Optimal generation of single-dispersion precompensated 1-fs pulses by molecular phase modulation

We show how an optimal control approach can be combined with the pump-probe technique for pulse compression by molecular phase modulation in hollow-core fibers to generate single 1-fs pulses in the visible. Varying the intensity and duration of the Gaussian-shaped pump pulse at the input induces optimal rotational response of the molecules. The probe pulse, which scatters off of the resulting time variation of the refractive index, is shaped at the input for optimal compression at the output, including dispersion to all orders.     

Laser transitions under resonant optical pumping of donor centres in Si:P

Terahertz stimulated emission of phosphorus donors in silicon optically excited by radiation from the free-electron laser FELIX has been studied. It is found that a spectral line of the Si:P laser emission depends on pump frequency. Stimulated emission arises on the 2p₀→1s(E) intra-centre transition (21.2 meV) under resonant pumping of the 2p₀ state and on the 2p₀→1s(T₂) transition (22.3 meV) under pumping of the 2p_± or higher odd-parity donor states. The line shift is attributed to the Auger redistribution of the 1s(E)- and 1s(T₂)-state populations. Received: 7 November 2002 / Revised version: 7 April 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/6705-55-07, E-mail: sergeij.pavlov@dlr.de RID="**" ID="**"On leave from: The Institute for Physics of Microstructures, Nizhny Novgorod, Russia     

Self-consistent expansion of a multicomponent plasma into a vacuum

The problem of the one-dimensional collisionless expansion of a multicomponent plasma into a vacuum is considered. In the hydrodynamic approximation, an approximate analytical solution for an arbitrary set of ion species with masses M₁,..., M_n and charge numbers Z₁e, Z₂e,..., Z_ne is found by using the technique of self-similar variables employed by Gurevich, Pariiskaya, and Pitaevskii for the case of single-species ions. A numerical iterative algorithm is developed in which the analytical solution is used as a first approximation.     

<Emphasis Type="Italic">N</Emphasis>-shaped voltage-current characteristic and current oscillations in Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganite

The voltage-current characteristics (VCC) of Sm_1?xSr_x MnO₃ samples with x=0.425 and x=0.450 were experimentally studied at a temperature of 77 K in pulsed and constant electric (E) and magnetic (H) fields up to 10 kOe for the H‖E and H⊥E orientations. N-shaped VCCs and high-frequency (up to 3 MHz) current oscillations were observed. It was found that the effect of colossal magnetoresistance had a threshold character and was smoothly reduced to zero with E→0.     

Molecule without electrons: binding bare nuclei with strong laser fields

We show how a carefully chosen combination of strong linearly and circularly polarized laser fields can bind two same-sign charges, not only suppressing their Coulomb repulsion in all three spatial dimensions, but also creating an effective attraction. As an example, we show how a molecule HD2+ stripped of both electrons can be kept bound by the laser fields.     

Experimental evidence of high-beta plasma confinement in an axially symmetric gas dynamic trap

In the axially symmetric magnetic mirror device gas dynamic trap (GDT), on-axis transverse beta (ratio of the transverse plasma pressure to magnetic field pressure) exceeding 0.4 in the fast ion turning points has been first achieved. The plasma has been heated by injection of neutral beams, which at the same time produced anisotropic fast ions. Neither enhanced losses of the plasma nor anomalies in the fast ion scattering and slowing down were observed. This observation confirms predicted magnetohydrodynamic stability of plasma in the axially symmetric mirror devices with average min-B, like the GDT is. The measured beta value is rather close to that expected in different versions of the GDT based 14 MeV neutron source for fusion materials testing.     

Anisotropy of the heat capacity of a mixed-state superconducting Nd<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> single crystal for various magnetic field orientations with respect to the crystallographic axes

The anisotropy in the superconducting properties of single-crystal Nd_1.85Ce_0.15CuO₄ was studied from measurements of the heat capacity within the temperature interval 2–40 K in zero magnetic field and in a magnetic field of 8 T. We report on the first observation of heat capacity jumps occurring at the superconducting transition for various magnetic field orientations with respect to the crystallographic axes and on a strong anisotropy of the magnetic contribution to heat capacity in magnetic fields oriented in the a-b plane and perpendicular to it. These measurements yielded the anisotropy in the electronic heat capacity coefficient γ_n(H) and in the superconducting transition temperature T_c(H). The angular dependence of the Sommerfeld coefficient γ_n in the a-b plane observed in a magnetic field of 8 T exhibits four-lobe symmetry and zero gap direction of the order parameter. A comparison of the results obtained on the Nd_1.85Ce_0.15CuO₄ single crystal with the data available for La_1.85Sr_0.15CuO₄ permits one to conclude that the mechanisms of superconductivity in the electron-and hole-doped superconductors are similar.     

Resonant Raman scattering spectra in a ZnCdSe/ZnSe structure with a quantum well and open nanowires

Resonance Raman scattering (RS) spectra of a ZnCdSe/ZnSe sample containing a single quantum well and quantum well-based open nanowires were studied at T=300 K. The longitudinal optical (LO) phonons involved in the formation of the observed spectra of the quantum-well and nanowire regions differ noticeably in energy. The LO phonon energies in the structures under study were calculated taking into account the compositional effect (doping of Cd into ZnSe) and biaxial strain. When excited in the exciton resonance region, RS is shown to occur via free (extended) excitonic states with the involvement of LO phonons of the ZnCdSe strained layer with final wave vectors near the Brillouin zone center. When excited below the excitonic resonance in the ZnCdSe layer, resonance scattering via localized exciton states provides a noticeable contribution to the observed RS lines. Because of the finite size of a localized state, phonons with large wave vectors are involved in these scattering processes. The RS lines produced under excitation in the excitonic region of the thick barrier layers are due to scattering from the ZnSe barrier phonons.     

Numerical Simulation of Ne–Tl and Ne–Ga Ion Hollow-Cathode Lasers

Results of theoretical and experimental studies of the behavior of the level populations and gain factors for the np ^1,3 P – ns ^1,3 S and np ^1,3 P –> (n-1)d ^1,3 D transitions in Tl II (n = 7) and Ga II (n = 5) are presented. These levels are filled due to the charge exchange in thermal collisions of neon ions with thallium and gallium atoms in thallium--neon and gallium--neon mixtures, respectively. The total pumping rate of all levels of a metal due to the charge exchange in the hollow-cathode discharge plasma at the operating concentrations of metal vapors was shown to be equal to the gas ionization rate. The latter is defined by the number of fast electrons and is independent of the charge-exchange cross section. The exciting and deexciting collisions with slow electrons and gas atoms were taken into account for all Tl II and Ga II levels related to the laser ones, and the radiation trapping by the resonance transitions of metal ions was considered. The partial pumping rates by the charge exchange were found for levels with 0 < E() < 1.5 eV. Theoretical results are compared with experimental data. The behavior of the lasing characteristics is explained for the known laser transitions. The parameters of the as yet unknown transitions in the IR range are predicted.