Saturation of the two-plasmon decay instability in long-scale-length plasmas relevant to direct-drive inertial confinement fusion

Measurements of the hot-electron generation by the two-plasmon-decay instability are made in plasmas relevant to direct-drive inertial confinement fusion. Density-scale lengths of 400 μm at n(cr)/4 in planar CH targets allowed the two-plasmon-decay instability to be driven to saturation for vacuum intensities above ~3.5×10(14) W cm(-2). In the saturated regime, ~1% of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to 90 keV as the laser beam intensity is increased from 2 to 7×10(14) W cm(-2). This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.     

Generating energetic electrons through staged acceleration in the two-plasmon-decay instability in inertial confinement fusion

A new hot-electron generation mechanism in two-plasmon-decay instabilities is described based on a series of 2D, long-term (~10 ps) particle-in-cell and fluid simulations under parameters relevant to inertial confinement fusion. The simulations show that significant laser absorption and hot-electron generation occur in the nonlinear stage. The hot electrons are stage accelerated from the low-density region to the high-density region. New modes with small phase velocities develop in the low-density region in the nonlinear stage and form the first stage for electron acceleration. Electron-ion collisions are shown to significantly reduce the efficiency of this acceleration mechanism.     

Origin of third-order optical nonlinearity in Au:SiO(2) composite films on femtosecond and picosecond time scales

Three sorts of probe laser, which have pulse durations of 200 fs, 35 ps, and 70 ps, were employed in the measurement of the third-order nonlinear optical susceptibility x((3)) in Au:SiO(2) composite films in a degenerate four-wave mixing scheme. We found that the composite films at their absorption peak (~550 nm) had a maximum x((3)) , which depends strongly on the pulse width of the probe laser. The value of x((3)) measured with a 70-ps laser was ~30 times larger than that measured with a 200-fs laser. The time-resolved measurements revealed that the optical nonlinearity on the femtosecond time scale is attributable mainly to contributions from the interband electric-dipole transition (especially at low concentrations) and partly to those from hot electrons rather than being dominated by hot-electron excitation in the picosecond regime.     

Enhanced photoinduced desorption from metal nanoparticles by photoexcitation of confined hot electrons using femtosecond laser pulses

Strong fluence dependence of photodesorption cross sections is observed in femtosecond laser photodesorption of NO from (NO)2 on silver nanoparticles, in contrast to femtosecond photodesorption on bulk metals. The time scale of excitation buildup is found to be equal or less than the pulse duration of ～100 fs; NO translational energies are independent of fluence and pulse duration. We propose a nanoparticle-specific nonlinear mechanism in which, due to confinement, strongly nonthermal hot-electron distributions are maintained during the femtosecond pulses, enhancing the normal desorption pathway.     

Role of hot-electron preheating in the compression of direct-drive imploding targets with cryogenic D2 ablators

The compression of direct-drive, spherical implosions is studied using cryogenic D2 targets on the 60-beam, 351-nm OMEGA laser with intensities ranging from approximately 3x10(14) to approximately 1x10(15) W/cm2. The hard-x-ray signal from hot electrons generated by laser-plasma instabilities increases with laser intensity, while the areal density decreases. Mitigating hot-electron production, by reducing the laser intensity to approximately 3x10(14) W/cm2, results in areal density of the order of approximately 140 mg/cm2, in good agreement with 1D simulations. These results will be considered in future direct-drive-ignition designs.     

纳秒强光场下碘甲烷的激光电离中高价离子产生研究

利用25纳秒脉冲Nd-YAG 532 nm的激光,在1011 W cm-2的光场强度下,用飞行时间质谱对不同载气条件下碘甲烷的激光电离过程进行了研究.当利用氩作为载气时,除观察到H+,C+,CH+,CH+3,I+,CH3I+等离子外,还观察到很强的C2+,I2+和I3+离子信号.这些高价离子的最可几平动能分别为55.5 eV,9.5 eV和27 eV.质谱峰形的分裂现象以及不同载气的实验结果表明这些高价离子可能来源于碘甲烷团簇的库仑爆炸过程.     

高位K2(1Λg) 与基态原子的碰撞转移

在K原子密度约为0.5～5×1016cm-3的样品池中,脉冲激光710 nm线双光子激发K2基态到高位1Λg态,研究了K2(1Λg)+ K(4S)碰撞转移过程.K原子密度由测量KD2线蓝翼对白光的吸收得到.测量不同K密度下1Λg态发射的时间分辨荧光强度,它是一条指数衰减曲线,由此得到1Λg态的有效寿命,从描绘出的有效寿命倒数与K原子密度关系直线的斜率得到1Λg态总的碰撞猝灭截面为(2.1±0.2)×10-14cm2,从截距得到的辐射寿命为(22±2)ns.测量了K的6S →4P3/2和4D→4P3/2在不同K密度下的时间积分荧光强度,得到了K2(1Λg)+K→K2(11∑ +g)+K(6S,4D)碰撞转移截面为(1.5±0.3)×10-15cm2(对转移到6S)和(8.5±3.0)×10-15cm2(对转移到4D).     

高位K_2分子与基态K原子及H_2分子间的激发转移

激光双光子激发K2至1Λg高位态,利用分子荧光光谱方法,研究了1Λg-3Λg间的碰撞转移截面。在纯K实验中,池温控制在553至603 K之间,K原子密度由光学吸收法测量得到。探测1Λg-11Σu+的直接时间分辨荧光的光强,它是一条纯指数衰减曲线,由此得到1Λg+态的有效寿命,有效寿命的倒数与K密度成线性关系,从直线的斜率得到1Λg态的猝灭截面为(2.5±0.3)×10-14cm2,从截距得到辐射寿命为(20±2)ns。由3Λg→13Σu+转移荧光的时间分辨谱,用类似的方法得到3Λg的猝灭截面为(2.5±0.6)×10-14cm2,辐射寿命为(16.0±3.2)ns。由1Λg→11Σu+与3Λg→13Σu+的时间积分强度比得到K2(1Λg)+K→K2(3Λg)+K的转移截面为(1.1±0.3)×10-14cm2。在K2-H2碰撞实验中,池温保持在553 K,K密度为5×1015cm-3,H2气压在40~400 Pa之间,其中K2-K碰撞效应是不能略去的,但可以用纯K结果扣除,得到K2(1Λg)+H2→K2(3Λg)+H2的碰撞转移截面为(2.7±1.1)×10-15cm2。K2(3Λg)+H2→K2(3Λg)以外态+H2的猝灭截面为(6.8±2.7)×10-15cm2。     

The cross sections and their temperature dependence of collisional energy transfer from high vibrationally excited ethylene molecules to electronic ground state Rubidium atoms

The cross sections and their temperature dependence of collisional energy transfer from the high vibrationally excited C_2H_4 to the electronic ground sate of Rb (V-E) have been studied by the method of IR laser pump combined with fluorescence probe. The fluorescence of rudidium atoms due to V-E energy transfer was detected. The measured rate constants of this V-E energy transfer is about 10~(-10)cm~2molec~(-1)s~(-1) and the corresponding cross section varies from 60 (?)~2 to 90 (?)~2. The cross sections decrease with increasing temperature over the temperature range of 393-683 K.     

High-Yield High-Efficiency Positron Generation in High-Z Metal Targets Irradiated by Laser Produced Electrons from Near-Critical Density Plasmas

An improved indirect scheme for laser positron generation is proposed. The positron yields in high-Z metal targets irradiated by laser produced electrons from near-critical density plasmas and underdense plasma are investigated numerically. It is found that the positron yield is mainly affected by the number of electrons of energies up to several hundreds of MeV. Using near-critical density targets for electron acceleration, the number of high energy electrons can be increased dramatically. Through start-to-end simulations, it is shown that up to 6.78 x 10~(10) positrons can be generated with state-of-the-art Joule-class femtosecond laser systems.     

碰撞诱导振动能量转移：Cs2[B1∏u(v′=5)]+ N2→Cs2[B1∏u(v′=4，6)]+ N2

利用激光诱导荧光方法研究了Cs_2B~1Π_u[(v′=5)]与N2的碰撞能量转移.脉冲激光激发Cs_2基态至B~1Π_u[(v′=5)]态,池温保持在410 K,N_2气压在1.5×10~2 Pa～2.5×10~3 Pa之间变化.荧光中含有直接荧光和碰撞转移荧光成分,记录直接荧光B~1Πu(v′=5)→Χ~1∑~+_g(v″=0)的时间分辨强度.从荧光强度的对数值给出的直线斜率得到B~1Π_u(v′=5)→Χ~1∑~+_g(v″=0)的有效寿命,由Stern-Volmer方程,得到B~1Π_u(v′=5)→Χ~1∑~+_g(v″=0)的辐射寿命为(45±9)ns.B~1Π_u(v′=5)态与N_2碰撞的猝灭总截面为(9.8±1.5)×10~(-15)cm~2.用类似的方法得到B~1Π_u(v′=4,6)能级的辐射寿命.在不同的N_2气压下,测量B~1Π_u(v′=5,4,6)→Χ~1∑~+_g(v″=0)的时间积分荧光强度,首次得到v′=5→v′=4及v′=5→v′=6的碰撞转移截面分别为(3.9±0.8)×10~(-15) cm~2和(4.1±0.8)×10~(-15)cm~2.     

Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere

X ray radiation as high as 50 keV, including K(α) of Ba and Mo, have been observed from a solid target during the interaction of low energy ~0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.     

Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator

Single-mode X couplers and three-dimensional waveguides are fabricated in transparent glasses by use of an unamplified femtosecond laser generating energies of up to 100 nJ. Changing fabrication parameters such as power and scanning speed permits creation of waveguides with a wide range of structures and refractive-index difference. Optical coherence tomography shows large refractive-index changes of up to ~10(-2) in the waveguides; these changes are consistent with guided mode analysis.     

Light scattering by single magnons in FeF3

Scattering of laser light from single magnons has been observed in the canted antiferromagnet FeF₃ as a function of temperature (260 – 360°K) and wave-vector (2 × 10⁵ ? 3 × 10⁴cm^?1). Measured magnon energies ranging up to 3.3 cm^?1 (260°K) and line widths are in good agreement with data from microwave resonance work.     

多组态Dirac-Fock方法与准相对论组态相互作用方法的比较研究

利用多组态Dirac-Fock(MCDF)方法与准相对论组态相互作用方法,分别详细计算了低Z、中Z和高Z原子(离子)各个壳层上电子的束缚能、平均轨道半径、总束缚能、激发能、精细结构能级以及类Ne等电子系列离子的2p53s 1,3P1-2p6 1S0跃迁能,并且对这两种方法的结果进行了数值比较.     

Laser-energy transfer and enhancement of plasma waves and electron beams by interfering high-intensity laser pulses

The effects of interference due to crossed laser beams were studied experimentally in the high-intensity regime. Two ultrashort (400 fs), high-intensity (4 x 10(17) and 1.6 x 10(18) W/cm(2)) and 1 microm wavelength laser pulses were crossed in a plasma of density 4 x 10(19) cm(3). Energy was observed to be transferred from the higher-power to the lower-power pulse, increasing the amplitude of the plasma wave propagating in the direction of the latter. This results in increased electron self-trapping and plasma-wave acceleration gradient, which led to an increased number of hot electrons (by 300%) and hot-electron temperature (by 70%) and a decreased electron-beam divergence angle (by 45%), as compared with single-pulse illumination. Simulations reveal that increased stochastic heating of electrons may have also contributed to the electron-beam enhancement.     

Application of snow nanograin targets for the generation of fast ions in femtosecond laser plasma

We have measured the energy of the directed motion of multiply charged ions produced when solid targets are exposed to low-contrast (10^?3–10^?2) femtosecond laser pulses with intensities 10¹⁵–10¹⁶ W cm^?2. The measurements are based on the recording of spatially resolved X-ray spectra for H-and He-like oxygen ions in the target plane. Analysis of the He_β and Ly_α line profiles has revealed fractions of accelerated ions in plasma with energies from several to several tens of kiloelectronvolts. We show that using a layer of frozen nanometer-size water droplets as the targets leads to an effective absorption of laser pulses and a twofold rise in the energy (to 0.1 MeV) of He-like oxygen ions compared to the use of solid targets.     

High power-efficiency terahertz quantum cascade laser

We demonstrate continuous-wave(CW) high power-efficiency terahertz quantum cascade laser based on semiinsulating surface-plasmon waveguide with epitaxial-side down(Epi-down) mounting process.The performance of the device is analyzed in detail.The laser emits at a frequency of ~3.27 THz and has a maximum CW operating temperature of ~ 70 K.The peak output powers are 177 mW in pulsed mode and 149 mW in CW mode at 10 K for 130-μm-wide Epi-down mounted lasers.The record wall-plug efficiencies in direct measurement are 2.26% and 2.05% in pulsed and CW mode,respectively.     

Photonuclear physics when a multiterawatt laser pulse interacts with solid targets

When a laser pulse of intensity 10(19) W cm(-2) interacts with solid targets, electrons of energies of some tens of MeV are produced. In a tantalum target, the electrons generate an intense highly directional gamma-ray beam that can be used to carry out photonuclear reactions. The isotopes 11C, 38K, (62,64)Cu, 63Zn, 106Ag, 140Pr, and 180Ta have been produced by (gamma,n) reactions using the VULCAN laser beam. In addition, laser-induced nuclear fission in 238U has been demonstrated, a process which was theoretically predicted at such laser intensities more than ten years ago. The ratio of the 11C and the 62Cu beta(+) activities yields shot-by-shot temperatures of the suprathermal electrons at laser intensities of approximately 10(19) W cm(-2).     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).