光泵磁共振实验中小信号的讨论

光泵磁共振实验中铷原子光谱具有超精细结构,在磁场中原子的超精细能级产生塞曼分裂,在垂直于产生塞曼分裂的磁场方向,加一射频磁场,并且满足一定条件时在塞曼子能级之间产生磁共振,实验发现除了出现符合跃迁条件的磁共振信号外,还有附加小信号,可用多量子跃迁解释。     

二维磁光阱产生铯原子束的数值模拟

通过分析铯原子在σ+-σ-组态的圆偏振光照射下塞曼子能级的分布情况,构造铯原子在二维磁光阱(2D-MOT)中的受力模型,利用龙格-库塔方法求解铯原子的运动方程,实现原子束产生过程的三维模拟。得出原子束流量随小孔半径、铯原子蒸汽压、激光光强、激光失谐量、磁场梯度等的变化规律。与实验数据进行比较表明受力模型的正确性, 该方法能直观模拟原子束的产生,准确揭示原子束流量随各项参数的变化规律,为实验提供理论指导。     

提高激光抽运铯原子磁力仪灵敏度的研究

本文报道了一种基于激光抽运射频共振的铯原子磁力仪. 通过圆偏振光将铯原子抽运到暗态, 实现偏极化. 外磁场存在时, 原子磁矩将以拉莫尔频率绕外磁场进动. 在共振射频磁场的作用下, 原子被去极化而重新吸收光子. 通过探测出射光光谱可以测得拉莫尔频率进而得到外磁场的信息. 本文通过运用自制的894 nm 外腔半导体激光器, 建立了激光稳频装置和低噪声磁场测量环境, 实现了一种基于铯原子激光抽运射频共振的磁力仪. 通过磁力仪参数优化以及闭环测量, 磁力仪测量的外磁场达到了19 fT/Hz^1/2的极限灵敏度和1.8 pT/Hz^1/2的本征灵敏度, 空间分辨率小于2 cm. 关键词： 光抽运 塞曼效应 光探测磁共振 磁力仪     

二维磁光阱产生铯原子束的数值模拟

通过分析铯原子在σ~+-σ~-组态的圆偏振光照射下塞曼子能级的分布情况,构造铯原子在二维磁光阱(2D-MOT)中的受力模型,利用龙格-库塔方法求解铯原子的运动方程,实现原子束产生过程的三维模拟.得出原子束流量随小孔半径、铯原子蒸汽压、激光光强、激光失谐量、磁场梯度等的变化规律.与实验数据进行比较表明受力模型的正确性,该方法能直观模拟原子束的产生,准确揭示原子束流量随各项参数的变化规律,为实验提供理论指导.     

Rb87基态超精细塞曼能级跃迁的偏振激发特性

本论述了Rb^87基态超精细塞曼能级光泵磁共振的偏振激发特性,给出了塞曼能级跃迁谱线随抽运光偏振特性的变化。     

用于全光铯原子磁力仪的激光器稳频技术研究

全光铯原子磁力仪是采用光学的方法实现微弱磁场检测,激光频率稳定性直接影响磁力仪的灵敏度。分析了二向色性原子蒸气激光频率锁定(Dichroic atomic vapor laser lock DAVLL)技术用于稳定激光器频率的原理,及其在全光原子磁力仪中的应用优势,发现通常的二能级原子模型不适用于分析铯原子D2线的稳频。实验测量了不同磁场下铯原子D2线基态Fg=4和Fg=3跃迁的DAVLL光谱,发现16mT是实现DAVLL稳频的最佳磁场;在此磁场附近,基态Fg=4跃迁鉴频曲线零点相对于Fg=4→Fe=5跃迁会产生6MHz/mT的线性频移,基态Fg=3跃迁鉴频曲线零点相对于Fg=3→Fe=4线会产生-9MHz/mT的线性频移。     

分布式反馈激光抽运铯磁力仪灵敏度相关参数研究

本文报道了分布式反馈(简称DFB)激光抽运铯磁力仪实验系统,研究并优化了激光光强、激光频率、射频强度、铯吸收泡温度、吸收泡缓冲气体压强等5类参数,测算得到不同情况下铯原子塞曼跃迁谱谱线宽度△f与信噪比S/N的比值,并利用公式δB=1γ/Cs△f/S/N(γCs为铯原子旋磁比)将其转化为磁力仪灵敏度,得到最优灵敏度为2.5 pT/Hz1/2.实验表明,本文所研究的5类参数都会不同程度提高磁力仪灵敏度,其中,激光频率、射频强度、铯吸收泡温度更是将灵敏度提高了10倍以上,这为更高性能、实用化半导体激光抽运铯磁力仪的研制提供了重要参考.     

基于铯原子光泵磁共振的磁场传感实验系统设计

给出了铯原子光泵磁共振测量磁场的原理.利用MEMS封装与制造技术,设计了一种芯片级的激光铯磁场传感系统,可实现对微弱磁场的检测.     

利用原子的塞曼光谱对半导体激光器进行稳频

中性原子的超精细能级在磁场中产生塞曼分裂 ,另外 ,左旋和右旋圆偏振光激发下原子的跃迁选择定则不同 ,因此 ,原子在超精细塞曼能级间的吸收谱峰相对无磁场条件下的吸收谱峰有一定的移动。利用这一点 ,验证了一种简单、灵活的方法对半导体激光器进行稳频 ,使激光器的线宽稳到小于 1MHz。通过对实验结果的分析 ,发现由左旋和右旋圆偏振光激发引起的原子吸收谱峰移动之和与饱和吸收峰半高宽相等时 ,稳频效果最好     

铯原子气室中D2线泵浦探测光谱的实验观测

在铯原子气室中采用偏振方向相互垂直且同向传播的线偏泵浦光和探测光，研究了铯原子D2线的泵浦探测光谱。由于在6 S1/2 F=3 – 6 P3/2 F’=2 超精细跃迁中存在多个L型塞曼子能级结构，从而产生了电磁诱导透明导致的吸收减弱；而在6 S1/2 F=4 – 6 P3/2 F’=5 超精细跃迁中则观测到了电磁诱导吸收。通过改变泵浦光的失谐量，在电磁诱导透明形成的吸收减弱凹陷和电磁诱导吸收产生的吸收增强峰内部均观察到了反常的吸收信号反转。     

Narrow spectral lines of modulation harmonics of gamma radiation in unordered magnetic materials

The spectra of gamma-resonance lines excited using the amplitude modulation of the pumping field of the Mössbauer effect were studied in unordered magnetic materials with a strong spread of magnetic hyperfine fields at nuclei of Mössbauer isotopes. The analysis demonstrates that the spectra of modulation harmonics consist of narrow lines, and their form and intensity are determined by the contribution of magnetic local hyperfine fields, which are excited at a pumping frequency. The inversion of intensities for odd harmonics with the change in sign of detuning of the pumping frequency relative to the average Larmor frequency of the Mössbauer nucleus was noted. The application of this method to the spectral analysis of inhomogeneous magnetic materials is discussed.     

强磁场下Cs原子超精细塞曼能级上的光泵浦研究

强磁场中的Ｃｓ原子有较大的超精细塞曼分裂，实验用频率可调谐的窄线宽半导体激光调谐到各超精细塞曼能级上进行光泵浦，利用稳态吸收谱方法研究了原子的光泵浦。表明基态超精细相互作用的碰撞修正项导致的驰豫跃迁是谱形状和电子自旋极化新特征的根缘。同时提出了强场下极化度的一种测量方法。     

相干布居数拍频信号与基态超精细子能级相干性关系的研究

相干布居数拍频(coherent population beating, CPB)现象, 产生于一个Λ型三能级原子系统中, 当双色相干激光场的频率差和两基态能级频率间隔近失谐的时候, 原子在激发态能级上的布居数会产生一个弛豫振荡, 且振荡频率等于失谐量. 当将此现象运用于原子标准频率的提取时, CPB频标的稳定度与CPB信号的幅度及信噪比直接相关. 本文理论推导了描述CPB 现象的表达式, 数值模拟并实验研究了⁸⁷Rb基态超精细子能级的相干性对CPB信号的影响, 通过控制与基态子能级共振相干激光场的抽运时间来控制能级的相干程度, 观测不同相干程度对CPB信号质量的影响. 研究结果表明CPB信号振荡的幅度与基态子能级相干程度成正比关系. 要改善CPB信号信噪比、提高原子频标稳定度, 建立、提高和保持基态超精细能级的相干性是关键. 本文还讨论了CPB现象用于弱磁场测量及其他方面应用的可行性.     

Determination of the structure of hyperfine sublevels of Rb in strong magnetic fields by means of the coherent population trapping technique

The splitting of hyperfine sublevels of the ⁸⁵Rb atom in strong magnetic fields has been studied by means of the coherent population trapping technique. Narrow resonances with a high signal-to-noise ratio have been detected in a 30-μm-thick spectroscopic cell. The magnetic field in the direction transverse to the laser beams has been created by permanent magnets and has reached 1600 G. Owing to the exclusive narrowness of the cell, the field in it is almost uniform. The break of the coupling between the electronic and nuclear moments, as well as the transition to the Paschen-Back regime in magnetic fields above 600 G, has been observed. The derivatives of the frequency shifts of the observed resonances and their asymptotic values in strong magnetic fields have been determined in terms of the magnetic field strength. The experimental results have been interpreted within a theoretical model based on the known constants of the hyperfine structure of the Rb atom.     

The role of spin-orbit coupling and hyperfine coupling in optical pumping ofF-centres

Optical pumping experiments onF-centres in potassium halides are described. The ground state polarization of the electronic spins achieved by optical pumping of isolatedF-centres is strongly dependent on the magnetic field and the wave-length of the pumping light. Experimentally it does not show the simple relationship to the magnetic circular dichroism (MCD) that has generally been assumed. A closer theoretical analysis shows indeed that the phenomena depend critically on the kind of spin mixing that prevails in the pumping cycle.If spin orbit coupling in the absorption band is the dominant spin mixing mechanism the sign of the pumping effect will be the same everywhere in the absorption band, if some other mechanism like spin mixing by radiationless transitions or hyperfine coupling to the surrounding nuclear spins prevails, the sign will follow the MCD. In strong magnetic fields experiments argue in favour of the first alternative, in low fields spin mixing by hyperfine coupling becomes important. On this basis the mechanism of optical pumping ofF-centres is discussed, and rate equations are given.     

Cs 5D_(5/2)-6F 728 nm Laser Spectroscopy with Single Pumping Laser

The sub-Doppler absorption laser spectroscopy at 728 nm transition from the 5D5/2 state to the 6 F state of cesium with linewidth near 10 MHz is first experimentally performed with indirect pumping from the ground state 6S_(1/2)to the state 7P_(3/2)by a 455.5 nm diode laser.Using a 455.5 nm diode laser as an indirect pump laser,several excited states will be populated due to spontaneous decay from the 7P state.We first implement the sub-Doppler absorption laser spectroscopy at 728 nm from the 5D_(5/2)state to the 6F state when Cs atoms within thermal glass cell decay to the 5D_(5/2) state.Due to velocity transfer effect,the hyperfine structure of 5D_(5/2)shows a mixed and complicated pattern but very clear structure when the 455.5 nm pumping laser is counter-propagating(or co-propagating)with the 728 nm probing laser.     

铯原子7S1/2态磁偶极超精细常数的测量

在室温下的原子气室中, 基于铯原子6S_1/2-6P_3/2-7S_1/2(852.3 nm+1469.9 nm) 阶梯型能级系统, 利用电光调制器的主频和±1级边带分别产生的三套双共振吸收光谱, 当驱动电光调制器的信号源频率严格等于7S_1/2态超精细分裂的能级间隔时, 三套谱线中的一些超精细跃迁谱线重叠且线宽最窄, 利用这一现象很好地避免了激光器频率扫描时非线性效应的影响, 测量出了7S_1/2 态超精细分裂能级间隔: 2183.72 MHz±0.23 MHz, 并计算出该态的磁偶极超精细常数: A_hfs= 545.93 m MHz±0.06 MHz, 与文献中报道的测量结果一致.     

A laser-pumped magnetometer for the mapping of human cardiomagnetic fields

Magnetic fields produced by biological organisms contain valuable information on the underlying physiological processes and their pathologies. Currently, superconducting detectors cooled far below room temperature are required to measure these generally weak biomagnetic signals. We have developed a sensitive laser magnetometer based on optical pumping of cesium atoms that makes it possible to map the magnetic field produced by the beating human heart. A gradiometer formed by two identical sensors greatly reduces the influence of external stray magnetic fields. The magnetometer operates at room temperature and therefore opens the way to affordable and convenient monitoring of biomagnetic fields in research and medical diagnostics. Received: 8 January 2003 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch     

Magnetic properties, Mössbauer effect and first principle calculations study of laves phase HfFe2

The magnetic properties and hyperfine interaction parameters for Laves phase HfFe₂ with C14 type structure are studied using SQUID magnetometer and M?ssbauer measurement. The saturation magnetization, remanent magnetization, coercive field, magnetic moment per unit formula and the hyperfine magnetic field at Fe site are reported. In addition, a detailed theoretical study of the electronic structure and hyperfine magnetic fields of the two possible HfFe₂ structures, C15 and C14, is presented. Using the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN 97 package, the equilibrium volume, bulk moduli, magnetic moments and hyperfine magnetic fields for the two structures are calculated. The obtained results are compared with the measured data.