Photon echo in the presence of a magnetic field: New prospects for data accumulation and processing

The possibility for echo signal switching-off and the switching of echo polarization between the ±45° positions is demonstrated for the photon echo (PE) and stimulated photon echo (SPE) generated in Yb vapor at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition by two pulses of the resonant linearly polarized radiation at appropriate experimental parameters in the weak magnetic field limit. The experimental data are in qualitative agreement with the theoretical analysis and the calculations made for the 1 ? 0 transition. The strong magnetic field limit leads to unpolarized PE and SPE signals generated by linearly polarized radiation pulses. The possibility of the generation of a long-lived echo in ytterbium vapor due to the magnetic field induced mixing of the upper working level with the metastable level is discussed. The results can be employed for the optical data storage and processing.     

Coherent control of optical transient processes in a gas

The possibility of controlling the processes of the free-polarization decay and photon echo in a gas by means of resonant CW radiation has been shown. The photon echo and free-polarization decay are formed using the method of the Stark switching of levels in a low-intensity radiation field in the presence of an orthogonally polarized strong field. The experiments in the gas at the R(4, 3) transition in the vibrational band 0 ? 1 v₃ ¹³CH₃F in the presence of radiation from a CW CO₂ laser and the calculations by the evolution operator method show a strong effect of the high-intensity radiation field on the photon echo and free-polarization decay to its complete suppression.     

Behaviours of excitonic optical nonlinearity associated with a large detuning rate photonic echo in biased asymmetric coupled quantum well structure

The third-order optical nonlinear susceptibility ?? ⁽³⁾ responsible for phase-conjugate beam in non-degenerate four-wave mixing for a biased asymmetric coupled quantum well (ACQW) structure are given by the doubled-side Feynman diagram technique. It is shown that the photonic echo (PE) in excess of one terahertz pump-probe detuning rate can be achieved under low temperature condition due to electronic wavepaket oscillation when two laser beams interacts with the delocalized excitons in the ACQW structure. Also, the resonant peak position in the frequency domain of the large detuning rate PE depends strongly on an external inverse electric field strength but its peak intensity is insensitive to this one, resulting in the frequency conversion peak maintains a constant in a small bias range.     

B(1)(+)/actual flip angle and reception sensitivity mapping methods: simulation and comparison

Knowledge of the spatial distribution of transmission field B₁⁺ and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B₁⁺ and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo.Several B₁⁺ mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B₁⁺ mapping techniques.This study introduces a quantitative comparison between two known methods for B₁⁺/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results.     

Implementation of a double-scanning technique for studies of the Hanle effect in rubidium vapor

We have studied the resonance fluorescence of a room-temperature rubidium vapor exited to the atomic ⁵P_3/2 state (D₂ line) by powerful single-frequency cw laser radiation (1.25 W/cm²) in the presence of a magnetic field. In these studies, the slow, linear scanning of the laser frequency across the hyperfine transitions of the D₂ line is combined with a fast linear scanning of the applied magnetic field, which allows us to record frequency-dependent Hanle resonances from all the groups of hyperfine transitions including V- and Λ-type systems. Rate equations were used to simulate fluorescence signals for ⁸⁵Rb due to circularly polarized exciting laser radiation with different mean frequency values and laser intensity values. The simulation show a dependence of the fluorescence on the magnetic field. The Doppler effect was taken into account by averaging the calculated signals over different velocity groups. Theoretical calculations give a width of the signal peak in good agreement with experiment.     

Averaged local field intensities in composite films

The photophysical properties of molecules adsorbed in composite films (e.g. surface island films) depend on the local electromagnetic field within the film. The ratio between the average field intensity 〈|E|²〉 in the film and the intensity |E_I|² associated with the incident field is a measure of the electromagnetic contribution to the surface influence on molecular photophysical phenomena. This ratio depends on the film composition and morphology, on the dielectric properties of the pure components making the film and on the frequency, direction and polarization of the incident radiation. Calculations of this ratio as a function of these parameters for several models of composite films are presented. Image interactions and retardation effects as well as radiative damping and finite size contributions to the dielectric response of the films are taken into account. In addition, an estimate of the field inhomogeneity within the film is obtained by calculating also the ratio 〈|E|²〉_shell/|E_I|² associated with the field in thin shells surrounding the dielectric particles which constitute the film.     

Effect of the Excitation Radiation Coherence on Oscillations of the Photon Echo Intensity

The physical reasons for observing the splitting of optical lines several orders of magnitude smaller than the spectral width of a laser pulse are investigated. A theory of coherent and incoherent photon echo (PE) in an external static magnetic field and in the presence of a pulsed magnetic field, which causes oscillations of the PE intensity, is elaborated. It is shown that the periods of oscillations in the echo intensity, the echo duration, and the dimensions of the regions in the inhomogeneous line, where the excited ions are coherent, do not depend on the degree of coherence of the laser pulse and on the external static magnetic field. As follows from the theory, in the case of the coherent excitation of the echo, the amplitude of the intensity oscillations is independent of the external static magnetic field if the inhomogeneous line is symmetric. It is shown that the amplitude of the oscillations at the incoherent excitation of the echo is equal to the autocorrelation function of the distribution function of the transition frequency along the inhomogeneous line with the argument equal to the Zeeman splitting of the optical line in the external magnetic field. In this case, the experimental values of the oscillation amplitude are in good agreement with the calculated values of the autocorrelation function for the total inhomogeneous line in LuLiF₄:Er³⁺ (⁴I_15/2?F_9/2 transition). In the same way, the autocorrelation function has been obtained for YLiF₄:Er³⁺ on the same transition.     

Normal modes of a probe field in the pulsed regime of electromagnetically induced transparency in a Λ-scheme of degenerate quantum transitions

The effect of separation of linearly polarized short probe pulses of electromagnetically induced transparency in the field of linearly polarized coupling radiation is modeled numerically. It is shown that the input-probe pulses polarized parallel or perpendicular to the input-probe field propagate in the medium without changing the state of their polarization. If the input-probe radiation is weak compared to the coupling radiation, then the probe field inside the medium is the sum of two independently propagating linearly polarized normal modes, which are excited by the projections of the input-probe pulse onto the direction of polarization of the coupling radiation and onto the perpendicular direction, respectively. The normal modes have the same phase velocities, but different velocities of their real envelopes. This circumstance leads to the rotation of the plane of polarization of the total probe field at short distances and to its separation into two pulses with mutually perpendicular directions of polarization at long distances. At a high intensity of the probe radiation, the input-probe pulse decays into pulses the planes of polarization of which are not mutually perpendicular. Under these conditions, it is impossible to represent the probe radiation as a sum of normal modes. The modeling is performed in the scheme of degenerate quantum transitions between states of levels ³ P ₀, ³ P ₁ ⁰, and ³ P ₂ of isotope ²⁰⁸Pb taking into account the Doppler broadening of spectral lines.     

Exchange charge model and analysis of the microscopic crystal field effects in KAl(MoO4)2:Cr

In the present paper, we report on consistent crystal field calculations of the Cr³⁺ ions energy levels in KAl(MoO₄)₂ using actual D_3d site symmetry of the Cr³⁺ position and employing the exchange charge model (ECM) of the crystal field. In addition to the energy level calculations, the Huang-Rhys factor S=5.7 and effective phonon energy ?ω=268 cm^-1 were evaluated in the single configurational coordinate model. Detailed treatment of the microscopic crystal field effects in the ECM framework allowed to obtain analytical dependence of the crystal field strength 10Dq on the Cr-O interionic distance and extracting from it the values of some parameters of the electron-vibrational interaction (EVI) in the KAl(MoO₄)₂:Cr³⁺ system. All obtained results are compared with experimental data and discussed; agreement between the calculated and experimental parameters is good.     

Generation of characteristic x rays by a terawatt femtosecond chromium-forsterite laser

Characteristic K _α x rays arising when a metallic target is irradiated by femtosecond infrared pulses that are generated by a terawatt chromium-forsterite laser system (1240 nm, 90 mJ, 80 fs) are studied. The absolute yield (up to 3 × 10⁸ photons/sr pulse) and the coefficient of the transformation of laser radiation to K _α radiation (maximum value ≈0.03%) are measured for an iron target. The dependence of the radiation intensity on the angle of incidence of p polarized laser radiation is analyzed. The mechanisms of the production of fast electrons responsible for generating characteristic x rays are discussed.     

Modulation spectroscopy using the beam-foil light source

When a beam of ions passes through a thin exciter foil, certain radiation emitted by the beam particles can exhibit time-periodic intensity variations. These variations can be induced by external E and H fields, or they can be the result of the field-free atomic structure itself.Intensity modulations observed so far in beam-foil spectroscopy can be divided into three classes: (1) Quantum mechanical interference of fine structure levels. This is a QM resonance arising from time-dependent populations of emitting states having different transition probabilities. The resonance is induced by external constant electric fields. (2) Initial coherent superpositions of radiating states. This results from the creation of M_L alignment at the instant of excitation by the foil. The modulations are field free and are observed in polarized light. (3) Rotating electric dipole in a magnetic field. When alignment occurs, the intensity of the beam radiation after excitation satisfies the relation I(t) = = I₀[1+Acos(2γHt)] e^?αt. The modulation will be a function of the magnetic field H and the gyromagnetic ratio γ.These effects can be used to study Lamb shifts, g-values, fine structure levels, and interaction processes.     

Influence of J-mixing on the phenomenological interpretation of the Eu3+ ion spectroscopic properties

Two sets of B^k_q crystal field parameters and Bγ_kq intensity parameters have been calculated from the transition occuring between the Stark levels of Eu³⁺ in KY₃F₁₀. One of the calculations includes the J-mixing; the other not. The influence of the J-mixing on the intensities of the “forbidden” transition ⁵D₀ → ⁷F₀ and on the hypersensitive transitions are analysed.     

Relativistic theory of atom-laser interactions at very high laser intensities

The high-frequency approximation of Kristi? and Mittleman is considered in detail as a basis for the relativistic theory of atom-laser interactions. The properties of the 3D potentials are discussed. Within a one-dimensional model similar to that employed by Kylstra, Ermolaev, and Joachain in ab initio calculations on the time-dependent Dirac equation, the electron mass-shift due to dressing by a superstrong laser field is investigated. In the full domain of the laser parameters, the frequency ω and the peak field strength ?₀, the 1D bound states exhibit remarkable features. The numerical calculations show the existence of a very wide intermediate range of the field strengths where, in the zeroth order of the high-frequency approximation, the binding is stabilized by the field.     

Laser-induced effects for overlapping autoinoizing rydberg states of xenon

The effect of a laser field on the cross section of photoionization of an atom by probe radiation in the region of closely lying autoionizing resonances is studied theoretically. The cross sections in the region of the overlapping autoionizing Rydberg states 5p ⁵(² P _1/2)6d′ J = 1 and 5p ⁵(² P _1/2)8s′ J = 1 resonantly coupled with the discrete state 5p ⁵(² P _3/2)7p[1/2]₁ in the xenon atom are calculated. The behavior of the shape and position of resonances in relation to the frequency and intensity of laser radiation is studied.     

Crystal field energy levels of the laser crystal Gd3Ga5O12: Nd

The crystal field energy levels of laser crystal Gd₃Ga₅O₁₂: Nd³⁺ are calculated using the diagonalization (of energy matrix) method. From the calculations, the 93 observed crystal field energy levels are explained reasonably and the root-mean-square (r.m.s.) deviation σ(≈25.6 cm⁻¹) and the scalar crystal-field strength parameter Nv (≈3847 cm⁻¹) are obtained. The results are discussed.     

Electromagnetically induced transparency in a molecular gas

The dynamic Stark effect on the as Q(5,4) transition of the 0 → 1 vibrational band of the ν₂ mode of ammonia ¹⁵NH₃ was studied by the method of copropagating waves of radiation of two ¹³CO₂ lasers operating at the R(18)_I line. For waves with mutually orthogonal polarizations, which propagated in a waveguide cell, the intensity of the saturating radiation reached 225 W/cm², and the absorption line of the probing radiation had the form of a two-hump curve with a splitting that was consistent with the Rabi frequency for the ammonia transition being investigated. At the maximum intensity of the saturating radiation, complete transparency was observed at the center of the absorption line of ¹⁵NH₃. When identically polarized saturating and probing waves propagating in a cell at a small angle with respect to each other were used, the splitting was more weakly manifested. The effect of a spatial inhomogeneity of the optical fields on the shape of the line of saturated absorption is discussed.     

Degradation of superconducting properties in MgB2 by Cu addition

The (MgB₂)_2−xCu_x (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E_F, and the corresponding velocity, V_F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB₂. Two different impurity phases, MgCu₂ and CuB₂₄, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T_c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x=0.0 samples, the transition is found to be sharp, ΔT∼1 K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x=0.0 sample the n was calculated to be 12×10²¹ cm⁻³, but for the x=0.5 sample it decreased to 1.3×10²¹ cm⁻³. We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, E_F, calculations the decrease of the carrier concentration by the additions of Cu into MgB₂ gives a decrease in E_F and this could be attributed to a shift of the Fermi level towards the top of the σ-hole band.     

Weak magnetic field behavior of photon echo in a LuLiF4:Er3+ crystal

We report the first observation and study of the photon echo in Er³⁺:LuLiF₄. The energy transition is ⁴ I _15/2 → ⁴ F _9/2 (λ = 6536 Å). The density of ErF₃ is 0.025 wt %. The operation temperature is 1.9 K. Measurements were made at low (up to 1200 Oe) and even zero external magnetic fields. We studied the behavior of the photon echo intensity vs. the magnetic field magnitude and direction about the crystal axis C and vs. the laser pulse separation t ₁₂ and observed an exponential growth and then, after a certain plateau, an exponential decrease in the photon echo intensity as a function of magnetic field upon increasing the magnetic field from zero. The parameters describing the exponential growth and decrease are independent of the direction of magnetic field. The value of the magnetic field (～20–200 Oe) at which the echo intensity is maximal and the value of the maximum itself decrease with increasing pulse separation t ₁₂ and the angle Θ between the magnetic field and crystal axis. The echo intensity decreases exponentially with increasing Θ. The parameter describing the exponential decrease is independent of the magnitude of the field. The echo intensity as a function of pulse separation shows exponential decay. The phase relaxation time depends on the magnitude and direction of the magnetic field. T ₂ is equal to 202 ± 16 ns at zero magnetic field. A phenomenological formula is suggested, which qualitatively presents the mentioned dependences, and the polarization properties of the backward photon echo in this crystal are studied. Because the ion of trivalent erbium is an optimum data carrier, the above results show that fine control of the multichannel transfer of processed optical information may be achieved by weak magnetic fields.     

Effect of magnetic field on luminescence of MgS,CaS and CaO phosphors activated with Bi3+ ions

The magnetic field dependence of the intensity I(A₀) of the zero-phonon line of the ³A_1u → ¹A_1g emission band is measured at 4.2 K for MgS, CaS and CaO phosphors activated with Bi³⁺ ions. The increase in I(A₀) is proportional to the square of the external magnetic field. The results are compared with those for Pb²⁺-activated phosphors.     

The electronic structure of CuCl: An “ab initio” Hartree-Fock SCF-MO study of the CuCl43− cluster

Results from restricted Hartree-Fock SCF-MO calculations on the CuCl₄^3? cluster occurring in solid CuCl are reported. The chemical bonding is discussed on the basis of a population analysis of the ground state orbitals. Calculations on the first ionized states show the Cu 3d electrons to be less tightly bound than the Cl 3p electrons in contrast to Koopmans' theorem predictions and in agreement with experiment. The high degree of localization of the 3d hole orbitals obtained is in conflict with earlier estimates of d-bond covalency. It is suggested that the “Ham effect” rather than covalency is responsible for the reduced spin orbit splitting that can be deduced from the observed exciton absorption. The potential surface for a Cu⁺ ion moving in the field of neighbouring Cl^? ions, has been determined from calculations on the CuCl₄^3? cluster in which the metal ion has been displaced towards a face, an edge or a vertex of the surrounding tetrahedron. The vibration frequency and the activation energy for the diffusion of Cu⁺ ions obtained are in fair agreement with the experimental data. These results support the viewpoint that the anomalous change in scattering intensity with temperature observed in X-ray and neutron diffraction studies is due to the presence of a substantial fraction of the Cu⁺ ions on interstitial lattice sites.