Photo induced collisions with laser cooled He* atoms

This paper presents an experimental investigation of cold collisions between metastable helium atoms in an optical trap at 1 mK. Penning (PI) and associative (AI) ionization reactions are distinguished using a mass spectrometer and studied under influence of near resonant laser light. Dramatic enhancement, by more than a factor 15, of the ion rate is observed when the laser is tuned close to resonance. Experimental findings are well-described, on an absolute scale, by model predictions. Received: 9 February 1998 / Revised: 4 May 1998 / Accepted: 14 May 1998     

Search for permanent EDM using laser cooled Fr atoms

The existence of a non-zero electric dipole moment (EDM) implies the violation of time reversal symmetry. As the time-reversal symmetry violation predicted by the Standard Model (SM) for the electron EDM is too small to be observed with current experimental techniques and any a non-zero EDM would indicate new physics beyond the SM. The tiny signal from the electron EDM is enhanced in the heavy atoms such as francium (Fr). We are constructing the laser-cooled Fr factory to search for the electron EDM.     

衍射极限尺度下的亚波长电磁学

作为波的本性之一,衍射是现代物理学的重要研究内容.衍射导致自由空间中波的能量不能被无限小地聚集,从而为成像、光刻、光存储、光波导等技术设定了一个原理性的障碍——衍射极限.对于电磁波和光波而言,尽管通过提高介质的折射率可以压缩衍射效应,但由于自然界中材料的折射率有限,该方法存在很大限制.近年来,随着表面等离子体光学的兴起,表面等离子体在超越传统衍射极限方面的能力和应用前景受到了学术界的关注.本文从亚波长电磁学的角度出发,介绍衍射极限研究的历史,综述了突破衍射极限的理论方法.首先,利用金属介质表面等离子体激元的短波长特性,可将等效波长压缩一个数量级以上,在纳米尺度实现光波的聚焦或定向传输;更进一步,通过人为设计超构材料和超构表面,利用结构化金属和介质中的局域谐振、耦合等特殊电磁响应,可实现亚波长局域相位调制、超宽带色散调控、近完美吸收、光子自旋轨道耦合等,从而突破传统理论的诸多局限,为下一代电磁学和光学功能器件奠定重要基础.     

第五讲 基于激光冷却原子的超冷分子制备与外场操控

超冷分子的理论和实验研究近年来取得了令人瞩目的巨大成就,极大地拓展了原子分子光物理的研究范畴。围绕超冷分子的制备与应用开创了很多全新的研究领域,如超高分辨分子光谱、分子量子态操控、精密测量以及量子模拟等。当前超冷分子的高效密集制备主要采用基于激光冷却的超冷原子缔合技术来实现。文章综述了超冷分子缔合制备的研究现状,阐述了光缔合、Feshbach共振缔合、受激拉曼绝热跃迁以及超短脉冲光缔合产生超冷分子的物理机制与实验进展,对外场操控超冷分子的实验结果及其潜在应用做了概要展示。     

Electron collisions with laser cooled and trapped metastable helium atoms: total scattering cross sections

Absolute measurements of total scattering cross sections for low energy (5-70 eV) electrons by metastable helium (2(3)S) atoms are presented. The measurements are performed using a magneto-optical trap which is loaded from a laser-cooled, bright beam of slow He(2(3)S) atoms. The data are compared with predictions from convergent close coupling and R matrix with pseudostate calculations, and we find good agreement between experiment and theory.     

Relativistic and interchannel coupling effects in photoionization angular distributions by synchrotron spectrocopy of laser cooled atoms

We investigate the angular distribution of photoionization fragments at low photon energies (12-40 eV) in an open shell atom, by synchrotron radiation recoil ion momentum spectroscopy in a laser cooled and trapped sample. For cesium atoms, for which relativistic effects play an important role and the ion recoil is relatively small, we could determine large and rapid changes of the asymmetry parameter beta from two, observed for s electrons outside resonances and far from the Cooper minimum. They can be explained by relativistic effects and interchannel coupling arising from final state configuration mixing.     

Condensation of laser cooled gases

We study a dynamical scheme for condensation of bosonic trapped gases beyond the Lamb-Dicke limit, when the photon-recoil energy is larger than the energy spacing of the trap. Using quantum master equation formalism we demonstrate that dark-state cooling methods similar to those designed for a single trapped atom allow for the condensation of a collection of bosons into a single state of the trap, either the ground, or an excited state. By means of Monte-Carlo simulations we analyse the condensation dynamics for different dimensions, and for different cooling schemes. Received 30 November 1998 and Received in final form 20 March 1999     

Mössbauer study of the microscopic surrounding of co atoms implanted in si and ge below the full amorphization limit

Mössbauer spectroscopy on ⁵⁷Co implanted into Si and Ge at doses between 10¹¹ and 10¹⁵ atoms/cm² supports the single track amorphization model. Calculations based on linear cascade theory do not give quantitative agreement with the experiment.     

Observation of doubly charged lead clusters below the critical size limit for Coulomb explosion of sphere configurations

Doubly charged lead clusters Pb₇²⁺, Pb₉²⁺, Pb₁₁²⁺ and Pb₁₃²⁺, far below the critical size for Coulomb explosion of par ticles with sphere like configurations (Pb₃₀) have been detected in time of flight mass spectra. Their existence can be explained by chain-like clusters.     

Doppler cooling to the quantum limit

Doppler cooling on a narrow transition is limited by the noise of single scattering events. It shows novel features, which are in sharp contrast with cooling on a broad transition, such as a non-gaussian momentum distribution, and divergence of its mean square value close to the resonance. We have observed those features using 1D cooling on an intercombination transition in strontium, and compared the measurements with theoretical predictions and Monte Carlo simulations. We also find that for very a narrow transition, cooling can be improved using a dipole trap, where the clock shift is canceled.     

Interferometry below the standard quantum limit with Bose-Einstein condensates

We discuss a scheme for using entangled Bose-Einstein condensates to detect phase differences with a resolution better than the standard quantum limit. To date, schemes have shown that the enhancement in phase resolution gained by entangling condensates is lost when dissipation is present. Here we show how this can be overcome by using number correlated condensates, as have been produced recently in the laboratory. We also outline a scheme for measuring this phase that is not destroyed when the effects of finite detector efficiency are considered.     

Ferromagnetism below the stoner limit in La-doped SrB6

Spin-polarized band calculations for LaSr7B48 show a weak ferromagnetic state. This is despite a low density of states (DOS) and a low Stoner factor. The reason for the magnetic state is found to be associated with a gain in potential energy in addition to the exchange energy, as a spin splitting is imposed. A DOS with an impuritylike La band is essential for this effect. It makes a correction to the Stoner factor and provides an explanation of the recently observed weak ferromagnetism in doped hexaborides.     

用脉冲半导体激光观察铷原子基态塞曼能级的相干共振

用半导体激光脉冲序列泵浦汽泡铷原子,观察其基态能级的△m=1和△m=2的塞曼(Zeeman)相干.利用直接调制激光二极管注入电流的增益开关技术,在波长780nm处产生约200ps的激光脉冲.对△m=1,用偏振光谱学方法检测;而对△m=2,则直接使用前向散射光谱学方法检测.     

Conductively cooled all-solid-state zigzag slab laser

A diode pumped, Q-switched Nd：YAG zigzag slab laser is developed using passive conduction cooling. Flat-flat and unstable resonators are adopted in this experiment. The 150-mJ multi-mode and 100-mJ single-mode laser outputs with pulse width of 10 ns are achieved, corresponding to optical efficiencies of 19% and 13%, respectively. The experimental result demonstrates that the laser has the property of compact structure, high efficiency, reliability, and high beam quality. The design of laser has a potential application in space environment.     

Cooling of atoms below the recoil energy under multizone Raman excitation

Cooling of atoms below the temperature determined by the recoil energy is proposed on the basis of the Raman excitation of three-level atoms by the field of standing waves with a relative spatial shift. The advantage of this cooling mechanism is a weak sensitivity to the shape and duration of light pulses used for the transfer of population under Raman excitation. It is shown that the effectiveness of such cooling increases sharply when multizone excitation is used and already several interaction zones are enough for deep transverse cooling of an atomic beam to a temperature significantly lower than the recoil energy.     

Spin-exchange collisions of submerged shell atoms below 1 Kelvin

Angular momentum changing collisions can be suppressed in atoms whose valence electrons are submerged beneath filled shells of higher principle quantum number. To determine whether spin-exchange collisions are suppressed in these "submerged shell" atoms, we measured collisional rates for six hyperfine states of Mn at T < 1 K. Although the 3d valence electrons in Mn are submerged beneath a filled 4s orbital, we find spin-exchange rate coefficients similar to Na and H (both nonsubmerged shell atoms).