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PubMed2006

Tear meniscus height, lower punctum lacrimale, and the tear lipid layer in normal aging.

PURPOSE: The purposes of this study are to investigate the tear meniscus height (TMH), lipid layer, and critical dimensions of the lower punctum lacrimale (DPL) in normal human subjects over a large age range; and to determine the shape and general characteristics of the lower punctum lacrimale in a normal population. METHODS: TMH and DPL were measured using a graticule set at the eyepiece of a slit lamp biomicroscope (magnification 32x). Lipid layer was assessed by interferometry (Tearscope, Keeler). Only one eye (right) of each subject was assessed. All subjects were normals without any tear-related disorders and/or symptoms. RESULTS: Four hundred forty-four subjects were assessed (268 females, age range, 6-91 years; 176 males, age range, 6-91 years). Mean (+/-standard deviation) TMH in females was 0.19 mm (+/-0.11) and in males was 0.19 mm (+/-0.10). TMH was related to age (x): in females, TMH=0.13+0.0009x (r=0.2393, p<0.001); in males, TMH=0.14+0.0008x (r=0.2492, p<0.001). TMH increased from 0.15 mm (+/-0.06) in the young (<20 years) to 0.21 mm (+/-0.10) in the elderly (>80 years). The punctum was closed in 10.8%, slit shaped in 24.3%, oval in 5.4%, and round in 59.5% of all cases. Mean age and TMH of those presenting with closed puncta was significantly higher than those presenting with open puncta (p<0.001). Mean (+/-standard deviation) area of open puncta was 0.008 mm (+/-0.013) in females and 0.010 mm (+/-0.018) in males. The difference was not significant. Diameter of round puncta was inversely related to age in females (r=-0.4985, n=149, p<0.001) but not within the males. A total of 14.6% of females and 12.5% of males presented with no discernible lipid layer. Lipid layer tended to be thinner in older subjects (one-way analysis of variance, F=6.667, p<0.001). There was no clear relationship between TMH and the lipid layer. CONCLUSIONS: There is a gradual increase in TMH and shift in size and shape of the punctum lacrimale with advancing years. Age-related changes in the lipid layer are expected to reduce tear volume by way of increased evaporation. On balance, it would appear any effects on tear volume by a thinning lipid layer are outweighed by changes in the puncta.

Adolescent↗
PubMed2006

Extending the dynamic range of phase contrast magnetic resonance velocity imaging using advanced higher-dimensional phase unwrapping algorithms.

Phase contrast magnetic resonance velocity imaging is a powerful technique for quantitative in vivo blood flow measurement. Current practice normally involves restricting the sensitivity of the technique so as to avoid the problem of the measured phase being 'wrapped' onto the range -pi to +pi. However, as a result, dynamic range and signal-to-noise ratio are sacrificed. Alternatively, the true phase values can be estimated by a phase unwrapping process which consists of adding integral multiples of 2pi to the measured wrapped phase values. In the presence of noise and data undersampling, the phase unwrapping problem becomes non-trivial. In this paper, we investigate the performance of three different phase unwrapping algorithms when applied to three-dimensional (two spatial axes and one time axis) phase contrast datasets. A simple one-dimensional temporal unwrapping algorithm, a more complex and robust three-dimensional unwrapping algorithm and a novel velocity encoding unwrapping algorithm which involves unwrapping along a fourth dimension (the 'velocity encoding' direction) are discussed, and results from the three are presented and compared. It is shown that compared to the traditional approach, both dynamic range and signal-to-noise ratio can be increased by a factor of up to five times, which demonstrates considerable promise for a possible eventual clinical implementation. The results are also of direct relevance to users of any other technique delivering time-varying two-dimensional phase images, such as dynamic speckle interferometry and synthetic aperture radar.

Algorithms↗
PubMed2006

Numerical modelling and data assimilation of the Larsen B ice shelf, Antarctic Peninsula.

In this study, the flow and rheology of pre-collapse Larsen B ice shelf are investigated by using a combination of flow modelling and data assimilation. Observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method. In particular, the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow. The application to the Larsen B ice shelf shows that a strong weakening of the ice in the shear zones, mostly along the margins, is necessary to fit the observed shelf flow. This pattern of bands with weak ice is a very robust feature of the inversion, whereas the ice rheology within the main shelf body is found to be not well constrained. This suggests that these weak zones play a major role in the control of the flow of the Larsen B ice shelf and may be the key to understanding the observed pre-collapse thinning and acceleration of Larsen B. Regarding the sensitivity of the stress field to rheology, the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments.

Journal Article↗
PubMed1999

Electrostriction of a near-critical fluid in microgravity.

We used interferometry to measure the electric-field-induced (i.e., electrostrictive) increase of the density of sulfur hexafluoride (SF6) near its critical point. The results at three temperatures (T(c)+5.0 mK, T(c)+10.0 mK, T(c)+30.0 mK with T(c)=319 K) agree with a calculation based on the Clausius-Mossotti relation and the restricted cubic model equation of state. To measure electrostriction, an inhomogeneous electric field (< or =26 kV/cm) was applied to the SF6 sample by charging a fine wire that passed through it. These measurements were performed in microgravity so that the small electrostrictive density changes (< or =3.5% in this paper) would not be complicated by stratification of the fluid's density induced by the Earth's gravity. The predicted shifts of the critical temperature and density resulting from the electric field were too small to detect.

Journal Article↗
PubMed1999

Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet.

Using interferometry, we investigate the dynamics of interaction of a relativistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 microm in diameter, with a radial gradient of electron density approximately 5 x 10(22) cm(-4) and with an on-axis electron density approximately ten times less than its maximum value of 8 x 10(19) cm(-3). A high radial velocity of the surrounding gas ionization of approximately 3.8 x 10(8) cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas excitation and ionization. Comparing the model predictions with the interferometric data, we reconstructed the axial profile of laser channel and on-axis laser intensity. The estimated maximum energy of accelerated ions is about 500 keV, and the total energy of the fast ions is 5% of the laser pulse energy.

Journal Article↗
PubMed1999

Formation of plasma channels in the interaction of a nanosecond laser pulse at moderate intensities with helium gas jets.

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].

Journal Article↗
PubMed1999

Nonlinearity of Pancharatnam's geometric phase in polarizing interferometers.

Earlier investigations show a time-variable nonlinear shift of the fringe pattern in a polarizing interferometer while rotating a polarizer at the exit. This effect was identified as Pancharatnam's geometrical phase and proposed for applications in interferometry and fast optical switching devices. A heterodyne analysis attributes moving fringes to a frequency difference between the interfering beams; thus changing fringe velocities point to a dynamic frequency development within the period of the uniformly rotating analyzer. This explanation offends the intuition and we undertake an experimental and theoretical investigation of the effect to solve the paradox. We determine, e.g., the complete frequency and mode spectrum of an arbitrary state of polarization P0 behind a rotating linear analyzer and behind a rotating arbitrary linear birefringent plate. We find that, in spite of a fast changing phase in the interferometer, no other (higher) frequency components appear in the spectral distribution of the intensity at the exit than the double of the rotary frequency of the analyzer: phase nonlinearities are compensated for by intensity changes. Only a phase-sensitive detector like an array of photodetectors is able to observe the nonlinearity of Pancharatnam's geometrical phase. A single detector only finds a sinusoidal intensity variation. Our insight into these relations leads us to two new applications of Pancharatnam's phase: supersensitivity of a polarizing double beam interferometer with a video camera acting as a phase detector and external tuning of a Fizeau interferometer.

Journal Article↗
PubMed2000

Comparison between the switching dynamics of homeotropic and planar cells of chiral smectic-C liquid crystals

The molecular dynamics during the switching of homeotropic and planar cells of chiral smectic-C liquid crystals has been investigated using electrooptic measurements and second-harmonic-generation interferometry. Two ferroelectric liquid crystals with great differences in their molecular structures have been studied. It has been found that the molecular motion is rather different depending on the type of cell used. For planar cells, the azimuthal rotation of the molecules is limited within half the smectic cone and the molecules rotate in opposite directions in the two halves of the chevron structure. In contrast, for homeotropic cells there is no chevron structure and a uniform macroscopic optical indicatrix can be defined all over the sample during the whole switching process. However, the molecular reorientation takes place within numerous microdomains of size smaller than the optical wavelength. There are two types of microdomains that occur with different probabilities depending on the material. Within each domain the molecules rotate in opposite directions, and the molecules describe a complete cone during a whole switching period.

Journal Article↗
PubMed2000

Observation of spatial asymmetry of THz oscillating electron plasma wave in a laser wakefield

The asymmetric spatial distribution of electron density perturbation is observed by using a frequency-domain interferometry technique. The wake amplitude of the outside bump is enhanced by the elliptical distribution of the pump laser pulse. This asymmetry can be explained with a two-dimensional analytical model expanded from cylindrically symmetric linear theory.

Journal Article↗
PubMed2001

Static and dynamic transport of light close to the Anderson localization transition.

Anderson localization of light refers to an inhibition of wave transport in scattering media due to the interference of multiple scattered waves. We present wavelength dependent midinfrared optical transport measurements in slabs of randomly packed germanium (Ge) micron-sized particles, using a free electron laser as a tunable source of pulsed radiation. Because of their high refractive index and low absorption, Ge and similar semiconductors are excellent systems to study Anderson localization of light. To characterize the samples fully, we have employed several complementary optical techniques: total diffuse transmission, total diffuse reflection, coherent transmission, and time-resolved speckle interferometry. In this way we obtained the scattering (l(s)) and transport (l) mean free paths, the absorption coefficient (alpha), the diffusion constant (D), and the energy transport velocity (v(e)). These measurements have been made as a function of midinfrared wavelength, so that the scattering cross section and absorption coefficients can be varied in the same samples. We found that the Ge samples are close (kl(s) approximately 3) to the localization transition, but still above it. Our measurements of l(s) and l suggest that l is renormalized due to interference at the proximity of the localization transition. We also found that the diffusion constant is significantly reduced in samples thinner than approximately 7l.

Journal Article↗
PubMed2001

Propagation of a randomized 600-ps laser beam in a helium gas jet over long scale lengths.

The propagation and interaction of a randomized 600-ps laser with a helium gas jet were studied experimentally for laser intensities of 10(14) W/cm(2). Such a study is of interest for the indirectly driven inertial confinement fusion scheme, where a randomized laser beam propagates into a gas-filled cavity over a distance of a few millimeters. The dynamics of ionization was studied using time resolved interferometry. Maps of electronic density n(e)(z,t) were retrieved from time resolved interferograms. The plasma temperature was studied using Thomson scattering. The results show that the laser diffracts while propagating, leading to a decrease in laser intensity and causing ionization to occur later in time. An ionization front, moving at a velocity of about v(f) approximately 2.8 x 10(6) m/s, was observed. Beam diffraction also causes a nonhomogeneous heating of the plasma: the entrance of the plasma is hotter than the exit. A one-dimensional model was used to fit the results. It takes into account collisional ionization and heating by inverse bremsstrahlung. The model shows very good agreement with the experiment.

Journal Article↗
PubMed2002

Wetting transitions at soft, sliding interfaces.

We observe (by optical interferometry) the contact of a rubber cap squeezing a nonwetting liquid against a plate moving at velocity U. At low velocities, the contact is dry. It becomes partially wet above a threshold velocity V(c1), with two symmetrical dry patches on the rear part. Above a second velocity V(c2), the contact is totally wet. This regime U>V(c2) corresponds to the hydroplaning of a car (decelerating on a wet road). We interpret the transitions at V(c1), V(c2) in terms of a competition between (a) liquid invasion induced by shear (b) spontaneous dewetting of the liquid (between nonwettable surfaces).

Journal Article↗
PubMed2003

Dynamics of supercooled liquids confined to the pores of sol-gel glass: a dynamic light scattering study.

Dynamics of low molecular weight and polymeric glass forming liquids in confined geometries has been studied by means of depolarized dynamic light scattering: photon correlation spectroscopy and Fabry-Perot interferometry. The pore size of the glassy matrix amounted to 2.5, 5.0, and 7.5 nm. The glass transition temperature T(g) of these liquids in confined geometries has been measured using differential scanning calorimetry. A systematic decrease of T(g) (up to 25 K) with decreasing pore size has been observed. The relaxation times of the alpha process at constant temperature were decreasing with decreasing pore size (up to 6 orders of magnitude at T(g)), while the width of their distribution was increasing. The change of the relaxation times can be assigned to the change of T(g) in confined geometries. After correcting the activation plots for the shift of T(g) a master curve was obtained for all pore sizes and the bulk material. The effect of chemical modification of the surface of the porous matrix on the dynamics of ortho-terphenyl has also been studied. These dramatic changes of the T(g) and the relaxation time of confined liquids can be explained by simple thermodynamic arguments. There is no indication that they are related to the change of the correlation length of cooperative dynamics.

Journal Article↗
PubMed2003

Time-resolved pulse propagation in a strongly scattering material.

Light transport in macroporous gallium phosphide, perhaps the strongest nonabsorbing scatterer of visible light, is studied using phase-sensitive femtosecond pulse interferometry. Phase statistics are measured at optical wavelengths in both reflection and transmission and compared with theory. The diffusion constant of light is measured in both reflection and transmission as a function of thickness and compared with theories for diffusive transport and localization. An unusually high energy velocity due to the bicontinuous structure of the porous network is reported. For such strongly scattering samples, we show that surface properties and the effective index of refraction need to be treated carefully.

Journal Article↗
PubMed2004

Interplay of impurities and solution flow as determinants of step pattern dynamics.

The first theory of step pattern evolution, the kinematic wave theory, employed the assumption of impurity effects on step kinetics. On the other hand, recent results have been considered within a framework linking step patterns to the mutual orientation of the solution flow and step motion directions in arbitrarily pure solutions. We explore the consequences of combining impurity and solution flow effects on the dynamics of the surface morphology of the (101) face of potassium dihydrogen phosphate (KDP) crystals. We employ phase-shifting interferometry for real time in situ monitoring of these dynamics. We find that, at solution supersaturations sigma</=0.035, step bunches form on all three vicinals of the (101) face regardless of the mutual orientation of the step motion and solution flow directions. Testing the mechanism of impurity-step pattern interactions, we show that bunching is caused by impurity molecules that adsorb on the surface and slow down and destabilize step trains without inducing growth cessation, i.e., the mechanism is inherently different from the one established for the (100) KDP face. We show that at sigma>0.040 impurities do not affect step bunching, and it is controlled by the direction of the solution flow, i.e., two distinct regimes of step bunching exist. The transition between the two regimes is governed by the exposure times of the terraces between steps tau : shorter tau's at higher growth rates lead to lower surface concentration of impurities and suppress the impurity effects on step kinetics and bunching.

Journal Article↗
PubMed2004

Preplasma conditions for operation of 10-Hz subjoule femtosecond-laser-pumped nickel-likex-ray lasers.

We present measurements of electron densities of plasmas with femtosecond (fs) temporal resolution. The plasmas are generated by laser pulses with different intensities at different time delays. Such plasmas are of great interest as preplasmas for transient, collisionally excited x-ray lasers. The laser pulses producing the plasmas are generated by stretching part of a 130-fs laser pulse of the ATLAS titanium-sapphire laser of our institute and focusing this radiation to a line on molybdenum and silver slab targets. The electron density is measured as a function of distance from the target by interferometry using a Wollaston prism. Using an ultrashort probe pulse allows one to obtain data extremely close, about 10 microm, to the target surface. Experimental data are compared with simulations using the MULTI hydrocode. The results allow comparison of the ablation from a hard (Mo) and a soft (Ag) material, optimization of prepulse-main pulse delay times, and selection of the best pump geometry allowing for propagation of the pump and x-ray beams. These points are key elements for the development of a high-repetition-rate soft-x-ray laser.

Journal Article↗
PubMed2004

Reflectivity-based evaluation of the coalescence of two condensing drops and shape evolution of the coalesced drop.

Image analyzing interferometry is used to study the details of the evolving shapes and coalescence of two condensing drops of 2-propanol on a quartz surface. The measured thickness profiles give fundamental insights into the transport processes within the drops before and after coalescence and the evolution of the coalesced drop from asymmetric to symmetric shape. The results indicate that the constant value of the adsorbed film thickness between the drops and profiles of the local thickness, slope angle, curvature, and curvature gradient govern the pressure fields in the coalescing drops. The shape evolution after coalescence is found to be driven by the capillary forces within the drop. Using the experimental data, we find that the calculations of the average shear stress for the fluid flow between the drops, the decrease in the interfacial excess energy, and the positions of the center of mass of the drops explain the physics of the coalescence phenomenon. However, the flow field is found to be complex because the pressure field indicates that there are complicated flows within the drop.

Journal Article↗
PubMed2005

Control of relative radiation pressure in optical traps: application to phagocytic membrane binding studies.

We show how to control the relative radiation pressure and thereby the stable trap position of an optically trapped bead by variation of the mean incident axial photon momentum. The thermal position fluctuations of a trapped bead are recorded by a three-dimensional back-focal-plane interferometry. The interferometric detection signals are in agreement with predictions based on an extended Mie theory. Depending on the application, the unique and linear range of such a detection system can be optimized by controlling the trap position of the bead. We use this method to investigate in three dimensions the binding of beads to membranes of living cells during phagocytosis. We found that independent of the bead coating (IgG, complement, LPS, avidin) the most frequent initial mechanical response of the cell was a downward pulling of the bead into the cell. The time delay between binding and response was on average 2 s.

Animals↗