Femtosecond luminescence decay due to exciton energy transfer in single-walled carbon nanotube bundles

We investigated luminescence decay kinetics in single-walled carbon nanotubes in large bundles excited by femtosecond pulses. The time constant of luminescence decay becomes longer with decrease in photon energy: 40 fs at 1.2 eV and 380 fs at 0.6 eV. This behavior is explained by exciton energy transfer from an excited to neighboring tubes.     

Time-resolved luminescence spectroscopy of LiF(U) crystals

We studied the luminescence spectra and decay kinetics of LiF(U) crystals excited by pulsed laser radiation (λ = 337 nm) in temperature range of 15–300 K. Two groups of lines with different exponential decay times are revealed in spectra. Based on comparative analysis of fast and slowly decaying luminescence spectra, temperature dependences of decay time constants, and line intensities, we conclude that electronic transitions from radiation levels of the impurity luminescence center can occur, not only in a vibrationally relaxed state, but also in a state with an excited high-frequency mode in O-U-O bonds.     

电致发光衰减测量系统的搭建及应用

发光衰减是发光的重要过程,测量发光寿命对研究发光机理十分重要,但传统研究在概念和方法上存在两个差错：(1)概念上认为衰减等同于激发态数目的减少,而忽视了衰减是发光强度的下降,两者是不同的概念;(2)方法上基于激发态规律推导,假设的边界条件不符合实际,没有实验的支持。同时,传统方法对设备的要求很高,且只限于光致发光。为了纠正差错,降低成本,搭建了一套全新的电致发光衰减测量系统,可用于所有可以周期激发的发光类型。从能量转换原理出发,采用周期激发,用脉冲间隔时间作为时间尺度来度量发光衰减持续的时间,通过脉冲间隔时间与发光寿命的对比,相应地发光强度有不同的变化,根据该现象简便地测量出发光寿命。基于该原理搭建的发光衰减测量系统,实验结果表明了发光强度随着激发频率,先保持不变然后逐渐下降,通过测量下折点即能够推算出发光衰减寿命,而且还发现发光衰减寿命与初始发光强度呈正相关的关系。认为发光寿命是发光强度的变化,是区别于传统研究以激发态数目为研究对象的一大创新,同时通过实验证明了发光寿命与初始亮度相关,也拓展了对发光寿命的新认识。     

Time resolved photoluminescence near the “band gap” in amorphous silicon

Two broad luminescence bands in weakly hydrogenated (glow discharge) undoped amorphous silicon have been observed using time resolved spectroscopy on a nanosecond timescale. Whereas the luminescence decay of the low energy band has been found to be sample independent, the peak position of the luminescence does show a sample dependence. We propose an intrinsic origin of this luminescence.     

Excitation wavelength and intensity dependence of luminescence decay in a-Si(H)

We present results on the dependence of the initial luminescence decay in hydrogenated amorphous Si on temperature, excitation photon energy and excitation intensity. We find that the initial decay becomes faster with increasing temperature and excitation intensity but slower with increasing excitation phonon energy. The total luminescence intensity varies sublinearly with excitation intensity at high intensities, showing that nonradiative processes become more important at high intensities. These results are discussed in terms of current models.     

The luminescence of ZnO ceramics

The luminescence properties of ZnO ceramics with grains 100–5000 nm sintered by different techniques from nanopowders were studied. The luminescence decay times were compared with that obtained for ZnO single crystal. The temperature dependence of non-exponential decay of defect luminescence (2.0–2.6 eV) was measured in wide time, intensity and temperature range. The luminescence decay kinetic at T ≤ 20 K shows the decay close to I(t) ～ t^?1 dependence. At temperature region 50–250 K the decay kinetics is more complicate since the TSL was observed in this temperature region. It is shown that the luminescence properties of NP and ceramics strongly depend on defect distribution on grains surface and the volume/surface ratio determine the luminescence decay in ZnO nanostructures and ceramics.     

Yb:YAG晶体性能实验测量

为了测量脉冲时间宽度小于20 ns时的射线时间分辨图像,发展了新型无机闪烁体Yb:YAG,并实验测量了晶体的发光衰减时间、X射线激发发光光谱、相对发光效率和空间分辨等性能,研究了Yb:YAG晶体的发光性能。实验表明,Yb:YAG发光有三种衰减成分,快成分衰减常数为1.2 ns,慢成分衰减常数与射线种类有关;X射线激发发光光谱在250~800 nm范围,有三个发光峰,分别为320,380和500 nm,且320 nm处强度最大;相对发光效率为1900 ph/MeV;使用钨分辨卡测得Yb: YAG空间分辨能力为2 lp/m,使用刀口法测得空间调制传递函数为0.5时的频率为0.7 lp/mm。结果说明Yb:YAG晶体性能能够满足所需测量要求。     

Tsallis distribution and luminescence decays

Usually, the Kohlrausch (stretched exponential) function is employed to fit the luminescence decays. In this work we propose to use the Tsallis distribution as an alternative to describe them. We show that the curves of the luminescence decay obtained from the Tsallis distribution are close to those ones obtained from the stretched exponential. Further, we show that our result can fit well the data of porous silicon at low temperature and simulation result of the trapping controlled luminescence model.     

Cooperative luminescence and absorption in ytterbium doped aluminosilicate glass optical fibres and preforms

The cooperative luminescence and absorption properties of Yb³⁺ doped aluminosilicate glass optical fibres and preforms are investigated in detail. In accordance with previous investigations, both the visible cooperative luminescence and the infrared luminescence decay measurements have been resolved into a single exponential decay component. We show that for a glass with similar Yb³⁺ dopant concentration but more Al³⁺, the glass emits less visible luminescence. Absorption loss measurements completed on fibre samples revealed a broad absorption in the 350-500 nm range, which we propose is due to a combination of Yb²⁺ absorption and cooperative absorption from Yb³⁺ ion pairs.     

Photoluminescence and photocatalytic activity of zinc tungstate powders

ZnWO₄ powders with grain size in range 20 nm–10 μm have been synthesized by a simple combustion method and subsequent calcinations. The photocatalytic activities of powders were tested by degradation of methylene blue solution under UV light. The luminescence spectra and luminescence decay kinetics were studied and luminescence decay time dependence on average powder-grain size was obtained. The correlation between self-trapped exciton luminescence decay time and photocatalytic activity of ZnWO₄ powders was shown. A model explaining the excitonic luminescence decay time correlation with photocatalytic activity was proposed.     

两种可溶性聚乙炔衍生物的飞秒发光动力学研究

采用飞秒频率上转换技术测量了两种可溶聚乙炔衍射生物ｐｏｌｙ（ｐｈｅｎｙｌａｃｅｔｙｌｅｎｅ（ＰＰＡ）和ｐｏｌｙ（１－ｐｈｅｎｙｌ－１－ｂｕｔｙ１）ＰＰＢ）的发光动力学过程。采用非均匀展宽态密我规行走理论对上述结果进行了分析。     

A geminate recombination model for photoluminescence decay in plasma-deposited amorphous Si:H

We develop a model of geminate electron-hole recombination, including tunnelling and diffusion, to account for photoluminescence decay in amorphous semiconductors. The model correctly predicts the shape of the decay, the luminescence quantum efficiency, and the microscopic electron mobility.     

Distribution of recombination lifetimes in amorphous silicon

Light-induced ESR (LESR) and luminescence in aSi:H are studied in order to resolve a recent controversy over whether the low temperature recombination is geminate or non-geminate. New transient measurements find that at moderate excitation intensities, luminescence is predominately monomolecular, whereas LESR is not. We show that the different behavior occurs because there is a very broad distribution of decay times for geminate electron-hole pairs. LESR is dominated by a small fraction of the pairs which have a very long decay time and which therefore slowly generate a non-geminate distribution. Luminescence is dominated by the closer geminate pairs that recombine quickly.     

能量传递过程的计算机模拟

通过对供体(D)—受体(A)间传递速率静态分布的模拟,得到了静态能量传递情况下D的发光衰减曲线,与Inokuti-Hirayama模型结果一致.模拟并讨论了D—D传递与D—A传递相关和非相关情况下D—D传递对D发光衰减的影响. 关键词：     

Luminescence of different modifications of crystalline silicon dioxide: Stishovite and coesite

Luminescence of very small samples of single crystals of coesite and stishovite has been studied. The spectra were detected under ionizing radiation (X-ray and electron beam) and the decay kinetics of cathodoluminescence in the range of time from 10 ns to 3 ms was measured. The coesite luminescence possesses a broad band at 3 eV with exponential decay about 680 μs at 80 K. The nature of this luminescence was explained as a self-trapped exciton creation in tetrahedron framework. The stishovite luminescence possesses two bands—blue (2.8 eV) and UV (4.7 eV). The UV band intensity grows more than 20 times with irradiation dose from initial level. This shows that the corresponding luminescence centers could be induced by the radiation. The decay of the UV band possesses a fast and a slow component. The determination of the fast decay parameters is beyond the capabilities of our apparatus (less than 10 ns), whereas the slow decay of the UV is non-exponential and takes place in the range of hundreds of microsecond. The blue band decay kinetics can be well approximated by power law ∼t⁻², which may correspond to recombination of defects created by radiation. The stishovite single crystal luminescence is very similar to that of germanium dioxide single crystal of rutile structure. The nature of the stishovite luminescence is explained as recombination of defects created by irradiation in octahedron-structured lattice.     

Minority carrier lifetime in p-type gallium phosphide

The decay time of the luminescence involving bound minority carriers shortens above some temperature with increasing temperature owing to thermal quenching. However, above some higher temperature there will be a plateau since the luminescence decay cannot be faster than the decay of the excited free minority carriers.     

Upconversion luminescence of CdTe nanocrystals by use of near-infrared femtosecond laser excitation

We study the steady-state and time-resolved luminescent properties of CdTe nanocrystals by one- and two-photon excitation with a femtosecond laser. We observe that 1208 nm excitation causes a shift of the emission peak of about 20 nm to the infrared compared with 400 nm laser excitation. It is found that upconversion luminescence is composed of a photoinduced trapping and a band edge excitonic state and produces the observation of biexponential decay kinetics. We conclude that the redshift of the emission peak is caused by the relative change in luminescence intensity between excitonic and trapping states.     

On the potential for molecular imaging with Cerenkov luminescence

Recent observation of optical luminescence due to beta decay from suitable radiotracers has led to the possible development of new preclinical optical imaging methods. The generation of photons that can be detected using instrumentation optimized for bioluminescence imaging has been putatively associated with the Cerenkov effect. We describe the simultaneous utilization of fluorescence reporters to convert Cerenkov luminescence to longer wavelengths for better tissue penetration and also for modulating the luminescence spectrum for potential molecular imaging strategies.     

Kinetics of photoluminescence decay of KCl-Yb

We have investigated the kinetics of the luminescence decay of a KCl-Yb crystal excited in one of the long-wavelength absorption bands of the Yb²⁺ ion by nitrogen laser radiation with a power density less than 10³ W/cm². We found that, in both luminescence bands, both the one associated with the allowed transition (399 nm) and the one associated with the forbidden transition (430 nm), there are two stages of decay—an initial exponential stage that gives way to a hyperbolic stage. Non-exponential decay due to the optical electron tunneling back to the luminescence center is treated as evidence of delocalization of the electron upon intracenter excitation and is explained by the formation of an excimer-like molecular bond upon photoexcitation of the crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 1763–1765 (October 1999)     

Transformation of resonant Raman scattering into luminescence in CdxZn1−xTe mixed crystals: Time-resolved spectroscopy

We report an experimental study of time characteristics of secondary emission in Cd_xZn_1-xTe mixed crystals (x = 0.32) under resonant excitation with a picosecond dye laser. When the incident laser frequency is tuned on to the luminescence maximum of localized excitons, the decay curve of the intensity of “Raman-like” lines exhibits a single exponential decay. Off resonance, however, a short-lived component corresponding to Raman scattering appears in addition to the long-lived component. The intensity of the Raman component relative to that of the luminescence component increases with increase of the off-resonance frequency. From these temporal behaviors, we have found, for the first time, the transformation of resonant Raman scattering into luminescence in mixed crystals as a function of incident frequencies.