are observed.     

Electronic structure and simulation of the dielectric function of β-FeSi2 epitaxial films on Si(111)

The optical functions of iron disilicide (β-FeSi₂) thin epitaxial films are calculated from the reflectance spectra in the energy range 0.1–6.2 eV with the use of the Kramers-Kronig (KK) integral relations. A comparison of the results of calculations from the transmittance and reflectance spectra and the data obtained from the reflectance spectra in terms of the Kramers-Kronig relations indicates that the fundamental transition at an energy of 0.87±0.01 eV is a direct transition. An empirical model is proposed for the dielectric function of β-FeSi₂ epitaxial films. Within this model, the specific features in the electronic energy-band structure of the epitaxial films are described in an analytical form. It is shown that the maximum contributions to the dielectric function and the reflectance spectrum in the energy range 0.9–1.2 eV are made by the 2D M ₀-type second harmonic oscillator with an energy of 0.977 eV. This oscillator correlates with the second direct interband transition observed in the energy-band structure of β-FeSi₂.     

First-principles study of electron dynamics with explicit treatment of momentum dispersion on Si nanowires along different directions

ABSTRACT

In this research, ground-state electronic structure and optical properties along with photoinduced electron dynamics of Si nanowires oriented in various directions are reviewed. These nanowires are significant functional units of future nano-electronic devices. All observables are computed for a distribution of wave vectors at ambient temperature. Optical properties are computed under the approximation of momentum conservation. The total absorption is composed of partial contributions from fixed values of momentum. The on-the-fly non-adiabatic couplings obtained along the ab initio molecular dynamics nuclear trajectories are used as parameters for Redfield density matrix equation of motion. The main outcomes of this study are transition energies, light absorption spectra, electron and hole relaxation rates, and electron transport properties. The results of these calculations would contribute to the understanding of the mechanism of electron transfer process on the Si nanowires for optoelectronic applications.     

Controlling two-dimensional electron localization via phase-controlled absorption and gain in the three-coupled quantum wells

We investigate two-dimensional (2D) electron localization via phase-controlled absorption and gain of a weak probe field in an asymmetric semiconductor three-coupled quantum well (TCQW) with a closed loop under the action of two orthogonal standing-wave fields. It is found that we can achieve high-precision and high-resolution 2D electron localization via properly varying the parameters of the system. The influences of direct one-photon transition and indirect three-photon transition on the precision of probe absorption–gain spectra are also discussed in details. Thus, the proposed scheme shows the underlying probability for the formation of the 2D electron localization in a solid.     

A Comprehensive Therotical Investigation of Intramolecular Proton Transfer in the Excited States for Some Newly-designed Diphenylethylene Derivatives Bearing 2-(2-Hydroxy-Phenyl)-Benzotriazole Part

This article presents a comprehensive therotical investigation of excited state intramolecular proton transfer (ESIPT) for some newly-designed diphenylethylene derivatives containing 2-(2-hydroxy-phenyl)-benzotriazole moiety with various substituted groups. The calculation shows the structural parameters and Mulliken charges of phototautomers enol (E) and keto (K) of these compounds exhibit no or tiny changes from S₀ to S₁. The calculated results suggest that HOMO and LUMO + 1 of the compounds displays excellent overlapping nature, and thus the absorption and emission could be from the electron transition of HOMO→LUMO + 1. The electron density distribution in the frontier orbital of E and K are influenced remarkably by various substituted groups in S₀ and S₁ states. Electron density distribution deficiency in 2-(2-hydroxy-phenyl)-benzotriazole part is observed in L + 1 for these derivatives. The calculation also suggests the potential energy curves of ESIPT are shown to be a strong relationship with electron donor-acceptor groups. The absorption spectra, normal emission spectra and ESIPT spectra of the derivatives were also calculated.     

PLE spectra analysis of the sub-structure in the absorption spectra of CdSeS quantum dots

The semiconductor CdSeS quantum dots (QDs) embedded in glass are analysed by means of absorption spectra, photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra. The peaks of absorption spectra shift to lower energies with the size of QD increasing, which obviously shows a quantum-size effect. Using the PLE spectra, the physical origin of the lowest absorption peak is analysed. In PLE spectra, the lowest absorption peak can be deconvoluted into two peaks that stem from the transitions of 1S_3/2--1S_e and 2S_3/2--1S_e respectively. The measured energy difference between the two peaks is found to decrease with the size of QD increasing, which agrees well with the theoretical calculation for the two transitions. The luminescence peak of defect states is also analysed by PLE spectra. Two transitions are present in the PLE, which indicates that the transitions of 1S_3/2--1S_e and 2S_3/2--1S_e are responsible for the defect states luminescence.     

Structural and optoelectronic properties of amorphous GaN thin films

Amorphous gallium nitride (a-GaN) thin films were deposited on glass substrate by electron beam evaporation technique at room temperature and high vacuum using N ₂ as carrier gas. The structural properties of the films was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). It was clear from XRD spectra and SEM study that the GaN thin films were amorphous. The absorbance, transmittance and reflectance spectra of these films were measured in the wavelength range of 300–2200 nm. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct band gap of E _g = 3:1 eV. The data analysis allowed the determination of the dispersive optical parameters by calculating the refractive index. The oscillator energy E ₀ and the dispersion energy E _d, which is a measure of the average strength of inter-band optical transition or the oscillator strength, were determined. Electrical conductivity of a-GaN was measured in a different range of temperatures. Then, activation energy of a-GaN thin films was calculated which equalled E _a = 0:434 eV.     

Electron refraction and light polarization effects in angle-resolved photoemission spectra of Cu surfaces using 45 eV radiation

Angle-resolved photoemission spectra of the low Miller-index faces of copper were taken with polarized 45 eV light for both surface normal and off-normal photoemission orientations. They reveal that low-energy photoelectrons emitted off-normal to the surface are strongly refracted toward the surface at the solid-vacuum interface; measured refraction angles are significantly (5̃0%) larger than predicted from a simple plane wave model. Furthermore, the spectra are found to be very sensitive to the orientation of the polarization of the exciting radiation with respect to the crystalline axes and the sample surface. Spectral dependence on light polarization is discussed in terms of direct transition selection rules for primary Mahan cones, influence of directionality on matrix elements, and direct transition contributions to the spectra from surface photoemission.