Nonlinear luminescence phenomena in fullerene crystallites

Time-resolved photoluminescence studies in powders and single crystals of C₆₀ are reported. The observed emission spectra, resulting from excitation above the HOMO-LUMO transition energy, are consistent with those reported for steady state conditions which may be associated with an intramolecular process and the observed luminescence decay times are of the order of 1 ns. No long lived decay from the triplet is observed at low intensities. A dramatic increase in the lifetime as well as a redshift and broadening of the spectrum is observed at high excitation densities. The luminescence intensity is seen to increase with the cube of the input intensity. The phenomenon is interpreted as an abrupt onset of emission from the highly populated excited state resulting from an interaction of the intramolecular states, a process which is dependent on a critical excited state density. Furthermore, the phenomenon may be photoexcited at photon energies which lie below the HOMO-LUMO transition energy, under which conditions an intensity dependence of the output luminescence on input to the sixth power is observed. Under off-resonant conditions the process is interpreted as two-photon assisted nonlinear emission.     

Broadband electroluminescent emission from fullerene crystals

The observation of electroluminescence from crystalline fullerenes is described and discussed for the first time. Using gold and aluminium electrodes as contacts, a broad band emission spectrum, extending from 400 nm to 1100 nm is observed. The spectrum has a primary maximum at 920 nm and a weaker feature centered on 420 nm. The spectral characteristics are independent of the applied field and the longer wavelength region is identical to that measured in the high excitation density photoluminescence spectrum. In addition, the electroluminescent output intensity increases with the cube of the injection current, strengthening the association to the nonlinear phenomena observed in the highly excited state of fullerenes.     

Raman studies of nonlinear phenomena in fullerene crystallites

The resonant Raman scattering of C₆₀ crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm^–1 is observed. No feature at 1459 cm^–1 is observable. With increasing intensities, the 1468 cm^–1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the -electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed.     

Effect of collective response on electron capture and excitation in collisions of highly charged ions with fullerenes

Projectile deexcitation Lyman x-ray emission following electron capture and K excitation has been studied in collisions of bare and Li-like sulphur ions (of energy 110 MeV) with fullerenes (C(60)/C(70)) and different gaseous targets. The intensity ratios of different Lyman x-ray lines in collisions with fullerenes are found to be substantially lower than those for the gas targets, both for capture and excitation. This has been explained in terms of a model based on "solidlike" effect, namely, wakefield induced stark mixing of the excited states populated via electron capture or K excitation: a collective phenomenon of plasmon excitation in the fullerenes under the influence of heavy, highly charged ions.     

Linear and non-linear behavior of cavity polaritons

We report on the linear and non-linear emission of cavity-polaritons under resonant excitation. At low excitation density, in addition to polariton photoluminescence, strong Rayleigh scattering is observed. At higher excitation densities, a sudden transition to a highly emissive state is observed, accompanied by spatial patterning. We attribute such phenomena to a combination of nonlinear cavity-polariton relaxation mechanism and nonlinear response of the cavity, leading to transverse pattern formation. A careful analysis of near- and far-field emission patterns with spatial filtering as well as reflectivity shows that an inhomogeneous situation develops, the center of the excited region undergoing a strong to weak coupling transition while the periphery is still in strong coupling. Despite the complex non-linear behavior we observe no signatures of Bose–Einstein condensation or Boser action.     

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm² excitation and recovers with a fast time constant 34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS ₁ and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S ₁) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.     

温度梯度法生长的Ce: YAlOZr3高温闪烁晶体的光谱分析

采用温度梯度法生长了熔体掺杂Ce浓度为1at%的YAlO3晶体，对于其吸收光谱、荧光光谱和x射线激发发射谱进行了表征分析.根据吸收光谱提出了一个色心模型，从而成功的解释了为什么刚生长出的晶体为粉红色，而分别经氢气和空气在1400℃退火后均能变为无色的现象和退火以后吸收光谱发生的显著变化.温度梯度法生长的Ce:YAP在330nm处存在着一定程度的自吸收和自激发.光致激发发射谱的发射主峰在368nm，而x射线激发发射谱的主发射峰红移至391nm，这表明在x射线激发下，晶体对发射光的自吸收将会减少.另外在x射线 关键词： 温度梯度法 Ce: YAP 色心 光谱     

Eu3+激活的硼钒酸钇的光致发光

找出了合成Eu3+激活的硼钒酸钇铕发光粉的适宜条件和发光最佳化学组分,它是(Y0.94Eu0.06)(V0.69B0.31)O3.7.对其发光光谱的温度依赖关系的研究指出,发光最佳温度在350℃左右.发光的这种温度依赖关系与基质和Eu3+对能量的吸收,以及基质向Eu3+能量传递效率的提高有关.此外,试验结果还表明,在365nm激发下,少显Gd2O3、Zn2GeO4·GeO2:Mn2+等添加剂的加入提高了样品的发光亮度.     

Time dependence of the luminescence excited by rectangular pulses

The theory of the time dependence of luminescence, during and after excitation by rectangular pulse, is discussed. Some phenomena, which cannot be described as direct excited luminescence, will be mentioned. In order to explain these phenomena, the model for energy transfer between two monomolecular centres has been used. On the basis of this model, the theoretical time dependence of the luminescence is obtained. The theoretical formula describe qualitatively very well the experimental observations in ZnSMn and other compounds.     

深能级杂质对光导半导体开关非线性特性的影响

建立了非线性GaAs光导开关深能级杂质瞬态模型的基本方程,获得了与实验现象定性吻合的电流输出,给出了平均载流子随时间演化的情况.分析结果表明,在考虑了深能级杂质的俘获、发射和碰撞电离后,有可能对非线性光导开关中发生的一系列现象做出解释,进一步的仔细分析将对非线性光导开关的设计和制作提供理论指导.     

Inhomogeneous Broadening of the Electronic Spectra of Laurdan

The excitation spectra and dependence of the luminescence spectra on the energy of excitation quanta were investigated for a laurdan molecule in glycerin. The most appreciable displacement of the emission spectra has been recorded for the most longwave luminescence band with a maximum in the region 500–510 nm. The relationship between the luminescence components of two emission bands with maxima at 425 and 500 nm depends strongly on the excitation energy. The dependences obtained are explained by the simultaneous existence of LE (localexcited) and TICT (twisted internal charge transfer) states and by the influence of the properties of the solvation sheath of laurdan, which invariably causes inhomogeneous broadening of the spectra of the polar solution molecules.     

Zum Lumineszenzmodell manganaktivierter Titanate

The emission spectra and the temperature dependence of the relative intensity of luminescence of three manganese activated titanate phosphors [Zn₂TiO₄, (Mg, Zn)₂TiO₄, Mg₂TiO₄] after optical excitation with radiation from 300–500 nm were measured for various temperatures above 4,2 °K. The luminescence of the Mn⁴⁺-centre is influenced by the cations (Mg²⁺ or Zn²⁺) of the lattice. The temperature dependence of the relative intensity of the luminescence depends on the excitation wavelength. The results are discussed and found to be in good agreement with those of aC-C-model which assumes two energy parabolas for the excited state.     

PbWO4闪烁晶体的发光动力学模型

在对PbWO₄闪烁晶体的光谱特性、发光衰减及其温度依赖以及热释光的研究基础上,并结合理论计算,提出了PbWO₄晶体发光的动力学模型,给出了PbWO₄晶体的基本能带结构及激子发光中心能态、陷阱能级在能隙中的位置。用此模型可以完整说明PbWO₄的发光过程,特别是导致室温下发光效率低的原因。最后还对其主发射成分蓝、绿发光中心的起源作了简要讨论。     

Visible nonlinear band-edge luminescence in ZnSe and CdS excited by a mid-infrared free-electron laser

Visible nonlinear band-edge luminescence in ZnSe and CdS bulk crystals was observed upon excitation by a mid-infrared free-electron laser (mid-IR FEL) at approximately 9 mm. The emission intensity is proportional to the 74th and 45th powers of the excitation intensity for ZnSe and CdS, respectively. For ZnSe, the temporal profile of the emission intensity does not follow the profile of the excitation macropulse of the FEL, but sharply rises and decays only at the maximum of the macropulse profile. These features are in marked contrast to those of a previous report, where the emission profile follows that of the macropulse, and the emission intensity scales with the 4th power of the excitation intensity. The experimental observations were reproduced by a numerical simulation based on impact ionization and avalanche ionization by electrons accelerated by the optical electric field of the FEL. The large nonlinearity in the bandedge emission comes from the macropulse temporal structure, which consists of micropulses densely spaced to allow excited carriers to survive when the next micropulse arrives. They work as seed carriers in the next carrier multiplication step.     

Nonlinear emission of semiconductor microcavities in the strong coupling regime

We report on the nonlinear laserlike emission from semiconductor microcavities in the strong coupling regime. Under resonant continuous wave excitation we observe a highly emissive state. The energy, dispersion, and spatial extent of this state is measured and is found to be dispersionless and spatially localized. This state coexists with luminescence that follows the usual cavity-polariton dispersion. It is attributed to the amplification of luminescence by a parametric gain due to cavity-polariton scattering. Despite the resonant excitation at 1.6 K, we observe no sign of Bose-Einstein condensation nor Boser action.     

Brighter glow in ZnS nanocrystals with polarized light

No known reports exist on luminescence enhancement under polarized light excitation. In this study, ZnS nanocrystals have been observed to produce brighter luminescence when excited by polarized light. ZnS:Mn bulk and nanocrystals have shown fivefold to tenfold increase in photoluminescence (PL) intensity when excited with linearly polarized light at 305 nm and 340 nm. Luminescence enhancement to a lesser degree was observed with linearly polarized light excitation for ZnS:Cu, Al and ZnS:Ag, Al nanocrystals. The observations suggest emission intensity dependence on the degree of anisotropy, which could be correlated mainly with the symmetry of the luminescence center and also to a lesser extent with nanoparticle shape asymmetry.     

Effects of solvent on nonlinear absorption properties of tetraphenylporphyrin compounds in the nanosecond regime

The nonlinear absorption properties of tetraphenylporphyrins (TPP) in different solvents and excitation intensities are investigated with nanosecond pulses by the Z-scan technique. The nonlinear absorption behaviour observed presents a distinct difference between in solvent mixtures and in pure solvents. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and then again to RSA is observed with the increases of excitation intensity in chloroform and pyridine mixed solvents. However, porphyrin molecules in pure solvents show good RSA behaviour under the studied intensity range, no saturation absorption is observed with nanosecond excitation. Excited state absorption parameters and two-photon absorption coefficients are obtained by theoretical fit using rate equations for population densities in a seven-level energy scheme.     

Gain mechanisms in field‐free InGaN layers grown on sapphire and bulk GaN substrate

The gain mechanisms and recombination dynamics of InGaN layers strongly depend on the structural properties of the substrate material. The 4.5 nm and 9.5 nm thick layers were grown by metal organic chemical vapor deposition on two different substrates (sapphire and GaN) with different dislocation densities. Time‐resolved photoluminescence spectroscopy at high excitation densities identifies the saturation of nonradiative recombination centers through excited carriers as a major gain mechanism. The prime argument is an unusual nonexponential luminescence decay. This was confirmed by a lower threshold of the optical gain for the structures grown on GaN with lower dislocation densities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Different processes responsible for blue pumped, ultraviolet and violet luminescence in high-concentrated Er:YAG and low-concentrated Er:YAP crystals

Ultraviolet and violet upconverted luminescence in high-concentrated (8.4 at%) Er³⁺:YAG and low-concentrated (1 at%) Er³⁺:YAP crystals has been investigated under blue laser excitation of the ⁴F_7/2 multiplet. The upconversion mechanisms were studied in detail based on upconversion luminescence intensity dependence and decay curves. Upconversion luminescence was attributed to energy transfer processes in Er³⁺:YAG and excited state absorption processes in Er³⁺:YAP, respectively.