偏置电场对聚对苯乙烯激发态弛豫特性的影响

为探讨洞悉电场对有机发光二极管电致荧光量子效率的影响,通过激发-探测超快光谱技术研究了激子在电场下的瞬态行为.与单重态激子相应的激发态在230 μJ/cm2激发强度下,显示了快慢两个弛豫过程. 快慢组分的权重因子及快组分弛豫时间常数是电场相关的, 在6.4×10⁵ V/cm的电场下,与无偏置电场相比,激子的快组分弛豫时间加速,快组分的权重因子由22%增加为72%,约50%的初始激子又通过电场而离解. 慢组分是电场无关的,其弛豫时间常数为890 ps. 实验结果还揭示了由激发光所产生的长程声学声子,其声速为17 /ps.     

电场调谐InAs量子点荷电激子光学跃迁

在低温5 K下, 采用光致发光光谱及外加偏压调谐量子点电荷组态研究了InAs单量子点的精细结构和对应发光光谱的偏振性、不同带电荷激子的圆偏振特性. 得出如下结果: 1) 指认InAs单量子点中不同荷电激子的发光光谱和对应的激子本征态的偏振特性; 2) 外加偏压可以调谐量子点的荷电激子的发光光谱; 3) 伴随着电子、空穴的能量弛豫, 电子的自旋弛豫时间远大于空穴的自旋弛豫时间. 关键词： InAs量子点 激子 荧光光谱 电场调谐     

LaⅡ5d2 1G4→4f5d 1F3超精 细结构光谱测量

利用少有的共线快离子激光光谱学方法研究了LaⅡ的超精细结构,测量了5d² < sup>1G₄态→4f5d ¹F₃态的超精细结构光谱,分 别得出了相应的磁偶极矩和电四极矩的超精细相互作用常数. 关键词： 共线快离子束激光光谱学 超精细结构 共振激发     

WS_2与WSe_2单层膜中的A激子及其自旋动力学特性研究

单层过渡金属硫化物由于其特有的激子效应以及强自旋-谷耦合性质,在光电子学及谷电子学等方面有着很广阔的应用前景.利用超快时间分辨光谱,本文系统地比较了两类钨基单层硫化物(WS_2和WSe_2)的A-激子动力学和谷自旋弛豫特性.实验结果表明, WS_2单层膜的A-激子弛豫表现为双指数过程,而对于WSe_2,其A-激子衰减表现为三指数过程,且激子的寿命远长于前者. WS_2谷自旋极化弛豫表现为单指数衰减,其寿命约0.35 ps,主要由电子-空穴交换作用所主导.而对于WSe_2,谷自旋弛豫表现出双指数弛豫特性:一个寿命为0.5 ps的快过程和一个寿命为28 ps的慢过程.快过程的弛豫来源于电子-空穴交换作用,而慢过程则由于自旋晶格散射形成暗激子的过程.通过调谐抽运光波长,进一步证实WSe_2较WS_2更容易形成暗激子.     

飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置,并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲“抽运-探测”方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm^-1的光谱分辨率,光谱检测范围为300—4000cm^-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明,β-胡萝卜素被激发到S₂态后,经由寿命约为0.3ps的中间态S_X态实 关键词： 飞秒时间分辨拉曼光谱 β-胡萝卜素 激发态内转换 振动弛豫     

LA声子对激子qubit纯退相干的影响

在一个抛物量子点中,以激子的真空态和基态作为量子比特(qubit),采用求密度矩阵元的方法,计算了由形变势下声学声子引发的激子量子比特纯退相干.找到了激子量子比特纯退相干因子对时间、温度和量子点受限长度的依赖关系.研究发现,激子量子比特的退相干因子在2.5ps的时间范围内随时间的增加而迅速增加,其纯退相干时间在ps量级;在温度即使为绝对温度0K时由LA声子引发的退相干依然存在,在温度大于3K后退相干因子随温度的增大而开始迅速增大;并同时发现量子点受限长度对退相干因子有重要影响,激子越受限退相干越快.研究结果表明,对激子量子比特使用适当大小量子点,且保持环境低温,并采用低能超快光学操作可以有效地抑制声子对激子量子比特纯退相干的影响. 关键词： 量子点 量子信息 量子比特     

CdSeS合金结构量子点的多激子俄歇复合过程

多激子效应通常是指吸收单个光子产生多个激子的过程,该效应不仅可以为研究基于量子点的太阳能电池开拓新思路,还可以为提高太阳能电池的光电转换效率提供新方法.但是,超快多激子产生和复合机制尚不明确.这里以CdSeS合金结构量子点为研究对象,研究了其多激子生成和复合动力学.稳态吸收光谱显示, 510, 468和430 nm附近的稳态吸收峰,分别对应1S_(3/2)(h)-1S(e)(或1S), 2S_(3/2)(h)-1S(e)(或2S)和1P_P(3/2)(h)-1P(e)(或1P)激子的吸收带.通过飞秒时间分辨瞬态吸收光谱和纳秒时间分辨荧光光谱两种时间分辨光谱技术对CdSeS合金结构量子点的超快动力学进行了探究,结果显示, 1S激子的双激子复合时间大概是80 ps,这一时间比传统量子点的双激子复合时间(小于50 ps)延长了近一倍,结合最近发展的超快界面电荷分离技术,在激子湮灭之前将其利用起来,这一时间的延长将有很大的应用前景;其中,在2S和1P激子中除上述双激子复合外,还存在一个通过声子耦合路径的空穴弛豫过程,时间大概是5—6 ps.最后,利用纳秒时间分辨荧光光谱得到该样品体系单激子复合的时间约为200 ns.     

激子复合区厚度对有机磁效应的影响

在常规型有机发光二极管的基础上, 通过改变发光层tri-(8-hydroxyquinoline) aluminum (III) (Alq₃)厚度, 研究了激子复合区厚度对有机发光二极管磁效应的影响.测量了器件在不同温度及偏压下电致发光及注入电流在外加磁场作用下的变化, 着重研究了低温下的有机磁电导效应和有机磁电致发光效应.实验发现, 低温(50 K)高磁场 (500 mT)下, 器件表现出随Alq₃厚度的减薄, 磁电导值由正到负再到正的非单调变化.利用磁场调控的超精细相互作用、 磁场抑制的三重态激子-电荷反应以及激子在界面的淬灭效应, 对有机磁电导在低温下表现出的现象进行了定性的解释.实验结果表明, 通过改变激子复合区的厚度, 可以实现对激子浓度的有效调节, 进而实现对有机磁电导和磁电致发光效应的调节. 该研究进一步丰富了有机磁效应的实验现象, 同时提供了一种调控有机磁效应的手段. 关键词： 激子复合区 激子浓度 有机磁电导 有机磁电致发光     

pH刺激响应水凝胶微观结构动力学的超快红外光谱研究

本文合成了甲基丙烯酸二甲氨基乙酯/丙烯酸共聚水凝胶,并在不同pH条件下表征其宏观溶胀行为. 以硫氰酸根阴离子作为局部红外探针,结合傅里叶变换红外光谱和超快红外光谱学研究了pH刺激响应水凝胶的微观结构动力学. 超快红外光谱数据表明,当水凝胶体系的pH从2.0变为12.0时,硫氰酸根探针的振动弛豫时间常数从(14±1) ps增加到(20±1) ps. 转动各向异性测量结果进一步表明,硫氰酸根探针的转动受到水凝胶中形成的三维网络结构的限制. 尤其是在pH为7.0时,SCN探针的旋转不会衰减到零.     

ZnCdSe量子阱/CdSe量子点耦合结构中的激子隧穿过程

用室温光致发光谱和飞秒脉冲抽运探测方法对不同垒宽的ZnCdSe量子阱/ZnSe/CdSe 量子点新型耦合结构中激子隧穿过程进行研究，观察到激子从量子阱到量子点的快速隧穿过 程.在ZnSe垒宽为10nm, 15nm, 20nm时，测得激子隧穿时间分别为1.8ps, 4.4ps, 39ps. 关键词： ZnCdSe量子阱 CdSe量子点 激子 隧穿     

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots （QDs） were investigated by two-colour femtosecond （fs） pump-probe （400/800 nm） and picosecond time-resolved photoluminescence （ps TRPL） experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.     

Direct time-domain observation of ultrafast dephasing in adsorbate-substrate vibration under the influence of a hot electron bath: Cs adatoms on Pt(111)

The stretching vibration of Cs adsorbed on Pt(111) is excited coherently by resonant stimulated Raman excitation and its time evolution is probed by fs time-resolved second harmonic generation as a function of pump laser fluence I. As I increases, a fast dephasing component with the lifetime of tau=0.7 ps grows, being superimposed on a slow one with tau=1.9 ps. The fast component is a manifestation of pure dephasing caused by elastic and inelastic scattering of hot electrons at adsorbate.     

Carrier transport effects and dynamics in multiple quantum well optical amplifiers

The gain recovery dynamics of multiple quantum well semiconductor optical amplifiers, following gain compression caused by ultrashort optical pulse excitation, have been studied for several devices of different structures. Fast, slow, and intermediate time constants are identified. The fast component (0.6 to 0.9 ps) corresponds to cooling of the dense, inverted electron-hole plasma. The slow component (150 to 300 ps) corresponds to replenishment of carriers from the external bias supply, with the dynamics dominated by spontaneous recombination (primarily Auger) of the electron-hole plasma. The intermediate time constant (2 to 14 ps) is caused by carrier capture by the quantum wells and is structure-dependent. For most of the devices, the capture process is dominated by diffusion-limited transport in the cladding/barrier region. The variation of carrier density and temperature also affects the refractive index profile of the devices and, hence, affects the waveguiding properties. Dynamical variation of the mode confinement factor is observed on the fast and slow timescales defined above.     

混合分子聚集体有机纳米材料中相干与非相干皮秒脉冲累积光子回波

用双调制(斩波器和驻波调制器)和外差检测累积光子回波技术,研究了PIC-I和azaPIC-I混合分子集体有机纳米材料中一维Frenkel激子的位相弛豫过程.观测到激子失相时间T2随azaPIC-I克分子数的增大从60ps变为224ps.这与在含能量陷阱的分子聚集体中激子失相时间T2随陷阱的增加而变短的现象刚好相反.并且对在脉宽为4ps和相于时间为1ps的相干脉冲和非相干脉冲的不同激发条件下的实验结果进行了分析和讨论.     

Investigation of non-equilibrium electron-hole plasma in nanowires by THz spectroscopy

Efficient emission of THz radiation by AlGaAs nanowires via excitation of photocurrent by femtosecond optical pulses in nanowires was observed. Dynamics of photoinduced charge carrier was studied via influence of electron-hole plasma on THz radiation by optical pump THz probe method. It was found that characteristic time of screening of contact field is about 15 ps. Recombination of non-equilibrium occurs in two stages: fast recombination of free electron and holes (with relaxation time about 700 ps), and slow recombination (with relaxation time about 15 ns), which involves a capture of electrons and holes on the defects of crystalline structure of nanowires.     

Ultrafast quenching of ferromagnetism in InMnAs induced by intense laser irradiation

Time-resolved magneto-optical Kerr spectroscopy of ferromagnetic InMnAs reveals two distinct demagnetization processes--fast (<1 ps) and slow (approximately 100 ps). Both components diminish with increasing temperature and are absent above the Curie temperature. The fast component rapidly grows with pump power and saturates at high fluences (>10 mJ/cm(2)); the saturation value indicates a complete quenching of ferromagnetism on a subpicosecond time scale. We attribute this fast dynamics to spin heating through p-d exchange interaction between photocarriers and Mn ions, while the approximately 100 ps component is interpreted as spin-lattice relaxation.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

Radiative recombination of free excitons in GaAs quantum wells

We have measured the radiative lifetime of excitons in GaAs quantum wells under resonant excitation at 10 K using time resolved luminescence spectroscopy with 6 ps time resolution. The luminescence decay has two components: a fast one with a time constant τ₁ (∼ 17 - 40 ps) and a slow one with a time constant τ₂ (∼ 80 - 300 ps). τ₂ is the lifetime of thermalised excitens at 10 K. τ₁ is due to two mechanisms in parallel: the radiative recombination of excitons with k_u < K₀ and the scattering by acoustical phonens into non radiative exciton states (k_u > k₀ and J = 2). The variation with temperature of τ₁ gives the lifetime of the excitons at k = 0, τ₀, which varies between 20 and 50 ps depending on the sample.     

Ultra-fast all-optical switching in MEH-PPV photonic crystals with resonantly enhanced nonlinearity

Photonic crystal all-optical switching is realized in two-dimensional nonlinear photonic crystals made of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]. Organic polymer films are fabricated by a slow evaporation method. Under resonant excitation, the organic matrix provides very large third-order nonlinear optical susceptibility. An operating pump energy as low as 514 pJ/cm² and a high switching efficiency of 70% are achieved for the photonic crystal optical switching. A switching time of 45.6 ps is maintained due to fast Forster transfer of excitons.