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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 La1/3NbO3 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 red...

The response of a SERF atomic magnetometer to a repetitive short-pulsed pump was investigated. Quantum sub-resonances at a repetition rate of $1/n$ of the Larmor frequency of the magnetic field inside the shield are experimentally observed and theoretically explained. This is a type of synchronization phenomenon. Investigations in single alkali atoms cells as well as mixed alkali atoms of K and Rb are presented. In the later, one species is pumped while the probe is on the other specie polarized by spin exchange. The effect of spin destruction, spin exchange and collisions are studied in order to account for the width of the resonances. Quantum calculations of a three levels $\Lambda$ model for this phenomenon exhibit a dip at the resonance frequency in the absorption spectrum for both cases of pulsed and CW pump modes and an evidence for EIT., Comment: 14 pages, 11 figures

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 divided into four areas: gain-inversion, absorption-no inversion, gain without inversion and absorption with inversion. The first two are usual regimes of incoherently pumped lasers, while the last two stem from the quantum interference phenomena. We analyse the parameter areas where such quantum regimes are possible. The exact numerical solution of the time-dependent master equation shows that keeping counter-rotating terms (no rotating wave approximation) gives rise to new spectral features in the form of additional peaks at combination frequencies. Actually the considered system works as a coherent frequency up-converter.

This article presents a method to determine the magnetic field distribution within the vapor cell of a spin-exchange relaxation-free (SERF) atomic magnetometer with a sensitivity of the order of 10 femtoTesla and a bandwidth of DC to 100 Hz, in the presence of an uncompensated ambient magnetic field of up to several nanoTesla. The method is based on the analysis of the atomic polarization in a multichannel pump–probe configuration, in which a spatially selective optical pumping enables to probe specific layers of the atomic vapor contained in a gas-buffered cell. An SERF magnetometer is inherently sensitive to one component of the magnetic field, orthogonal to the pump and probe laser beams. The sensor’s performance can be drastically degraded by the other uncompensated components of the magnetic field. Typically, SERF magnetometry requires very good magnetic shielding and active compensation of residual magnetic field to properly function; this is commonly achieved by applying a complex design of Helmholtz coils in a sophisticated compensation procedure. The method suggested in this article eliminates the influence of non-uniform residual magnetic fields on the accuracy of measurements without precise compensation of the interfering field components. This procedure is used to simplify the measurements of magnetic field distribution in a large vapor cell with required accuracy. This is critically important for precise multichannel magnetic field mapping used for localization of the magnetic dipole-field source.

The high-pressure behavior of Li x La1/3NbO3 (x = 1/6, 1/3, 1/2, 2/3) perovskites where Li cations were substituted for the existing vacancies was studied using synchrotron X-ray diffraction. It was shown that all these materials undergo irreversible pressure-induced amorphization around 14.5 GPa regardless of the Li concentration. The Li-inserted materials were found to exhibit a standard pressure response (bulk modulus pressure derivative B 0′ ~4) when in the crystalline phase, whereas La1/3NbO3 shows a linear volume contraction versus pressure, i.e., B 0′ ~(−1). These results suggest that the structural collapse is not a consequence of cation disorder resulting from the Nb atoms (B-site) migrating to the A-site vacancies. The observed pressure response can be understood by increased occupancy of the A-sites opposing the tilting of the NbO6 octahedra. The pressure evolution of the Nb oxidation state is discussed.

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.

La1/3TaO3, an A-site cation deficient perovskite, has been studied under pressure by synchrotron X-ray powder diffraction and Raman spectroscopy. It undergoes irreversible pressure induced amorphization at P=18.5 GPa. An almost linear unit cell volume decrease vs. pressure is observed from ambient pressure up to the phase transition. The Raman spectroscopy also shows amorphization at the same pressure, with positive shifts of all modes as a function of pressure. The pressure dependence of the Eg and A1g Raman modes arising from the octahedral oxygen network is discussed.

In a recent publication we have studied counterintuitive pulse sequence applied to a three level ladder scheme, utilizing rectangular-shaped pulses. In this research we investigate amplification without population inversion in a three level ladder system interacting with two electromagnetic fields, namely, the probe and coupling fields. We take both probe and drive fields in the form of Gaussian shaped pulses. In a counterintuitive sequence scheme, the short probe pulse is introduced prior to the application of the coupling field, in contrast to a regular sequence scheme, where both fields are introduced at the same time. The influence of varying the probe pulse width and time delay between the initiation times of the probe and coupling fields on transient probe gain is investigated. It is found that the system exhibits a kind of memory about previously applied weak probe pulse. This may allow for detection of the past event by applying a strong drive pulse in the future.

La 1/3 NbO 3 , an A-site cation deficient double perovskite, has been studied under pressure by synchrotron X-ray powder diffraction and Raman spectroscopy. In contrast to other highly distorted perovskites, transforming to post-perovskite on compression, La 1/3 NbO 3 undergoes irreversible pressure induced amorphization at ~ 14.5 GPa. Over a wide range of pressure before that transition it presents an almost linear volume decrease versus pressure, accompanied by convergence form orthorhombic to a cubic unit cell. The Raman spectroscopy also shows an amorphization at the same pressure. However, where upon full decompression the X-ray diffraction pattern remains amorphous, new Raman peaks become active.

A three level scheme interacting with three laser fields is considered. It is found that two competing processes contribute to the absorption/dispersion on a particular transition: (i) one-photon processes governed by the field applied to that transition, and (ii) two-photon processes governed by the two laser fields applied to two other transitions. Interference between the two processes can be both constructive and destructive. It is shown that the net absorption/gain and dispersion can be controlled by the relative phases of the laser fields.

In this research we study lasing without population inversion from a three-level atom interacting with two laser fields, in the ladder or cascade scheme. We investigate counterintuitive sequencing as well as regular sequencing of the time of laser fields application. In a counterintuitive sequence scheme a short probe pulse is introduced prior to the application of the coupling field, in contrast, to a standard sequence scheme, where both fields are introduced at the same time. The influence of varying the probe pulse width and time delay between the initiation of probe and coupling fields on transient probe gain is investigated. The calculations indicate the potential of producing gain without inversion via counterintuitive sequence scheme.

Ultrashort pulse counterintuitive sequencing of three level lambda scheme shows five fold enhancement of the amplification without inversion on the probe transition. Introducing a short pulse weak probe field prior to the strong pump field results in an enhanced dispersion and stronger amplification on the probe field. The presence of an incoherent pump populating the upper level is crucial to the amplification. Parametric study of this effect is briefly presented.

We show that a single-cell spin-exchange relaxation-free (SERF) atomic magnetometer can be used for 3-D field measurements. The 3-D operation is achieved by optically pumping the magnetometer cell in successive layers perpendicular to the laser probe beam and by collecting the signals with a photodiode array. The vapor cell is thus divided into voxels acting as local atomic magnetometers. Each voxel's contribution to the probe linear polarization rotation is proportional to the convolution of the local polarization and the magnetic field. The layers are pumped in sequence without waiting for the atomic polarization in the previously pumped layer to relax to zero, consequently, reducing the total measurement time. The photodiode signals are the sum of accumulated linear polarization rotation caused by all the layers. These signals are processed by a deconvolution algorithm that extracts the information originating from each individual voxel. Our theoretical analysis and experimental results show that the time of a single-layer pumping can be reduced below the atomic-cell relaxation time constant, with almost no loss of precision.

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 approximate analytic solutions for dressed-state populations and coherences are derived, within the so called “secular approximation,” under resonant conditions. We also present time dependent numerical solutions for population and coherences in the off-resonance regime. A spectral analysis is also performed revealing the structure of various dressed states transitions. These are shown to be composed of quintets centered about the frequencies of the coupling and probe laser fields and having sidebands located symmetrically at positions shifted from line center at the Rabi and double Rabi frequencies.

A dressed-state study of lasing without population inversion from a three-level atom interacting with a bichromatic laser field in the ladder configuration is being formulated. The system is allowed to be dressed by both laser-field photons (double dressing). The temporal behavior of the system under consideration is being explored numerically. Although the use of dressed-basis density matrix equations introduces major complexity into the equations, this procedure has crucial importance to the understanding of the atomic spectrum and the underlying physics of the processes involved. The system is found to possess a gain without population inversion, as well as a regular gain with inversion for various transitions, both in the transient and steady-state regimes. Enhancement of the refractive index accompanied by vanishingly small absorption was also found. The frequency response of various transitions is explored via numerical Fourier analysis. Some transitions are found to exhibit a dispersion-like line shape, clear evidence of a quantum-coherent interference effect. The spectrum consists of two distinct quintets centered about the coupling and probe-laser frequencies. The spectral features consisting of each quintet are located symmetrically (at resonance conditions only) around the line center at positions shifted from the center by the generalized Rabi and double Rabi frequencies.

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.

A scheme for obtaining a laser without inversion in the deep X-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in the Ar8+ system. In this system, conventional lasing with population inversion in a neonlike scheme is used. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. The ground level is incoherently pumped into the highest level, while the latter is incoherently depumped into the ground level, at a different adjustable rate. A detailed density matrix calculation of the ladder configuration is performed [3], and the conditions under which the system exhibits inversionless gain, as well as regular gain, are derived. Numerical calculations in the bare state picture exhibits gain without inversion both at resonance and at the generalized Rabi side bands. The probe lineshape is shown to be constructed of two symmetrical dispersionlike features each centered at the generalized Rabi side bands. This is evidence of the quantum interference occurring at these locations and the basis for the gain without inversion.

A scheme for obtaining up-conversion lasing without inversion in the visible range is proposed using a Doppler-broadened He-Ne mixture as active medium. We consider a cascade scheme (2s2↔2p2↔1s3) with a frequency up-conversion ratio of 1.91. The upper transition is inverted and driven by an external field and lasing without inversion occurs at 616.4 nm provided that the collisional decay rates of the 1s3 and 1s5 levels overcome a certain threshold.

The factorial design technique is implemented to achieve greater accuracy in the determination of magnetic field distribution within a single cell of spin-exchange relaxation-free atomic magnetometer. Three-dimensional magnetic field distribution within a single vapor cell can be found by consecutively pumping, layer by layer, all the cell volumes perpendicular to the probe laser beam, detected by a photodiode array. Thus each element of the array collects information about the magnetic field in the small volume (voxel) which forms when the corresponding part of the probe beam and optically pumped layer cross. One of the most effective ways to enhance measurement accuracy is repeated pumping of the layers and averaging the measured results. However, the measurement time is multiplied several times due to the repeated scanning of the cell volume. The suggested technique enables increased measurement accuracy of each voxel while preserving the number of measurements. Magnetic field distribution is determined by the illumination of the cell layers one by one or simultaneously, according to a special algorithm, with subsequent multifactorial analysis of the obtained results.

We propose a concept of fractional quantum resonances based on an idea of driving a quantum system by an external field in the form of a repetitive pulse sequence. We show that, in addition to intuitively expected resonance at repetition rate ${\ensuremath{\omega}}_{\text{Rep}}$ equal to the internal quantum frequency $\ensuremath{\omega}$, the system exhibits less intuitive fractional quantum resonances at ${\ensuremath{\omega}}_{\text{Rep}}=\ensuremath{\omega}/n$, with $n$ being an integer. It is a nonintuitive phenomenon since the external repetition rate has no quantum character, yet the atom responds to it, if the rate is equal to $1/n$ of its eigenfrequency. This result is in good agreement with recent magnetometer experiments. It is also found that other resonances are possible at pulse-repetition-rate frequencies equal to $1/n$ of combination frequencies involving the internal system frequencies and Rabi frequencies. We show that quantum-interference-related phenomena of gain without population inversion and absorption with inversion are observed at these unique resonances. Fractional quantum resonances provide a spectroscopy tool, allowing us to measure optical frequency on one transition through detection of resonances on another transition. We believe that our results will have implications in other quantum-related processes, such as laser-assisted resonant enhancement of chemical reactions and biological processes.

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.

Dynamic properties of closed three-level laser systems are investigated. Two schemes of pumping\char22{}$\ensuremath{\Lambda}$ and V\char22{}are considered. It is shown that the nonlinear behavior of the photon number as a function of pump both near and far above threshold is crucially different for these two configurations. In particular, it is found that in the high pump regime laser can turn off in a phase-transition-like manner in both $\ensuremath{\Lambda}$ and V schemes.

A scheme for active frequency doubling is suggested. The system comprises N semiconductor quantum wells situated into resonant cavity. Theoretical model for such systems is based on three-level virtual atoms in ladder configuration. It is found that the system can lase in either superradiant or subradiant regime, depending on the number of atoms N. When N passes some critical value the transition from the super to subradiance occurs in a phase-transition-like manner. Stability study of the steady state supports this conclusion.

An explicit relationship between the Lorenz-Mie resonances and nonlinear optics of a microsphere has been found. We have constructed an approximate analytic method which allows an adequate assessment of the threshold for stimulated Raman scattering in microspheres. Systematic investigation of the characteristic equation, using approximate analytic expressions, reveals that not all modes have the same quality. Some resonances with very high quality correspond to a physical situation in which the spherical waves can be envisioned as whispering-gallery light waves that are totally internally reflected by the sphere boundary, without suffering almost any loss. \textcopyright{} 1996 The American Physical Society.

A scheme for active second harmonics generation is suggested. The system comprises N three-level atoms in ladder configuration, situated into a resonant cavity. The system generates the field whose frequency is twice the frequency of the pumping laser, and the field phase is locked to the phase of the pumping field. It is found that the system can lase in either superradiant or subradiant regime, depending on the number of atoms N. When N passes some critical value the transition from the super to subradiance occurs in a phase-transition-like manner. Stability study of the steady state supports this conclusion. For experimental realization of the super/subradiant second harmonics generation we propose semiconductor quantum well structures, superconducting quantum circuits, and evanescently coupled waveguides in which equally spaced levels relevant to this study exist.

A model of a superradiant laser comprising of N three-level systems driven by two pumping lasers is considered in semiclassical approximation. Steady-state equations for density matrix elements are solved both analytically and numerically. Various properties of generated light are investigated. It is found that such a laser can exhibit both superradiance, when the intensity scales as N(2), and subradiance when the intensity does not depend on N. The phase of the laser field is locked to the relative phase of the pumping lasers.

The dressed-atom approach to the fluctuation problem in lasers is described. Such an approach allows one to analyze the fluctuations in various kinds of laser models. The intensity fluctuations in a regularly pumped laser are considered in the framework of the dressed-atom approach. The Fokker-Plank equation for the field density matrix is derived from first principles without heuristic assumptions. An alternative interpretation of the noise suppression in the regularly pumped laser is given. It is based upon the nonstationary picture for atomic variables. In such a regime the statistics of the output laser field may be controlled by the inuput statistics of the atomic initial conditions. It is the regular pump that provides the best input statistics

The intrinsic property of laser systems with many internal degrees of freedom to generate a squeezed light is studied. The dressed-atom approach to fluctuations, described in the preceding paper [Khazanov, Koganov, and Shutter, Phys. Rev. A 48, 1661 (1993)], is employed. A minimization principle which governs noise quenching in laser systems is described comprehensively. This principle allows one to avoid complicated quantum-mechanical calculations to assess the squeezing capacity of the system. It is shown that noise in a laser system can be decomposed into noise states. These states interact coherently. For instance, each nonactive level in a multilevel lasing scheme may represent separate noise states under certain conditions while both lasing levels make only one noise state

The dynamics of a photoemitted charged pulse in a one dimensional photocathode is investigated. At a very short pulse duration, on the order of a few picoseconds, short with respect to the transit time, interesting features are revealed. After a transient period, the charged pulse spreads and becomes uniformly distributed. The pulse spreads approximately to the extent of the transit time in the gap. This behavior is the result of the self-action electric field and depends on the total charge emitted under the laser power, on the pulse length and on the electric field. A one dimensional simulation and an analytic model describe the pulse propagation and are in excellent agreement. At high acceleration fields, in the relativistic regime, different time evolutions result. In the present simulation we make the simplified assumption that the relativistic effect only changes the electron mass. The result is that the self-field is overcome by the relativistic dynamics, and the electron pulse steepens.

In this paper we present the preliminary results from a new experimental technique to synchronize the pulses from a mode-locked Nd-YAG laser to the light pulses in the VUV storage ring at the National Synchrotron Light Source (NSLS). We describe a method to electronically change the delay time between the laser pulses and the synchrotron pulses. We also illustrate a method to overlap the synchrotron pulses with the laser pulses in space and time. Preliminary results will be presented for two experiments.

The amplitude fluctuations in a single-mode laser for the case of regular pumping when the atoms are injected in the cavity at fixed, unfluctuating rate are discussed. The dressed atom approach is employed. A comparison is made of such pumping scheme with the conventional one, when the number of active atoms is fixed. The results of the present work agree well with other approaches.

A new principle of laser coherence found recently provides an example of the most correlated quantum state. We discuss the relation between squeezing phenomena and quantum correlations (QC). These new results can be understood only in the framework of a special QC approach.

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 cold part of the cell. In this configuration there is no differential pressure on the hot windows and thus can be used also in high‐pressure studies. Moreover, thin windows can be used to minimize their effect on absorption measurements. A continuous independent adjustment of the absorption length and the vapor density can be achieved by simple adjustment of the hot windows spacing. This is accomplished by moving the inner pipes from outside the cell without opening it. A movable inner thermocouple is also added to control and measure the temperature and its distribution along the absorption region.

Laser schemes with one and two lasing transitions are compared with respect to amplitude squeezing capacity. It is shown that the squeezing capacity of schemes with one lasing transition is larger. The hierarchy of the two different photon statistics is established.

Utilization of polarizing optics for dynamic attenuation of intense unpolarized laser beams is reported. The new family of variable attenuator is composed of a retardation plate mounted between two identical polarization splitters placed in tandem. By varying the retardation, or by rotating the plate, the transmission is varied. These attenuators are characterized by a high transmission and a high optical damage threshold. A transmission dynamic range as large as 1 to 10(-5) is achievable with polarization splitters made of crystals such as sapphire. These attenuators are applicable in laser material processing and in Q switching of unpolarized lasers.

The change in the reflection behavior of VO2 films for the 1.06 μm 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.

We show that initially dipole-forbidden atomic transitions can be effectively controlled by bichromatic laser fields. Such transitions become dynamically allowed due to ac-Stark effect. Gain, dispersion and spectral properties of these transitions are discussed.

Charge transfer was predicted in the Zn‐Ar system. We have shown experimentally the validity of this prediction and that in fact Ar+ excites the Zn+ 2P3/2,1/2 states. Moreover, we find that this process preferentially excites the J= (1)/(2) state in this mixture. This is inferred from the behavior of the intensity ratio of the transition from these two states in correlation with the intensity of the 811.5‐nm line of Ar i, where the latter is a measure for Ar ionization. Our model is based on the molecular states of Zn+Ar and Ar+Zn. The model displays a plausible curve crossings leading to the preference 2P1/2 excitation.

We present partial density of states measurements for the occupied electronic levels around oxygen, strontium and bismuth in Bi2Sr2CaCu3O8. The data were obtained using soft X-ray emission. We find that the oxygen 2p density of states peaks 4.7eV below EF and is roughly symmetric in appearance. We also observe a p-type density of states around the strontium sites centered near 5.3eV below EF. States of d-type symmetry are observed around the bismuth near the binding energy of the bismuth 6s level. We compare the measurements with calculations that have been presented in the literature and with published photoemission results.

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.

We have observed an insulator to metal transition in amorphous silicon monoxide at a critical pressure of ${P}_{c}\ensuremath{\sim}12\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ as deduced from the temperature dependence of the electrical resistance and ir and Raman spectroscopies. X-ray scattering data indicate a disordered material at pressures below and above the transition to the metallic state and up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Above ${P}_{c}$, $dR∕dT$ becomes positive, and the ir and Raman spectra flatten, indicating delocalization of nonbonding states. The temperature dependence of the conductivity is in very good agreement with the Anderson model for amorphous materials.

The dynamics and quantum statistical properties of the three-level laser ladder model are investigated. A comparison between this model to the other models, V and {lambda} ones, is presented. We found that the ladder-pumped lasers have both the highest intensity and the lowest linewidth, whereas the maximum noise reduction is more pronounced in the V and {lambda} models. We also show that at high pumping rate the ladder scheme behaves as the two-level one.

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

We describe an autodyne lidar with multiple external mirrors by using an analytical approach presented previously [ Appl. Opt.41, 7087 ( 2002)]. The use of multiple target mirrors allows one to detect and locate the concentration of species between any pair of neighboring mirrors.

A scheme for obtaining laser in the deep x-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in ${\mathrm{Ar}}^{+8}$ system. In this system, conventional lasing with population inversion has been obtained in a neonlike fashion. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. This will be selected to best match the frequency needed for the transition from lower level of the present x-ray lasing to the ground state. Numerical calculations in the bare state picture exhibit possible gain at resonance, and at the Rabi side bands, for appropriate choice of system parameters, i.e., the strength of the driving field, the strength of the incoherent pump that provides the required energy needed for the inversionless amplification and the deexcitation rate. The latter rate is detrimental to lasing as it increases. It is quite interesting to note that the system exhibits surprisingly high stability for gain without inversion even for quite high rate of the incoherent deexcitation processes. Real and imaginary parts of the coherences were also obtained for detuned drive and probe fields. These provide insight into the role played by the quantum interference. The present theoretical calculations give high confidence in the quantum interference process as a candidate for realizing very short wavelength laser.

An analytical approach for a calculation of the parameters of autodyne lidar is presented. Approximate expressions connecting the absorption coefficient and the distance to the remote target with both the lidar parameters and the measured quantities are obtained. These expressions allow one to retrieve easily the information about the atmosphere from the experimental data.