Control of group velocity of light via magnetic field

We have theoretically studied the effects of quantum coherence in a driven quasi-degenerate two-level atomic system. We have shown that the quantum interference, which can be destructive or constructive, can be controlled by an externally applied magnetic field allowing one to implement both electromagnetically induced transparency and electromagnetically induced absorption in the same atomic system. Determined by frequency dispersion of the index of refraction of the system, the group velocity of light pulses ranges from ultra-slow to superluminal with changing of the magnitude of the magnetic field.     

Frequency Stability of Atomic Clocks Based on Coherent Population Trapping Resonance in ^85Rb

An atomic clock system based on coherent population trapping （CPT） resonance in 85Rb is reported, while most past works about the CPT clock are in ST Rb. A new modulation method （full-hyperfine-frequency-splitting modulation） is presented to reduce the effect of light shift to improve the frequency stability of the CPT clock in SSRb. The experimental results show that the short-term frequency stability of the CPT clock in SSRb is in the order of 10^-10/s and the long-term frequency stability can achieve 1.5 × 10^-11/80000s, which performs as well as 87 Rb in CPT resonance. This very good frequency stability performance associated with the low-cost and low-power properties of SSRb indicates that an atomic clock based on CPT in SSRb should be a promising candidate for making the chip scale atomic clock.     

Magneto-Optical Trapping of Ytterbium Atoms with a 398.9nm Laser

We report the realization of ytterbium magneto-optical trap （MOT） operating on the dipole-allowed ^1S0 - ^1P1 transition at 398.9nm. The MOT is loaded by a slowed atomic beam produced by a Zeeman slower. All seven stable isotopes of Yb atoms could be trapped separately at different laser detuning values. Over 10^7 174 Yb atoms are collected in the MOT, whereas the atom number of fermionic isotope ^171Yb is roughly 2.3 × 10^6 due to a lower abundance. Without the Zeeman slower the trapped atom numbers are one order of magnitude lower. Both the even and odd isotopes are recognized as excellent candidates of optical clock transition, so the cooling and trapping of ytterbium atoms by the blue MOT is an important step for building an optical clock.     

利用补偿线圈提高塞曼减速器效率的理论及实验研究

研究了用于锶原子光晶格光钟原子冷却的塞曼减速器,应用增添补偿线圈的方法可以延长减速器的有效减速距离和增大减速器末端的磁场梯度,进而增加一级冷却俘获锶原子的数目,理论分析采用该方法实现的塞曼减速器较使用单一线圈塞曼减速器可以增加31.17%的俘获原子数目;飞行时间法测量了减速前后原子束中原子的速度分布,原子的最可几速度由380m/s降为43m/s,分布线宽相应变窄。荧光法测量俘获原子数目表明在相同实验条件下,应用补偿线圈后磁光阱俘获原子数目从1.26×106提高到1.81×106,增加30.4%。     

Polarisation-dependent optical pumping for interrogation of a magnetic-field-independent “clock” transition in laser-cooled trapped 87Sr+

We discuss the prospect of using the ⁸⁷Sr⁺ ion as an optical frequency standard. The ion offers a narrow electric quadrupole clock transition which has no first-order Zeeman shifts, and the required wavelengths can be generated with convenient solid-state laser systems. We describe how to cool and probe the ion in zero magnetic field by employing polarisation modulation of the cooling light to avoid coherent population trapping in dark states. The polarisation modulation scheme also provides optical pumping of the ion into the initial state of the narrow clock transition.     

Magnetooptical compression of atomic beams

We consider the propagation of an atomic beam in a quadrupole magnetic field under transverse irradiation by a cooling laser field. The cooling laser field was chosen in the form of a two-dimensional σ⁺-σ^? configuration. We show that the sub-Doppler resonance in the radiation force can be used to reduce the diameter of the atomic beam to a value on the order of 10 mm. We establish that the simultaneous transverse cooling and compression of the atomic beam allow its phase density to be increased to values of the order of 10^?4–10^?3. The dipole interaction of an atom with the cooling and compressing laser field in a quadrupole magnetic field is analyzed in terms of a simple (3 + 5)-level model atom.

     

Magneto-Optic Fiber Gratings Useful for Dynamic Dispersion Management and Tunable Comb Filtering

Intelligent control of dispersion management and tunable comb filtering in optical network appfications can be performed by using magneto-optic fiber Bragg gratings （MFBGs）. When a nonuniform magnetic field is applied to the MFBG with a constant grating period, the resulting grating response is equivalent to that of a conventional chirped grating. Under a linearly nonuniform magnetic field along the grating, a linear dispersion is achieved in the grating bandgap and the maximal dispersion slope can come to 1260ps/nm2 for a l0-mm-long fiber grating at 1550nm window. Similarly, a Gaussian-apodizing sarnpled MFBG is also useful for magnetically tunable comb filtering, with potential application to clock recovery from return-to-zero optical signals and optical carrier tracking.     

Velocity spectrometer for a neutral atomic beam

A velocity spectrometer for a neutral atomic beam has been developed to prepare a monovelocity atomic beam. In order to disperse a relatively fast atomic beam according to its longitudinal velocity, a magnetic quadrupole lens with a large magnetic gradient has been used. The device was made using NdFeB permanent magnets. The magnetic gradient in the quadrupole was measured to be sufficiently large (1 T/cm) and uniform along the radial direction. The resonance fluorescence spectra of the Li atomic beam after passing through the magnetic quadrupole lens have been measured by using a single-mode tunable laser. From the peak shift of the fluorescence spectrum, the exerted force on Li atoms by the magnetic lens was 8160 times as large as the acceleration of gravity. Using the present spectrometer, we have measured the longitudinal velocity distribution of the thermal Li beam at 800 °C, which was in good agreement with the result from the fluorescence spectra. It is expected that a monovelocity Li atomic beam of 1000 m/s with a velocity spread narrower than 1% can be easily formed with a compact experimental arrangement using the developed velocity spectrometer. In this case, the available flux given as a fraction of the incident Li flux is estimated to be about 1%. A high-velocity resolution of 0.03% is expected at the available fractional flux of about 0.01%. Received: 5 June 2000 / Revised version: 14 August 2000 / Published online: 8 November 2000     

Bright atomic beam by a temporal Zeeman acceleration

A novel method to produce a slow, monochromatic, and bright pulsed atomic beam from a magneto-optical trap by switching the magnetic field of the trap is proposed. A pulsed lithium atomic beam with a brightness of 1.1×10¹⁵ /sr s and a velocity of 13 m/s was produced as an experimental proof of this technique. The conversion efficiency from the trap into the atomic pulse was nearly 100%. Received: 22 October 1999 / Revised version: 15 November 1999 / Published online: 8 March 2000     

STUDY OF OPTICAL PUMPING OF ALKALI ATOMIC BEAM IN STRONG MAGNETIC FIELDS

We have gained a very comprehensive set of results for infrared diffraction (10.6μm wavelength) from fabricated single grooves of reflection with widths in the range of 5μm to 65μm. The experimental results show that the diffraction can still occur when the groove width becomes comparable to or less than the wavelength of the probing light. Variations of the diffraction intensities have a very regular sinusoidal relationship with the polarization angle of the incident light; and the diffraction intensities are the when the incident light is TM polarized. However, the diffraction intensities will be a minimum when the incident light is TE polarized; and the biger the diffraction angle, the higher the rate of change of the diffractive intensity with increasing polarization angle of the incident light.     

激光减速原子束频标的建议

本文提出一个激光减速的碱金属原子束频标的方案。用共振激光束对原子束同时进行减速和选态,使原子速率降到10m/s以下,而光抽运作用使原子自动集中到基态超精细结构中具有最大磁量子数的塞曼子能级上。为避免重力场中束轨迹下垂,用偏转磁铁或多束激光使束由水平转成垂直向上,然后用级联磁共振使原子过渡到频标所需的m_F=0能级。利用原子上升和重力场中自由下落两次通过单个微波谐振腔而取得Ramsey共振信号,线宽约为¹Hz。信号用另一束激光检测。予期这种频标的稳定度和准确度可比现有束型频标提高一个数量级以上。文中详细讨论了激光减速和选态的方法,克服横向加热效应的措施,实现级联磁共振的办法,以及获得垂直束装置的设计等。     

High-resolution atomic channeling using. velocity-selected atomic beam

High-resolution atomic channeling using velocity-controlled atoms may be able to overcome precision limitations of the conventional atom lithography. We have experimentally clarified the dependence of line width and contrast of atomic patterns in the channeling region on the velocity spread of the atomic source for the first time. Thermal or velocity-selected atomic beams prepared with a one-dimensional magneto-optical trap were employed as the atomic sources. We investigated the channeling characteristics by measuring the frequency shifts of the atomic absorption spectra in an intense standing wave light field. As a result, we can show that narrower line width and higher contrast atomic patterns are obtained as the velocity spread becomes narrower. An atomic pattern with an estimated line width of 57 nm was generated when the velocity spread of the atomic source was almost 50 m/s, that is, 1/6 that of the thermal beam. Received: 16 June 2001 / Published online: 7 November 2001     

Observation of collimation and decollimation of an atomic beam in a misaligned standing wave

This paper reports an experimental study on the collimation and decollimation of an atomic beam in a misaligned standing wave, in which the effective detuning caused by the Doppler effect is affected by the longitudinal velocity of the atomic beam. The experiment shows that in a strong field with red detuning between laser field and atomic transition frequency, laser heating in a normal standing wave becomes laser cooling in a misaligned standing wave for an approriate misalignment angle. For blue detuning, laser cooling in a standing wave can also become laser heating in a misaligned standing wave for an appropriate condition. These results ca be used in controling atomic motion.     

A Radio Frequency Field Guide Using Field Induced Adiabatic Potential

We study the behaviour of atoms in a field with both static magnetic field and radio frequency （rf） magnetic field. We calculate the adiabatic potential of atoms numerically beyond the usually rotating wave approximation, and it is pointed that there is a great difference between using these two methods. We find the preconditions when RWA is valid. In the extreme of static field almost parallel to rf field, we reach an analytic formula. Finally, we apply this method to 87 Rb and propose a guide based on an rf field on atom chip.     

抽运-检测型非线性磁光旋转铷原子磁力仪的研究

报道了一种抽运-检测型的非线性磁光旋转铷原子磁力仪.其原理是线偏振光通过处于外磁场环境中被极化的原子介质后,由于原子对线偏振光中左、右圆偏成分不同的吸收和色散,导致光的偏振方向会产生与磁场相关的转动.分析了该磁力仪的工作原理,并测试了它对不同磁场大小的响应.测试结果表明,磁力仪测量范围为100—100000 nT,极限灵敏度为0.2 p T/Hz~(1/2),磁场分辨率为0.1 p T.进一步研究了不同磁场下原子系综极化态的横向弛豫时间,讨论了原子磁力仪高磁场采样率的获得方法.本文的原子磁力仪在5000—100000 n T的磁场测量范围内磁场采样率可实现1—1000 Hz范围内可调,能够测量低频的微弱交变磁场.本文的研究内容为大磁场测量范围、高灵敏度、高磁场采样率的原子磁力仪研制提供了重要参考.     

Atomic beam guide by a one-dimensional magneto-optical trap

An atomic beam has been collimated, compressed, and deflected simultaneously by an atomic beam guide based on an inclined one-dimensional magneto-optical trap (1D-MOT). Isotope-selected rubidium atoms were extracted from the naturally-mixed thermal atomic beam with this method. We could manipulate the transverse displacement of the deflected beam precisely by adjusting the current in the copper rods to generate the quadrupole magnetic field. We could extract more than 50% of the incident atoms as a deflection beam when we combined this deflection technique with the atomic deceleration using a broadband spectral light. Received: 10 December 1998 / Published online: 24 June 1999     

Eine Quelle für polarisierte Lithiumkerne

A source for nuclear polarized lithium-6 and -7 ions was built following the method of separation of the atomic hyperfine states in a strong inhomogeneous magnetic sextupole field and ionisation of the selected atoms by electron bombardment in a weak homogeneous field. The velocity distribution of the atomic beam formed by a Laval nozzle and the focussing properties of the sextupole magnet were determined using a mechanical velocity selector. After separation of the two isotopes the ion beam has an intensity of 10^?9 ampere. As no background of unpolarized lithium ions was found, a vector polarisation of (32±1)% and (24±1)% for lithium-6 and -7 and a tensor polarisation of (?32±1)% for lithium-6 can be expected.     

Influence of atomic alignment on crossover signals in saturation spectroscopy

The crossover resonance between the atomic transitions² S _1/2(F=2)→² P _1/2(F′=1, 2), observed in the saturation spectrum of sodium, was found to be sensitive to the atomic orientation of the ground state, produced by velocity selective optical pumping. For zero magnetic field and the same linear polarization of saturating and probing beam, a negative signal was recorded for the crossover, corresponding to an increase in absorption of the probing beam. Application of a sufficiently strong magnetic field perpendicular to the polarization vector causes the destruction of the alignment, leading to a change in sign for the crossover intensity, i.e., to an enhanced transmission of the probing beam. It was shown experimentally that an increase in the atomic transit time has the same effect. Using optical pumping theory all qualitative features could be accounted for.     

激光冷却在原子束六极磁透镜技术中的应用

分析了由六极磁场构成了原子束磁透镜的原理及像差。在比较了热原子束和单色冷原子束的会聚效果后得出结论：影响它成像的主要因素是色差。应用激光冷却技术能使这一影响减小,从而使磁透镜在原子光学领域得到更广泛的应用。