Collective excitations of harmonically trapped ideal gases

We theoretically study the collective excitations of an ideal gas confined in an isotropic harmonic trap. We give an exact solution to the Boltzmann-Vlasov equation; as expected for a single-component system, the associated mode frequencies are integer multiples of the trapping frequency. We show that the expressions found by the scaling ansatz method are a special case of our solution. Our findings are most useful in case the trap contains more than one phase: we demonstrate how to obtain the oscillation frequencies in case an interface is present between the ideal gas and a different phase.     

Search for scalar and pseudoscalar bosons emitted in nuclear decays via their interactions

A search for light penetrating scalar and pseudoscalar bosons has been performed at the Jülich nuclear-reactor FRJ-1 (Merlin). In nuclear deexcitation processes pseudoscalar particles should be emitted in magnetic transitions and scalar particles in electric transitions. The experiment relies on the detection of the interaction products from the conversion of bosons in the field of an electron or a nucleus with a rotatable multi-scintillator system. The geometrical arrangement of the scintillators allows to distinguish between scalar and pseudoscalar particles via the different angular distributions of the final state photons. We obtain as upper limits for the product of production ratio and interaction cross sectionW for pseudoscalar particles W}<6.9>^–40 cm², and for scalar particles W}<1.3>^–39 cm² with 90% confidence.We gratefully acknowledge the support of this experiment by the KFA Jülich and we are very grateful for most valuable discussions with H. Bechteler, H.R. Koch, O.W.B. Schult, H. Seyfarth and A. Zehnder. We would like to express our thanks to the personnel of the reactor Merlin. We also wish to acknowledge the excellent work of the technical staff of our institute in particular to E. Hermens and R. Pahlke. Thanks to H. Geller and C. Latsch who assisted with the runs.     

Realization of an ultra-flat silica surface with angstrom-scale average roughness using nonadiabatic optical near-field etching

We propose a new method of optical near-field etching where a nonadiabatic process is applied to a synthetic silica substrate using a continuum wave laser (λ=532 nm) with a Cl₂ gas source. Because the absorption band edge energy of Cl₂ is higher than the photon energy of the light source, we preclude the conventional adiabatic photochemical reaction. An optical near field, generated on the nanometrically rough substrate, induces the nonadiabatic chemical reaction to the Cl₂ molecules and thereby selectively etches away the roughness, leaving an ultra-flat synthetic silica surface with a minimum average surface roughness R _a of 1.37 Å.     

Investigation of toluene LIF at high pressure and high temperature in an optical engine

Toluene laser-induced fluorescence (LIF) emission spectra were acquired in an optical engine with excitation at 248 nm. Toluene was homogeneously seeded in pure nitrogen and air which were used as intake gases. Data were acquired during the compression phase without ignition leading to simultaneous increases in temperature and pressure from 20°C and 1 bar to 500°C and 23.6 bar. Compared to LIF emission spectra at high temperature and atmospheric pressure reported in the literature, the toluene-LIF emission signal shifts to longer wavelengths when temperature and pressure increase simultaneously, whereas the spectrally integrated emission intensity is slightly affected by the pressure level.     

Dark resonances for ground-state transfer of molecular quantum gases

One possible way to produce ultra-cold, high-phase-space-density quantum gases of molecules in the rovibronic ground state is given by molecule association from quantum-degenerate atomic gases on a Feshbach resonance and subsequent coherent optical multi-photon transfer into the rovibronic ground state. In ultra-cold samples of Cs₂ molecules, we observe two-photon dark resonances that connect the intermediate rovibrational level |v=73,J=2〉 with the rovibrational ground state |v=0,J=0〉 of the singlet X ¹ Σ _g ⁺ ground-state potential. For precise dark resonance spectroscopy we exploit the fact that it is possible to efficiently populate the level |v=73,J=2〉 by two-photon transfer from the dissociation threshold with the stimulated Raman adiabatic passage (STIRAP) technique. We find that at least one of the two-photon resonances is sufficiently strong to allow future implementation of coherent STIRAP transfer of a molecular quantum gas to the rovibrational ground state |v=0,J=0〉.     

Bose-Einstein correlations of neutral gauge bosons in pp collisions

The theory for Bose-Einstein correlations in case of neutral gauge bosons in pp collisions at high energies is presented. Based on quantum field theory at finite temperature, the two-particle Bose-Einstein correlations of neutral gauge bosons are carried out for the first time. As a result, the important parameters of the correlation functions can be obtained for the Z ⁰ Z ⁰ pairs. The article is published in the original.     

Spin 3/2 fermions with attractive interactions in a one-dimensional optical lattice: phase diagrams,entanglement entropy,and the effect of the trap

We study spin 3/2 fermionic cold atoms with attractive interactions confined in a one-dimensional optical lattice. Using numerical techniques, we determine the phase diagram for a generic density. For the chosen parameters, one-particle excitations are gapped and the phase diagram is separated into two regions: one where the two-particle excitation gap is zero, and one where it is finite. In the first region, the two-body pairing fluctuations (BCS) compete with the density ones. In the other one, a molecular superfluid (MS) phase, in which bound-states of four particles form, competes with the density fluctuations. The properties of the transition line between these two regions is studied through the behavior of the entanglement entropy. The physical features of the various phases, comprising leading correlations, Friedel oscillations, and excitation spectra, are presented. To make the connection with experiments, the effect of a harmonic trap is taken into account. In particular, we emphasize the conditions under which the appealing MS phase can be realized, and how the phases could be probed by using the density profiles and the associated structure factor. Lastly, the consequences on the flux quantization of the different nature of the pairing in the BCS and MS phases are studied in a situation where the condensate is in a ring geometry.     

Density,phase and coherence properties of a low dimensional Bose-Einstein systems moving in a disordered potential

We present a detailed numerical study of the dynamics of a disordered one-dimensional Bose-Einstein condensates in position and momentum space. We particularly focus on the region where non-linearity and disorder simultaneously effect the time propagation of the condensate as well as the possible interference between various parts of the matter wave. We report oscillation between spatially extended and localized behavior for the propagating condensate which dies down with increasing non-linearity. We also report intriguing behavior of the phase fluctuation and the coherence properties of the matter wave. We also briefly compare these behavior with that of a two-dimensional condensate. We mention the relevance of our results to the related experiments on Anderson localization and indicate the possibility of future experiments     

Towards a monolithic optical cavity for atom detection and manipulation

We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs substrate. We experimentally determined the propagation losses in the waveguide at 780 nm, the wavelength of Rb atoms. We have also made a numerical and analytical estimate of the losses induced by the presence of the gap which would allow the interaction of cold atoms with the cavity field. We found that the intrinsic finesse of the gapped cavity can be on the order of F∼30, which, when one takes into account the losses due to mirror transmission, corresponds to a cooperativity parameter for our system C∼1.     

An electrosprayed coating process for fabricating hemispherical PMMA droplets for an optical diffuser

A modified electrospraying process is proposed for fabricating uniform microsized poly(methyl methacrylate) (PMMA) hemispherical droplets on poly(ethylene terephthalate) films for use as optical diffusers. In this process, an electrode controlled by an electric field is attached to a rotating collector, and various electric field conditions are applied to the droplets ejected from a positively charged nozzle. The distribution of small hemispherical droplets and the surface roughness resulting from this modified electrospraying process are more uniform than those achieved in the normal electrospraying process. The frequency of the field applied to the electrode is not critical to the production of stable PMMA droplets. The optical diffusivity of the PMMA film fabricated using this process is greater than that of normally fabricated film, demonstrating the feasibility of using this process to create newly designed optical diffusers.     

A study of KCL lattice defects by thermoluminescence experiments

The thermoluminescent emission of X-irradiated potassium chloride is recorded simultaneouslyvs. temperature and wavelength. Samples of different origin and prepared through different processes, including thermal treatments, are examined. Most records show essentially two glow peaks, the wavelength of the one at higher temperature being slightly shifted toward the red. On the whole, experimental results suggest that luminescent centres are originated by potassium ion vacancies lying at a variable distance from interstitial potassium ions. The observed red shift is ascribed to the Coulomb energy of the pairs of these point defects of opposite charges.     

On the competiting role of mean-field and residual interactions in flow observables

Theoretical analyses of heavy-ion reactions are performed in the framework of the semi-classical Landau-Vlasov approach. The incident energies are investigated in the range from intermediate to low energy regimes, where transverse collective motion has been experimentally evidenced. The influence of the equation of state (E.O.S.) parameters on various collective observables is studied in relation with the action of the residual interactions. From the sensitivity to both aspects, and taking into account the experimental biases limitations, our investigation indicates that E.O.S. signatures should be more expected at energies below 100 MeV per nucleon.     

High-efficiency,high-speed VCSELs for optical interconnects

High-efficiency, high-speed, tapered-oxide-apertured vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm have been demonstrated. By carefully engineering the tapered oxide aperture, the mode volume can be greatly reduced without adding much optical scattering loss for the device sizes of interest. Consequently, these devices can achieve higher bandwidth at lower current and power dissipation. In addition, the parasitics are reduced by implementing deep oxidation layers and an improved p-doping scheme in the top mirror. Our devices show modulation bandwidth exceeding 20 GHz, a record for 980 nm VCSELs. Moreover, 35 Gb/s operation has been achieved at only 10 mW power dissipation. This corresponds to a data-rate/power-dissipation ratio of 3.5 Gbps/mW. Most importantly, our device structure is compatible with existing manufacturing processes and can be easily manufactured in large volume making them attractive for optical interconnects.     

Hadron production inpBe interactions at 14.6 GeV/c

Invariant cross sections for the production of ^±,K ^±,p, andd inpBe interactions at 14.6 GeV/c are well reproduced by calculations carried out in the framework of the Rossendorf collision model.Communicated by: F. Lenz     

Shrinkage stress and mechanical properties of photoactivated composite resin using the argon ion laser

The objective of this study was to verify the influence of photoactivation with the argon ion laser on shrinkage stress (SS), followed by evaluation of Vickers microhardness (VM), percentage of maximum hardness (PMH), flexural strength (FS), and flexural modulus (FM) of a composite resin. The study groups were: L1-laser at 200 mW for 10 seconds; L2-laser at 200 mW for 20 seconds; L3-laser at 250 mW for 10 seconds; L4-laser at 250 mW for 20 seconds; H-halogen light at 275 mW for 20 seconds. Data were analyzed by ANOVA/Tukey’s test (α=5%). The values of SS (MPa) were statistically lower for the group L3 (1.3)c, followed by groups L1 (2.7)b, L4 (3.4)a, b, L2 (3.7)a, and H (4.5)a. There was no difference in the values of VM when the same time of photoactivation was used, with respective values being L1=70.1a, L2=78.1b, L3=69.9a, L4=78.1b and H=79.9b. All groups showed a PMH of at least 80%. Only the group L1 showed differences in FS (MPa) and FM (GPa), the respective values of 86.2 and 5.4 being lower. Therefore, the use of argon ion laser had influenced the composite resin polymerization. The L3 group presented adequate mechanical properties and minimum SS, reducing the clinical working time for photoactivation of restorations with the tested resin by 50%.     

The role of bound states in time-dependent quantum transport

Charge transport through a nanoscale junction coupled to two macroscopic electrodes is investigated for the situation when bound states are present. We provide numerical evidence that bound states give rise to persistent, non-decaying current oscillations in the junction. We also show that the amplitude of these oscillations can exhibit a strong dependence on the history of the applied potential as well as on the initial equilibrium configuration. Our simulations allow for a quantitative investigation of several transient features. We also discuss the existence of different timescales and address their microscopic origin.     

Interference-induced enhancement of intensity and energy of a quantum optical field by a subwavelength array of coherent light sources

Recently, we have shown a mechanism that could provide great resonant and nonresonant transmission enhancements of the classical (nonquantum) light waves passed through subwavelength aperture arrays in thin metal films not by the plasmon–polariton waves, but by the constructive interference of diffracted waves (beams generated by the apertures) at the detector placed in the far-field zone. We now present a quantum reformulation of the model. The Hamiltonian describing the phenomenon of interference-induced enhancement and suppression of both the intensity and energy of a quantum optical field is derived. The basic properties of the field energy determining by the Hamiltonian are analyzed. Normally, the interference (addition) of two or more waves causes enhancement or suppression of the light intensity, but not the light energy. The model shows that the phenomenon could be observed experimentally, for instance, by using a subwavelength array of the coherent quantum light-sources (one- and two-dimensional subwavelength apertures, fibers, dipoles, and atoms).     

Fabrication of three-dimensional photonic crystal structures containing an active nonlinear optical chromophore

Direct laser writing by two-photon polymerization of photosensitive materials has emerged as a very promising technique for rapid and flexible fabrication of photonic crystals. In this work, a photosensitive silica sol-gel containing the nonlinear optical chromophore Disperse Red 1 is synthesized, and the two-photon polymerization technique is employed to fabricate three-dimensional photonic crystals with stop-gaps in the near-infrared. The composite material exhibits minimal shrinkage during photopolymerization, eliminating the need for shrinkage compensation or the fabrication of support structures.     

New optical diagnostics of the VEPP-4M collider

The experiments in measuring the dynamic aperture and beam energy spread of the VEPP-4M collider [1] are described. The optical diagnostics of the accelerator were applied for these purposes. The text was submitted by the authors in English.     

Influence of the pump threshold on the single-frequency output power of singly resonant optical parametric oscillators

We demonstrate that for a given pump source, there is an optimum pump threshold to achieve the maximum single-frequency output power in singly resonant optical parametric oscillators. Therefore, cavity losses and parametric amplification have to be adjusted. In particular, continuous-wave output powers of 1.5 W were achieved with a 2.5 cm lithium niobate crystal in comparison with 0.5 W by a 5 cm long crystal within the same cavity design. This counter-intuitive result of weaker amplification leading to larger powers can be explained using a model from L.B. Kreuzer (Proc. Joint Conf. Lasers and Opt.-Elect., p. 52, 1969). Kreuzer also states that single-mode operation is possible only up to pump powers which are 4.6 times the threshold value. Additionally, implementing an outcoupling mirror to increase losses, single-frequency waves with powers of 3 W at 3.2 μm and 7 W at 1.5 μm could be generated simultaneously.