Slow spin relaxation of Rb atoms confined in glass cells filled with dense 4He gas at 1.85 K

At temperatures below 2.1 K, long-lived gaseous Rb atoms in glass cells have been generated with a simple method: irradiating the cells, containing 4He gas and Rb metal, with a cw laser. The obtained atomic Rb density ( approximately 10(8) cm(-3)) decreases with a 1/e time constant of about 10 s at 1.85 K. We have performed optical pumping of the Rb atoms and measured the longitudinal electronic spin relaxation time at 1.85 K as well. For processes (such as Rb-He collisions) which do not remove the atomic Rb from the vapor, this relaxation time is found to be about 60+/-15 s.     

Improved stabilization of a 1.3 microm femtosecond optical frequency comb by use of a spectrally tailored continuum from a nonlinear fiber grating

We report significant enhancement (+24 dB) of the optical beat note between a 657 nm cw laser and the second-harmonic generation of the tailored continuum at 1314 nm generated with a femtosecond Cr:forsterite laser and a nonlinear fiber Bragg grating. The same continuum is used to stabilize the carrier-envelope offset frequency of the Cr:forsterite femtosecond laser and permits improved optical stabilization of the frequency comb from 1.0 to 2.2 microm. Using a common optical reference at 657 nm, a relative fractional frequency instability of 2.0 x 10(-15) is achieved between the repetition rates of Cr:forsterite and Ti:sapphire laser systems in 10 s averaging time. The fractional frequency offset between the optically stabilized frequency combs of the Cr:forsterite and Ti:sapphire lasers is +/-(0.024 +/- 6.1) x 10(-17).     

Accurate monitoring of an interferometric fiber-optic sensor with a multimode semiconductor laser

A technique for monitoring interferometric sensors by correlation of the optical signal from the sensor with the signal from a phase-modulated reference interferometer is described. The technique is applied to temperature measurement with a fiber Fabry-Perot sensing head. We extend the 7 degrees C unambiguous measurement range obtained with a 1.3-mum laser diode as the light source to 43 degrees C by adding a second laser diode emitting at 1.55mum . Experimentally, a rms phase-measurement error of +/-0.28rad , corresponding to a temperature error of +/-0.32 degrees C , was achieved with a low-quality multimode Fabry-Perot laser. The phase error was improved to +/-0.14rad , corresponding to a temperature error of +/-0.16 degrees C , by use of a distributed-feedback laser diode.     

Gas phase activation energy for unimolecular dissociation of biomolecular ions determined by focused RAdiation for gaseous multiphoton ENergy transfer (FRAGMENT)

We present a novel approach for the determination of activation energy for the unimolecular dissociation of a large (>50 atoms) ion, based on measurement of the unimolecular dissociation rate constant as a function of continuous-wave CO(2) laser intensity. Following a short ( approximately 1 s) induction period, CO(2) laser irradiation produces an essentially blackbody internal energy distribution, whose 'temperature' varies inversely with laser intensity. The only currently available method for measuring such activation energies is blackbody infrared radiative dissociation (BIRD). Compared with BIRD, FRAGMENT: (a) eliminates the need to heat the surrounding ion trap and vacuum chamber to each of several temperatures (each requiring hours for temperature equilibration); (b) offers a three-fold wider range of effective blackbody temperature; and (c) extends the range of applications to include initially cold ions (e.g., gas-phase H/D exchange). Our FRAGMENT-determined activation energy for dissociation of protonated bradykinin, 1.2 +/- 0.1 eV, agrees within experimental error to the value, 1.3 +/- 0.1 eV, previously reported by Williams et al. from BIRD experiments. Copyright 1999 John Wiley & Sons, Ltd.     

Correlations and counting statistics of an atom laser

We demonstrate time-resolved counting of single atoms extracted from a weakly interacting Bose-Einstein condensate of 87Rb atoms. The atoms are detected with a high-finesse optical cavity and single atom transits are identified. An atom laser beam is formed by continuously output coupling atoms from the Bose-Einstein condensate. We investigate the full counting statistics of this beam and measure its second order correlation function g((2))(tau) in a Hanbury Brown-Twiss type experiment. For the monoenergetic atom laser we observe a constant correlation function g((2))(tau)=1.00 +/- 0.01 and an atom number distribution close to a Poissonian statistics. A pseudothermal atomic beam shows a bunching behavior and a Bose distributed counting statistics.     

RbH(X1∑+,v=O～2)能级与H2碰撞振动能量转移

利用积分时间分辨荧光光谱方法,研究了RbH(X1∑+,v=0～2)与H2间的振动碰撞能量转移.在Rb-H2混合样品池中,泵浦激光双光子激发Rb原子至6D态,Rb(6D)与H2反应生成RbH(x1∑+)分子,探测激光延迟泵浦激光20 ns,通过激光感应荧光光谱(LIF)的测量,确定了X1∑+(v=0～2,J)原生态的转动...     

Polarimetry of short-pulse gamma rays produced through inverse Compton scattering of circularly polarized laser beams

We have developed a polarimetry of ultrashort pulse gamma rays based on the fact that gamma rays penetrating in the forward direction through a magnetized iron carry information on the helicity of the original gamma rays. Polarized, short-pulse gamma rays of (1.1+/-0.2)x10(6)/bunch with a time duration of 31 ps and a maximum energy of 55.9 MeV were produced via Compton scattering of a circularly polarized laser beam of 532 nm off an electron beam of 1.28 GeV. The first demonstration of asymmetry measurements of short-pulse gamma rays was conducted using longitudinally magnetized iron of 15 cm length. It is found that the gamma-ray intensity is in good agreement with the simulated value of 1.0x10(6). Varying the degree of laser polarization, the asymmetry for 100% laser polarization was derived to be (1.29+/-0.12)%, which is also consistent with the expected value of 1.3%.     

均衡功率的超宽带可调谐掺铒光纤激光器

报道了一种功率均衡的超宽带可调谐掺铒光纤激光器.该激光器采用窄线宽可调谐FP滤波器、掺铒光纤、波分复用器和耦合器构成光纤环形腔激光器，并通过部分反馈光功率自动控制，对输出激光功率进行均衡.实现了87 nm带宽（1525~1612 nm）范围内，输出激光功率波动小于0.2 dB的均衡结果.输出激光3 dB线宽为0.26 nm，边模抑制比大于55 dB.     

Photoluminescence kinetics of multiperiod GaAs/AlGaAs structures with asymmetric barriers promising for making unipolar lasers

Photoluminescence kinetics in multiperiod GaAs/AlGaAs quantum-well structures with asymmetric barrier heights has been investigated. Owing to the barrier asymmetry, a time constant of nonradiative recombination between the laser subbands of 9 ps has been achieved, which is record long for unipolar lasers and exceeds the nonradiative relaxation time constant of the lower laser subband. This provides population inversion in the system. Efficient suppression of an unwanted cascade transition between the laser subbands via quasi-continuum states has been demonstrated.     

Efficient propagation of polarization from laser photons to positrons through compton scattering and electron-positron pair creation

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2 x 10(4) e+ /bunch. The magnitude of positron polarization has been determined to be 73 +/- 15(stat) +/- 19(syst)% by means of a newly designed positron polarimeter.     

Measurement of the absolute energy level and hyperfine structure of the 87Rb 4D5/2 state

We have measured the absolute frequency of the excited state transition 5P(3/2)-4D(5/2) in a (87)Rb atom with a femtosecond frequency comb, utilizing the recently developed spectroscopic technique of the double resonance optical pumping method. The absolute energy level of the 4D(5/2) state is determined by measuring the absolute frequency of the 5S(1/2)-5P(3/2) transition simultaneously. The hyperfine structure constants of the 4D(5/2) state are obtained by using the measured frequency. The magnetic dipole constant, A, is determined to be (-16.747+/-0.010) MHz with an uncertainty reduced 60-fold compared with a previous result. The electric quadrupole constant, B, is determined, for what is to our knowledge the first time, to be (4.149+/-0.059) MHz.     

Collision-induced vibrational energy transfer: K2[1(1) sigmau+ (nu'=2)]+He, H2-->K2[1(1) sigmau+(nu'=1,3)]+He, H2

Collisional energy transfer processes K2[1(1) sigmau+ (nu'=2)]+He, H2-->K2[1(1) sigmau+(nu'=1,3)]+He, H2 were studied by laser induced fluorescence under gas cell conditions. During the experiments, the cell temperature was kept constant at 420K. The buffer gas pressure was varied over the range from 40 to 250 Pa, Kz molecules were irradiated with pulses of radiation from an OPO laser, populating K2[1(1) sigmau+ (nu'=2)] by photon absorption. The resulting fluorescence included the direct component emitted in the decay of the optically excited state and the sensitized components arising from collisionally populated states. The decay signal of time-resolved fluorescence from 1(1) sigmau+ (nu'=2)]-->1(1) sigmag+(nu"=0) transition was monitored. In the early period after excitation, only very little population in states v'= 1 or 3 had yet accumulated, the rate of collisional activation to the state nu'=2 was negligible. The decay curve of the nu' = 2-->nu" = 0 was treated as a single exponential function. From the measurement of the time-resolved fluorescence, the semilog plot was shown. The slope yielded the effective lifetime of the nu' = 2--> nu' = 0 transition. Based on the Stern-Volmer equation, the radiative lifetime (36 +/- 7) ns was obtained. The total cross sections for deactivation of 1(1) sigmau1 (nu'=2) state by means of collisions with He and H2 are (3.0 +/- 0.5) x 10(-16) cm2 and (6.4 +/- 1.2) x 10(-15) cm2, respectively. The radiative lifetimes of 1(1) sigmau+(nu' = 1,3) states can also be determined through time-resolved fluorescence in pure K vapor. The time-integrated intensities of 1(1)sigmau+(nu' = 1,2,3)-->1(1) sigmag+(nu"=0) transition at different He or Hz pressure were measured. The ratio of fluorescence intensities versus 1/P(He, H2) can be fitted by a straight line. The slopes yield the cross sections sigma(nu'=2-->nu'=1) = (1.4 +/- 0.5) x 10(-16) cm2 and (3.2 +/- 1.0) x 10(-15) cm2; sigma(nu'=2-->nu'=3)= (1.2 +/- 0.4) x 10(-16) cm2 and (2.6 +/- 0.9) x 10(-15) cm2 for He and H2, respectively. Cross sections for the effective quenching of the nu' = 1,2,3 states were also determined. To our knowledge, the cross-sections for these processes are reported for first time.     

Emittance measurements of a laser-wakefield-accelerated electron beam

The transverse emittance of a relativistic electron beam generated by the interaction of a high-intensity laser with an underdense plasma has been measured with the "pepper-pot" method. For parameters pertaining to the forced laser wakefield regime, we have measured an emittance as low as (2.7+/-0.9) pi mm mrad for (55+/-2) MeV electrons. These measurements are consistent with 3D particle-in-cell simulations of the experiment, which additionally show the existence of a relatively large halo around the beam core.     

Imaging electron trajectories in a laser-wakefield cavity using betatron X-ray radiation

We demonstrate that betatron x-ray radiation accurately provides direct imaging of electrons trajectories accelerated in laser wakefields. Experimental far field x-ray beam profiles reveal that electrons can follow similar transverse trajectories with typical excursions of 1.5 microm+/-0.5 microm in the plane of laser polarization and 0.7 microm+/-0.2 microm in the plane perpendicular.     

Precision microwave measurement of the 2 3P1-2 3P2 interval in atomic helium

The 2 3P1-to- 2 3P2 interval in atomic helium is measured using a thermal beam of metastable helium atoms excited to the 2 3P state using a 1.08-&mgr;m diode laser. The 2 3P1-to- 2 3P2 transition is driven by 2.29-GHz microwaves in a coaxial transmission line. Our result of 2 291 174.0+/-1.4 kHz is the most precise measurement of helium 2 3P fine structure. This measurement plays a key role in obtaining a new value for the fine-structure constant.     

Excitation and relaxation of metastable state NaK(1 3pi) at high vibrational levels

The authors have investigated collision vibrational energy transfer rate constants in NaK[1 3pi(v)] and He system. Pump laser excitation of the spin-forbidden band was used to produce very highly vibrationally excited metastable state NaK[1 3pi (v = 22, 21, 20)]. The probe laser was used to excite the 1 3pi (v = 22, 21, 20) to 5 3pi(v'). Laser induced fluorescence (LIF) from 5 3pi --> 1 3sigma+ transition was used to follow the collision dynamics. The semilog plots of time-resolved LIF was obtained. The slopes yielded the effective lifetimes. From such data several Stern-Volmer plots could be constructed and the relaxation rate constants could be extracted for the sum of all processes that give rise to the decay of the prepared vibrational state. The rate constants (in units of 10(-11) cm3 x s(-1)) for v being 22, 21 and 20 are 1.4 +/- 0.1, 1.2 +/- 0.1 and 1.0 +/- 0.1, respectively. The vibrational relaxation rate is increasing with vibrational quantum number. In order to determine the importance of multiquantum relaxation, it is necessary to measure the relative population of both the prepared state and collisionally populated states. By the kinetic equations governing up to delta(v) = 2 transitions, the time dependence of populations of the vibrational states were obtained. With the help of the integrating the population equations over all time, the importance of the two-quantum relaxation could be studied experimentally. By varying the delay between the pump and the probe laser, the He pressure dependent vibrational state specific decay could be measured. The time evolutions and relative intensities of the three states v = 22, 21 and 20 by preparing v = 22 were obtained. Using experimental data the rate constants (in units of 10(-11) cm3 x s(-1)) for v = 22 --> 21 and v = 22 --> 20 are 0.67 +/- 0.15 and 0.49 +/- 0.12, respectively. The single quantum relaxation accounts for only about 48% of the total relaxation out of v = 22. Multi-quantum relaxation (delta(v) > 1) was found to be important at high vibrational states.     

Ultrafast dynamics process of platinum nanoparticles under femtosecond laser

The energy relaxation dynamics process of Pt nanoparticles with a diameter of 25 nm after a 400-nm femtosecond laser excitation was investigated using pump-probe technology with 130 fs laser pulse. Experimental results show that the process includes a fast decay with time constant of 850 fs and a slow decay with time constant of 9 ps. Theoretical discussion indicates that the former results from electron-phonon coupling process, while the latter is from phonon-phonon coupling process.     

Laser spectroscopic determination of the 6He nuclear charge radius

We have performed precision laser spectroscopy on individual 6He (t(1/2)=0.8 s) atoms confined and cooled in a magneto-optical trap, and measured the isotope shift between 6He and 4He to be 43 194.772+/-0.056 MHz for the 2(3)S1-3(3)P2 transition. Based on this measurement and atomic theory, the nuclear charge radius of 6He is determined for the first time in a method independent of nuclear models to be 2.054+/-0.014 fm. The result is compared with the values predicted by a number of nuclear structure calculations and tests their ability to characterize this loosely bound halo nucleus.     

热物性参量对飞秒激光烧蚀金属影响的分子动力学模拟

利用结合双温模型的分子动力学模拟方法,研究了飞秒激光与金属相互作用的烧蚀机制.采用中心波长为800 nm,能量密度从0.043 J·cm~(-2)到0.40 J·cm~(-2)不等,脉宽分别为70 fs和200 fs的激光烧蚀金属镍和铝材料.靶材的温度、原子位型以及内部压力随时间的演化展示了材料热物性参量特性和激光参量对烧蚀结果的影响.结果显示材料电子热传导率对飞秒脉宽激光下的影响仍然较大;对比铝和镍的结果可知,铝的电子晶格耦合系数比镍的小,故电子晶格间的温度梯度持续时间较长;铝的电子热传导系数比镍的大,所以材料上下表面电子温度耦合的时间缩短.铝薄膜表面在能量密度为0.40 J·cm~(-2)激光烧蚀下呈现纳米尺寸的晶体结构.     

Observation of laser-pulse shortening in nonlinear plasma waves

We have measured the temporal shortening of an ultraintense laser pulse interacting with an underdense plasma. When interacting with strongly nonlinear plasma waves, the laser pulse is shortened from 38 +/- 2 fs to the 10-14 fs level, with a 20% energy efficiency. The laser ponderomotive force excites a wakefield, which, along with relativistic self-phase modulation, broadens the laser spectrum and subsequently compresses the pulse. This mechanism is confirmed by 3D particle in cell simulations.