Controlling the width of picosecond laser pulses

The relaxation time τ_R of the saturable dye used to mode-lock a Nd: YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Δω changed by the insertion of an etalon. The pulse duration τ_p was approximately transform-limited for τ_R<2π/Δω but increased to about twice this value when 2π/Δω<τ_p<τ_R. No significant increase in pulse duration was observed for τ_R?2π/Δω but multiple pulses were generated within each round-trip-transit-time.     

Study of polarized luminescence in erbium-doped laser glasses

A partial polarization of luminescence in laser phosphate and silicate erbium-doped glasses was found to take place for the fundamental laser transition ⁴ I _13/2→⁴ I _15/2 (λ=1.55 μm) under excitation by linearly polarized laser radiation (532 and 790–990 nm). The shape of the luminescence spectrum depends on the wavelength of the exciting light and on the composition of the glass matrix. The degree of polarization of the luminescence depends on the spectral range of both the excitation and the detection, attaining a maximum of ～1%. The concentration dependence of the degree of polarization is studied.     

Study of excited states in49V with the48Ti (p, γ)49V reaction

In the⁴⁸Ti(p, γ)⁴⁹V reaction gamma decays of thirteen resonances betweenE _p =960 and 1570 keV are investigated. Level energies within ±0.5–±2.0 keV andQ-value 6756.8±1.5 keV are obtained. Branching ratios for the resonance states and strongly populated bound states are given. Gamma-ray angular distribution measurements yield the followingJ(keV) assignments of⁴⁹V bound and resonance states:J(1140)=5/2,J ^π(2235)=5/2^(?),J(2264)=(3/2),J(2308)=3/2,J(3912)=3/2,J(8105,Ep=1374keV)=(1/2) andJ ^π(8289,E _p =1564keV)=3/2^(?). Multipolarity mixing ratios for all measured primary and secondary gamma rays are tabulated. Dopplershift attenuation measurements yield the mean lifetimes τ _m (keV) of the following bound states in⁴⁹V:τ _m (748)=(200± ₁₀₀ ⁴⁰⁰ )fs, τ _m (1140)=(250± ₁₀₀ ⁵⁰⁰ )fs, τ _m (1155)>400 fs, τ _m (1515)=(45± ₂₀ ³⁰ )fs, τ _m (1644)=(55± ₂₀ ³⁰ )fs, τ _m (1661)=(25±5)fs, τ _m (1994)>400 fs, τ _m (2235)=(30± ₁₅ ³⁰ )fs, τ _m (2264)=(45± ₁₅ ³⁰ )fs and τ _m (2308)=(20±10)fs.     

Resonant Raman scattering of polymeric sulfur nitride modified by iodine

Resonance Raman spectra of (SNI_y)_x crystals have been measured at 150 K. The energies of the principal Raman lines of the chromophore (109 and 154 cm^?1) and their intensity behaviour with exciting laser frequency are consistent with the formation of a charge-transfer complex in which iodine enters the interfiber regions of the (SN)_x lattice as I₅^- or I₃^- linked to distorted I₂ units. This structural model is supported by comparison of these Raman data with the excitation profiles of the chromophoric group in starch-iodine and α-cyclodextrine-iodine complexes. Possible mechanisms for the conductivity increase on the basis of the proposed charge transfer model are also discussed.     

Dislocation dynamics in pulse-loaded NaCl crystals

The mean path l of freshly introduced dislocations in NaCl crystals under the effect of triangular loading pulses τ(t) is shown to depend only on the pulse amplitude τ_m and be insensitive to the rate of stress growth. The replacement of triangular pulses with trapezoidal ones with a constant-load plateau (τ=const) extension of up to 60 min only insignificantly changes the l(τ_m) dependence. The data obtained are interpreted on the basis of the concept of quasi-static relaxation in a nonequilibrium system of dislocations subject to a combined effect of time-dependent applied stresses τ(t), coordinate-dependent internal stresses τ_i(x), and “dry friction” τ_p due to the pinning of dislocations by point defects. In such a model, the l(τ_m) dependence should saturate at τ_m<2τ_p; this is in fact observed in the range of 0.2τ_m<τ_m<0.3τ_y (here, τ_y is the yield stress), which gives an estimate for the pinning stress τ_p≈0.1τ_y. Based on the model suggested, a series of experimentally confirmed predictions were obtained, e.g., a recipe of “switching-off” of anomalies. Thus, a preliminary treatment of the samples by a series of stress pulses or holding in a magnetic field, which transforms the system of fresh dislocations into a more equilibrium state, sharply decreases the density of mobile dislocations quasi-statically responding to a pulsed load. It is shown that the discussed anomalies of dislocation mobility should be observed only in sufficiently pure crystals, where , and should be absent in contaminated crystals, where τ_p ～ τ_y.     

Exciton molecule in semiconductors by phonon-assisted two-photon absorption

Direct creation of bi-excitonic states by photon-assisted two-photon absorption in indirect-gap semiconductors is investigated theoretically. The symmetry of the indirect bi-exciton states and of the phonon used are given for the case of Ge. A numerical application to the case of Si shows that the indirect two-photon absorption coefficient for bi-excitonic α²_I (bi-ex) transitions is several orders of magnitude larger than both indirect two-photon interband, α²_I (band), and excitonic, α²_I (exc), transitions. It becomes smaller than both indirect one-photon interband, α¹_I (band), and excitonic, α¹_I (exc), transitions for available laser intensities. The essential contribution to this enhancement of α²_I (bi-ex) is found to be from the resonance effect in the first process and from both the resonance effect and matrix elements included in the second process.     

Attosecond manipulation of ionization and efficient broadband supercontinuum generation in stretched molecules

<正>We theoretically investigate the high-order harmonic generation from stretched molecules in a linearly polarized intense field.By adopting an infrared pulse combined with an ultraviolet(UV) attosecond pulse,the ionization process can be controlled effectively.In this excitation scheme,the harmonic spectrum beyond I_p+3.17U_P is significantly enhanced by two orders,where I_p and U_p=e~2E_0~2/(4m_eω~2) are the ionization and ponderomotive potential,then smooth broadband supercontinuum with the bandwidth of about 120 eV is obtained,which leads to an isolated sub-60- as attosecond pulse with a high signal-noise ratio.Moreover,the bandwidth of the supercontinuum is weakly dependent on the location and pulse duration of the UV pulse.     

Second order logarithmic corrections to the Drell-Yan cross-section

We present a complete order α _s ² calculation of the large logarithmic terms of the type ln ⁱ (1?x)/(1?x) (x=Q ²/s), which appear in the Wilson coefficient of the total and differential DY cross-sections. These terms are computed using renormalizationgroup methods. It is shown that besides the well known constant part, they constitute the bulk of the radiative correction. This in particular holds for the higher τ-region which is still accessible to experiment. The large logarithmic corrections determine the shape of theK-factor and give a partial explanation of the phenomenon of anomalous scaling.     

Der Faraday-Effekt an amorphen Selenschichten

Cl^38m was produced by neutron activation of chlorine containing samples in a pneumatic irradiation facility. The decay of the Cl³⁸-isomer was investigated by various scintillation spectrometer technics. The half-life and the internal conversion coefficient α of the 0.66 MeV isomeric transition was found to beT _1/2=0.74±0.03 sec and α=(5.0±1.2)×10^?4. These data suggest aM-3 transition between an isomeric state with the spinI=5^? and the ground-state of Cl-38 (I=2^?).     

Photoinduced semiconductor-metal phase transition in a peierls system

The dynamical equation for the order parameter of the metal-semiconductor phase transition, as well as the kinetic equation for the density of nonequilibrium electron-hole pairs of a Peierls system in a light field, has been derived. An expression for the time τ of the nonthermal photoinduced semiconductor-metal phase transition has been obtained from these equations for the case of an ultrashort light pulse. It has been shown that, to initiate the phase transition, the energy density W of the light pulse must be higher than the critical value W _c. The W _c, τ, and optical absorption coefficient γ₀ that are calculated in the framework of the proposed model are in agreement with the experimental data (W _c ≈ 12 mJ/cm², τ ≈ 75 fs, and γ₀ ≈ 10⁵ cm^?1) on the irradiation of a vanadium dioxide film by a laser pulse with a duration of τ_p ≈ 15 fs, a photon energy of ?θ₀ = 1.6 eV, and an energy density of W = 50 mJ/cm².     

Evolution of a turbulent cascade on the surface of liquid hydrogen under a change in the spectral characteristic of an exciting force

The modification of the turbulent cascade in a system of capillary waves on the surface of liquid hydrogen under a change in the spectral width of exciting noise has been experimentally studied. The correlation function I _ω of the deviations of the surface of liquid hydrogen from equilibrium under broadband excitation is a monotonically decreasing function of frequency. The distribution I _ω in the inertial range is well described by the power function ω^?m with the exponent m close to 17/6. In the presence of narrowband excitation, a chain of peaks appears on the cascade I _ω; the positions of the peaks are described by a power function of frequency with the exponent m = 3.8 ± 0.1.     

An effective lagrangian for multiphoton processes and a non-linear Born-Infeld lagrangian

An effective lagrangian for multiphoton processes (of Euler-Heisenberg type) is computed for an electromagnetic field interacting with massive scalar, fermion and vector particles. It is found that if the charges g_I, masses m_I and numbers N_I of massive particles satisfy the “supersymmetric” conditions, N_Sm_S⁴+3N_Vm_V⁴?2N_Fm_F⁴=0 and g_I²/m_I⁴ is I-independent, where I=S, F, V stands for scalars, fermions and vectors respectively, then not only quartic divergences (poles at n=0 in dimensional regularization) proportional to constant terms cancel out but also maximally helicity changing interaction terms vanish in the effective lagrangian. A possibility to relate a non-linear Born-Infeld lagrangian to the effective lagrangian is examined.     

Light Scattering Studies on Phase Behavior of PP/PcBR Blends During Melt‐Mixing

Phase formation, morphology, and their evolution of binary blends of polypropylene with poly(cis‐butadiene) rubber were investigated by a back small angle laser scattering (BSALS) on‐line system and online sampling. The morphology formation process can be divided into three stages: early stage, intermediate stage, and late stage. Phase contrast microscopy (PCM) and small angle laser scattering (SALS) measurements have been introduced to compare with the results of BSALS and the corresponding phase morphology was also observed using scanning electron microscopy (SEM). Structure parameters such as average chord length l¯₁ and integral invariant Q were calculated to describe the relationship between phase evolution and processing conditions. Furthermore the velocity constant of the dispersed phase dimension variation k=dQ/dt was calculated at the early stage to describe the relationship with different volume fractions of dispersed phase. The characteristic wavevector q _m , and its corresponding maximum intensity I _m , increase monotonically with time and vary exponentially with time at the early stage of phase dispersion; the slope yields the change rate constants of domain size for q _m and I _m , α and β, respectively. The rate constants α and β increase with increasing content of dispersed phase, and α/β ≈1.     

Temperature dependent time delay of the voltage response to supercritical current pulses in superconducting aluminium

Current pulses larger than the critical current I_c are passed through narrow superconducting aluminium strips. The time delay between the start of the pulse and the onset of the voltage response is measured as a function of current amplitude for various temperatures (0.84<T/T_c<0.98). We find the delay time to be dependent on the temperature as (1 - T/T_c)^α with α changing from a value of - 0.5 for I/I_c close to 1 to a value of - 1.0 for I/I_c larger than 3.     

Relaxation of the analyzing power of the (p, τX) and (p, αX) reactions in the continuum

The observed continuous decrease of the analyzing power for the reactions ($\text{p}$, τX) and ($\text{p}$, αX) on ⁹⁰Zr and ²⁰⁹Bi at E_p = 72 MeV as a function of the excitation energy of the final nucleus is described by a phenomenological relaxation process.     

Electron-positron pair production by electromagnetic pulses

Electron-positron pair production in vacuum by a single focused laser pulse and by two counter-propagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic three-dimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with a transverse electric or magnetic vector (e-or h-polarized wave, respectively). The dependence of the number of produced pairs on the radiation intensity and focusing parameter is studied. It has been shown that the number of pairs produced in the field of a single e-polarized pulse is many orders of magnitude larger than that for an h-polarized pulse. The pulse-intensity dependence of the number of pairs produced by a single pulse is so sharp that the total energy of pairs produced by the e-polarized pulse with intensity near the intensity I _S = 4.65 × 10²⁹ W/cm² characteristic of QED is comparable with the energy of the pulse itself. This circumstance imposes a natural physical bound on the maximum attainable intensity of a laser pulse. For the case of two colliding circularly polarized pulses, it is shown that pair production becomes experimentally observable when the intensity of each beam is I ～ 10²⁶ W/cm², which is one to two orders of magnitude lower than that for a single pulse.     

Low-frequency noise in monodisperse platinum nanostructures near the percolation threshold

The percolation threshold p ₀ ≈ 0.6 is determined for monodisperse platinum nanostructures with 1.8-nm metallic particles deposited in a monolayer onto an insulating substrate through laser electrodispersion. It is shown that, in the “metallic” state (for p > p ₀), both the magnitude of the noise and its temperature dependence are close to those of pure metallic Pt layers. The frequency dependence of the normalized noise power spectral density is described by the relationship S _I /I ² ～ 1/f ^γ with the exponent γ close to unity. For current densities j ≥ 10⁷?10⁸ A/cm², the noise power spectral density S _I increases more rapidly with a further increase in the current as compared to I ² because of the current generating excess defects. For p < p ₀, the dependence of the conductivity σ on the temperature is adequately described by the standard relationship σ ～ exp[?(T ₀/T)^1/2]. The normalized noise power spectral density S _I /I ² exceeds the corresponding value for a quasi-metallic structure by many orders of magnitude. The noise power spectral density S _I is approximately proportional to the square of the current only for very low currents and increases steeply with a further increase in the current.     

Experimental study of a broad-band XeCl double-pass amplifier with SBS mirror

An oscillator-amplifier XeCl laser system has been used to experimentally investigate the effectiveness of a liquid stimulated Brillouin scattering (SBS) mirror for correcting the spatial aberrations of broad-band laser radiation in a double-pass amplifier. It has been found that the SBS amplifier performance is strongly dependent on the intensity I _p exciting the Brillouin medium. A good beam reconstruction has been attained at I _p1GW/cm², whereas highly aberrated output beams have been delivered by the SBS amplifier for I _p>1 GW/cm². By comparing the broad-band SBS amplifier performance to that of the same amplifier with a dielectrically coated flat mirror at one end, it has been found that the use of a SBS mirror is advantageous to obtain lower divergence output beams only for low energy pump beams (<1 mJ).     

IR Laser Control of the Clustering of CF<Subscript>3</Subscript>Br Molecules during the Gas-Dynamic Expansion of a CF<Subscript>3</Subscript>Br/Ar Mixture: Bromine Isotope Selectivity

The infrared (IR) laser radiation control of the clustering of CF3Br molecules during the gas-dynamic expansion of a CF₃Br/Ar mixture at the exit from a nozzle is investigated. Prominence is given to studying the possibility of bromine-isotope-selective suppression of the clustering of CF₃Br molecules due to their resonance vibrational excitation in the gas-dynamic expansion zone near the nozzle. A continuous CO₂ laser is used in experiments to excite molecules and clusters in a beam, and a quadrupole mass spectrometer is used to detect them. The experimental setup and the experimental technique are described. The dependences of the efficiency of molecule clustering suppression on the exciting laser radiation parameters, the gas parameters (composition, pressure) above the nozzle, and the distance from the nozzle exit section to a molecule irradiation zone are obtained. Bromine-isotope-selective suppression of molecule clustering is shown to occur at the exit from the nozzle due to the resonance vibrational excitation of gas-dynamically cooled CF₃Br molecules. When CF₃Br/Ar mixtures are used at pressure ratios p(CF₃Br): p(Ar) = 1: 10 and 1: 30, the enrichment of a cluster beam by bromine isotopes are K_enr(⁸¹Br) ≈ 1.18 ± 0.09 and 1.12 ± 0.07 during the 9R(30) laser line (1084.635 cm^–1) irradiation of a jet. The clustering suppression selectivity is α ≈ 1.18 when the mixture at the pressure ratio p(CF₃Br): p(Ar) = 1: 10 is used. These results suggest that the proposed method can selectively control the clustering of the molecules containing the heavy element isotopes that have a small isotope shift in IR absorption spectra (OsO₄, WF₆, UF₆).     

Upconversion-pumped population kinetics for4I13/2 and4I11/2 laser states of Er3+ ion in several host crystals

Population kinetics of the upper⁴I_11/2 and lower⁴I_13/2 laser states of the Er³⁺ ion were studied experimentally and theoretically in (Er)BaY₂F₈, (Er)YLF, (Er)YSGG, (Er)CaF₂ and (Er)YALO. Fluorescence from these states to the⁴I_15/2 ground state was excited through upconversion simultaneously with the⁴I_11/2 →⁴I_13/2 lasing using 1.53 µm radiation from an erbium : glass laser for optical pumping. Lifetimes of both states are altered during lasing by co-operative energy transfer processes: the lifetime of the lower state τ₁ is shortened and that of the upper state τ₂ increased with the resultant ratio τ₂/τ₁>1. After lasing the lifetime ratio returns to the ‘normal’ value τ₂/τ₁ <1; that is, one obtained under weak ultraviolet excitation. Kinetic rate equations for the population density functions for both laser states were set up and solved by approximation in three time domains. It was assumed that only one co-operative energy transfer process operates in the laser crystals and determines the population inversion kinetics. Consistency relationships for comparison of the theoretical results with the experiment were developed. Only (Er)BaY₂F₈ spectral features showed close agreement with theory, resulting in a high score of 94% for the overall correlation in the consistency test, whereas all other crystals scored <50%. As a result of this high correlation, a close match between theoretical and experimental population decay curves was shown for (Er)BaY₂F₈. Most probably, more than one energy transfer process shapes the decay curves and determines the population inversion kinetics for the other laser crystals. (Er)YALO showed little lifetime change for the laser states, apparently due to inefficient co-operative energy transfer processes. As a result it probably lased in a self-terminating short-pulse mode.