Auditory detection of paired pulses by a dolphin in the presence of a pulse jam

From behavioral studies of a bottlenose dolphin (Tursiops truncatus), the audibility thresholds were measured for a single pair of equal-amplitude pulses, i.e., clicks, presented to the dolphin in combination with a pulse jam. The pulse jam consisted of pairs of identical pulses with a pulse spacing τ_j within the pairs and a pair repetition rate f _j. Series of pulses were interrupted by a pause R>1/f _j, within which the pulse jam was absent while a pair of test pulses was supplied to one of the two channels at random. Each series had a duration T, and the total stimulation cycle was J=T+R. The dependence of the test pair detection threshold on the pulse spacing τ_j was studied at different fixed values of the pulse spacing in the test pair: τ_t=50, 100, 200, and 500 µs. Preliminary measurements performed with τ_j=τ_t=100 µs were used to adjust the parameters of the pulse jam. The threshold shift at τ_j=τ_t=100 µs reached 35 dB above the audibility threshold of the test pair in the absence of the pulse jam. On both sides of the point τ_j=τ_t=100 µs the thresholds decreased with varying τ_j to approximately 20 dB above the detection threshold of the test pair in the absence of the jam. However, in the course of training, the threshold curves gradually shifted downwards approaching the detection level of the test pair in the absence of the jam and becoming progressively flatter (the selectivity with respect to the pulse jam vanished). A decrease in the pause duration R restored the dependence of the test pair detection threshold on τ_j. In this case, a statistically significant maximum was obtained at τ_j=τ_t for τ_j within the critical interval (for τ_t<500 µs). Beyond the critical interval (for τ_t>500 µs), even with the smallest pause duration (R=15 ms), no dependence of the test pair detection thresholds on τ_j could be observed.     

Influence of the shape of the exciting laser pulse on fluorescence saturation in the quantitative analysis of dissolved trace organic substances

Theoretical investigations of the influence of the exciting laser pulse shape on fluorescence saturation in the quantitative analysis of a single-component fluorescing solution, when the Raman signal of the solvent is usedas a reference, are carried out. A quantity α is introduced, responsible for the influence of laser pulse shape on the precision of the analysis. The dependences of α on the exciting radiation photon flux density I_m in the range I_m = 10²² ? 10²⁸ photons/cm²?s and on the duration of the exciting pulses τ_p in the range τ_p = 0.6 ? 60 ns are computed for different pulse shapes in the case of rhodamine 6G dissolved in water. Conditions when α is, in practice, independent of τ_p and of the pulse shape are found. A general analytical expression is found for α, which produces errors not greater than several percent when compared to the computed values of α. An experimental verification of the theoretical results in the case of a rectangular-shaped pulse in the spatial domain and a Gaussian-shaped pulse in the temporal domain is realized.     

Numerical studies on wakefield excited by Gaussian-like microwave pulse in a plasma filled waveguide

Based on a differential equation derived analytically in terms of wakefield potential ?_MW in a plasma filled rectangular waveguide, we investigate the wakefield (E_MW) generated with the help of Gaussian-like microwave pulse under the effect of microwave frequency (f), pulse duration (τ), waveguide width (b), equilibrium plasma density (n₀) and microwave intensity (I). The study conducted for three cases of τ > 1/f_p, τ = 1/f_p and τ < 1/f_p, where f_p is the plasma frequency, reveals that the amplitude of the wakefield is increased for the large pulse duration and higher microwave intensity but is decreased with the waveguide width and microwave frequency for all these cases. The wakefield shows stronger dependence on the microwave frequency when the microwave with larger intensity is used. The wakefield decreases at a faster rate with the waveguide width for the case of τ > 1/f_p.     

Transient Raman gain stimulated by a noise pump

We predict a strong transient Raman gain stimulated by a Gaussian noise field in dispersive media. Our theory is based on the method of successive approximations, the first of which is the Markovian approximation. It is shown that the transient behavior will take place near critical pump intensity both for “short” (τ_p > T₂) and “ultrashort” (τ_p < T₂) laser pulses (τ_p is the pump pulse duration, T₂ is the relaxation time of optical phonons). Solutions for average intensities of the Stokes wave and optical phonons are obtained for arbitrary form of the given pump pulse. We show that transient Raman gain allows to form a very steep leading edge of the Stokes pulse.The steady-state increments of the second approximation describe the effects of a strong noise field. We show that after the intensity of the pump has exceeded a value beyond which the framework of validity of the Markovian approximation does not hold anymore, the steady-state Raman gain is saturated.We give a review of experimental results and make estimations for conditions of observing transient scattering in the field of a noise pump. Methods of obtaining the powerful femtosecond pulses based on stimulated Raman scattering of broad-band excimer lasers and supercontinuum emission are suggested.     

一维复合势垒中的Larmor钟

利用Larmor钟方法计算了一维复合势垒系统中的隧穿时间．从计算结果，对由Larmor钟所定义的Larmor时间在复合势垒中的有效性、可加性，以及它们与透射时间τ_T、反射时间τ_R、逗留时间τ_D的关系作了讨论．6个Larmor时间中，τ_xT,τ_xR,τ_yT和τ_yR都不是有效的时间量，它们不能为隧穿时间提供有效的描述，只有τ_yT和τ_yR满足可加性和判据τ_D＝Tτ_yT+Rτ_yR，它们最有可能给出透射时间τ_T、反射时间τ_R．τ_yT,τ_yR与势垒的对称性有关，等式τ_yT＝τ_yR是否成立与势垒是否对称有关． 关键词：     

Laser Ablation of Metals by Low-Density Picosecond Pulses

Thresholds of laser ablation ofmetals for the picosecond range of laser pulse duration are determined. Within the two-temperature metal model, the space-time dependences of the electron and lattice temperature are obtained. It is shown that ablation parameters become dependent on the pulse duration at the pulse duration τ _p ≥ τ _ie , where τ _ie is the cooling time of the electron gas. For noble metals, such pulse durations are τ _p ≈ 3 ? 10 ps.     

Dislocation dynamics in pulse-loaded NaCl crystals

The mean path l of freshly introduced dislocations in NaCl crystals under the effect of triangular loading pulses τ(t) is shown to depend only on the pulse amplitude τ_m and be insensitive to the rate of stress growth. The replacement of triangular pulses with trapezoidal ones with a constant-load plateau (τ=const) extension of up to 60 min only insignificantly changes the l(τ_m) dependence. The data obtained are interpreted on the basis of the concept of quasi-static relaxation in a nonequilibrium system of dislocations subject to a combined effect of time-dependent applied stresses τ(t), coordinate-dependent internal stresses τ_i(x), and “dry friction” τ_p due to the pinning of dislocations by point defects. In such a model, the l(τ_m) dependence should saturate at τ_m<2τ_p; this is in fact observed in the range of 0.2τ_m<τ_m<0.3τ_y (here, τ_y is the yield stress), which gives an estimate for the pinning stress τ_p≈0.1τ_y. Based on the model suggested, a series of experimentally confirmed predictions were obtained, e.g., a recipe of “switching-off” of anomalies. Thus, a preliminary treatment of the samples by a series of stress pulses or holding in a magnetic field, which transforms the system of fresh dislocations into a more equilibrium state, sharply decreases the density of mobile dislocations quasi-statically responding to a pulsed load. It is shown that the discussed anomalies of dislocation mobility should be observed only in sufficiently pure crystals, where , and should be absent in contaminated crystals, where τ_p ～ τ_y.     

Isospin violation in τ → 3πντ

Isospin violating signals in the τ^? → (3π)^?ν_τ decay mode are discussed. For the τ^? → π^?π^?π⁺ν_τ decay mode, isospin violation arises from the vector current contribution in the τ^? → ωπ^?ν_τ decay with the subsequent isospin violating ω decay into π⁺π^?. We demonstrate that such effects may be observed in presently available data through the measurement of the interference effects of these vector current contributions with the dominating axial vector current,i. e. through a measurement of the structure functionsW _F, WG, WH andW _I. In the case of the τ^? → π⁰π⁰π^?ν_τ decay mode, a vector current contribution is generated by ηπ⁰ mixing in the decay chain τ^? → η?^?ν_τ → π⁰π⁰π^?ν_τ. We find that this effect is rather small, the magnitude of the associated interference terms being too low for present statistics.     

Upconversion-pumped population kinetics for4I13/2 and4I11/2 laser states of Er3+ ion in several host crystals

Population kinetics of the upper⁴I_11/2 and lower⁴I_13/2 laser states of the Er³⁺ ion were studied experimentally and theoretically in (Er)BaY₂F₈, (Er)YLF, (Er)YSGG, (Er)CaF₂ and (Er)YALO. Fluorescence from these states to the⁴I_15/2 ground state was excited through upconversion simultaneously with the⁴I_11/2 →⁴I_13/2 lasing using 1.53 µm radiation from an erbium : glass laser for optical pumping. Lifetimes of both states are altered during lasing by co-operative energy transfer processes: the lifetime of the lower state τ₁ is shortened and that of the upper state τ₂ increased with the resultant ratio τ₂/τ₁>1. After lasing the lifetime ratio returns to the ‘normal’ value τ₂/τ₁ <1; that is, one obtained under weak ultraviolet excitation. Kinetic rate equations for the population density functions for both laser states were set up and solved by approximation in three time domains. It was assumed that only one co-operative energy transfer process operates in the laser crystals and determines the population inversion kinetics. Consistency relationships for comparison of the theoretical results with the experiment were developed. Only (Er)BaY₂F₈ spectral features showed close agreement with theory, resulting in a high score of 94% for the overall correlation in the consistency test, whereas all other crystals scored <50%. As a result of this high correlation, a close match between theoretical and experimental population decay curves was shown for (Er)BaY₂F₈. Most probably, more than one energy transfer process shapes the decay curves and determines the population inversion kinetics for the other laser crystals. (Er)YALO showed little lifetime change for the laser states, apparently due to inefficient co-operative energy transfer processes. As a result it probably lased in a self-terminating short-pulse mode.     

用超短光脉冲简并四波混频测量介质弛豫率的研究

本文讨论用ps光脉冲简并四波混频测量非线性介质弛豫率和在适度或强的相位调制情况下测量激光脉冲宽度的方法,通过用计算机模拟及与实验结果比较,得出结论:对t_p>>τ_r和t_p<<τ_r两种极限情况,该方法是足够精确的.对τ_r/t_p的中间情况,该方法只能给出一个估计.     

高次谐波辐射发射特性研究

报告由不同脉冲宽度(半高宽,FWHM)和不同载波-包络相位(CEP,Φ)的激光产生的高次谐波辐射能量输出时间特性即发射特性的研究结果. 计算表明,由宽度为几个周期的激光产生的高次谐波辐射的截止能量明显低于由无限长脉冲宽度激光产生的截止能量ω_max=3.17U_p+I_p(其中ω_max为光子角频率,U_p和I_p分别为激光有质动力势和原子的电离能). 例如,由两周期(FWHM),Φ＝15°的激光产生的高次谐波辐射的截止能量为ω_max=2.90U_p+I_p,此时发射特性单脉冲(即分布单脉冲)具有最大的能量带宽0.86U_p. 脉冲中心位置的载波相位和时间宽度分别为0.94rad(弧度)和1.29rad. 而该激光脉冲在Φ＝-75°时能产生截止能量为ω_max=2.70U_p+I_p,最大能量带宽为0.70U_p的双分布脉冲,其中心位置分别为-0.58rad和2.43rad,宽度分别为1.22rad和1.33rad. 随着激光脉冲宽度的增加,分布单脉冲的能量带宽比时间宽度下降得更快. 对于一定宽度的激光脉冲,所产生的分布单脉冲的能量带宽和时间宽度的CEP依赖性显示出180°的周期结构. 利用这个有趣的特点,在实验上可以通过调节CEP来选择分布脉冲的能量参数,也可用来定位和控制阿秒脉冲的时间参数. 理论分析指出,只要选择合适的阿秒X射线能量带宽,CEP不稳定性对于光电子谱和测量结果的影响将大为降低,甚至在最大程度上消除这种影响. 这些研究结果不仅有助于在物理上深入了解高次谐波辐射的动力学过程,而且对于进一步在实验上优化和选择阿秒单脉冲和双脉冲具有重要的参考和指导意义. 关键词： 高次谐波产生 鞍点方法 谐波发射特性 分布脉冲     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

高能电子与超强激光束作用产生的阿秒脉冲列

利用非线性汤姆孙散射的理论，从理论和数值模拟上研究了单电子在横向穿越高斯激光束束 腰时所辐射的x射线阿秒脉冲列的性质. 主要分析了电子以初始能量γ₀=1M eV—100M eV横向穿越激光振幅参数为a₀=1—10的高斯光束束腰获得的阿秒辐射脉冲的 时间 和空间性质. 计算表明，辐射呈现脉冲列的形式. 脉冲列的包络宽度取决于激光强度、束腰 的宽度以及入射电子能量. 电子的初始能量比激光强度对电子辐射脉冲的影响更大. 辐射脉 宽、脉冲间隔和脉冲包络宽度都正比于1/γ²₀，辐射功率正比于 γ⁶₀，辐射能 量正比于γ⁴₀. 当改变激光振幅a₀时，辐射功率正比 于a²₀、辐射包络中单 个脉冲脉宽正比于1/a₀、脉冲之间的间隔正比于a₀. 当保持激光强 度不变，而改变光束 束腰半径w₀时，辐射的脉冲数量、包络和辐射能量正比于w₀. 当 激光功率保 持不变时而改变激光强度和束腰半径时，脉冲包络宽度和最大辐射能量都基本不变. 当激光 振幅参数a₀=1，电子初始能量为10MeV时，激光束腰为两个激光波长时，电子 辐 射脉冲包络宽度只有14×10^-3τ₀（τ₀为入 射激光周期），达到几个阿秒的量级. 关键词： 阿秒脉冲 非线性汤姆孙散射 高斯激光光束     

Thermal diffusivities of group V semimetals

A flash method, employing a pulsed flashlamp-pumped dye laser has been used to determine the thermal diffusivities of elemental arsenic, antimony, and bismuth. Measurements were made both on poly-crystalline samples and on oriented single crystal (111) wafers in the form of small disks 3.0?7.0 mm in diameter, and 0.43–1.33 mm thick. Thermocouples (0.076 mm dia) monitor the back surface temperature as a function of time after the front of the sample is exposed to a single laser pulse of pulse width (τ) <1.0 μsec. The room temperature thermal diffusivities α (cm²/sec) of polycrystalline As, Sb and Bi are respectively: 0.234, 0.182 and 0.0651. The diffusivities parallel to the trigonal crystal axis α₁ are: 0.119,0.142 and 0.0350. Measurements on polycrystalline Cu, Al and Ti yield 1.13, 0.96 and 0.085 cm²/sec, in agreement with literature values.     

Tau hadronic branching ratios

From 64492 selected τ-pair events, produced at the Z⁰ resonance, the measurement of the tau decays into hadrons from a global analysis using 1991, 1992 and 1993 ALEPH data is presented. Special emphasis is given to the reconstruction of photons and π⁰s, and the removal of fake photons. A detailed study of the systematics entering the π⁰ reconstruction is also given. A complete and consistent set of tau hadronic branching ratios is presented for 18 exclusive modes. Most measurements are more precise than the present world average. The new level of precision reached allows a stringent test τ-μ, universality in hadronic decays, ${cal G}_{?u}/{cal G}_{?}=1.0013pm 0.0095$, and the first measurement of the vector and axial-vector contributions to the non-strange hadronic τ decay width: R_τ,V = 1.788 ± 0 0.025 and R_τ,A = 1.694 ± 0.027. The ratio (R_τ,V ? R_τ,A)/(R_τ,V + R_τ,A), equal to (2.7 ± 1.3) %, is a measure of the importance of QCD non-perturbative contributions to the hadronic τ decay width.     

Spin of the τ-neutrino and suppression of the decay-mode τ → ντπ

It is pointed out that a zeromass spin $\text{3}\text{2}\text{τ}$-neutrino naturally suppresses τ → ν_τπ and is not in contradiction with known data.     

Threshold dependence of the vibrational excitation of molecules on laser radiation intensity

The collisionless vibrational excitation of a polyatomic molecule in an IR laser radiation field has been theoretically studied. It has been shown that (i) the degree of vibrational excitation (namely, number 0000 of vibrational quanta of a molecular mode near-resonant with the IR laser field that are absorbed by the molecule) is low if laser pulse intensity P (energy flux density in the laser beam) is lower than a certain critical value P _cr; (ii) the degree of excitation abruptly increases after crossing the boundary where P = P _cr; (iii) this effect is attributed to two properties inherent in polyatomic molecules, namely, the anharmonicity of the vibrational mode interacting with the laser field and the energy exchange with other modes; and (iv) at P > P _cr, number 00000 is determined only by energy density Φ = Pτ_P, where τ_P is the laser pulse duration, 00000 monotonically increases with increasing Φ. The model is in good agreement with the experimental data.     

Two-photon pumping of random lasers by picosecond and nanosecond lasers

We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency. PACS 33.50.Dq; 42.55.Mv; 42.55.Zz     

Theory of synchronously pumped dye lasers

An analysis of cw synchronously pumped dye lasers is presented. Under the assumption that the cavity (tuning element) bandwidth is much wider than the bandwidth of the transform limited pulses generated, the pulse forming dynamics is rigorously treated. It is shown that for a finite mismatch between the lengths of the dye and the pump lasers, a steady-state pulse develops in the dye laser cavity with a conserved pulseshape. The characteristics (energy, shape, peak power, duration) of these pulses of ultimate width are quantitatively determined as a function of cavity mismatch. An analytical solution for the pulse envelope is determined, which yieldsI(t)∝Sech²(t/t _p ) to a good approximation.     

Efficient lasing action from Rhodamine-110 (Rh-110) impregnated sol-gel silica samples prepared by dip method

Rhodamine-110/sol-gel samples are prepared by sol-gel technique using dip method. Concentration dependent photophysical studies of these samples have indicated about the least possibility of aggregate formation. The lasing action of Rh-110 in silica samples is studied as a function of dye concentration. An efficient laser emission is observed when the samples are transversely pumped at 337.1 nm and 1.5 Hz repetition rate using a nitrogen laser (400 μJ energy/pulse and 4 ns pulse duration). The maximum of 166% laser efficiency of dye doped sol-gel samples compared to Rhodamine-6G (Rh-6G) in methanol is achieved. The photostability is also measured by using N₂ laser at 1 Hz and it is found nearly 165 pulses. The possible reasons for the photodegradation of the dye molecules are discussed in detail.