Experimental determination of spin-rotation and spin-spin magnetic interactions in by nuclear spin conversion

When a gas sample of ¹³CH₃F is prepared with a population of isomers (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon called nuclear spin conversion is mainly governed by intramolecular spin-spin and spin-rotation interactions. In the quantum relaxation model [P.L. Chapovsky, Phys. Rev. A 43, 3624 (1991)], two pairs of ortho-para levels (J = 9, K = 3; J' = 11, K' = 1) and (J = 20, K = 3; J' = 21, K' = 1) are principally responsible for the conversion. The levels of the second pair are coupled by both spin-spin and spin-rotation interactions. The application of an electric field (up to 10 kV/cm) induces a crossing of the Stark components of this pair, which is observed for the first time. A specific experimental set-up based on an electric field of alternating triangular shape is used, which allows the determination of the strength of both interactions via the measurement of the spin conversion decay rates. This work yields the first experimental value for the electronic contribution to the spin-rotation interaction in ¹³CH₃F. Received 23 May 2002 / Received in final form 18 September 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: Patrice.Cacciani@univ-lille1.fr     

Continuous-flow optical pumping NMR in a closed circuit system

In a typical continuous-flow optical pumping setup, the chemical shift of xenon in the adsorbed phase depends on the gas flow rate due to warming of the sample surface by the gas stream. Calibration of the system using the (207)Pb resonance of solid lead nitrate is necessary to determine the actual sample temperature. Optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate alters signal intensity ratios and may lead to the complete suppression of signals.     

Efficient population transfer by delayed pulses despite coupling ambiguity

In traditional schemes of multilevel multilaser excitation, each laser pulse interacts with only one pair of states, and the rotating wave approximation (RWA) is applicable. Here we study the population transfer process in a three-state system when each of the two lasers interacts with each of the pair of states and when the Rabi frequencies characterizing the interaction strengths of the system are comparable to or larger than the difference of the transition frequencies. We show that complete and robust population transfer is possible under conditions more general than those hitherto considered necessary for stimulated Raman adiabatic passage (STIRAP) or for successive π pulses. Using adiabatic Floquet theory we show that successful population transfer can be interpreted as adiabatic passage by means of a transfer state which connects the initial and final states. The Floquet picture offers a convenient interpretation of the population transfer as accompanied by multiple absorption of photons from or emission into the laser fields.     

Yb离子抽运动力学及脉冲储能特性研究

从准三能级Yb离子的能级结构出发，建立了Yb离子的抽运和激光速率方程，结合解析和数值方法，研究了Yb激光介质的抽运动力学过程，包括抽运激发效率、最低抽运强度、激光能量提取效率等关键参数.比较了三类典型的Yb激光介质性能：Yb:S-FAP,Yb:YAG以及Yb:FP-glass.以放大自发辐射(ASE)为设计判据，重点研究了脉冲储能型Yb激光器的设计准则，包括增益介质的厚度与掺杂浓度.最后利用此模型给出了基于Yb:S-FAP以及Yb:YAG的100J级二极管抽运固体激光器(DPSSL)的总体设计参数.将对基于Yb激光介质的脉冲储能型DPSSL的设计提供有益的参考. 关键词： Yb离子 速率方程 抽运动力学 二极管抽运固体激光器     

离轴抽运对Nd:YAG激光器振荡模式影响的研究

以激光二极管端面抽运固体激光器为研究对象,通过实验研究了离轴抽运条件下,离轴量的大小对Nd:YAG激光器的振荡光阈值、振荡模式、光束质量以及激光器的斜效率的影响,分析了在同一抽运位置改变LD电流时输出光位置及模式的变化情况.实验结果表明,当抽运光离轴量增大时,斜效率增大,但输出光依次出现从低阶模到高阶模的变化,激光器的振荡阈值升高,光束质量变差;在同一抽运位置增大LD电流时,输出光从高阶模向低阶模转化,光斑中心向此时的抽运中心轴靠拢.     

890 mW Diode Pumped Nd:YVO4 Intracavity-doubled Red Light Laser

A diode-pumped Nd3+:YVO4 emitting at 1342 nm and 671 nm was developed. Low concentration neodymium doped Nd:YVO4 shows excellent stability at 1342 nm. With a type-Ⅱ noncritical phase-matched LBO crystal as the intracavity frequency doubler, 890 mW of 671 nm light was obtained at 11.2 W incident pumping power, the optical-optical conversion efficiency is 7.9%.     

内壳层跃迁--实现超短波长激光的途径

内壳层跃迁机制是实现超短波长激光的一种很有潜力的方案，随着近年来超短超强激光技术和X射线激光实验方法的进展，实现内壳层跃迁机制的超短波长硬X射线激光不再是遥不可及的梦想，文章详细介绍了内壳层跃迁机制X射线激光的原理，并讨论了开展内壳层跃迁机制X射线激光实验的一些实际相关问题。     

Comparison of characteristics of an x-ray preionized XeCl laser, pumped by water transmission line and low inductance capacitor bank

With a high uniformity of x-ray preionization, a long pulse duration and a high specific energy (9 J l^-1), an XeCl excimer laser output has been obtained by using a compact low inductance capacitor bank as the laser excitation circuit. The superior features of the low inductance capacitor bank (LICB) compared to the water transmission line (WTL) are attractive for repetitively pulsed XeCl excimer lasers.     

Optical pumping and coherence effects in fluorescence from a four level system

An effective four-level system around the D2 line of ⁸⁵Rb at room temperature, is experimentally investigated by fluorescent studies under the action of two driving fields L1 and L2. This system exhibits unique features in fluorescence as a function of frequency separation between L1 and L2. In particular, at two-photon resonance, when the Rabi frequency of L1 exceeds that of L2, signatures of Electromagnetically Induced Transperancy effect (EIT) arising from the three-level Λ sub-system is present as a sub-natural dip in fluorescence from the fourth level. At comparable strengths of L1 and L2 the fluorescence features indicate a regime, where the effects arising from optical pumping and EIT effect due to ground hyperfine level coherence coexist. We see in the coexistence regime, saturation effects arising from difference frequency crossing (DFC) resonances and optical pumping around the EIT window. At low strengths of L1, all signs of coherence vanishes from the system and the fluorescent features result from incoherent optical pumping through the Autler-Townes split states of the excited state hyperfine levels, which are split due to the stronger L2 laser. The dominant role of the L1 laser in creating a robust transparency signal even in the presence of an off-resonant excitation is brought out. The results are supported by density matrix calculations.     

Experimental study of non-linear energy pumping occurring at a single fast frequency

Experimental verification of passive non-linear energy pumping in a two-degree-of-freedom system comprising a damped linear oscillator coupled to an essentially non-linear attachment is carried out. In the experiments presented the non-linear attachment interacts with a single linear mode and, hence, energy pumping occurs at a single ‘fast’ frequency in the neighborhood of the eigenfrequency of the linear mode. Good agreement between simulated and experimental results was observed, in spite of the strongly (essentially) non-linear and transient nature of the dynamics of the system considered. The experiments bear out earlier predictions that a significant fraction of the energy introduced directly to a linear structure by an external impulsive (broadband) load can be transferred (pumped) to an essentially non-linear attachment, and dissipated there locally without spreading back to the system. In addition, the reported experimental results confirm that (a) non-linear energy pumping in systems of coupled oscillators can occur only above a certain threshold of the input energy, and (b) there is an optimal value of the energy input at which a maximum portion of the energy is absorbed and dissipated at the NES.