Optical limiting and pulse reshaping of picosecond pulse trains by fullerene C60

We present a dynamical theory of nonlinear absorption and propagation of a laser pulse train that contains 20 subpulses with an individual pulse width of 100 ps. It is shown that the accumulative nonlinearity and the reverse saturation absorption play important roles in the optical limiting performance and pulse shaping. When the incident field is not too strong, the population transfer reveals a slow response process, and the periodic sequence of short light pulses can be regarded as a continuous long pulse. The general theory is applied to fullerence C₆₀, which is a popular reverse saturable absorption material and a good limiter because of its larger excited-state absorption cross-section compared with that of the ground state. The propagation of the front subpulses is mainly affected by the linear transition between the ground state and the first excited singlet state, while the latter subpulses are attenuated by the excited-state absorption. Moreover, these two different kinds of absorption mechanisms result in different radial distributions for different subpulses. The pulse propagation is studied by solving numerically the coupled rate equations and the propagation equation of the optical pulse intensity, using experimental parameters as input. We suggest a new method to measure the lifetime of the triplet state.     

Optical operation of ultracold atomic quasi-clusters

We report several exact solutions of a two-dimensional (2D) Gross-Pitaevskii equation with an optical lattice potential, which describe the motion of an array of ultracold atomic quasi-clusters in a Bose-Einstein condensate. The velocity of the atomic quasi-clusters can be controlled by adjusting the optical potential strength so that one can stop or drive them by the optical brake. The atomic quasi-clusters form a superfluid for the propagation state or a critical insulator for the non-propagation one, and the brake and drive are associated with the quantum phase transitions between the insulator and superfluid.Received: 3 February 2003PACS: 03.75.-b Matter waves - 05.70.Jk Critical point phenomena - 05.30.Jp Boson systems - 67.90. + z Other topics in quantum fluids and solids; liquid and solid helium     

Numerical simulation of melting and evaporation of a cold foil target irradiated by a pre-pulse

Melting and evaporation of matter due to pre-pulse irradiation of a high-peak-power ultra-short pulse laser onto a cold foil target and the expansion of laser-produced plasma are numerically evaluated using a hydrodynamic code based on CIP (cubic-interpolated propagation) and modified C-CUP (CIP-combined unified procedure) methods. The material properties of the solid, equation of state, laser absorption coefficient, skin depth, and thermal conductivity are consistently implemented. The formation and propagation of laser-produced plasmas are obtained with good numerical stability. PACS 02.70.-c; 52.38.Mf; 52.38.Kd     

Highly efficient inelastic four-wave mixing using dual induced transparency and coherently prepared states

We investigate a life time broadened and coherently prepared five-state system for multi-wave mixing processes. We show that very efficient wave mixing occurs, producing an unconventional mixing wave that has the characteristics of both conventional four-wave mixing (FWM) and stimulated hyper-Raman (SHR) emission. In addition, we show interesting multiple simultaneous multi-photon interference effects at large propagation distances and demonstrate more than 10 orders of magnitude suppression of populations of the probe wave terminal state and the near three-photon resonance mixing wave generating state. These new type of multi-photon interference based induced transparency effects, which are critically dependent on two distinctive relaxation processes involving both an external supplied and an internally generated fields, are fundamentally different from the conventional three-state electromagnetically induced transparency effect which does not depend on propagation. As a consequence, both the probe and the wave-mixing field to propagate nearly free of absorption and distortions in a highly dispersive medium.     

均匀展宽增益介质中超光速饱和现象的研究

根据饱和增益理论分析可知,不同抽运光功率对应着介质不同的吸收和增益状态. 在吸收介质中,相干布居振荡效应导致光脉冲经历饱和吸收,光脉冲传输延迟;在增益介质中, 相干布居振荡效应导致光脉冲经历增益饱和,光脉冲传输超前. 本文由铒离子的亚稳态速率方程出发,建立了掺铒光纤中超光速传输的理论模型, 同时得到时间超前的数值解析表达式;讨论了低频率段超光速的饱和现象, 即超光速在低频段并不随着抽运光功率的增强而加强,而在高频率段随抽运光功率的增加而加强.     

Acoustic boundary layer influence on scale model simulation of sound propagation: Theory and numerical examples

Scale model simulation of sound propagation above a solid surface will give a systematic and calculable error in the predicted sound field, because the acoustic boundary layer above the surface has an apparent admittance which is not invariant under scaling. The typical error is approximately 5 dB depending on the geometrical configuration, scale and frequency. The effect of the acoustic boundary layer admittance is negligible for sound propagation above an acoustically soft surface (e.g., grassland). One may moreover note, with reference to scale model simulation of concert hall acoustics, that the absorption coefficient of a solid surface increases with frequency.     

Spectroscopy of highly luminescent 1,3,5-triazapentadiene complexes of platinum(II) in solution and in the adsorbed and solid states and quantum-chemical interpretation

We have measured the absorption, luminescence, and luminescence excitation spectra, as well as the excited-state lifetimes and luminescence quantum yields, of 1,3,5-triazapentadiene complexes of platinum(II) in a solution, in the solid state, in an adsorbed state on a SiO₂ surface, and in a polystyrene matrix at room temperature and at 77 K. We also have performed quantum-chemical calculations of the equilibrium geometry of the ground and excited states of the complexes and of the nature and structure of molecular orbitals.     

4-四氢喹啉酮的合成及发光性能

对喹吖啶酮结构改造合成了新型结构荧光体4-四氢喹啉酮,对其吸收光谱及荧光光谱进行了研究,发现该化合物的吸收峰及荧光发射峰随溶剂极性的增加而不断红移,表明荧光来源于分子内激发态电荷转移发光.该化合物在固态即显示荧光,表明经结构改造后,消除了喹吖啶酮类的固态荧光猝灭效应.4-四氢喹啉酮结构简单,易于合成和结构修饰.这对于寻找新的主体荧光发射材料具有重大意义.     

The Role of Intermolecular Interactions in Solid State Fluorescent Conjugated Polymer Chemosensors

A functionalized fluorescent conjugated polymer, tolylterpyridine poly(p-phenyleneethynylene-thienyleneethynylene (ttp-PPETE), was designed and synthesized to detect trace amounts of toxic transition metal pollutants in ground water. Photophysical studies in tetrahydrofuran (THF) successfully demonstrated this polymer as a selective and sensitive chemosensor for Ni²⁺ and Co²⁺ in aqueous solution. Solid state composites of these chemosensors have now been prepared which can be modified to provide for inexpensive and portable field based chemical detection. A solid composite of ttp-PPETE, blended with poly (methyl methacrylate) shows UV–vis absorption and fluorescence emission spectra which are red- shifted when compared to solution phase spectra, suggesting an increase in conjugation in the solid state. An additional absorption peak, not present in solution, is also observed in the solid state. The presence of this new peak provides evidence of interacting FCP chains in the solid state. Concentration dependent experiments were done on the solid composite showing red-shifted emission peaks accompanied by a significant reduction in the fluorescent quantum yield. These observations are consistent with the formation of aggregated polymer species in the solid state. Intermolecular interactions of this type can be manipulated in the design of sensitive and selective solid state fluorescent conjugated polymer sensors.     

溶液相变对β胡萝卜素溶质的紫外-可见吸收和共振拉曼光谱的影响

测量了β-胡萝卜素溶解在环己醇中65~2℃范围内的紫外-可见吸收与共振拉曼光谱。溶液在20℃时转为固态。随着温度的降低,相变时吸收光谱产生较大蓝移。虽然相变后吸收光谱、拉曼光谱仍红移,拉曼散射截面仍增加,拉曼谱带线宽仍减小,但相变后的光谱随温度的变化却大幅增加。这主要是由于固相中的β胡萝卜素分子的π电子能隙对碳碳键振动调制作用增强所致。文中对这些物理现象给予了解释。     

Nondispersive electromagnetic beams in plasmas

We prove that different modes of nondispersive electromagnetic beams can propagate in a stationary isotropic plasma. But, a stationary plasma in a uniform magnetic field may only support a mode at frequencies less than the angular cyclotron frequency.Received: 12 June 2003, Published online: 9 September 2003PACS: 43.20.Bi Mathematical theory of wave propagation - 41.20.Jb Electromagnetic wave propagation; radiowave propagation - 52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)     

Finite size effects on structure and magnetism in mass-selected CoPt nanoparticles

In this paper, the one-way absorption property in one-dimensional dielectric/metal photonic crystal structure with a dielectric defect layer is studied. The effects of incident angle and state of polarization on one-way absorption behaviour of the anti-resonant (AR) mode are investigated. The normally incident wave from left to right propagation is totally allowed to penetrate to the structure but right to left propagation totally reflected at the same wavelength. It is found that, with increasing of the incident angle, the AR mode shifts to the lower wavelengths and its intensity decreases. Simultaneously, another AR mode with reversed one-way property appears at higher wavelength. The one-way behaviour on absorption is observed at the both states of polarizations but is localized on different wavelengths. Those effects, are suggesting that the proposed structure can be used as a direction sense polarization splitter or reflector/antireflector device.     

强非线性吸收下Z扫描衍射模型及实验研究

根据光的衍射理论,建立了非线性光学介质对高斯光束的开孔Z 扫描理论模型.数值计算表明,当相移比值Γ≤0.15,出射光功率随入射光功率的增加出现振荡衰减现象;当Γ>0.15,在输入光强低于某一个值时,输出随输入的增加而线性增加;当输入光强超过该值后,输出功率不再增加甚至趋于零.实验采用Nd∶YAG脉冲激光,在波长为532 nm,脉宽8 ns条件下,对含3.5%和7.0%碳纳米管的聚苯胺/碳纳米管复合体薄膜（PANI/MWCNT）的非线性吸收特性进行了研究.计算表明,该类化合物的激发态吸收截面积远大于其基态吸收截面积,具有很强的反饱和吸收.     

Doppler-free spectroscopy in driven three-level systems

We demonstrate two techniques for studying the features of three-level systems driven by two lasers (called control and probe), when the transitions are Doppler broadened as in room-temperature vapor. For -type systems, the probe laser is split to produce a counter-propagating pump beam that saturates the transition for the zero-velocity atoms. Probe transmission then shows Doppler-free peaks which can even have sub-natural linewidth. For V-type systems, the transmission of the control beam is detected as the probe laser is scanned. The signal shows Doppler-free peaks when the probe laser is resonant with transitions for the zero-velocity group. Both techniques greatly simplify the study of three-level systems since theoretical predictions can be directly compared without complications from Doppler broadening and the presence of multiple hyperfine levels in the spectrum.Received: 10 September 2003, Published online: 6 January 2004PACS: 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 42.50.-p Quantum optics     

Finite difference method calculations of X-ray absorption fine structure for copper

The finite difference method is extended to calculate X-ray absorption fine structure (XAFS) for solid state copper. These extensions include the incorporation of a Monte Carlo frozen phonon technique to simulate the effect of thermal vibrations under a correlated Debye–Waller model, and the inclusion of broadening effects from inelastic processes. Spectra are obtained over an energy range in excess of 300 eV above the K absorption edge—more than twice the greatest energy range previously reported for a solid state calculation using this method. We find this method is highly sensitive to values of the photoelectron inelastic mean free path, allowing us to probe the accuracy of current models of this parameter, particularly at low energies. We therefore find that experimental data for the photoelectron inelastic mean free path can be obtained by this method. Our results compare favourably with high precision measurements of the X-ray mass attenuation coefficient for copper, reaching agreement to within 3%, and improving previous results using the finite difference method by an order of magnitude.     

Optical limiting properties of zinc phthalocyanines in solution and solid PMMA composite films

The nonlinear absorption and optical limiting (OL) performance of tetra- and octasubstituted zinc phthalocyanine complexes were described in solution and in the solid state using the open-aperture Z-scan technique. The measurements were performed using collimated 4 ns pulses generated from a frequency-doubled Nd:YAG laser at 532 nm wavelength. The polymeric films exhibit a much larger effective nonlinear absorption coefficient in comparison with solution. However, the parameters of the ratio of the excited to ground state absorption cross section and energy-dependent saturation in solution are much better compared to properties in the polymeric film. In terms of the ratio of the excited to ground state absorption cross section, the peripherally substituted complexes show better OL performance than the non-peripherally substituted derivative.     

不同表压下芪盐单层LB膜二次谐波的研究

在不同表压下制备芪盐的单层LB薄膜,测量LB膜的二次谐波信号.在低表压得到增强的二次谐波信号.对比芪盐的透射光谱,认为在LB膜中,芪盐以固相态存在,低表压时的固相态聚集体的吸收峰351nm使四阶非线性极化率x(4)(-2ω;ω,ω,ω,-ω)产生共振增强,四阶光学非线性现象对二次谐波信号产生影响.     

Two-photon absorption of optical and x-ray quanta

Two-photon absorption of an optical (laser) and an x-ray quantum is considered within the context of the Born approximation. A simple expression relating the two-photon and one-photon photoionization absorption coefficients is obtained. If the upper state is one of the conduction bands of a solid, the two-photon absorption coefficient is shown to peak at points in the zone for which the curvature of the band is a maximum. Numerical estimates show that with a 100 kW Nd laser, the intensity of the two-photon absorption process is a few percent of that of the one-photon process.