Generating high-current monoenergetic proton beams by a circularly polarized laser pulse in the phase-stable acceleration regime

A new ion acceleration method, namely, phase-stable acceleration, using circularly-polarized laser pulses is proposed. When the initial target density n(0) and thickness D satisfy a(L) approximately (n(0)/n(c))D/lambda(L) and D>l(s) with a(L), lambda(L), l(s), and n(c) the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively, a quasiequilibrium for the electrons is established by the light pressure and the space charge electrostatic field at the interacting front of the laser pulse. The ions within the skin depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a high-intensity monoenergetic proton beam can be generated. The proton dynamics is investigated analytically and the results are verified by one- and two-dimensional particle-in-cell simulations.     

Near-Field Diffractive Optical Pumping of a Laser Medium

A Nd:YAG rod active element placed inside a resonator was pumped by the second harmonic radiation (the wavelength =0.532 m) of a Q-switched Nd:YAG laser diffracted at a circular aperture. Various distributions of pumping intensity in the Fresnel (near-field) region along the direction of the pump beam diffraction were produced. Inversion profiles with maxima or minima at the resonator axis were formed in the active element, depending on its distance from the circular aperture. Gain-switched operation of the Nd:YAG rod laser at =1.064 m is reported at the fundamental mode and also at the TEM₀₁, TEM_01* and other higher-order modes for different positions of the active element in the near-field region. Applications of such diffractive optical pumping for the concentration of the pumping radiation into the active media and for laser beam profling are discussed. © 2004 The Optical Society of Japan     

High-power continuous-wave upconversion fiber laser at room temperature

We report cw laser emission of a Pr, Yb-doped ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF fiber in the red spectral region. Laser emission was achieved on the transition P(0)(3)?F(2)(3)(lambda(L)=635 nm) with a Ti:sapphire pump laser tuned to lambda(p)=850 nm . A maximum output power of P(out)=675 mW was obtained at an incident pump power of P(in)=3.37 W . The output power was increased to P(out)=1020 mW when pumping with P(in)=5.51 W was provided by two Ti:sapphire lasers. A photon avalanche process was found to contribute strongly to the population of the upper laser level.     

Finite-size scaling and universality of the thermal resistivity of liquid 4He near Tlambda

We present measurements of the thermal resistivity rho(t,P,L) near the superfluid transition of 4He at saturated vapor pressure and confined in cylindrical geometries with radii L=0.5 and 1.0 microm [t identical with T/T(lambda)(P)-1]. For L=1.0 microm measurements at six pressures P are presented. At and above T(lambda) the data are consistent with a universal scaling function F(X)=(L/xi(0))(x/nu)(rho/rho(0)), X=(L/xi(0))(1/nu)t valid for all P (rho(0) and x are the pressure-dependent amplitude and effective exponent of the bulk resistivity rho, and xi=xi(0)t(-nu) is the correlation length). Indications of breakdown of scaling and universality are observed below T(lambda).     

Field and temperature dependence of the superfluid density in LaFeAsO1-xFx superconductors: a muon spin relaxation study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.     

Luminescence of Higher Plants upon Excitation by Laser Radiation with Different Energy and Time Parameters

The luminescence response of the leaves of higher plants exposed to laser radiation with different energy and time parameters has been investigated at a wavelength of 685 nm. The plants were irradiated by laser radiation at 1060, 632, and 530 nm. Radiation at = 632 nm was used as the control. It is shown that radiation at = 1060 nm suppresses photosynthesis, whereas that at = 530 nm stimulates it.     

Photoemission electron microscopy as a tool for the investigation of optical near fields

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO(x)). Photoemission was induced by illumination with a Hg UV lamp (photon energy cutoff homega(UV) = 5.0 eV, wavelength lambda(UV) = 250 nm) and with a Ti:sapphire femtosecond laser (homega(l) = 3.1 eV, lambda(l) = 400 nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.     

Dynamics of the Emission of Light by C2 and C3 Molecules in a Laser Plasma Produced by Two-Pulse Irradiation of the Target

The laser plasma produced by irradiation of a graphite target simultaneously by the first and second harmonics of two Nd³⁺:YAG lasers has been studied by emission spectroscopy methods. The delay between radiation pulses () varied from 0 to 700 sec. It is established that in the absence of a delay between pulses ( = 0) the increase in the intensity of plasma emission at the wavelengths corresponding to the radiative transitions of the C₂ and C₃ molecules is of nonradiative character. The plasma produced by laser radiation at the wavelength = 1064 nm exerts its influence on the radiation spectrum of the plasma produced by laser radiation at the wavelength = 532 nm, if the magnitude of the delay between laser pulses does not exceed 30 sec. The most probable reason for this character of influence of the prepulse on the laser plasma radiation spectrum is sublimation of soot particles caused by laser radiation at = 532 nm.     

Production of WZ events in pp collisions at square root(s) = 1.96 TeV and limits on anomalous WWZ couplings

We present results from a search for WZ production with subsequent decay to l nu l' l' (l and l' = e or mu) using 0.30 fb(-1) of data collected by the D0 experiment between 2002 and 2004 at the Fermilab Tevatron. Three events with WZ decay characteristics are observed. With an estimated background of 0.71 +/- 0.08 events, we measure the WZ production cross section to be 4.5(-2.6)(+3.8) pb, with a 95% C.L. upper limit of 13.3 pb. The 95% C.L. limits for anomalous WWZ couplings are found to be -2.0 < delta kappaZ < 2.4 for form factor scale lambda = 1 TeV, and -0.48 < lambdaZ < 0.48 and -0.49 < delta g(1)(Z) < 0.66 for lambda = 1.5 TeV.     

An integrated organic photoconductive detector for optoelectronics

In the system (poly-(N-vinyl carbazole) (PVK): trinitrofluorenone (TNF) a photoconductive complex is formed. Thin metal films are sputtered on BK7 glass substrates, forming two electrodes with 1 mm separation. The photoconductive complex is deposited from a solution in chlorobenzene/tetrahydrofurane on the glass substrate between the electrodes, forming a photoconductive detector. Photoconductivity of these planar detectors is studied using He-Ne laser light (=633 nm) as a function of electric field and for different TNF concentrations. An increase of photosensitivity is found for high TNF concentrations.Time resolution of the photoconductive PVK:TNF detector is investigated using a ruby pulse laser (=694 nm). The possible use of such detectors in combination with other polymer lightguides is demonstrated. Compared to most polymer waveguide materials PVK exhibits a rather high refractive index ofn=1.7. Thus the coupling of light into the photoconductive film is achieved directly. The planar structure allows further integration of polymeric components for optoelectronics.     

Novel waveguide design for high brightness optoelectronic applications

We describe a novel compound ridge waveguide in which the highest order propagating mode is guided in the low index region of the structure and exhibits a narrow single lobe. For a prototype AlGaAs/GaAs was waveguide designed for operation at = 980 nm we show that due to the narrow lobed emission this mode produces significant improvements in source brightness and hence fibre coupling efficiency. In the case of a 61 m aperture laser diode operating in this mode 20% coupling efficiency is predicted between a structure and standard single mode fibre designed for = 980 nm operation.     

Reducing the first-order Doppler shift in a Sagnac interferometer

We demonstrate a technique to reduce first-order Doppler shifts in crossed atomic/molecular and laser beam setups by aligning two counterpropagating laser beams as part of a Sagnac interferometer. Interference fringes on the exit port of the interferometer reveal minute deviations from perfect antiparallelism. Residual Doppler shifts of this method scale with the ratio v/(4d) of the typical atomic/molecular velocity v and the laser beam diameter d. The method is implemented for precision frequency calibration studies at deep-UV wavelengths, both in one- and two-photon excitation schemes: the 6s(2) --> 6s30p(3/2)J=1 line in Yb at 199 nm and the 4p(6) --> 4p(5)p[1/2](0) transition in Kr at lambda=212 nm. The achieved precision of 6 x 10(-10) is limited by the characteristics of the laser system.     

Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Laser-induced desorption of metal atoms at low rate has been studied for pulsed excitation with wavelengths of = 266, 355, 532 and 1064 nm. For this purpose sodium adsorbed on quartz served as a model system. The detached Na atoms were photo-ionized with the light of a second laser operating at = 193 nm and their kinetic energy distribution was determined by time-of-flight measurements. For = 1064 nm a distribution typical of thermal bond breaking is observed. If desorption, however, is stimulated with light of = 266 or 532 nm, the kinetic energy distribution is non-thermal with a single maximum atE _kin = 0.16 ± 0.02 eV. For = 355 nm the non-thermal distribution is even bimodal with maxima appearing atE _kin = 0.16 ± 0.02 and 0.33 ± 0.02 eV. These values of the kinetic energies actually remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of different binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanism is discussed in the framework of the Menzel-Gomer-Redhead scenario. The transition from non-thermal to thermal desorption at large fluentes of the laser light could also be identified.     

Orange,red and deep-red flashlamp-pumped Pr3+:LiYF4 laser with improved output energy and efficiency

Three laser transitions (607.2 nm, 639.5 nm, 720.9 nm) in a flashlamp-pumped Pr³⁺:LiYF₄ laser at room temperature are investigated. Cerium ions (Ce³⁺) doped into the fused-quartz envelope of the flashlamp efficiently transfer strong UV radiation from pumping light into the absorption region of Pr³⁺ ions ( = 420–480 nm), so that the slope efficiency of all three laser emissions could be increased by almost 100% compared to excitation with a pure quartz flashlamp. At a pump energy of 30 J the output energy of these three laser emissions reached 4.7 mJ, 87 mJ and 30 mJ.     

Diode-pumped Tm: YAG solid-state lasers with indirect and direct manifold pumping

We study the continuous-wave (cw) characteristics of both two-manifold and three-manifold Tm: YAG laser pumped at _p 1.8 µm or _p = 0.785 µm and lasing at ₁ = 2.02 µm. The three-manifold rate equations are adiabatically reduced to their two-manifold form. For each pumping scheme, the steady-state rate equations are combined with the cw differential equations for the forward- and reverse-lasing fields and the pump-depletion differential equation. These three coupled cw differential equations are solved analytically. This gives the linear flux-conservation law between the input pump and the laser output, the minimum crystal length, and optimal output couplings. We show that the major difference between these two pumping schemes is due to the different pump effective absorption cross sections and not the two-for-one cross relaxation. Our example shows that the minimum intensity threshold and optimal crystal length are smaller for pumping at _tp = 0.785 µm than pumping at _p 1.8 µm.     

Gradient-Field Sorting of Microparticles

The method of spatial sorting of different types of microparticles in a liquid with the aid of a gradient laser field has been proposed and implemented. The method is based on linear scanning of the interference fringes of the field imposed on a suspension. It is shown theoretically that at given parameters of the particles and field it is possible to select such a rate of scanning of interference fringes at which only the particles of one species will be captured and carried in the wake of the field, while others are left in place. The efficiency of the proposed method of selection of particles is confirmed experimentally with an example of a water suspension of polymeric small balls of diameter 5.8 and 10 m illuminated by interference fringes of He–Ne laser radiation ( = 632.8 nm). The implemented method of control of microparticles can find application in physics, biology, medicine, microtechnology, and also in a number of other fields.     

Investigation of emission characteristics of a photodissociation thallium laser

Results are reported of investigations of the spectral and energy properties of the emission of a thallium laser on the transitions 7S_1/2-6P_3/2 ( = 535 nm) and 7S_1/2-6P_1/2 ( = 377.6 nm). Excited thallium atoms are produced in dissociation of the TII molecule by radiation from an ArF laser (I = 193 nm). The experiments were performed with two cavity types, to make possible operation with lasing at one or two wavelengths. In the case of operation with one wavelength, the emission spectrum at = 535 nm consists of the hyperfine structure components of two isotopes T1²⁰³ and T1²⁰⁵ whose frequency difference depends on the emission flux density in the cavity. At the = 377.6–nm wavelength the isotopic structure of the line is not manifested, since the isotope shift is small compared with the Doppler width. When two wavelengths are generated, it is observed that when a certain flux density is reached the emission structure is qualitatively altered, with unequal changes in the character of the emission spectra of the different lines. The results are discussed.Translated from Lazernye Sistemy, pp. 58–66, 1982.     

Submillimeter-wave dielectric measurements using an open-resonator

The open resonator method for dielectric measurements is developed in the submillimeter-wave region. This method, different from usual laser methods is based on the sweeping of resonator length. The conventional theory developed in the millimeter-wave region is improved so as to make measurements possible with resonable precision even in this frequency region and the design criterion of an optimum resonator at around 890 GHz (₀=337m) is presented. Finally, the measurements are performed for several kinds of low loss material and it is confirmed that the present method becomes a useful one in the submillimeter-wave region.     

Dynamics of the Trapping of Microparticles by an Interference Field on Exposure to Continuous and Pulsed Radiation

The dynamics of motion of microparticles in a gradient light field depending on the acting radiation time sequences has been studied theoretically and experimentally. A water suspension of small polymeric balls illuminated by interference fringes of the radiation from a cw He–Ne laser ( = 632.8 nm) and also pulse-periodic YAG:Nd³⁺ laser ( = 532.1 nm; _p 17 nsec; f 50 Hz) was used. It has been established that the gradient field generated by continuous radiation ensures a higher velocity for transporting particles and, consequently, a more effective change in their local concentration and acceleration of their diffusion. In contrast to this, interference irradiation of particles by short repetitive pulses is not very effective for their transportation in space but ensures much higher instantaneous velocities and a higher kinetic energy. As applied to biological objects, this regime has a slight effect on the local concentration and diffusion of particles, but can cause conformational changes in biological structures.     

利用遗传算法设计可见光波段全能反射器

介绍了遗传算法在设计宽频带全能反射器中的应用.设计的关键是通过遗传算法寻找到不同一维准周期光子晶体间的最佳组合方式以及光学厚度以便形成一个光子晶体异质结.利用传输矩阵法分析了一维准周期系统中的电磁传输特性.计算结果表明,准周期光子晶体的全方向反射带宽受晶胞单元以及周期数的影响.根据这些规律,用遗传算法成功地优化了光子晶体异质结的结构并得到两种适用于可见光波段的高性能全能反射器.例如结构为(HLLHL)13(HL)13( LHL)15 ［注: nL=1.46,nH=2.6,dL=0.218λ0/nL且dH=0.201λ0/nH］的反射器在可见光波段内的全方向反射带宽达到了88.42% (446 nm~779 nm).