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We study the propagation of 0.05-1 TW power, ultrafast laser pulses in a 10 meter long rubidium vapor cell. The central wavelength of the laser is resonant with the $D_2$ line of rubidium and the peak intensity in the $10^{12}-10^{14} ~W/cm^2$ range, enough to create a plasma channel with single electron ionization. We observe the absorption of the laser pulse for low energy, a regime of transverse confinement of the laser beam by the strong resonant nonlinearity for higher energies and the transverse broadening of the output beam when the nonlinearity is saturated due to full medium ionization. We compare experimental observations of transmitted pulse energy and transverse fluence profile with the results of computer simulations modeling pulse propagation. We find a qualitative agreement between theory and experiment that corroborates the validity of our propagation model. While the quantitative differences are substantial, the results show that the model can be used to interpret the observed phenomena in terms of self-focusing and channeling of the laser pulses by the saturable nonlinearity and the transparency of the fully ionized medium along the propagation axis., Comment: 12 pages, 10 Figures

We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path. Simulation results agree with the experimental results reported in arXiv:2007.14894v2: with negative gradients, the charge of the modulated bunch is lower than with positive gradients. In addition, the bunch modulation frequency varies with gradient. Simulation results show that dephasing of the wakefields with respect to the relativistic protons along the plasma is the main cause for the loss of charge. The study of the modulation frequency reveals details about the evolution of the self-modulation process along the plasma. In particular for negative gradients, the modulation frequency across time-resolved images of the bunch indicates the position along the plasma where protons leave the wakefields. Simulations and experimental results are in excellent agreement., Comment: 13 pages, 11 figures

A precise characterization of the incoming proton bunch parameters is required to accurately simulate the self-modulation process in the Advanced Wakefield Experiment (AWAKE). This paper presents an analysis of the parameters of the incoming proton bunches used in the later stages of the AWAKE Run 1 data-taking period. The transverse structure of the bunch is observed at multiple positions along the beamline using scintillating or optical transition radiation screens. The parameters of a model that describes the bunch transverse dimensions and divergence are fitted to represent the observed data using Bayesian inference. The analysis is tested on simulated data and then applied to the experimental data. A precise characterization of the incoming proton bunch parameters is required to accurately simulate the self-modulation process in the Advanced Wakefield Experiment (AWAKE). This paper presents an analysis of the parameters of the incoming proton bunches used in the later stages of the AWAKE Run 1 data-taking period. The transverse structure of the bunch is observed at multiple positions along the beamline using scintillating or optical transition radiation screens. The parameters of a model that describes the bunch transverse dimensions and divergence are fitted to represent the observed data using Bayesian inference. The analysis is tested on simulated data and then applied to the experimental data.

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400\,GeV proton bunch. Results show that a positive/negative gradient in/decreases the number of micro-bunches and the relative charge per micro-bunch observed after 10\,m of plasma. The measured modulation frequency also in/decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradient adding to the slow wakefields' phase velocity during self-modulation growth predicted by linear theory., Comment: 14 pages, 4 figures

We study experimentally the longitudinal and transverse wakefields driven by a highly relativistic proton bunch during self-modulation in plasma. We show that the wakefields' growth and amplitude increase with increasing seed amplitude as well as with the proton bunch charge in the plasma. We study transverse wakefields using the maximum radius of the proton bunch distribution measured on a screen downstream from the plasma. We study longitudinal wakefields by externally injecting electrons and measuring their final energy. Measurements agree with trends predicted by theory and numerical simulations and validate our understanding of the development of self-modulation. Experiments were performed in the context of the Advanced Wakefield Experiment (AWAKE)., 4 figures

Background Fluid resuscitation remains a cornerstone in the management of acute bleeding. According to Starling's “Three-compartment model”, four-times more crystalloids have the same volume effect as colloids. However, this volume-replacement ratio remains a controversial issue as it may be affected by the degradation of the endothelial glycocalyx layer, a situation often found in the critically ill. Our aim was to compare colloid and crystalloid based fluid resuscitation during an experimental stroke volume index (SVI) guided hemorrhage and resuscitation animal model. Methods Anesthetized and mechanically ventilated pigs were randomized to receive a colloid (Voluven®,HES, n=15) or crystalloid (Ringerfundin®,RF, n=15) infusion. Animals were bled till baseline SVI (Tbsl) dropped by 50% (T0), followed by resuscitation until initial SVI was reached (T4) in four steps. Invasive hemodynamic measurements, blood gas analyses and laboratory tests were performed at each assessment points. Glycocalyx degradation markers (Syndecan-1/hematocrit ratio, Glypican/hematocrit ratio) were determined at Tbsl, T0 and T4. Results Similar amounts of blood were shed in both groups (HES group: 506±159 mls blood, RF group: 470±127 mls blood). Hemodynamic changes followed the same pattern without significant difference between the groups. Animals received significantly less resuscitation fluid in the HES compared to the RF-group: 425 [320-665], vs 1390 [884-1585] mls, p

We investigate the propagation of ultra-short laser pulses in atomic rubidium vapor. The pulses are intensive enough to ionize the atoms and are directly resonant with the 780 nm $D_2$ line. We derive a relatively simple theory for computing the nonlinear optical response of atoms and investigate the competing effects of strong resonant nonlinearity and ionization in the medium using computer simulations. A nonlinear self-channeling of pulse energy is found to produce a continuous plasma channel with complete ionization. We evaluate the length, width and homogeneity of the resulting plasma channel for various values of pulse energy and initial focusing to identify regimes optimal for applications in plasma-wave accelerator devices such as that being built by the AWAKE collaboration at CERN. Similarities and differences with laser pulse filamentation in atmospheric gases are discussed., 8 figures

BACKGROUND: Fluid resuscitation is still a major challenge. We aimed to describe changes in central venous oxygen saturation (ScvO2 ) and venous-to-arterial carbon dioxide gap (dCO2 ) during an experimental stroke volume (SV) index (SVI)-guided hemorrhage and fluid resuscitation model in pigs. METHODS: Twelve anesthetized, mechanically ventilated pigs were bled till baseline SVI (Tbsl ) dropped by 50% (T0 ), thereafter fluid resuscitation was performed with balanced crystalloid in four steps until initial SVI was reached (T4 ). Statistical analysis was performed with Statistical Program for Social Sciences version 18.0; data are expressed as mean +/- standard deviation. RESULTS: After bleeding, ScvO2 dropped (Tbsl = 78 +/- 7 vs. T0 = 61 +/- 5% P < 0.05) and oxygen extraction ratio increased (Tbsl = 0.20 +/- 0.07 vs. T0 = 0.36 +/- 0.05, P < 0.05). By T4 the ScvO2 normalized, but on average it remained 5% lower than at Tbsl (T4 = 73 +/- 9% P < 0.05) and oxygen extraction also remained higher as compared with Tbsl (T4 = 0.24 +/- 0.09 P = 0.001). ScvO2 showed significant correlation with SVI (r = 0.564, P < 0.001). dCO2 increased during hypovolemia (Tbsl :5.3 +/- 2.0 vs. T0 :9.6 +/- 2.3 mmHg, P = 0.001), then returned to normal by T4 = 5.1 +/- 2.6 mmHg, and it also showed significant correlation with SVI (R = -0.591, P < 0.001) and oxygen extraction (R = 0.735, P < 0.001). CONCLUSIONS: In this SV-guided bleeding and fluid resuscitation model, both ScvO2 and dCO2 correlated well with changes in SV, but only the dCO2 returned to its baseline, normal value, while ScvO2 remained significantly lower than at baseline. These results suggest that dCO2 may be a good hemodynamic endpoint of resuscitation, while ScvO2 is not strictly a hemodynamic parameter, but rather an indicator of the balance between oxygen delivery and consumption.

We solve the Maxwell–Bloch equations of resonant nonlinear optics using GPUs and compare the computation times with traditional single- and multithreaded programs. A detailed benchmarking of programs as a function of various parameters shows how the massive parallelism built into GPUs becomes more and more advantageous as the physical problem becomes more and more demanding. For the case of multimode light propagating through an inhomogeneously broadened medium of many-level quantum systems, the program executing on GPUs can be over 20 times faster than that executing on all cores of a modern CPU. The methods presented can be applied in a wide area of atomic physics where the time evolution of atomic ensembles is to be computed.

Monitoring hypovolemia is an everyday challenge in critical care, with no consensus on the best indicator or what is the clinically relevant level of hypovolemia. The aim of this experiment was to determine how central venous oxygen saturation (ScvO2) and central venous-to-arterial carbon dioxide difference (CO2gap) reflect hypovolemia-caused changes in the balance of oxygen delivery and consumption. Anesthetized, ventilated Vietnamese minipigs (n=10) were given a bolus followed by a continuous infusion of furosemide. At baseline and then in five stages hemodynamic, microcirculatory measurements and blood gas analysis were performed. Oxygen extraction increased significantly, which was accompanied by a significant drop in ScvO2and a significant increase in CO2gap. There was a significant negative correlation between oxygen extraction and ScvO2and significant positive correlation between oxygen extraction and CO2gap. TakingScvO2<73% and CO2gap >6 mmHg values together to predict an oxygen extraction >30%, the positive predictive value is 100%; negative predicted value is 72%. Microcirculatory parameters, capillary perfusion rate and red blood cell velocity, decreased significantly over time. Similar changes were not observed in the sham group. Our data suggest thatScvO2<73% and CO2gap >6 mmHg can be complementary tools in detecting hypovolemia-caused imbalance of oxygen extraction.

Intense laser light traversing a thin layer of nematic liquid crystal is an example of a simple, easy to realise physical system, that shows very complex behaviour. Light can turn the optical axis of this uniaxial medium while propagating through it, and the dynamical behaviour that results ranges from simple orientational instabilities, through low-dimensional chaotic behaviour, to spatiotemporal pattern formation. In this paper, we review recent advances in the theoretical description of the complex phenomena that light can induce in nematic liquid crystals. We discuss the various approximations made in the models, their range of applicability, and contrast their results with experimental observations. In particular, we discuss the achievements of the plane wave approximation in various geometries, and examine how the numerous bifurcation scenarios calculated from the models can be used to interpret observations. We treat with special emphasis the results achieved in the description of the strongly nonlinear regime, where experiments have revealed interesting bifurcations and chaotic behaviour. We also discuss the mostly open problems of transverse pattern formation and the effects that can be attributed to the finite cross-section of the laser beam.

Background Major abdominal surgery is associated with significant risk of morbidity and mortality in the perioperative period. Optimising intraoperative fluid administration may result in improved outcomes. Our aim was to compare the effects of central venous pressure (CVP), and central venous oxygen saturation (ScvO2)-assisted fluid therapy on postoperative complications in patients undergoing high risk surgery. Methods Patients undergoing elective major abdominal surgery were randomised into control and ScvO2 groups. The target level of mean arterial pressure (MAP) was ≥ 60 mmHg in both groups. In cases of MAP 3 % decrease indicated need for intervention, regardless of the actual MAP. Data are presented as mean ± standard deviation or median (interquartile range). Results We observed a lower number of patients with complications in the ScvO2 group compared to the control group, however it did not reach statistical significance (ScvO2 group: 10 vs. control group: 19; p = 0.07). Patients in the ScvO2 group (n = 38) received more colloids compared to the control group (n = 41) [279(161) vs. 107(250) ml/h; p

We investigate the behavior of Rb85 atoms in the field of a sequence of frequency-chirped short laser pulses for coherent manipulation of the Rb atomic beam. The analysis is based on numerical solution of equations for the density matrix elements of the hyperfine levels of the 5S1/2-5P3/2 transition in Rb85 atom. We analyze two different regimes of interaction including relatively short laser pulses (when the width of the pulse envelope spectrum is of order or exceeds frequency interval between the hyperfine levels resulting in effective mixing of them) and relatively long ones (when the ground hyperfine levels are resolved but the excited ones are not resolved). We show that in all regimes considered, the interaction of a frequency-chirped laser pulse with the multilevel Rb85 system is similar to the interaction with an effective two-level atom at sufficiently large peak intensities of the pulses. It allows us to perform efficient excitation of the multilevel atom by transferring populations of two hyperfine ground states to the excited ones and back to the ground states using a pair of frequency-chirped laser pulses. We present experimental results of utilizing this scheme of population transfer for coherent manipulation of an ensemble of Rb85 atoms in a magneto-optical trap.

We study theoretically the transitions induced by a circularly polarized plane light wave perpendicularly incident on a thin layer of homeotropically oriented nematic liquid crystal. There are three dynamical regimes separated by different types of transitions.

We report on efforts to develop a theory to understand director oscillations that are generated when a linearly polarized light-wave falls on a homeotropically aligned nematic at a slightly oblique angle. The results of various approaches are compared with each other and to experimental results.

Photon-echo based optical quantum memory schemes often employ intermediate steps to transform optical coherences to spin coherences for longer storage times. We analyze a scheme that uses three identical chirped control pulses for coherence rephasing in an inhomogeneously broadened ensemble of three-level $\Lambda$-systems. The pulses induce a cyclic permutation of the atomic populations in the adiabatic regime. Optical coherences created by a signal pulse are stored as spin coherences at an intermediate time interval, and are rephased for echo emission when the ensemble is returned to the initial state. Echo emission during a possible partial rephasing when the medium is inverted can be suppressed with an appropriate choice of control pulse wavevectors. We demonstrate that the scheme works in an optically dense ensemble, despite control pulse distortions during propagation. It integrates conveniently the spin-wave storage step into memory schemes based on a second rephasing of the atomic coherences., Comment: 11 pages, 9 figures

Fluid resuscitation is still a major challenge. We aimed to describe changes in central venous oxygen saturation (ScvO2 ) and venous-to-arterial carbon dioxide gap (dCO2 ) during an experimental stroke volume (SV) index (SVI)-guided hemorrhage and fluid resuscitation model in pigs.Twelve anesthetized, mechanically ventilated pigs were bled till baseline SVI (Tbsl ) dropped by 50% (T0 ), thereafter fluid resuscitation was performed with balanced crystalloid in four steps until initial SVI was reached (T4 ). Statistical analysis was performed with Statistical Program for Social Sciences version 18.0; data are expressed as mean ± standard deviation.After bleeding, ScvO2 dropped (Tbsl = 78 ± 7 vs. T0 = 61 ± 5% P0.05) and oxygen extraction ratio increased (Tbsl = 0.20 ± 0.07 vs. T0 = 0.36 ± 0.05, P0.05). By T4 the ScvO2 normalized, but on average it remained 5% lower than at Tbsl (T4 = 73 ± 9% P0.05) and oxygen extraction also remained higher as compared with Tbsl (T4 = 0.24 ± 0.09 P = 0.001). ScvO2 showed significant correlation with SVI (r = 0.564, P0.001). dCO2 increased during hypovolemia (Tbsl :5.3 ± 2.0 vs. T0 :9.6 ± 2.3 mmHg, P = 0.001), then returned to normal by T4 = 5.1 ± 2.6 mmHg, and it also showed significant correlation with SVI (R = -0.591, P0.001) and oxygen extraction (R = 0.735, P0.001).In this SV-guided bleeding and fluid resuscitation model, both ScvO2 and dCO2 correlated well with changes in SV, but only the dCO2 returned to its baseline, normal value, while ScvO2 remained significantly lower than at baseline. These results suggest that dCO2 may be a good hemodynamic endpoint of resuscitation, while ScvO2 is not strictly a hemodynamic parameter, but rather an indicator of the balance between oxygen delivery and consumption.

We present a theoretical study of the Light-Induced Freedricksz transition in two geometries where dynamical phenomena induced by the laser light have been observed. We consider a spatially extended system, i.e. we assume the laser beam to be much wider than the thickness of the cell. We suggest that pattern formation phenomena in the plane of the cell entirely due to the light-director interaction should be possible to observe.

We present a study of the passage of a monochromatic, linearly polarized cw laser beam through a cell of homeotropically aligned nematic liquid crystal at a slightly oblique angle. The light is polarized perpendicular to the plane of incidence. Experiments in this geometry have revealed the existence of complex, time-dependent dynamics of the director motion. We present a model for the director dynamics derived from the basic equations that shows both periodic and chaotic behavior at different light intensities as observed in the experiments. The model we derive exhibits a variety of complex behaviors, among them an uncommon route to chaos via gluing bifurcations that has not yet been observed in any real physical system.

We study the passage of monochromatic, cw laser light through a cell of homeotropically aligned nematic liquid crystal at a slightly oblique angle, polarized perpendicular to the plane of incidence. Experiments in this geometry have revealed complex, time-dependent dynamics of the director motion. We derive a model for the director dynamics that shows both periodic and chaotic behavior at different light intensities as observed in the experiments. It follows an uncommon route to chaos via gluing bifurcations which has not yet been observed in any real physical system.

An analytic theory of the adiabatic passage (AP) regime of interaction of short frequency chirped laser pulses with a two-level quantum system (QS) being initially in the superpositional quantum state is presented. We show that the initial value of the non-diagonal elements of the density matrix can influence the dynamics of the population transfer during the action of the laser pulse, but do not influence essentially the final populations of the states of the QS obtained at the end of the interaction near the AP regime. A novel solution to the Bloch equations is obtained in the form of a converging power series generalizing the ordinary AP solution.

We have utilized neural networks for fast evaluation of tomographic data on the MT-1M tokamak. The networks have proven useful in providing the parameters of a nonlinear fit to experimental data, producing results in a fraction of the time required for performing the nonlinear fit. Time required for training the networks makes the method worth applying only if a substantial amount of data are to be evaluated.

Photon-echo based quantum memories use inhomogeneously broadened, optically thick ensembles of absorbers to store a weak optical signal and employ various protocols to rephase the atomic coherences for information retrieval. We study the application of two consecutive, frequency-chirped control pulses for coherence rephasing in an ensemble with a 'natural' inhomogeneous broadening. Although propagation effects distort the two control pulses differently, chirped pulses that drive adiabatic passage can rephase atomic coherences in an optically thick storage medium. Combined with spatial phase mismatching techniques to prevent primary echo emission, coherences can be rephased around the ground state to achieve secondary echo emission with close to unit efficiency. Potential advantages over similar schemes working with $\pi$-pulses include greater potential signal fidelity, reduced noise due to spontaneous emission and better capability for the storage of multiple memory channels., Comment: 12 pages, 13 figures, accepted for publication in Phys. Rev. A

We consider the simultaneous propagation of a pair of Raman-resonant, frequency-modulated (chirped) laser pulses in an optically thick medium, modeled by an ensemble of $\ensuremath{\Lambda}$ atoms. A self-organization (``matching'') effect is shown for the chirped-pulse pair, which leads to a quasilossless propagation. Furthermore, we demonstrate that a well-defined coherent superposition of the atomic ground states and, correspondingly, a coherence are robustly created in the medium that can be controlled by amplitudes of the laser pulses. The proposed scheme can be applied to substantially increase the efficiency of the optical wave mixing processes, as well as in other nonlinear processes where the initial preparation of a spatially extended medium in a coherent superposition state is required.

We propose a scheme of transient laser manipulation and cooling of two-level quantum systems with narrow natural linewidths by a sequence of counterpropagating laser pulses with special frequency chirping. Interaction with a large number of laser pulses within the decay time decreases drastically the cooling time of such systems. \textcopyright{} 1996 The American Physical Society.

We theoretically analyze pulse propagation in a medium of inhomogeneously broadened two-level quantum systems, which have a vibrational degree of freedom with respect to the center-of-mass coordinate. This system mimics local mode oscillations of rare-earth-metal-ion dopants in dielectric crystals that are coupled to electronic transitions. We show the emergence of various nonlinear optical phenomena, such as self-induced transparency or the nonlinear interaction between two pulses coupling to different electrovibrational transitions. Interaction between the pulses makes it possible to generate various Raman sidebands of the incident fields and to tune the location where they are generated. We also demonstrate controlled population transfer between electrovibrational states of the ions at specific points along the propagation axis. Similarities and differences between our results and other pulse propagation phenomena of few-level quantum systems are discussed.

Red blood cell transfusion is done primarily as a means to improve oxygen delivery (DO₂). Current transfusion guidelines are based solely on hemoglobin levels, regardless of actual DO₂ need. As central venous oxygen saturation (ScvO₂) may reflect imbalances in DO₂ and consumption (VO₂) the aim of this study was to investigate the value of ScvO₂ as an indicator of oxygen balance in isovolemic anemia.After splenectomy, anesthetized Vietnamese mini pigs (n = 13, weight range: 18-30 kg) underwent controlled bleeding in five stages (T₀-T₅). During each stage approximately 10% of the estimated starting total blood volume was removed and immediately replaced with an equal volume of colloid. Hemodynamic measurements and blood gas analysis were then performed.Each stage of bleeding resulted in a significant fall in hemoglobin, T₀ : 125 (113-134) to T(5) : 49 (43-55) g/l [T₀ : 7.7 (6.9-8.2) to T₅ : 3.0 (2.6-3.4) mmol/l]. The O₂-extraction (VO₂/DO₂) increased significantly only from T₃ : 35 (21-40) %, P 0.05. The change of ScvO₂ showed a similar pattern and dropped below the physiological threshold of 70% at T₄ : 68 (61-76) %. At this point, hemoglobin was below the recommended transfusion trigger value, 59 (53-67) g/l [3.6 (3.3-4.1) mmol/l]. There was a strong significant association between ScvO₂ 70%) and VO₂/DO₂ 30%): r = -0.71, r² = 0.50, P 0.001.The results of this study show that ScvO₂ reflects changes of VO₂/DO₂ in isovolemic anemia better than Hb alone, therefore it may be used as an additional indicator of blood transfusion in clinical practice.

Anemia can cause an imbalance in oxygen delivery (DO2) and consumption (VO2), which may be difficult to detect. Recently the venous to arterial carbon dioxide difference has been shown to be increased (>5 mmHg) in certain critically ill conditions [1,2]. No study has yet investigated its significance in severe normovolemic anemia. Therefore, the aim of this study was to investigate the course of the central venous to arterial carbon dioxide gap (dcvCO2) in isovolemic anemia.

We study the quantum control of multilevel atoms with rotationally degenerate levels using short laser pulses. Various control schemes are considered, ones using {pi} pulses, frequency-chirped pulses, two consecutive pulses, or two pulses that overlap each other partially. We study the possibilities of controlling the quantum state of an ensemble of atoms distributed randomly over one or more rotationally degenerate levels initially. For the sake of concreteness we use the hyperfine level scheme of the {sup 85}Rb D line, but the results can easily be generalized for any of the alkali-metal atoms used in cooling and trapping experiments. We find that even though a number of difficulties arise, such as unequal coupling constants between rotational sublevels or dephasing between different hyperfine levels during the interaction, control schemes using simple or multiphoton adiabatic passage can be used to control the internal states of the atoms effectively as well as the center-of-mass motion. Furthermore, it is shown that in some cases it is possible to exploit the inequality of the coupling constants to entangle the rotational substates with specific distinct translational quantum states and hence separate these substates in momentum space.

We discuss the mechanical momentum transfer to two-level atoms by a pair of short, counterpropagating, frequency-chirped laser pulses, which partially overlap each other in the atomic medium. We show that such a pulse pair can induce a much greater change of momentum than pulses that do not overlap (interact separately with the atoms). The dispersive effect on the atomic velocity distribution is shown to be much smaller for the case of overlapping pulses. Furthermore, there are regimes where the method is not sensitive to the exact values of the pulse parameters or the initial velocity distribution. The interaction can be repeated for a cumulative effect, so a sequence of such pulse pairs can be used very effectively to manipulate the mechanical motion of atoms.

This paper shows that a single laser pulse with a chirped frequency can generate trapped superposition of the ground states in the /spl Lambda/-atom when the width of the transform-limited laser pulse envelope frequency spectrum (without chirp) is smaller but the peak Rabi frequency of the pulse is larger than frequency interval between the two ground states of the /spl Lambda/-atom. This study also analyzes the propagation of the frequency-chirped laser pulse in a medium consisting of /spl Lambda/-atoms analytically and by exact numerical solution of coupled Bloch-Maxwell equations. The results show that there is nearly absorptionless propagation of the single frequency-chirped laser pulse on distances more than 40 times the pulse duration, when the formulated above conditions of negligible excitation of the /spl Lambda/-atom are fulfilled. The described effect of absorptionless propagation of the single frequency-chirped pulse is referred to as electromagnetically self-induced transparency.

We investigate the director oscillations of a homeotropically aligned nematic liquid crystal that are generated by cw laser radiation incident at a slightly oblique angle. The full nematodynamic equations are solved numerically, and it is shown that the inclusion of backflow leads to a qualitative change of the theoretical bifurcation scenario at moderate to high intensities. Very good correspondence is achieved with recent observations. The route to chaos via a sequence of homoclinic gluing bifurcations, whose existence was suggested by simple models, but doubted by recent calculations and experiments, is shown to exist in a parameter region unexplored by experiments.

We investigate the dynamical phenomena induced by a circularly polarized plane wave incident perpendicularly on a homeotropically aligned nematic layer. We study theoretically the influence of the velocity field (backflow effect) on the bifurcation scenario. Whereas backflow leads to substantial quantitative changes of secondary bifurcation thresholds, the overall bifurcation scenario remains unchanged. In the regime of uniform precession of the director with large reorientation, an unanticipated spatial oscillation of the flow field across the layer is found. Quantitative comparison with experimental large-aspect ratio systems is now possible.

We investigate the behavior of ${}^{85}\mathrm{Rb}$ atoms in the field of a sequence of frequency-chirped short laser pulses. The analysis is based on a numerical solution of equations for the probability amplitudes of the hyperfine levels of the ${5S}_{1/2}{\ensuremath{-}5P}_{3/2}$ transition in the ${}^{85}\mathrm{Rb}$ atom and the dressed-states analysis. We analyze different regimes of interaction, including relatively short laser pulses (when the width of the pulse envelope spectrum is of the order of or exceeds the frequency interval between the hyperfine levels resulting in effective mixing of them) and relatively long ones (when the ground hyperfine levels are resolved but the excited ones are not resolved). In the latter case dependence of the population transfer efficiency on the initial coherence of the ground states is analyzed. The case of long laser pulses when all working hyperfine levels are resolved is also discussed using numerical simulations and a dressed-states analysis. We show that in all regimes considered, the interaction of a frequency-chirped laser pulse with the multilevel ${}^{85}\mathrm{Rb}$ system is similar to the interaction with an effective two-level atom at sufficiently large peak intensities of the pulses. It allows us to perform efficient excitation of the multilevel atom by transferring populations of two hyperfine ground states to the excited ones and back to the ground states using a pair of frequency-chirped laser pulses. We propose to utilize this scheme of population transfer for the coherent manipulation of a beam of ${}^{85}\mathrm{Rb}$ atoms using sequences of counterpropagating frequency-chirped short laser pulses.

We study theoretically the instabilities induced by a linearly polarized ordinary light wave incident at a small oblique angle on a thin layer of homeotropically oriented nematic liquid crystal with special emphasis on the dye-doped case. The spatially periodic Hopf bifurcation that occurs as the secondary instability after the stationary Freedericksz transition is analyzed., Comment: 8 pages, 7 figures, LaTeX, accepted to Phys. Rev. E

We study the propagation of coherent light pulses in a medium of three-level atoms with degenerate ground- and excited-state sublevels in an electromagnetically induced transparency (EIT)-type configuration. Both the strong control field and the weak probe pulse have elliptical polarization, which gives rise to concurrent multipath couplings between the ground-state sublevels and the auxiliary stable state. We derive the probe field susceptibility and show that in general, the probe field propagates in two separate polarization modes, one of which is attenuated, the other of which displays EIT. This generic result is valid provided the atomic medium is prepared to be in a pure quantum state over the ground-state sublevels initially. We also investigate the case when the initial state of the medium is described by an incoherent mixture of ground-state sublevels and show how EIT-like pulse propagation degrades. The possibility of controlling the probe susceptibility matrix with control field polarization provides a convenient tool for probing the quantum state of the medium on the degenerate ground-state sublevels.

In contrast to a few follicles arranged in a wedge-shaped segment at the periphery of a lymph node, a significant amount of thyroid tissue in a cervical lymph node is considered evidence of metastatic thyroid carcinoma. In a consecutive series of 243 patients with papillary carcinomas, 52 presented with lateral cervical masses that proved to be lymph nodes with metastatic thyroid carcinoma, in the absence of readily palpable thyroid nodularity. The metastatic disease was demonstrated by excisional biopsy in 40 patients and fine needle aspiration cytology in 12 patients. Thirty-two (of the 52) underwent further diagnostic work-up, consisting of radionuclide scintigraphy and ultrasonography, with the demonstration of abnormalities consistent with a thyroid neoplasm in 75 per cent (24/32) of these patients. The remaining 20 patients had no additional studies except for chemical thyroid function evaluation. The operative treatment in all 52 patients was total thyroidectomy and unilateral or bilateral modified neck dissections, when extensive cervical adenopathy was encountered. Papillary thyroid carcinoma, ranging in size from 2 to 14 mm, was found in the lobe ipsilateral to the presenting cervical node metastasis, in all specimens. In addition, contralateral cervical lymph node metastases were found in five (10%) of these patients, with no evidence of thyroid carcinoma in the corresponding thyroid lobe. With a mean follow-up period of 9 years, all patients are alive with no evidence of recurrence.(ABSTRACT TRUNCATED AT 250 WORDS)

Recurrent hyperparathyroidism (HPT) occurs in a small percentage of patients undergoing parathyroidectomy for primary HPT and is usually due to inadequate excision of hyperfunctioning parathyroid tissue in the neck, a missed ectopic and hyperplastic parathyroid, or, less commonly, parathyroid carcinoma and parathyroid autografts. In order to determine the incidence, clinical characteristics, and outcome of patients with recurrent HPT due to parathyroid autografts, we reviewed our experience with 604 consecutive patients operated on for primary HPT between 1965 and 1989. One hundred of these patients received parathyroid autografts consisting of portions of one or more parathyroid glands. Three patients with autografts, placed in the sternocleidomastoid muscle, developed recurrent HPT due to their autografts for an incidence of 3 per cent. Recurrent disease was diagnosed between 62 and 113 months with an average of 89 months. The autotransplants in all three of these patients were from hyperplastic or adenomatous parathyroid tissue. Two patients had a history of neck irradiation. Preoperative thallium scans accurately localized the hyperfunctioning parathyroid tissue in all three patients. At operation, the hyperfunctioning autografts had grown into a discrete mass with a single vascular pedicle and were resected. Histologic examination disclosed either hyperplastic or adenomatous tissue, and corresponded to the histology and location of the original tissue transplanted in each case. Follow-up ranges from 12 to 67 months, with an average of 48 months. All patients remain cured and none require oral calcium supplementation. We conclude that graft-dependent recurrent HPT is due to the autotransplantation of hyperplastic or adenomatous parathyroid tissue and that thallium scanning is instrumental for diagnosis and localization.(ABSTRACT TRUNCATED AT 250 WORDS)

The diagnosis of thyroid carcinoma during the course of lobectomy for a dominant nodule occasionally cannot be rendered on the basis of frozen section. Once the diagnosis of carcinoma is made, the question of completion thyroidectomy arises. The decision to perform completion thyroidectomy and the timing, safety, and efficacy of this procedure are reviewed.During the past 25 years (1965 to 1990), we operated on 351 consecutive patients with thyroid carcinoma. One hundred of these patients (84 women and 16 men) were initially treated by unilateral thyroid lobectomy for the previously stated reasons. Histopathologic examination of the permanent sections of the initial thyroid lobectomy specimen demonstrated papillary carcinoma in 70 patients and follicular carcinoma in 30 patients. Within a few months, a completion thyroidectomy was performed.The completion thyroidectomy specimen contained papillary carcinoma in 33 (47%) of the 70 patients with papillary carcinoma and 10 (33%) of the 30 patients with follicular carcinoma. Overall, 43 of these 100 patients harbored thyroid carcinoma in the contralateral lobe. Complications of completion thyroidectomy were transient recurrent nerve paresis in two patients and temporary hypoparathyroidism in three patients, requiring calcium and vitamin D therapy for a few months.Although the significance and treatment of papillary carcinoma are debated on the basis of size and grade of the primary lesion and age and sex of the patients, once the diagnosis is made in one lobe we believe that a completion thyroidectomy should be considered, not only for papillary carcinomas but also for follicular carcinomas because 47% (papillary) to 33% (follicular) of these patients will harbor the neoplasm in the contralateral lobe.