Interference enhancement of resonant magneto-optical effects

The influence of spectral interference of different nature on resonant Faraday rotation is studied using as examples the effect of electromagnetically induced transparency in pure discrete atomic transitions and the transparency windows of autoionization resonances of gaseous media. A common numerical criterion for a substantial effect of spectral interference and coherence of atomic transitions on magneto-optical phenomena is established. A direct relationship between the polarization ellipse rotation angle of cw monochromatic light and reduction of the light pulse group velocity resulting from a strong frequency dispersion of the resonant refractive index is found. The known experimental data on ultraslow light pulses indicate the possibility of interference enhancement of the Faraday effect by a factor of 10⁶?10⁷.     

Light drag in moving media with a high frequency dispersion

The influence of spectral interference of various nature on the effects of radiation drag is studied using as examples electromagnetically induced transparency in pure discrete atomic transitions and the transparency windows of autoionization resonances of gaseous media. The analysis is performed using the Maxwell equations and taking into account the equivalence of the corresponding spatial dispersion of a substance and the optical effects caused by uniform motion. The realization conditions of the drag coefficients in the Lorentz and Laub forms are found from the exact dispersion equations obtained. A direct relation between the monochromatic radiation drag coefficient and the decrease in the pulsed radiation group velocity due to high frequency dispersion of the resonant refractive index is found. The known experimental data on ultraslow light pulses indicate the possibility of interference enhancement of the light drag effect by a factor of 10⁶-10⁷.     

Autoionization resonances in the photoabsorption spectra of Fe<Superscript><Emphasis Type="Italic">n</Emphasis>+</Superscript> iron ions

The photoabsorption cross sections of a neutral iron atom, as well as positive Fe⁺ and Fe²⁺ ions, are calculated in the relativistic random-phase approximation with exchange in the energy range 20–160 eV. The wavefunctions of the ground and excited states are calculated in the single-configuration Hartree–Fock–Dirac approximation. The resultant photoabsorption spectra are compared with experimental data and with the results of calculations based on the nonrelativistic spin-polarized version of the random-phase approximation with exchange. Series of autoionization resonance peaks, as well as giant autoionization resonance lines corresponding to discrete transitions 3p → 3d, are clearly observed in the photoabsorption cross sections. The conformity of the positions of calculated peaks of giant autoionization resonances with experimental data is substantially improved by taking into account additionally the correlation electron–electron interaction based on the model of the dynamic polarization potential.     

Electromagnetically-induced transparency,slow light,and negative group velocities in a room temperature vapor of 4He1

⁴He¹ at room temperature is a particularly interesting system as velocity changing collisions (VCCs) are necessary to observe ultra-narrow (less than 10 kHz) EIT windows for purely electronic spins in the presence of Doppler broadening. Such narrow resonances are known to be linked to a dramatic reduction of the group velocity of a probe pulse, although the medium is transparent. The evolution of the delay is recorded with respect to the coupling beam intensity and to small Raman detunings. We also demonstrate that it is possible to use optically detuned resonances (Fano-like profiles) to see a transition from slow light to negative group velocity. All these measurements are found to be in good agreement with a simple model based on an effective homogeneous linewidth. To cite this article: F. Goldfarb et al., C. R. Physique 10 (2009).     

Interference effects in alignment and orientation processes of atoms excited by polarized radiation

The dynamics of the processes of alignment and orientation of atoms subjected to the action of polarized radiation has been studied theoretically in the energy range of excitation of autoionization resonances. The alignment parameters A ₂₀ for the 4p ⁴5p states of Kr II populated through the excitation and autoionization decay of Kr I 3d ⁹ np resonances depend on energy due to the interference between different resonance channels and the channel of direct photoionization. It is predicted that the orientation parameter O ₁₀ of some Kr II 4p ⁴5p states and the parameter of the angular distribution of photoelectrons β_e1 strongly depend on energy. The absolute photoionization cross sections are calculated for the population of the Kr II 4p ⁴5p states in the case where the energy of excitation photons corresponds to the first four 3d ⁹ np resonances. A good agreement between the calculated and measured photoionization cross sections proves that it is important to take into account the interference between different resonance channels in order to appreciate the dynamics of the Auger decay of Kr I 3d ⁹ np resonances.     

Observation of autoionization resonances in uranium by step-wise laser photoionization

A large number of autoionization resonances have been observed in uranium in the energy range 50,590–51,560cm^–1 by two-step three-photon ionization technique, using two copper vapor laser pumped dye lasers. A Rydberg series converging to the ionization limit of UII at 1749cm^–1 (⁶ L _13/2) has been identified. Some of these resonances are very narrow with a fwhm of 0.1cm^–1. Possible origins of these are discussed.     

Parametric transformation and spectral shaping of supercontinuum by high-intensity femtosecond laser pulses

The cascaded nonlinear-optical transformation of high-power ultrashort light pulses in an ionizing gas medium involving supercontinuum generation, followed by a frequency conversion of this radiation in the field of femtosecond laser pulses with an intensity of 10¹⁴–10¹⁵ W/cm² has been demonstrated. Parametric four-wave mixing is shown to allow a highly efficient spectral transformation and shaping of supercontinuum radiation. The maximum efficiency of a parametric frequency conversion of femtosecond laser pulses in an ionizing gas medium achieved under the conditions of our experiments is estimated as 1%.     

Laser optoacoustic measurement of paper porosity

The propagation of broadband ultrasonic pulses in combined media that consist of printing paper of different porosity saturated with different liquids is studied. The experiments are performed with three types of paper, namely, Zoom Ultra (Stora Enso, Finland) with surface densities of 80 and 100 g/m² and Data Copy (Mo Do, Sweden) with a surface density of 160 g/cm², and with two types of saturating liquids: ethanol and transformer oil. To excite ultrasonic pulses and to detect them with a high time resolution, the laser optoacoustic spectroscopy method is used. For each type of liquid-saturated paper, the phase velocity of ultrasound is measured in the frequency range of 5–35 MHz. The absence of any noticeable frequency dispersion of the phase velocity is revealed. The possibility of measuring the porosity of printing paper on the basis of the theoretical model of a two-phase medium with the use of the corresponding experimental data is demonstrated.     

Autoionization cross section of lithium atoms excited by electron impact

We measure the full autoionization cross section of lithium atoms excited by electrons in the energy range from the first autoionization threshold at 56.39 to 600 eV. Data are obtained by determining the total intensity of electron spectrum of autoionization states 1sn ₁ l ₁ n ₂ l ₂ detected at the “magic” observation angle of 54.7°. The cross section behavior is characterized by a sharp increase to a maximum value of 1.7 × 10⁻¹⁸ cm² in the energy interval of 56.4–60 eV and a subsequent monotonic drop to a value of 10⁻¹⁸ cm² at 600 eV. We have discovered a “thin” cross section structure that reflects the presence of strong resonances of Li⁻ ions in the near-threshold area of excitation of the lowest energy autoionization states (1s2s2)2S, (1s2s2p)⁴ P, 1s(2s2p ³ P)² P), and 1s(2s2p ¹ P)² P. We have established that the contribution of autoionization to the absolute cross section of single ionization of lithium atoms does not exceed 4%. We perform a comparative analysis of the data with analogous data for potassium and cesium atoms.     

Effect of the pulse leading-edge time on the dielectric strength of a vacuum gap

Optimal regimes for electrode conditioning in a vacuum by applying voltage pulses with different waveforms are considered. For nanosecond pulses with a constant duration (t _p = const), the impulse dielectric strength for an oblique voltage wave is shown to be more than four times higher than for a rectangular pulse with an infinitely short leading-edge duration. The dependences of the dielectric strength on the conditioning pulse duration in the range 10^?10 < t _p < 10^?3 s for pulses with different rise rates are obtained. The dielectric strength increases from 2 × 10⁷ V/m for microsecond pulses to 10¹⁰ V/m for subnanosecond pulses.     

Ultracold Rydberg atoms for efficient storage of terahertz frequency signals using electromagnetically induced transparency

Quantum communication with terahertz (THz) frequency signals has many advantages like reduced attenuation and scintillation effects in certain atmospheric conditions along with very high level of data security. In this work, we propose a scheme to realize Quantum Memory (QM) for efficient storage of terahertz (THz) frequency signals using Electromagnetically Induced Transparency (EIT) in an ultracold atomic medium of ⁸⁷Rb Rydberg atoms prepared in a Two Dimensional Magneto Optical Trap (2D-MOT). The uniqueness of our scheme lies in the choice of the energy levels involved in the EIT process, all three of which have been chosen to be the Rydberg levels (enabling signal beam to be in THz) in a lambda type arrangement. This first of its kind proposal reveals that atomic media are a potential candidate for devising QMs which can store THz frequency signals. We have estimated that the Optical Depth (OD) in our scheme can reach a very high value of 690, very high maximum obtainable storage efficiency (η) of ～99%, the group velocity ($v_g$) can be as low as 5.07 × 10³ m/s, and the Delay Bandwidth Product (DBP) can be as high as 9.5. All of these estimates emphasize the feasibility of our scheme as a QM device for efficient storage of THz pulses.     

Influence of the helium autoionization structures on the single/double ionization signal

Calculations of intense field (around 10¹⁶ W/cm²) single- and double-ionization processes in helium at XUV wavelengths are presented. The laser wavelength is chosen near the | 2s2p ¹ P autoionization structure and the dynamics are explored. Single and double ionization yields, as well as the photoelectron energy spectrum for photon energies around the autoionization structure are calculated. In the case of a pulse of few femtoseconds duration, no significant enhancement of the double ionization yield has been found in tuning the photon frequency around the peak of the resonance. It is also shown that in the case of a long pulse (and hence narrow compared with the relevant autoionization width), the branching ratio of double to single ionization yield can be relatively enhanced by tuning to the absorption minimum of the resonance. Received 19 February 2002 / Received in final form 2 May 2002 Published online 19 July 2002     

Study of postcollision interaction upon electron-impact excitation of the magnesium atom

The excitation functions of the spectral lines of the MgI principal series in the VUV region (for the principal quantum numbers n = 4–7) are studied in experiments on collisions of electrons with magnesium atoms in the range of electron energies from the excitation thresholds to ～25 eV. The scatter in the electron energy was ～0.7 eV. Above the ionization potential, some singularities attributed to autodetachment and autoionization states were found in excitation functions. Shifts to higher energies with increasing n, which take place for the most significant singularity (maximum) relative to the autoionization state 3p4s ¹ P ₁ ^o , are explained by the important role played by this state in the population of the initial levels through the mechanism of post-collision interaction. The theoretical analysis of the experimental results based on the classical model of the postcollision interaction estimates the width of the autoionization state as ～0.45 eV, which is close to the data of other researchers.     

A waveguide high-pass filter system for measuring the spectrum of pulsed terahertz sources

We propose a system for measuring spectra of terahertz (THz) pulses, including single pulses, which is based on high-pass filters (HPFs). The system consists of channels for measuring amplitudes of pulses (initial pulses and those transmitted via HPFs with different cutoff frequencies) and an algorithm for processing of the obtained data. The pulse spectrum is restored by using the iteration method or the amplitude–frequency method. The iteration method of spectrum restoration is applicable in the range of THz pulse durations from 10⁻⁹ s to 10⁻⁷ s. The amplitude–frequency method is applicable to THz pulses with durations exceeding 10⁻⁸ s. The system for measuring of THz pulse spectra was simulated by using the characteristics of specially developed waveguide HPFs. The relative simulation error of determining the central frequency by the amplitude–frequency method is equal to 2 · 10⁻⁶ for THz pulse durations of 10⁻⁵ s and longer.     

Parametric generation of tunable picosecond pulses in proustite between 1.2 and 8 μm

Infrared pulses were generated over a wide tuning range in a single-path system consisting of Nd:YAG laser pulses and two proustite crystals. A bandwidth of approximately 10 cm^-1 for idler pulses was observed. The energy conversion efficiencies varied strongly with frequency between 10^-2 and 10^-4. Bulk damage did not occur for pump intensities up to 6 GW/cm².     

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p ⁵ n ₁ l ₁ n ₂ l ₂ autoionizing states. The cross section has two maxima, 1.8 × 10^?16 and 2.2 × 10^?16 cm², at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.     

Comparison of the Effects of Different <Superscript>19</Superscript>F <Emphasis Type="Italic">π</Emphasis> Pulses on the Sensitivity and Phaseability of the <Superscript>19</Superscript>F-<Superscript>13</Superscript>C HSQC

The ¹⁹F-¹³C heteronuclear single quantum coherence (HSQC) experiment is vital for the structural elucidation of polyfluorinated organic species, yet its sensitivity and phaseability are limited by difficulties in uniform excitation of the widely disperse ¹⁹F spectral window. Adiabatic pulses of different types have previously been employed to generate effective π pulses for inversion and refocussing, but a systematic comparison of various adiabatic and other inversion pulses has not been published. In this work, it was observed that the use of a broadband inversion pulse (BIP) during the t ₁ evolution period resulted in properly phaseable spectra for experiments optimized to detect ¹ J _CF, in contrast to CHIRP or WURST adiabatic pulses. For the INEPT and reverse-INEPT transfer segments of the HSQC, optimal sensitivity for resonances distant from the transmitter frequency was afforded by optimized universal rotation (BURBOP) or CHIRP pulses. In HSQC experiments with delays optimized for two-bond correlations, only the use of BURBOP pulses in INEPT and reverse-INEPT sequences afforded spectra cleanly phaseable across the F ₂ and F ₁ spectral windows. This observation is supported by off-resonance pulsed field gradient spin-echo experiments.     

Second-harmonic generation taking into account dispersion of nonlinear susceptibility

Second-harmonic generation in the field of an ultrashort pulse and the propagation of extremely short pulses in a medium with quadratic nonlinearity are analyzed. Second-harmonic generation is analyzed taking into account the effect of second-and third-order group velocity dispersion and dispersion of nonlinear susceptibility up to the second order. Corrections, whose order of smallness is determined by the parameter (ω_L t _p)^?1, where t _p is the pulse duration and ω_L is the carrier frequency of the pump wave, are obtained. For a large phase mismatch, two new solutions are found that describe the stationary evolution of solitary pump and second-harmonic waves in the regions of both anomalous and normal group velocity dispersions.     

Two-color photoionization spectroscopy of uranium in a U–Ne hollow cathode discharge tube

A U–Ne hollow cathode discharge tube is used as a source of uranium atomic vapors as well as a photoelectron/photoion detector for carrying out two-color three-photon photoionization spectroscopy of uranium. Using the uranium excitation transition 0 cm⁻¹ (⁵L ₆ ⁰ ) → 16 900.38 cm⁻¹ (⁷M₇) at 591.5-nm laser wavelength as a first step transition and scanning the wavelength of a second laser from 558 to 568 nm, high-lying odd-parity atomic levels of uranium are studied in the energy region 34 500–34 813 cm⁻¹. All the expected 21 odd-parity atomic levels identified by various researchers in this region are observed in a single spectrum, demonstrating the high sensitivity achieved therein. In addition to this, we have identified eight autoionization resonances of uranium starting from its odd-parity atomic level at 33 801.06 cm⁻¹ pumped by two-photon excitation. Four out of these eight autoionization resonances are observed for the first time.     

Effect of Resonant Photon Emission in Radiative Heat Transfer and Generation

The effect of resonances in the photon emission rate in the radiative heat generation and transfer and the Casimir friction at the sliding of two polar dielectric plates has been studied. The resonances have different origin in the frequency ranges of the normal and anomalous Doppler effects. In the frequency range of the normal Doppler effect, the resonances are due to resonant photon tunneling between surface phonon/plas-mon polaritons of plates. Such resonances exist only at a relative sliding velocity v = 0 for two identical plates. However, the resonances may occur at v ≠ 0 for different plates. In the frequency range of the anomalous Doppler effect, the resonances are due to the excitation generation in both plates. While the resonances are finite in the frequency range of the normal Doppler effect, singular resonances are possible even in the presence of dissipation in the system in the frequency range of the anomalous Doppler effect. The resonances for identical and different sliding plates have been considered.