Synchrotron radiation sources have opened a new window on the century-old use of x rays as a scientific tool. X-ray fluorescence, excited by the photoabsorption process, has been a part of this research picture almost since the day that x... more
Synchrotron radiation sources have opened a new window on the century-old use of x rays as a scientific tool. X-ray fluorescence, excited by the photoabsorption process, has been a part of this research picture almost since the day that x rays were first discovered. However, the investigation of multi-photon processes in gases and solids had to wait until the second half of the 20th century. The advent of intense synchrotron radiation sources based on the use of specialized insertion devices will provide many new scientific opportunities for the 21st century. This presentation will outline some of the recent exciting discoveries in soft x-ray fluorescence spectroscopy and discuss a new type of laser-synchrotron hybrid experimental technique based on the time structure of the synchrotron radiation.
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Enhancement of the sensitivities of optical magnetometers, atomic clocks and atom interferometers and other quantum metrology devices requires introducing new physical processes to improve on their present achievements. Many body... more
Enhancement of the sensitivities of optical magnetometers, atomic clocks and atom interferometers and other quantum metrology devices requires introducing new physical processes to improve on their present achievements. Many body collective correlations among the atoms, spins or, in general, quantum systems may prove to be a suitable method. As these correlations introduce interference terms in the intensity of the scattering amplitudes, they may enhance the signal as $N(N-1)$ for N correlated quantum systems. These correlations enhance the signal to noise ratio by a factor of $N^2$ and contribute to better sensitivity in quantum metrology. Moreover atomic correlation may provide quantum noise limit, Heisenberg limit. In the present communication excitation exchange induced by photons in a cavity between two atoms is calculated and clearly exhibits correlation and collective effects. A novel operator is introduced that expresses photon-induced excitation exchange that takes in accou...
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ABSTRACT La K-edge X-ray absorption spectroscopy has been used to elucidate the changes in the local electronic and lattice structure that occur in the A-site deficient double perovskite LaNbO up to 6 GPa. The pressure evolution of the... more
ABSTRACT La K-edge X-ray absorption spectroscopy has been used to elucidate the changes in the local electronic and lattice structure that occur in the A-site deficient double perovskite LaNbO up to 6 GPa. The pressure evolution of the oxygen dodecahedrum around the A-site has been examined. XANES (X-ray absorption near edge structure) data show modifications ascribed to the increase of bands overlapping as a consequence of the bond distance contraction, which has been directly probed by EXAFS (extended x-ray absorption fine structure) spectra. The La–O Debye Waller factors (DWFs) tend to increase whereas the La–Nb bond DWFs show only a tendency to decrease indicating the robustness of the crystal lattice structure, even in presence of the oxygen disordering. This permits the system to reverse back to its original conditions in this pressure range as evident from the measurements upon pressure release. The present results have been interpreted in the light of charge transfer related to the two-step reduction mechanism acting at the Nb site (with niobium ions passing from Nb to Nb) which also results in the elongation of the Nb–O bond distances in the octahedra, in agreement with the Nb K-edge results reported earlier.
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ABSTRACT Not Available
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The conventional way of studying laser noise assumes analytic solution of the quantum problem for the laser field coupled to the active medium. A number of the laser schemes have been discussed, and predictions for noise reduction and for... more
The conventional way of studying laser noise assumes analytic solution of the quantum problem for the laser field coupled to the active medium. A number of the laser schemes have been discussed, and predictions for noise reduction and for the squeezed state of field have been made.1 On the basis of first principies of the quantum mechanies, the conventional approach exhibits the relatively rare situation in which the quantum equations admit compact analytic solution. At the same time, the difficulty in interpreting the results seems to be unresolved. Specifically, it is very difficult lo determine the relative contribution of each physical source of noise to the final fluctuation result.
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The behavior of a laser intensity as a function of a pump rate is studied. We consider a three-level lasing scheme with two different types of pumping. In the first case, the lower lasing level is depleted to the ground state, and then... more
The behavior of a laser intensity as a function of a pump rate is studied. We consider a three-level lasing scheme with two different types of pumping. In the first case, the lower lasing level is depleted to the ground state, and then the pumping is used to excite the atom from the ground state to the upper lasing level. In the second case the pumping is used to excite the atom directly from the lower lasing level to one of the excited states, which is then depleted to the upper lasing level. We refer these two types of pumping as type 1 and type 2, respectively. It is shown that although these two schemes are equivalent mathematically, i.e., they are formally described by the same set of equations, physically their dynamical properties are completely different. It is noted that the pumping of the type 1 does not affect either of lasing levels directly, whereas the pumping of the type 2 does connect the lower lasing level with one of the excited states. It is this difference of the two ways of pumping which is responsible for the crucial difference in the dynamic behavior of the lasing schemes of types 1 and 2
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Conical light emission is observed when an intense quasiresonant laser beam propagates in an atomic vapor.1-4 A parametric four-wave mixing process was suggested to explain this effect.1 However, this model is inconsistent with... more
Conical light emission is observed when an intense quasiresonant laser beam propagates in an atomic vapor.1-4 A parametric four-wave mixing process was suggested to explain this effect.1 However, this model is inconsistent with experiments both quantitatively in the value of the predicted cone angle and qualitatively due to the absence of the obligatory blue counterpart to the red-shifted cone photon.2-4 A model of Cherenkov radiation by a laser-induced moving polarization was proposed to account for the origin and angular/spectral dependencies of the conical emission.3
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ABSTRACT The observations of anomalous emission near the D1 transition of a dense (>or=1014 cm-3) sodium vapour illuminated by a laser detuned to the blue with respect to the D2 transition is studied and reported. A discussion of... more
ABSTRACT The observations of anomalous emission near the D1 transition of a dense (>or=1014 cm-3) sodium vapour illuminated by a laser detuned to the blue with respect to the D2 transition is studied and reported. A discussion of existing models is presented. The authors propose a type of parametric down-conversion process to explain this emission. This process involves a laser photon splitting into two photons by the three-level system of sodium, assisted by a magnetic dipole transition.
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The passage of a laser light detuned to the blue of a resonant atomic transition results in a conical emission around the laser beam [1, 2]. This conical emission shell has a half-angle of a few degrees around the laser axis, is red... more
The passage of a laser light detuned to the blue of a resonant atomic transition results in a conical emission around the laser beam [1, 2]. This conical emission shell has a half-angle of a few degrees around the laser axis, is red detuned from the transition and is spectrally broad (~ 10 cm-1).
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Research Interests: Engineering and Physics
Abstract High-pressure X-ray diffraction measurements have demonstrated that the cation-deficient perovskites Pr1/3NbO3, Pr1/3TaO3, Nd1/3NbO3, and Nd1/3TaO3 undergo irreversible pressure-induced amorphization (PIA). This occurs near... more
Abstract High-pressure X-ray diffraction measurements have demonstrated that the cation-deficient perovskites Pr1/3NbO3, Pr1/3TaO3, Nd1/3NbO3, and Nd1/3TaO3 undergo irreversible pressure-induced amorphization (PIA). This occurs near 14.5 GPa for the niobates and 18.5 GPa for the tantalates. The unit cell volumes of the four oxides show an almost linear decrease as the pressure is increased. It is concluded that the PIA transition occurs at higher pressures in the tantalates due to the lower MO6 initial tilting at ambient conditions, which is associated with the larger atomic mass of the tantalum. The behavior of these oxides is compared to that of CaTiO3, and the role of both the weakening of the M–O–M π-bonding and the cation vacancies on the observed structural changes is discussed.
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ABSTRACT The line profile of several Cu autoionization levels have been measured experimentally by the optogalvanic technique. The optogalvanic signals related to the autoionization states have a distinct signature, namely a fast initial... more
ABSTRACT The line profile of several Cu autoionization levels have been measured experimentally by the optogalvanic technique. The optogalvanic signals related to the autoionization states have a distinct signature, namely a fast initial response which is the result of an essentially immediate ionization of these levels. As a result, the initial response overlaps the temporal profile of the laser. This is followed by a usual optogalvanic signal (OGS) reflecting the relaxation of the plasma to its equilibrium conditions on plasma timescale. A tunable pulsed dye laser is used to obtain OGS and to scan the resonance profile of several Cu autoionization levels. The transitions show the characteristic asymmetric Fano line profiles. The detection of fast autoionization processes as well as measurements of saturation broadening in some of these levels are demonstrated.
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The change in the reflection behavior of VO2 films for the 1.06 mum wavelength at the semiconductor-metal phase transition was investigated. The nonlinearity caused by photoinduced transition was used for phase conjugation of a pulsed... more
The change in the reflection behavior of VO2 films for the 1.06 mum wavelength at the semiconductor-metal phase transition was investigated. The nonlinearity caused by photoinduced transition was used for phase conjugation of a pulsed Nd:YAG laser. At 60 Hz Q-switching rate and average intensities of the pump and probe waves at 1 and 0.3 W/cm2, respectively, the reflection coefficient of the conjugated wave was ~1%, the threshold energy density was 8 mJ/cm2, the rise time of the signal was 30 ns. The response rate of the phase conjugation via the VO2 films is discussed.
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... Harter and Boyd [2] measured the temporal character of the laser beam before and after the cell, finding no changes in a 3 ns pulse. The temporal characteristics of the cone emission is very important in order to determine its origin.... more
... Harter and Boyd [2] measured the temporal character of the laser beam before and after the cell, finding no changes in a 3 ns pulse. The temporal characteristics of the cone emission is very important in order to determine its origin. ... 2. Harter and RW Boyd, Phys. Rev. ...
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ABSTRACT Anisotropic thermoelectrical Bi0.4Sb1.6Te 3 samples were prepared by powder metallurgy procedures. A strong correlation was observed between the morphological anisotropy of the powder particles and the crystal anisotropy of the... more
ABSTRACT Anisotropic thermoelectrical Bi0.4Sb1.6Te 3 samples were prepared by powder metallurgy procedures. A strong correlation was observed between the morphological anisotropy of the powder particles and the crystal anisotropy of the compound. By using this correlation and taking a particle size range in which the morphological anisotropy was dominant, the thermoelectrical anisotropy was maximized. This desirable particle size was determined by SEM and XRD measurements, while sample anisotropy was determined by XRD measurements on sample facets that had been oriented in parallel and in perpendicular to the applied compression, respectively. Two types of samples, one anisotropic with a high orientation factor (~0.77) of the c-axis parallel to the compressing direction and the other pseudo-isotropic, were fabricated. This study is the first step toward a fabrication of functionally graded (FGM) thermoelectric samples, based on two Bi0.4Sb1.6Te3 ingots, each with a different carrier concentration. The first ingot was designed so that its maximum efficiency is at room temperature, while the second ingot, doped with Pb, is designed for a higher temperature. The results indicate that a power factor of 40 muWcm-1K-2 can be achieved for a sample aimed to operate at the lower temperature. This value is comparable to that observed in single crystals
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We show that it is possible to get gain without inversion at high frequency in a three-level ladder type scheme driven by a single driving laser. The transition from the ground state to first excited state is driven by a low-frequency... more
We show that it is possible to get gain without inversion at high frequency in a three-level ladder type scheme driven by a single driving laser. The transition from the ground state to first excited state is driven by a low-frequency resonant laser field. The next excited state lies far above the first one; therefore its resonant frequency can be much higher than that of the lower transition, and hence one should go beyond the most commonly used rotating wave approximation consisting of retaining only resonant terms (slowly oscillating) in the Hamiltonian. We solve the master equation, both analytically (under proper approximations) and numerically, with the full Hamiltonian keeping counter-rotating highly non-resonant terms. We show a dressed state picture explaining spectral features of gain-absorption profiles. Also we present a kind of phase diagram in the plane γ–Λ, where γ and Λ are spontaneous emission rate and incoherent pumping rate, respectively. The whole γ–Λ plane is di...
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A dressed-state study of lasing without population inversion from a three level atom interacting with a bi-chromatic laser field, in the ladder configuration, is formulated. We allow the atomic system to be dressed by both laser filed... more
A dressed-state study of lasing without population inversion from a three level atom interacting with a bi-chromatic laser field, in the ladder configuration, is formulated. We allow the atomic system to be dressed by both laser filed photons (double dressing). The evolution of the system under consideration is being explored both analytically and numerically, within the transient regime. Time dependent
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ABSTRACT The {ital L}{sub 2,3} emission spectrum of {ital c}-Si, excited by monochromatized synchrotron radiation, has been recorded with a 5-m Rowland spectrometer. Dramatic spectral changes are observed as the excitation energy is... more
ABSTRACT The {ital L}{sub 2,3} emission spectrum of {ital c}-Si, excited by monochromatized synchrotron radiation, has been recorded with a 5-m Rowland spectrometer. Dramatic spectral changes are observed as the excitation energy is varied from the 2{ital p} binding energy up to 144 eV. It is proposed that a spectator electron, close to the bottom of the conduction band, influences the emission spectrum. The observations suggest that intraband shakeup is important in the excitation process, and that a population of low-lying levels, via initial-state shakeup, influences the high-energy-excited Si {ital L} emission spectrum.
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There Is an ongoing interest in the conical emission resulting from the passage of a laser light tuned in the vicinity of a resonant atomic transition(1–2). We also have observed such emission in high density sodium vapor in the range of... more
There Is an ongoing interest in the conical emission resulting from the passage of a laser light tuned in the vicinity of a resonant atomic transition(1–2). We also have observed such emission in high density sodium vapor in the range of 1015 cm−3, when an intense laser radiation is blue detuned with respect to any of the D lines. This conical emission shell has half angle of a few degrees around the laser axis, is red detuned from the respective D line, is spectrally broad (3A) and is obtained in laser intensity and detunings and sodium vapor density ranges where self-focusing occurs. No symmetric blue shifted component was observed. Harter and Boyd(1) have obtained symmetrical Rabi side-band emission in the forward direction at sodium vapor density of 1014 cm−3.
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Nb K-edge x-ray absorption spectroscopy is utilized to investigate the changes in the local structure of the A-site deficient double perovskite La1/3NbO3 which undergoes a pressure induced irreversible amorphization. EXAFS results show... more
Nb K-edge x-ray absorption spectroscopy is utilized to investigate the changes in the local structure of the A-site deficient double perovskite La1/3NbO3 which undergoes a pressure induced irreversible amorphization. EXAFS results show that with increasing pressure up to 7.5 GPa, the average Nb-O bond distance decreases in agreement with the expected compression and tilting of the NbO6 octahedra. On the contrary, above 7.5 GPa, the average Nb-O bond distance show a tendency to increase. Significant changes in the Nb K-edge XANES spectrum with evident low energy shift of the pre-peak and the absorption edge is found to happen in La1/3NbO3 above 6.3 GPa. These changes evidence a gradual reduction of the Nb cations from Nb(5+) towards Nb(4+) above 6.3 GPa. Such a valence change accompanied by the elongation of the average Nb-O bond distances in the octahedra, introduces repulsion forces between non-bonding adjacent oxygen anions in the unoccupied A-sites. Above a critical pressure, the Nb reduction mechanism can no longer be sustained by the changing local structure and amorphization occurs, apparently due to the build-up of local strain. EXAFS and XANES results indicate two distinct pressure regimes having different local and electronic response in the La1/3NbO3 system before the occurence of the pressure induced amorphization at ∼14.5 GPa.
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ABSTRACT Red shifted conical emission induced by a laser blue shifted with respect to the D2 line of sodium is observed and interpreted in terms of Cherenkov emission. Measured cone angles and wavelength shifts agree well with model... more
ABSTRACT Red shifted conical emission induced by a laser blue shifted with respect to the D2 line of sodium is observed and interpreted in terms of Cherenkov emission. Measured cone angles and wavelength shifts agree well with model calculations taking into account saturation and dispersion effects.
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In a spectroscopic study carried out in this laboratory a new energy transfer mechanism from a noble gas molecule to an atom was identified. This transfer process is similar to the Penning ionization in the sense that it populates energy... more
In a spectroscopic study carried out in this laboratory a new energy transfer mechanism from a noble gas molecule to an atom was identified. This transfer process is similar to the Penning ionization in the sense that it populates energy levels of a guest atom selectively within a certain range. A mixture of Ar:Xe was studied. This report discusses these
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... 4 OPTICS COMMUNICATIONS 15 March 1984 TWO-PHOTON ENHANCED OPTOGALVANIC SIGNAL AND POPULATION INVERSION R. SHUKER, A. BEN-AMAR and G ... use approximate considerations to estimate An3(t). First, as rl is large =0. Then Art2 2 (ni - no)... more
... 4 OPTICS COMMUNICATIONS 15 March 1984 TWO-PHOTON ENHANCED OPTOGALVANIC SIGNAL AND POPULATION INVERSION R. SHUKER, A. BEN-AMAR and G ... use approximate considerations to estimate An3(t). First, as rl is large =0. Then Art2 2 (ni - no) and Ana S 2 ...
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ABSTRACT The onset of dimerization in organic mixed-stack compounds allows the appearance of the otherwise symmetric phonons in the absorption spectra. A general theoretical framework shows that the most sensitive effect arises when... more
ABSTRACT The onset of dimerization in organic mixed-stack compounds allows the appearance of the otherwise symmetric phonons in the absorption spectra. A general theoretical framework shows that the most sensitive effect arises when symmetric and antisymmetric modes have nearby frequencies; the growth of dimerization then leads to gradual transfer of absorption intensity, from the antisymmetric mode to the symmetric one. Absorption data on tetramethyl-p-phenylene-diamine chloranil shows this feature and thereby demonstrates the onset of dimerization below ~250 K. The transition involves a small and continuous change in ionicity.
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A three-level atom in the V-configuration is treated under two coherent laser fields at resonance with the two allowed transitions. The master equation for the system and full quantum mechanical Hamiltonian are derived, and equations of... more
A three-level atom in the V-configuration is treated under two coherent laser fields at resonance with the two allowed transitions. The master equation for the system and full quantum mechanical Hamiltonian are derived, and equations of motion for the elements of the density matrix are obtained by projecting into the bare- and dressed-states schemes. Analysis of light amplification without population
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When an intense laser beam is blue detuned with respect to the D2 transition of sodium, at sodium densities > 1013 cm−3 self-focusing occurs. At still higher densities and for laser detuning in the range 6–15 cm−1 to the blue side of... more
When an intense laser beam is blue detuned with respect to the D2 transition of sodium, at sodium densities > 1013 cm−3 self-focusing occurs. At still higher densities and for laser detuning in the range 6–15 cm−1 to the blue side of the D2 line a coherent emission in the forward direction is observed.1 This emission is shifted 1–3 cm−1 to the blue of the D1 transition, depending on the sodium density, and, within the instrumental resolution of our spectrometer of 1.2 cm−1, its frequency is independent on the laser detuning. This emission was observed earlier both in potasium2 and sodium3 vapors, but was not studied thoroughly, neither was a satisfactory explanation presented. We present experimental data of this emission and show that non of the ordinary mechanisms, such as collision induced population transfer, three-photon effect etc., is consistent with the experimental results. We propose a laser photon splitting by the doublet system of sodium as a possible explanation of the “blue peak” emission.
We present a device which makes possible accurate determination of the absorption length of saturated metal vapors inside a long cylindrical absorption cell functioning in the heat-pipe regime. The device is simple to construct and to... more
We present a device which makes possible accurate determination of the absorption length of saturated metal vapors inside a long cylindrical absorption cell functioning in the heat-pipe regime. The device is simple to construct and to operate. The absorption length is determined by the spacing between two movable hot windows welded to inner pipes which are open ended in the
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Spatially modulated gain of a distributed feedback dye laser (DFDL) is used to construct a laser-controlled reflective device. The mirror-type device has the tunability and spectral selectivity of the DFDL. The reflection occurs during... more
Spatially modulated gain of a distributed feedback dye laser (DFDL) is used to construct a laser-controlled reflective device. The mirror-type device has the tunability and spectral selectivity of the DFDL. The reflection occurs during DFDL pulse duration.
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ABSTRACT Not Available
There has been considerable interest in the conical emission resulting from the passage of a laser light detuned to the blue of a resonant atomic transition.1 This conical emission shell has half angle of few degrees around the laser... more
There has been considerable interest in the conical emission resulting from the passage of a laser light detuned to the blue of a resonant atomic transition.1 This conical emission shell has half angle of few degrees around the laser axis, is red detuned from the transition and is spectrally broad (~10 cm−1). It is obtained under conditions of laser detuning and atomic vapor density for which self-trapped filaments are generated. The main model that has been suggested for the ring emission is that of four-wave parametric amplification of Rabi red sideband generated in the self-trapped filaments leading to emission in the form of a cone surrounding the laser beam.2 The cone angle calculated according to this model is smaller than the measured one by almost a factor of 2. Moreover, the absence of a blue counterpart to the red-shifted ring emission in most of the experiments disagrees with this model.
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ABSTRACT A dielectric microsphere possesses natural modes of oscillation at characteristic frequencies, corresponding to specific size to wavelength ratios. These structure resonances are known to cause extremely large field intensities... more
ABSTRACT A dielectric microsphere possesses natural modes of oscillation at characteristic frequencies, corresponding to specific size to wavelength ratios. These structure resonances are known to cause extremely large field intensities and have extremely high quality. We investigate here systematically the characteristic equation, defining the natural frequencies, as a function of the change in the asymptotic behavior of spherical Bessel functions which reveals the fact that there exists resonances with very narrow linewidth. This characteristic has very important implications to nonlinear optics.
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