478 research outputs found
Apparatus for timeâresolved measurements of acoustic birefringence in particle dispersions
An apparatus for timeâresolved measurements of the birefringence induced in a particle suspension by an acoustic wave pulse is described. Efficient acoustic coupling is obtained by operating near the transducer resonant frequency and by matching the acoustic impedances of the cell constituents. An almostâoverdamped acoustic configuration can alternatively be employed whenever a faster response is needed. Careful design of the optical setup and of the detection unit minimize diffraction and stressâbirefringence parasitic effects and yields a good responsivity at fairly low acoustic intensities. A test of the apparatus on a colloidal suspension of PTFE rodlike particles is presented and discussed
Non-Markovian Decay and Lasing Condition in an Optical Microcavity Coupled to a Structured Reservoir
The decay dynamics of the classical electromagnetic field in a leaky optical
resonator supporting a single mode coupled to a structured continuum of modes
(reservoir) is theoretically investigated, and the issue of threshold condition
for lasing in presence of an inverted medium is comprehensively addressed.
Specific analytical results are given for a single-mode microcavity resonantly
coupled to a coupled resonator optical waveguide (CROW), which supports a band
of continuous modes acting as decay channels. For weak coupling, the usual
exponential Weisskopf-Wigner (Markovian) decay of the field in the bare
resonator is found, and the threshold for lasing increases linearly with the
coupling strength. As the coupling between the microcavity and the structured
reservoir increases, the field decay in the passive cavity shows non
exponential features, and correspondingly the threshold for lasing ceases to
increase, reaching a maximum and then starting to decrease as the coupling
strength is further increased. A singular behavior for the "laser phase
transition", which is a clear signature of strong non-Markovian dynamics, is
found at critical values of the coupling between the microcavity and the
reservoir.Comment: to appear in Phys. Rev. A (December 2006 issue
Turning light into a liquid via atomic coherence
We study a four level atomic system with electromagnetically induced
transparency with giant and susceptibilities of
opposite signs. This system would allow to obtain multidimensional solitons and
light condensates with surface tension properties analogous to those of usual
liquids
Polyelectrolyte Persistence Length: Attractive Effect of Counterion Correlations and Fluctuations
The persistence length of a single, strongly charged, stiff polyelectrolyte
chain is investigated theoretically. Path integral formulation is used to
obtain the effective electrostatic interaction between the monomers. We find
significant deviations from the classical Odijk, Skolnick and Fixman (OSF)
result. An induced attraction between monomers is due to thermal fluctuations
and correlations between bound counterions. The electrostatic persistence
length is found to be smaller than the OSF value and indicates a possible
mechanical instability (collapse) for highly charged polyelectrolytes with
multivalent counterions. In addition, we calculate the amount of condensed
counterions on a slightly bent polyelectrolyte. More counterions are found to
be adsorbed as compared to the Manning condensation on a cylinder.Comment: 5 pages, 1 ps figur
Frequency dependence of the dielectric and electro-optic response in suspensions of charged rod-like colloidal particles
We have performed an experimental investigation on the electrokinetic properties of charged rod-like fluorinated latex colloids. Systematic measurements of electrophoretic mobility, dielectric constant and electric birefringence have been performed as a function of the concentration of added nonionic surfactant and salt. In the investigated range of parameters, the zeta potential is a strongly decreasing function of the concentration of nonionic surfactant, while it is basically independent from ionic strength. We have obtained the frequency dependence of dielectric constant and Kerr constant as a function of zeta-potential and ionic strength. We observe the transition from a low frequency behavior, where both the dielectric constant and the Kerr constant are enhanced by the presence of the double layer, to a high frequency behavior, where both quantities take the value expected for unchanged particles in an insulating medium. The shape of the frequency dispersion of the Kerr constant coincides with that of the dielectric constant, but the cut-off frequencies are the same only when the zeta-potential of the particles is low
Characterization of anisotropic nano-particles by using depolarized dynamic light scattering in the near field
Light scattering techniques are widely used in many fields of condensed and
sof t matter physics. Usually these methods are based on the study of the
scattered light in the far field. Recently, a new family of near field
detection schemes has been developed, mainly for the study of small angle light
scattering. These techniques are based on the detection of the light intensity
near to the sample, where light scattered at different directions overlaps but
can be distinguished by Fourier transform analysis. Here we report for the
first time data obtained with a dynamic near field scattering instrument,
measuring both polarized and depolarized scattered light. Advantages of this
procedure over the traditional far field detection include the immunity to
stray light problems and the possibility to obtain a large number of
statistical samples for many different wave vectors in a single instantaneous
measurement. By using the proposed technique we have measured the translational
and rotational diffusion coefficients of rod-like colloidal particles. The
obtained data are in very good agreement with the data acquired with a
traditional light scattering apparatus.Comment: Published in Optics Express. This version has changes in bibliograph
Conical-emission and shock-front dynamics in femtosecond laser-pulse filamentation
We investigate both experimentally and numerically the space-time dynamics of an ultrashort laser pulse during self-focusing and nonlinear propagation in water by means of a time-gated angular-spectrum characterization.
The results identify the formation of shock fronts on both trailing and leading edges of the wave packet that are due to the formation of subluminal and superluminal group velocity intensity peaks, sustained by conical emission
Self-diffusion coefficients of charged particles: Prediction of Nonlinear volume fraction dependence
We report on calculations of the translational and rotational short-time
self-diffusion coefficients and for suspensions of
charge-stabilized colloidal spheres. These diffusion coefficients are affected
by electrostatic forces and many-body hydrodynamic interactions (HI). Our
computations account for both two-body and three-body HI. For strongly charged
particles, we predict interesting nonlinear scaling relations and depending on volume fraction
, with essentially charge-independent parameters and . These
scaling relations are strikingly different from the corresponding results for
hard spheres. Our numerical results can be explained using a model of effective
hard spheres. Moreover, we perceptibly improve the known result for of
hard sphere suspensions.Comment: 8 pages, LaTeX, 3 Postscript figures included using eps
Critical Behavior of Hadronic Fluctuations and the Effect of Final-State Randomization
The critical behaviors of quark-hadron phase transition are explored by use
of the Ising model adapted for hadron production. Various measures involving
the fluctuations of the produced hadrons in bins of various sizes are examined
with the aim of quantifying the clustering properties that are universal
features of all critical phenomena. Some of the measures involve wavelet
analysis. Two of the measures are found to exhibit the canonical power-law
behavior near the critical temperature. The effect of final-state randomization
is studied by requiring the produced particles to take random walks in the
transverse plane. It is demonstrated that for the measures considered the
dependence on the randomization process is weak. Since temperature is not a
directly measurable variable, the average hadronic density of a portion of each
event is used as the control variable that is measurable. The event-to-event
fluctuations are taken into account in the study of the dependence of the
chosen measures on that control variable. Phenomenologically verifiable
critical behaviors are found and are proposed for use as a signature of
quark-hadron phase transition in relativistic heavy-ion collisions.Comment: 17 pages (Latex) + 24 figures (ps file), submitted to Phys. Rev.
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