Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

太阳能电池陷光结构辐射特性研究

本文分析了不同微结构、电池厚度对太阳能电池光损失的影响,提出了一种新的电池陷光结构。运用时域有限差分法(FDTD)仿真了该类太阳能电池陷光结构中光传输过程,获得了不同波源入射下陷光结构电池的储能特性和光捕获特性,优化设计了最佳陷光结构并分析了其光捕获性能。     

Acceleration mechanism of atom clusters during radiationless transitions

We describe the mechanism of energy transition from an electron to an atom cluster during nonradiative electron relaxation. An assessment of this process is given, and it is shown that it could play an important role in accelerating the rebuilding of the atomic structure in amorphous alloys upon crystallization, chemical reactions in a condensed medium, and other processes associated with the rebuilding of atomic structures subjected to irradiation by light quanta.     

一维液态泡沫渗流实验研究及表面能和粘性耗散分析

液态泡沫由大量气泡密集堆积在微量表面活性剂溶液中形成，是远离平衡态的软物质. 泡沫强制渗流在微观上是指以恒定流率输入的液体在气泡间隙内的微流动过程，是影响泡沫稳定的主要因素之一. 采用在表面活性剂溶液中添加微量色素以显示泡沫中液体流动的方法，确定了透射率与液体分率的对应关系，测量得到了一维液态泡沫强制渗流中渗流波传播规律以及液体分率的演变规律；理论推导了泡沫基本单元，即开尔文单元结构(Kelvin cell)的粘性耗散能表达式，并依据Surface Evolver软件计算得到了不同液体分率时开尔文单元结构对应的的表面能，并计算出了与实验系统对应的开尔文单元结构的表面能和粘性耗散. 基于开尔文单元结构内液体分率演变的准静态假设，分析了表面能和粘性耗散的演变规律.     

Where is the energy of slow light stored?

We analyze the energy storage process of light propagating with slow group velocity in a sample where electromagnetically induced transparency (EIT) is created by a strong coupling field. We compare the formation of slow light in EIT and in self-induced transparency (SIT). For SIT, soliton-like propagation of light with essentially reduced group velocity takes place because of the temporary storage of an appreciable part of the pulse energy in the atoms. For EIT, no energy of the probe is stored in the atoms. This energy is transformed to the coupling field and leaves the sample with phase velocity c without absorption. Slow light is formed by a low frequency coherence induced at the input by the probe and coupling fields in a two-quantum excitation process. This coherence propagates as a “spin wave” with small group velocity, and at a large distance from the input, the coherence rules the process of the energy transformation from the coupling field to the probe, reproducing exactly the temporal profile of the probe at the input.     

Deflection of resonant multilevel particles in a standing wave light field

We study the deflection of an atomic system with a resonant multilevel energy structure in classical and quantized wave light fields. We obtain that the shape of the momentum distribution is very sensitive to the energy level structure, whereas its moments are not. Comparison with a two-level case is made.     

Manipulating the retrieved frequency and polarization of stored light pulses

We have experimentally demonstrated that stored and retrieved light pulses have different frequencies but maintain phase coherence; a stored light pulse is released with a different polarization. However, the manipulation process causes an energy loss of the retrieved pulse. We have discovered that Clebsch-Gordan coefficients among the Zeeman sublevels play an important role in the energy loss and have demonstrated a solution for avoiding the energy loss.     

脉冲激光沉积法制备的纳米硅薄膜的结构与光学性质

采用二次阳极氧化法,制备了多孔氧化铝模板。在真空背景下,用脉冲激光沉积法,在多孔氧化铝模板上沉积一层硅,制成了硅与多孔氧化铝的复合膜,然后用盐酸将多孔氧化铝模板完全腐蚀掉,制备均匀分布着硅纳米线的硅膜。用扫描电子显微镜、X射线衍射、光致发光对纳米硅的结构和光学性质进行了测试分析。结果表明:硅纳米线的直径约为67.5nm,长度约为100nm,数密度约10¹¹/cm²。光致发光谱是可见光范围内的一个宽发射峰,上面叠加了许多具有精细结构的尖峰,尖峰之间的波长间隔不相等,但能量间隔相等。分析了样品的结构特点,利用量子限制模型和表面发光中心模型对光谱进行了解释,提出了一个新的能级模型,求出了各个尖峰对应激发态能级的量子数。为探讨纳米硅的发光机制和实现硅发光器件的制备提供了实验依据。     

Non-linear optical processes involving excited subbands in laser-dressed quantum wires with triangular cross-section

The conduction subband structure of a triangular cross-section GaAs/AlGaAs quantum well wire under intense laser field is theoretically investigated by taking into account a finite confining potential. The calculation of the subband energy levels is based on a two-dimensional finite element method within the effective mass approximation. It is shown that a transversally polarized laser field non-uniformly shifts the subband energy levels and could be used for tuning the intersubband transitions and altering the related optical susceptibilities. We found that the non-resonant laser field allows the magnification and the red- or blueshift of the third-order non-linear susceptibility peaks for particular polarizations of the pump light and proper laser parameter values. The effects of the laser dressing field on the intersubband third harmonic generation and quadratic electro-optical process are discussed.     

NaYF4∶Er3+的水热合成及其紫外上转换发光性能

采用水热法制备了Er³⁺掺杂的NaYF₄上转换发光材料,X射线衍射结果表明,当反应温度为180 ℃和200 ℃时,晶体属于六方晶型和四角晶型混合相态;当反应温度为220 ℃时,该晶体属于纯六方晶型结构。SEM和TEM观察发现,晶粒为六角形,样品颗粒分散性好,平均粒径约为100 nm。荧光光谱测试结果表明,当激发波长为500 nm时,样品发射出紫外光。从Er³⁺能级图谱可以得出,Er³⁺基态电子⁴I_15/2首先跃迁到²H_11/2与⁴S_3/2能级上,随即经过能量转移上转换过程(ETU)分别发射出310 nm和340 nm的紫外光。结合Er³⁺发光机理可以推出上转换峰310 nm和340 nm均属于双光子过程。 研究结果表明,以NaYF₄为基质掺杂Er³⁺产生的紫外上转换光在生物成像、光催化发应及生物标记等方面有着广阔的应用前景。     

Self-steepening is an abrupt process

We discover that self-steepening occurs so fast with slight change in the input energy that it could be considered as an on-off process whose potential application could be far reaching. This observation was carried out by generating a stable uniform pattern of femtosecond filaments inside a methanol cell and measuring the evolution of the white light spectra. We found that the white light supercontinuum, a consequence of self-steepening, turned on almost instantaneously with respect to a very slight change in the energy.     

Wavelet analysis and Arnold web picture for detecting energy transfer in a Hamiltonian dynamical system

Our motivation is to understand how, in chemical reactions, the reaction coordinate effectively gains dynamical energy from the other degrees of freedom (i.e., bath coordinates) avoiding thermalization of the redistributed energy. In such a system, the phase space structure should be not homogeneous; i.e., the system is never ergodic. In this study, we introduce a way to capture the inhomogeneity of the phase space and to monitor energy transfers among their partial degrees of freedom in nonergodic systems using wavelet analysis and a picture of the Arnold web. First, we examine several simple energy transfer processes, i.e., a motion on a resonance line, between resonance lines, and around a resonance junction in a simple three-degree-of-freedom (DOF) system and show how the elemental processes of the intramolecular vibrational energy redistribution (IVR) are detected by our tools. We especially note that the structure of the higher order resonance of the system can be detected by wavelet analysis and motion in the action space. Next, we analyze a reaction process in a simple Hamiltonian system of 3 DOF with a double-well potential, i.e., a system with a transition state of the center-saddle-center type, and detect energy transfers in the reactive process. The aim of the study is to propose a way to characterize the inhomogeneity of the phase space, e.g., the reactive doorway, which leads to controllability of the chemical reaction by light, i.e., control of the reaction by selectively preparing an initial state in the reactive doorway by optical excitation.     

Singlets and triplets in hybrid nanodevices

An organic material thin layer can be used to resonantly absorb light and nonradiatively transfer excitation to an adjacent inorganic quantum well the optical nonlinearities of which can in this way be turned on more efficiently than by direct optical pumping. We theoretically consider this process in a hybrid structure based on crystalline tetracene in which the singlet exciton energy is close to twice the one of a triplet exciton and thermally activated singlet exciton fission into two triplets can be efficient. We investigate how the temperature dependence of the singlet exciton diffusion length affects the functional properties of such hybrid organic-inorganic nanostructures based on tetracene. We show how temperature activated fission opens a new possibility to turn on and off the indirect pumping due to energy transfer from the organic into the inorganic subsystem.     

Coherent transmission and angular structure of light scattering by monolayer films of polymer dispersed liquid crystals with inhomogeneous boundary conditions

We consider monolayer polymer films with oriented droplets of a nematic liquid crystal (LC). Relations for the coherent transmission coefficients of a layer of oriented ellipsoidal droplets and for the intensity of light scattered by monolayers of spherical and spheroidal droplets have been obtained. The amplitude-phase screen model and the interference approximation of the theory of multiple wave scattering have been used. To describe light scattering by an individual ellipsoidal droplet with inhomogeneous surface binding, we have developed an anomalous diffraction approximation. For monolayers of spherical LC droplets, the coherent scattering coefficients and the angular scattering structure have been analyzed. The internal structure of nematic droplets have been calculated by the relaxation method based on the solution of the minimization problem of the free energy volume density. We have studied basic regular features of light scattering by a monolayer with homogeneous and inhomogeneous boundary conditions at the LC-polymer interface. We show that, for films that contain droplets with inhomogeneous boundary conditions of the tangentially normal type, the angular structure of the scattered light is asymmetric with respect to the polar scattering angle.     

Phase-coupling-induced ultraslow light propagation in solids at room temperature

We show that the group velocities of light pulses can be decelerated dramatically by the use of a dispersive phase-coupling effect through a wave mixing process. We have observed experimentally such a phase-coupling-induced ultraslow light propagation with a group velocity as low as 0.05 m/s in a photorefractive Bi12SiO20 crystal at room temperature. Moreover, the ultraslow light is amplified in the Bi12SiO20 crystal because of the unidirectional energy transfer from a coupling beam to the ultraslow light. This technique to produce ultraslow light propagation is valid for all nonlinear wave mixing processes with a dispersive phase-coupling effect.     

Tuning visible light generation based on multi-channel quasi-periodically poled LiTaO3

We demonstrate broadband tuning visible light generation based on a multi-channel quasi-periodically poled LiTaO₃ crystal, in which a quasi-phase matched optical parametric generation process and a quasi-phase matched sum-frequency mixing process were achieved simultaneously. The conversion characters on spectrum and energy were studied by using a nanosecond pulse laser at 1.064 μm as pump light. We could tune the visible light over ~ 26 nm by means of changing the crystal's channel and temperature. The ratio of the output wavelength variation to that of temperature was ~ 0.07 nm/°C. The single-pass slope efficiency was 6.3% with the maximum output energy of 25 μJ.     

单束激光脉冲俘获及放大机理的理论分析与数值模拟研究

本文提出用固体靶前放置薄膜靶来实现激光场放大的新方案, 研究了针对单束激光脉冲条件的俘获及放大机理. 理论模型与数值模拟均表明入射激光能量可以部分地以驻波形式驻留在靶间区域并得到有效放大. 研究表明在入射激光光压、俘获激光光压和电荷分离场的共同作用下, 薄膜靶电子层压缩和膨胀是能量积累存在反复振荡过程的直接原因, 经过振荡后激光脉冲得到稳定俘获.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

Luminescence from hot electrons relaxing by LO phonon emission in p-GaAs and GaAs doping superlattices

Laser excited hot electrons in GaAs relax by LO phonon emission within a few hundred femtoseconds, leading to a series of peaks in the distribution of hot electrons in the conduction band, which we observe in luminescence. We find that the luminescence peaks shift according to the acceptor binding energy for C?, Ge?, Zn?, and Be-p-doped GaAs layers grown by MBE and LPE. Thus we prove that recombination is between hot electrons and neutral acceptors. The series of peaks due to electrons from the heavy hole band agree well with $\text{k}\text{.}\text{p}$ band structure, while peaks due to those from light holes are about 15 meV lower than expected from the band structure. We show that the discrepancy is not due to heating or surface fields. The peak separation in the luminescence ladder is about 6% larger than the LO energy suggesting emission of renormalised LO phonons. We find thermalisation by LO emission also in GaAs nipi doping superlattices. In nipi crystals the emission is shifted to higher energies (by 12 meV for light and by 6 meV for heavy holes) due to a change in band structure caused by the space charge fields.     

硅片绒面形貌影响光线反射的数值研究

太阳能硅片表面绒面的光陷阱可以使光在其中经历多次反射,从而尽量减少光的反射损耗.不同光陷阱的形貌决定了光的不同多次反射路径而具有不同的反射效果.为了研究光陷阱形貌及光线入射角对减反射效果的影响,提出了数值仿真计算的方法跟踪每一条光线的反射过程计算加权出射系数.从而可以计算分析复杂形貌绒面的减反射效果并给出合理的优化方法,为制备高性能绒面结构提供理论依据.当光陷阱尺寸小于入射光线波长时,发生镜反射,将该尺寸的结构平滑处理.然后从光陷阱的深径比、高度、密度等方面计算分析光在不同入射角的情形下的加权出射系数.提出了理想的绒面光陷阱形貌,及获得最佳反射效果的入射角度.最后计算碱腐蚀及电火花加工产生的两种典型绒面的加权出射系数,并利用实验测量值验证了该计算方法.