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At least 1,225 records · Page 68Linked to original sources
PubMed2005

High-frequency microrheology of wormlike micelles.

We have measured the frequency-dependent shear modulus of entangled solutions of wormlike micelles by high-frequency microrheology and have compared the results with those from macrorheology experiments done on the same samples. Using optical microrheology based on laser interferometry we have measured loss and storage moduli over six decades in frequency up to about 100 kHz. We present data over a decade in concentration in the entangled regime and find good agreement between micro- and macrorheology, thus validating recently developed microrheology techniques. By collapsing data for different concentrations, we furthermore determine both the concentration scaling of the plateau modulus and a power-law exponent of the complex shear modulus at high frequencies.

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PubMed2005

Characterization and control of plasma density distribution for the development of solid-target x-ray lasers.

By using deflectometry of a longitudinal probe pulse and reflective interferometry of a transverse probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target materials. Many important issues crucial to x-ray lasing such as electron density distribution, electron temperature, and the optimal timing between pumping pulses can be resolved with these methods.

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PubMed2005

Capillary discharge-driven metal vapor plasma waveguides.

We report the generation of dense plasma waveguides containing a large concentration of silver ions by means of a fast (approximately 55 ns first half-cycle) microcapillary discharge. Concave plasma density profiles with axial electron density > 1 x 10(19) cm(-3) were measured from discharge ablation of 330 or 440 microm diameter Ag2S capillaries with 3-5 kA peak amplitude current pulses. The dynamic of this plasma waveguide was studied with interferometry, absorption measurements, and hydrodynamic model simulations. The results are relevant to the development of efficient longitudinally pumped metal vapor soft x-ray lasers, in particular those employing transient excitation of Ni-like ions. An approach to the design of a gain saturated wave-guided 13.9 nm laser in Ni-like Ag is discussed.

Journal Article↗
PubMed2005

Plasma conditions for improved energy coupling into the gain region of the Ni-like Pd transient collisional x-ray laser.

We have directly probed the conditions in which the Ni-like Pd transient collisional x-ray laser is generated and propagates by measuring the near-field image and by utilizing picosecond resolution soft x-ray laser interferometry of the preformed Pd plasma gain medium. The electron density and gain region of the plasma have been determined experimentally and are found to be in good agreement with simulations. We observe a strong dependence of the laser pump-gain medium coupling on the laser pump parameters. The most efficient coupling occurs with the formation of lower density gradients in the preformed plasma and when the duration of the main heating pulse is comparable to the gain lifetime (approximately 10 ps for mid- Z Ni-like schemes). This increases the output intensity by more than an order of magnitude relative to the commonly utilized case where the same pumping energy is delivered within a shorter heating pulse duration (<3 ps) . In contrast, the higher intensity heating pulses are observed to be absorbed at higher electron densities and in regions where steep density gradients limit the effective length of the gain medium.

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PubMed2005

Plasma waveguides efficiently generated by Bessel beams in elongated cluster gas jets.

We demonstrate the efficient generation of plasma waveguides in elongated cluster gas jets using 100 ps axicon-generated Bessel beam pump pulses. The plasma waveguide space and time evolution is measured using picosecond interferometry. Small radius waveguides with central densities as low as approximately 10(18) cm(-3) can be generated with this technique. Despite the expected subpicosecond cluster disassembly time, we observe long pulse absorption efficiencies that can be more than a factor of 10 greater than in unclustered gas targets of the same volume average atomic density. The maximum long pulse absorption observed in cluster jets under our range of conditions was 35%. The explanation for the enhanced absorption is that in the far-leading edge of the laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then heated by the remainder of the pulse. From this perspective, the use of clustered gases is equivalent to a supercharged preionization scheme for long duration laser pulses.

Journal Article↗
PubMed2006

Measurements of the three-dimensional shape of ice crystals in supercooled water.

Experimentally grown ice crystals from ultrapure supercooled water are imaged by means of Mach-Zehnder interferometry. By analyzing the fringe patterns the phase information and thus the three-dimensional shape of the ice crystals is recovered quantitatively. The integral parameters height of the basal plane, volume, and surface of the crystals are measured as a function of time and supercooling. It is found that all measured parameters follow a power law as a function of time and the exponents are found to be independent of the supercooling. The shape transition from the prismatic to the basal face along the main growth direction of the ice dendrites as a function of the distance from the tip is found to be a power law as well. Our findings support the validity of universal growth laws in pattern forming systems.

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PubMed2006

X-ray spectroscopy of 1 cm plasma channels produced by self-guided pulse propagation in elongated cluster jets.

We diagnose the self-channeled propagation of intense femtosecond pulses over an extended distance in a N2O cluster gas target using high resolution kilovolt x-ray pinhole images of the channel and spatially resolved x-ray spectra. The x-ray images are consistent with femtosecond optical scattering, shadowgraphy, and interferometry images. We observe extended plasma channels (approximately 9 mm) limited either by the cluster jet length or by absorption, for injected laser intensities in the range of 10(16)-10(17) W/cm2. Spectral line shapes for the OVII 1s2-1s3p and OVIII 1s-2p transitions (at 1.8627 and 1.8969 nm, respectively) show significant broadening to the blue side and with truncated emission on the red side. We attribute this effect to Doppler blueshifted emission from fast ions from exploding clusters moving toward the spectrometer; redshifted emission from the opposite side of the cluster is absorbed.

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PubMed2006

Prediction and observation of tin and silver plasmas with index of refraction greater than one in the soft x-ray range.

We present the calculated prediction and the experimental confirmation that doubly ionized Ag and Sn plasmas can have an index of refraction greater than one for soft x-ray wavelengths. Interferometry experiments conducted using a capillary discharge soft x-ray laser operating at a wavelength of confirm that in few times ionized laser-created plasmas of these elements the anomalous dispersion from bound electrons can dominate the free electron contribution, making the index of refraction greater than one. The results confirm that bound electrons can strongly influence the index of refraction of numerous plasmas over a broad range of soft x-ray wavelengths confirming recent observations. The understanding of index of refraction at short wavelengths will become even more essential during the next decade as x-ray free electron lasers will become available to probe a wider variety of plasmas at higher densities and shorter wavelengths.

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PubMed2000

Thermalization of a two-phase fluid in low gravity: heat transferred from cold to Hot

We present an experimental study of the thermal response to a positive temperature quench in two-phase fluid SF6 in low gravity for temperature ranging from 10.1 to 0.1 K from the critical temperature. The temperature was measured simultaneously in the gas, the liquid, and the cell wall by thermistors and the density distribution was observed by interferometry. During the quench the gas temperature considerably exceeded the temperature of the heating walls (overheating up to 23%). This striking observation is discussed in the light of the adiabatic heat transfer in this highly compressible fluid while the key role of the localization in low gravity of the gas and liquid phases is revealed.

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PubMed2000

Dynamics of layering transitions in confined liquids.

Multiple beam interferometry and video microscopy were used to investigate the layering transition of thin liquid films of 1-undecanol confined between atomically smooth mica surfaces. The expulsion of a molecularly thin lubricant layer was followed directly in two dimensions. Overall, the dynamics of the transition follows theoretical predictions based on two-dimensional hydrodynamics. Frequently, pockets of liquid remain trapped inside the contact area at the end of the transition. The trapped pockets undergo shape transformations to minimize elastic and interfacial energy.

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PubMed2000

Probing possible decoherence effects in atmospheric neutrino oscillations.

It is shown that the results of the Super-Kamiokande atmospheric neutrino experiment, interpreted in terms of nu(mu)<-->nu(tau) flavor transitions, can probe possible decoherence effects induced by new physics (e.g., by quantum gravity) with high sensitivity, supplementing current laboratory tests based on kaon oscillations and on neutron interferometry. By varying the (unknown) energy dependence of such effects, one can either obtain strong limits on their amplitude or use them to find an unconventional solution to the atmospheric nu anomaly based solely on decoherence.

Atmosphere↗
PubMed2000

Measurement of shock wave rise times in metal thin films.

We have measured the rise time of laser-generated shock waves in vapor plated metal thin films using frequency-domain interferometry with subpicosecond time resolution. 10%- 90% rise times of <6.25 ps were found in targets ranging from 0.25 to 2.0 microm in thickness. Particle and average shock velocities were simultaneously determined. Shock velocities of approximately 5 nm/ps were inferred from the measured free surface velocity, corresponding to pressures of 30-50 kbar. Thus, the shock front extends only a few tens of lattice spacings.

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PubMed2000

Eliminating the mean-field shift in two-component bose-einstein condensates

We demonstrate that the nonlinear mean-field shift in a multicomponent Bose-Einstein condensate may be eliminated by controlling the two-body interaction coefficients. This modification can be achieved by engineering the environment of the condensate. We consider the case of a two-component condensate in a quasi-one-dimensional atomic waveguide, achieving modification of the atom-atom interactions by varying the transverse wave functions of the components. Eliminating the density-dependent phase shift represents a promising potential application for multicomponent condensates in atom interferometry and precision measurements.

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PubMed2001

Limits on the cosmological abundance of supermassive compact objects from a search for multiple imaging in compact radio sources.

Using very long baseline interferometry we have searched a sample of 300 compact radio sources for examples of multiple imaging produced by gravitational lensing; no multiple images were found with separations in the angular range 1.5--50 milliarcsec. This null result allows us to place a limit on the cosmological abundance of intergalactic supermassive compact objects in the mass range approximately 10(6)M( middle dot in circle) to approximately 10(8)M( middle dot in circle); such objects cannot make up more than approximately 1% of the closure density ( 95% confidence). A uniformly distributed population of supermassive black holes forming soon after the big bang does not, therefore, contribute significantly to the dark matter content of the Universe.

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PubMed2001

Magnetic conveyor belt for transporting and merging trapped atom clouds.

We demonstrate an integrated magnetic device which transports cold atoms near a surface with very high positioning accuracy. Time-dependent currents in a lithographic conductor pattern create a moving chain of potential wells; atoms are transported in these wells while remaining confined in all three dimensions. We achieve mean fluxes up to 10(6) s(-1) with a negligible heating rate. An extension of this device allows merging of atom clouds by unification of two Ioffe-Pritchard potentials. The unification, which we demonstrate experimentally, can be performed without loss of phase space density. This novel, all-magnetic atom manipulation offers exciting perspectives, such as trapped-atom interferometry.

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PubMed2001

Two-kaon correlations in central Pb+Pb collisions at 158 a GeV/c.

Two-particle interferometry of positive kaons is studied in Pb+Pb collisions at mean transverse momenta approximately 0.25 and 0.91 GeV/c. A three-dimensional analysis was applied to the lower p(T) data, while a two-dimensional analysis was used for the higher p(T) data. We find that the source-size parameters are consistent with the m(T) scaling curve observed in pion-correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.

Journal Article↗
PubMed2001

Observation of phase fluctuations in elongated Bose-Einstein condensates.

The occurrence of phase fluctuations due to thermal excitations in Bose-Einstein condensates (BECs) is studied for a variety of temperatures and trap geometries. We observe the statistical nature of the appearance of phase fluctuations and characterize the dependence of their average value on temperature, number of particles, and the trapping potential. We find pronounced phase fluctuations for condensates in very elongated traps in a broad temperature range. The results are of great importance for the realization of BEC in quasi-1D geometries, for matter wave interferometry with BECs, as well as for coherence properties of guided atom laser beams.

Journal Article↗
PubMed2001

Exciton Rabi oscillation in a single quantum dot.

A spectroscopic method, which enables characterization of a single isolated quantum dot and a quantum wave function interferometry, is applied to an exciton discrete excited state in an InGaAs quantum dot. Long coherence of zero-dimensional excitonic states made possible the observation of coherent population flopping in a 0D excitonic two-level system in a time-domain interferometric measurement. Corresponding energy splitting is also manifested in an energy-domain measurement.

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