Cooperative emission of dye molecules at high dephasing rates

The dependence of the emission of a concentrated (～10¹⁹ cm^?3 solution of Rhodamine C on the power density Φ of exciting laser radiation was studied. The emission intensity for the power density of exciting radiation above ～10²⁵ cm^?2 s^?1 was found to have a nearly quadratic dependence on the power density Φ, and this emission was interpreted as the cooperative spontaneous emission of a Dicke type. For Φ?10²⁵ cm^?2 s^?1, the emission intensity increased with increasing Φ according to the exponential law. This emission was interpreted as the amplified spontaneous emission. The spectra of cooperative emission depended on the pump radiation power only weakly. The absence of lasing in dye solutions at high concentrations, which is a well-known phenomenon, was shown to be caused by the development of the cooperative spontaneous emission and not by the concentration quenching, and the former process is more rapid than the latter.     

重频Yb:YAG片状激光器电光调$lt;i$gt;Q$lt;/i$gt;时空演化模拟计算和实验研究

基于Yb³⁺抽运动力学,结合光线追迹的方法,建立了抽运过程中的放大自发辐射模型,得到激光介质中三维含时储能分布.将速率方程理论和角谱传播理论结合,对谐振腔内调Q脉冲的形成、传播过程进行建模计算,得到激光脉冲的时间-空间分布和光束质量因子变化规律.同时进行了激光二极管抽运重频Yb：YAG片状激光器电光调Q实验,并与模拟计算的结果进行了对比校核,印证了计算模型的正确性.这为主动调Q固体激光器的设计提供了参考. 关键词： Yb：YAG激光器 Q')" href="#">调Q 时空演化     

Emission spectrum of concentrated dye solutions excited by high-power laser radiation

Emission spectra of concentrated (~10¹⁸–10¹⁹ cm^–3) ethanol solutions of Rhodamine C excited by high-power (~10²⁵–10²⁶ cm^–2s^–1) laser radiation were measured. The emission spectrum consists of two narrow bands (with half-widths of ~200–300 cm^–1). The long-wavelength band was interpreted as cooperative spontaneous emission, and the short-wavelength band was assigned to the amplified spontaneous emission. The ratio of their intensities depends on the dye concentration and the pump power density. The ratio of band intensities is assumed to be determined by the dephasing rate of excited molecules responsible for cooperative spontaneous emission.     

Parametric four-wave mixing in the 3d and 4d Rydberg states of NO

2 Σ⁺,H^′ 2Π^±(v^′=2), and 4dσ,πO²Σ⁺,O^′ 2Π^±(v^′=0) Rydberg states of NO molecules are described. The analyses of the two-photon excitation functions and infrared emission spectra revealed that only the ²Π^- component was involved in PFWM. This is in good accordance with the absence of amplified spontaneous emission (ASE) in the ²Σ⁺ and ²Π⁺ components due to their predissociative character. Results provide some support for the mechanism that the generation of the vacuum ultraviolet radiation was brought by PFWM in which ASE served as a third driving wave. Received: 6 January 1998     

Amplified spontaneous emission of F2− color center in LiF crystal

Intense, directional, and narrow-bandwidth 1130-nm emission was generated from LiF crystal at room temperature by the pumping with radiation in the wavelength region of the absorption band of the F₂ ⁻ color center. The emission was observed by the pumping with radiation of pulse energy above 5 mJ, and the divergence angle of the beam was about 16 mrad. This emission is attributed to the amplified spontaneous emission by the F₂ ⁻ center. Received: 8 April 1999 / Revised version: 18 May 1999 / Published online: 16 September 1999     

Parametric four-wave mixing in the H2Σ+,H′2Π(v′=0) states of NO

2 Σ⁺,H^′2Π(v^′=0)←X²Π(v^′′=0) two-photon transition of NO, both near-infrared and vacuum ultraviolet radiation were emitted along the laser propagation direction. The analyses of emission and excitation spectra revealed that the parametric four-wave mixing (PFWM) process coexisted with amplified spontaneous emission. Polarization properties of the IR radiation are found to be dependent on the rotational levels. Pressure and laser power behaviors of the generated waves were reported. The mechanism of PFWM was discussed in terms of selection rules of the relevant ro-vibronic transitions. Received: 19 September 1996/Revised version: 27 January 1997     

An approach to enhance the receiver sensitivity with SOA for optical communication systems

In this paper, we investigate an SOA (semiconductor optical amplifier) preamplifier structure by optimizing the carrier lifetime in order to reduce the amplified spontaneous emission (ASE) noise and crosstalk, with adequate gain increase. This proposed SOA optical preamplifier has no need of optical alignment and antireflection coating. This structure of SOA eliminates the need of optical filter, and exhibits large tolerance to the input light wavelength. The receiver sensitivity is investigated for single and multi channel transmission links. The received power of − 50.34 dBm is observed at bit error rate (BER) 10^− 12 for 10 Gb/s with PIN receiver. Further, the impact of gain, amplified spontaneous emission power and gain variation for different carrier lifetime with input power for OOK system is illustrated. The proposed SOA has constant gain of 30.06 dB up to gain saturation for carrier lifetime 0.18 ns. It is predicted that low value of carrier lifetime suffers less from ASE noise.     

Time-resolved photoluminescence on the stimulated emission in polycrystalline ZnO nanoparticles

The detailed room temperature stimulated emission including its optical characteristics from ZnO nanoparticles, which were prepared by a homogenous precipitation method, has been investigated by the time-resolved spectroscopy. The light emission originates from a free exciton recombination at a lower excitation level; the amplified spontaneous emission appears at a moderate excitation level, in which the threshold excitation intensity is 0.65 GW cm⁻². The resonant stimulated emission was observed in ZnO nanoparticles at a higher excitation intensity. Also, the emission lifetime is drastically reduced. Compared to the fluorescence decay curves, the time-resolved spectrum of the stimulated emission suggests the Gaussian-like decay time with only a few of picoseconds. The dynamic processes of lasing behavior and the characteristics of lasing emission in ZnO nanoparticles could provide the information on the crystal quality, the exciton and the lasing action in the particles.     

1313nm脉冲泵浦下掺Er3+光纤中多波长可见光的产生

本文报道了以1313nmNd:YLF激光脉冲泵浦掺饵(Er³⁺)单模GeO₂/SiO₂光纤而产生多波长可见光的实验,在该光纤中观察到(463～510)nm间的多波长放大自发辐射过程,此过程对应于Er³⁺离子的²G_7/2等几个激发态到⁴I_13/2亚稳态的跃迁.其激发过程是分步的四光子吸收过程,同时,掺饵光纤的红外发射峰1530nm(⁴I_13/2-⁴I_15/2)与1313nm泵浦光之间的三波混频过程也对这一区间的连续多峰的光谱结构有贡献.实验中还观察到540nm附近的绿光辐射.     

Suppression of amplified spontaneous emission in a femtosecond chirped-pulse amplifier

The suppression of amplified spontaneous emission (ASE) that arises during the femtosecond chirped-pulse amplification is presented. On the basis of the distinct differences in the spatial, temporal and spectral region between ASE and the amplified laser signal, the noise arising from ASE was effectively filtered out. The ratio between the amplified femtosecond pulse and the ASE peak power was higher than 10⁷. Pulses as short as 38 fs were amplified to peak power of 1.4 TW.     

Nonlinear emission in sodium vapour upon pulsed-laser excitation of the 42 P levels

Parametric four-wave mixing, stimulated electronic Raman scattering and amplified spontaneous emission cascades have been studied experimentally by single-photon resonant and near-resonant excitation of the 4² P levels of dense sodium vapour. The dependence of emission intensities on laser detuning and exciting laser power is discussed.     

Amplified spontaneous emission in neat films of arylene-vinylene polymers

Amplified spontaneous emission (travelling-wave lasing) was achieved for a set of 18 poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) related arylene-vinylene copolymers, a part of which substituted with additional phenyl groups at the vinyl double bond. Wave-guiding neat thin films on glass substrates were used. The samples were transversally pumped with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm). Travelling-wave emission occurred in the wavelength region between 485 and 650 nm. Repeat unit based absorption cross-section spectra, normalized fluorescence quantum distributions, and amplified spontaneous emission spectra are presented. Ground-state absorption cross-sections at the wavelengths of peak amplified emission are extracted from effective gain length measurements. Effective stimulated emission cross-sections are derived from pump pulse energy densities necessary for optical narrowing.     

High-gain erbium-doped fiber amplifier incorporating a double-pass amplification technique as a preamplifier

We present a high-gain erbium-doped fiber amplifier to be utilized as a preamplifier. A double-pass amplification technique is used in the first-stage amplifier together with a tunable bandpass filter. The secondstage amplifier is a counter-pumped configuration and another tunable bandpass filter is utilized to filter out amplified spontaneous emission from the first-stage amplifier. This design is able to produce a high gain of 55.6 dB and a noise figure of 6.02 dB at 1530 nm with a signal power of ?45 dBm. The receiver sensitivity measurement shows that the proposed amplifier improves the minimum detectable power from ?33.7 to ?40.8 dBm for a bit-error rate of 10^?11 at 155 Mbps.

     

碳纳米管阴极的强流脉冲发射性能研究

采用丝网印刷和涂敷方法制备了两种碳纳米管阴极，并研究了两种阴极的强流脉冲发射特性，表征了阴极表面碳纳米管的形貌及分布.研究结果表明在脉冲宽度为100 ns、电压为1.64×10⁶ V的脉冲电场下，涂敷法制备阴极的场发射电流最高达5.11 kA，最高发射电流密度达260 A/cm².丝网印刷法制备阴极的场发射稳定性优于涂敷法制备阴极，但其发射电流低.阴极表面发射体的形貌与分布影响了阴极的脉冲发射性能.碳纳米管阴极的脉冲发射机理为爆炸电子发射.碳纳米管阴极可以作为强 关键词： 碳纳米管 阴极 脉冲发射 强电流     

The Application of Mössbauer Emission Spectroscopy to Industrial Cobalt Based Fischer–Tropsch Catalysts

The application of Mössbauer emission spectroscopy to study cobalt based Fischer–Tropsch catalysts for the gas-to-liquids process was investigated. It was shown that Mössbauer emission spectroscopy could be used to study the oxidation of cobalt as a deactivation mechanism of high loading cobalt based Fischer–Tropsch catalysts. Oxidation was observed under conditions that are in contradiction with the bulk cobalt phase thermodynamics. This can be explained by oxidation of small cobalt crystallites or by surface oxidation. The formation of re-reducible Co³⁺ species was observed as well as the formation of irreducible Co³⁺ and Co²⁺ species that interact strongly with the alumina support. The formation of the different cobalt species depends on the oxidation conditions. Iron was used as a probe nuclide to investigate the cobalt catalyst preparation procedure. A high-pressure Mössbauer emission spectroscopy cell was designed and constructed, which creates the opportunity to study cobalt based Fischer–Tropsch catalysts under realistic synthesis conditions.     

Formation of the XeI excimer sustained by dielectric barrier discharge in Xe／I2 at low pressure

Dielectric barrier discharge is used to study the mechanism of XeI excimer formation in the mixture of Xe and I₂ at low pressures (<1330 Pa). Fluorescence emission in the spectral region of 200－260 nm is examined. We report on the characteristics of the 253 nm emission intensity which varied with different total pressure. The results indicate that under the present experimental conditions, electron impact is the major reaction producing the excimer XeI(B), interpreted as Xe⁺+I₂^-→XeI(B)+ I^*, then radiating 253 nm fluorescence from transition B→X. The 253 nm emission increases with the total gas pressure up to a maximum value at a pressure of about 540 Pa, then decreases as the gas pressure is further increased. The 206 nm emission is determined by I^* from ionic recombination between Xe⁺ and I₂^-. This result differs from previous works under other experimental conditions.     

Emission mechanism of radiophotoluminescence in Ag-doped phosphate glass

The objective of this study is to investigate the emission mechanism of radiophotoluminescence (RPL) in the Ag⁺-doped phosphate glass (glass dosimeter), which is now used as individual radiation dosimeter, because the emission mechanism of RPL in glass dosimeter was not fully understood. Optical properties such as optical absorption spectrum, RPL spectrum and change of RPL spectrum as a function of X-ray irradiation dose were measured for commercially available glass dosimeter. In this study, we discuss the emission mechanism of two RPL peaks at 460 nm and 560 nm, based on the fact that electrons and holes produced by X-ray irradiation are trapped at Ag⁺ ions to produce Ag⁰ and Ag²⁺ ions, respectively, when the Ag⁺-doped phosphate glass is exposed to X-ray. We would like to propose the emission mechanism of RPL peaks at 460 nm and 560 nm, concerning with Ag²⁺ and Ag⁰ ions.     

反铁电陶瓷的强电子发射特性研究

采用掺镧锆锡钛酸铅反铁电陶瓷作为阴极材料,研究了脉冲电压激励下陶瓷的电子发射特性.当激励电压为800V、抽取电压为0V时,得到1.27A/cm²的发射电流密度;当抽取电压增加到4kV时,获得1700A/cm²的发射电流密度.分析了发射电流随抽取电压的变化关系,讨论了反铁电陶瓷强电子发射的内在机理.结果表明：掺镧锆锡钛酸铅反铁电陶瓷能够在较低的激励电压(400V)下实现电子发射,发射电流远大于按照Child-Langmuir定律计算出的电流,三接点附近局域反铁电—铁电相变产生初始电子发射,初始电子电离中性粒子形成等离子体,增强了电子发射. 关键词： 铁电阴极 反铁电体 电子发射     

<Emphasis Type="Italic">T</Emphasis>-odd angular correlations in the emission of prompt gamma rays and neutrons in nuclear fission induced by polarized neutrons

Study of the T-odd three-vector correlation in the emission of prompt neutrons from ²³⁵U fission by polarized cold neutrons has been continued at the facility MEPHISTO of the FRM II reactor (Technical University of Munich). The sought correlation was not found within experimental error of 2.3 × 10⁻⁵. The upper limit for the asymmetry coefficient has been set to |D _n | < 6 × 10⁻⁵ at 99% confidence level, whereas for ternary fission correlation coefficient D _α = (170±20) × 10⁻⁵. This limit casts doubt on a model that explains the three-vector correlation in ternary fission by the Coriolis mechanism. At the same time, five-vector correlation in the emission of prompt fission neutrons has been measured, which describes the rotation of the fissioning nucleus at the moment it breaks (ROT effect). At the angle 22.5° to the fission axis, the correlation coefficient was found to be (1.57 ± 0.20) × 10⁻⁴, while at the angle of 67.5° it is zero within the experimental uncertainty. The existence of ROT effect in the emission of prompt fission neutrons can be explained by the anisotropy of neutron emission in the rest frame of the fragment (fission fragments are aligned with respect to the axis of deformation of the fissioning nucleus), similar to the mechanism of ROT effect in the emission of prompt γ-rays.     

Theoretical description of the towing mode through a time-dependent quantum calculation

The target emission for the reaction of ⁵⁸Ni and ⁵⁸Ni at 44 MeV/A is investigated in a time-dependent quantum model. We show that, besides the well-known mechanisms of particle emission, a new phenomenon which gives rise to a specific angular and energetic distribution of the particle emission, is also expected. Properties of this mechanism are similar to the so-called “towing mode”, recently observed experimentally in a similar reaction (J.A. Scarpaci et al., Phys. Lett. B 428 (1998) 241). Characteristics of this new emission type are described.