Electromagnetically induced diffraction in sodium vapor

By using a sodium atomic vapor we have observed a honeycomb-type multiple-diffraction pattern that was due to a grating induced optically by three noncollinear bichromatic excitation beams. In particular, when the two incident frequencies satisfy the two-photon resonance condition and the atomic density is high, the diffraction is enhanced by electromagnetically induced transparency, and strong Stokes and anti-Stokes components are generated in well-defined directions.     

Four-wave parametric oscillation in sodium vapor by electromagnetically induced diffraction

We have observed a novel type of parametric oscillation in sodium atomic vapor where four off-axis signal waves simultaneously build up under resonant and counterpropagating pump beams with elliptical beam profiles. The four waves, two of them Stokes shifted and the other two anti-Stokes shifted, have similar output powers of up to 10 mW with a conversion efficiency of 30% and are parametrically coupled by electromagnetically induced diffraction.     

Distortion-free gain and noise correlation in sodium vapor with four-wave mixing and coherent population trapping

We observed optical gain as great as 30 with nearly distortion-free beam propagation in optically dense sodium vapor, using four-wave mixing. Moreover, 15-dB classical noise correlations were seen in the amplified probe and conjugate beams. To achieve this performance in such a strongly absorbing medium, one must suppress unwanted absorption and self-focusing effects. This is accomplished with coherent population trapping.     

Observation of self-oscillation in ring cavities with sodium vapor phase conjugate mirrors

We present two kinds of ring cavities with phase conjugate mirrors. One cavity requires a tuning condition while the other one can oscillate independently of the cavity length. Both cavities have been experimentally studied using a sodium vapor phase conjugate mirror. Oscillations occur without any other gain medium inside the cavity. A strong correlation between the beams propagating in opposite directions in the cavity is observed.     

Generation of spatially broadband twin beams for quantum imaging

We generate spatially multimode twin beams using 4-wave mixing in a hot atomic vapor in a phase-insensitive traveling-wave amplifier configuration. The far-field coherence area measured at 3.5 MHz is shown to be much smaller than the angular bandwidth of the process and bright twin images with independently quantum-correlated subareas can be generated with little distortion. The available transverse degrees of freedom form a high-dimensional Hilbert space that we use to produce quantum-correlated twin beams with finite orbital angular momentum.     

Production of metastable hydrogen atoms by charge exchange of protons in potassium vapor

Fraction of H(2s) atoms in neutral hydrogen beams produced by passage of proton beams through a thin potassium vapor has been determined for proton energies between 0.4 and 3.0 keV by using H^? formation from H(1s) atoms in the second potassium vapor. It exhibits energy dependence similar to that obtained in cesium vapor.     

Sub-doppler absorption resonances of ring-shaped laser beams in a thin vapor cell

Experimental research is carried out into the features of sub-Doppler absorption resonances caused by optical pumping and transit relaxation of Cs atoms in a thin 120-μm cell (with rarefied Cs vapor) for ring-shaped laser beams. Such resonances are analyzed both in the transmission of the single running beam and at the interaction of counterpropagating beams for different degrees of their spatial overlapping in the cell.     

Pulse shortening in degenerate four wave mixing in atomic sodium vapor

Phase conjugation is observed in sodium vapor near the resonance lines using a monomode pulsed dye laser. The divergence of the beam strongly varies during the pulse. By combining the curvature of the pumping beams and the diameter of a pinhole, it is possible to generate pulses which are shorter than the incident pulse by a factor as large as 20.     

Evolution of finite energy Airy beams in cubic-quintic atomic vapor system

In a numerical investigation, we demonstrate the evolution of a one-dimensional and two-dimensional finite energy Airy beam in a Λ-type three-level atomic vapor with linear, cubic, and quintic susceptibilities considered simultaneously with the dressing effect. Quasi-solitons and soliton pairs are observed due to this competition mechanism. We find that the frequency detuning of the pump field and its power greatly affect the formation and evolution of generated solitons. In general, around the two-photon resonance point and for low intensities of the pump field, it is less difficult to form solitons. This investigation enriches the study of the propagation properties of Airy beams and soliton generation in atomic vapor.     

Quantized rotation of atoms from photons with orbital angular momentum

We demonstrate the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of variant Planck's over 2pi, using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate of sodium atoms. We show that the process is coherent by creating superpositions of different vortex states, where the relative phase between the states is determined by the relative phases of the optical fields. Furthermore, we create vortices of charge 2 by transferring to each atom the orbital angular momentum of two photons.     

电子束激发原子对金属蒸发的影响

在原子蒸气激光同位素分离工程(AVLIS)中,金属受电子束的加热而熔化,并向真空蒸发,蒸气原子通过电子束的过程中,可能与电子发生非弹性碰撞,被激发到高能级.在膨胀的过程中,高能级的原子通过与原子的非弹性碰撞消激发,将原子内的电子能量转换为蒸气的平动能.为了分析电子束与原子作用对蒸气的密度、速度和温度等物理特性的影响,采样直接模拟蒙特卡罗法(DSMC)模拟钆原子蒸发过程.模拟结果表明,电子束与原子的作用使得原子蒸气的速度增加,密度减小,温度升高 关键词： 金属蒸发 AVLIS DSMC 电子枪     

Infrared absorption in dense sodium vapor

The absorption spectra of a dense resonance medium were experimentally studied for the example of thermally heated dense sodium vapor. Several mechanisms that might cause substantial absorption and enhanced intensity of emission in the IR spectral region, λ τ; 0.9 μm, were considered. For the first time, a detailed study of the structure of the absorption spectra of sodium vapor in the specified wavelength range was performed to determine the influence of the kind and pressure of the buffer gas. It was found that buffer gas characteristics had a substantial effect on the absorption coefficient of vapor. The presence of the molecular component (dimers and trimers) in sodium vapor could not explain the experimental dependences of absorption in the infrared region. Possible influence of microparticles formed in condensation of convective sodium vapor flows in heated cells on the optical properties of vapor was considered. Microparticles could contribute to the observed absorption, but were incapable of explaining the substantial intensity of vapor radiation reported earlier. Possible many-particle effects on the absorption in the far spectral line wing were discussed. For the first time, the method of molecular dynamics was used to show for the example of the distribution function of ionic microfields in a dense plasma that such effects were in principle capable of substantially raising the profile of the line and increasing absorption in the region of large detunings from the resonance compared with the simple quasi-static model in the nearest-neighbor approximation.     

4Pi focusing of spatially modulated radially polarized vortex beams

We propose a method for generating focal beams with special intensity distributions using radially polarized vortex beams in a 4Pi configuration. A spherical dark-hollow beam and hollow beam array can be obtained by vortex beams with topological charge of m=1. A dark channel can be generated using vortex beams with topological charge of m=2. The length of the well-defined hollow beam array and the dark channel is about 30λ. These interesting beams are useful in optical trapping and manipulation.     

Delocalized correlations in twin light beams with orbital angular momentum

We generate intensity-difference-squeezed Laguerre-Gauss twin beams of light carrying orbital angular momentum by using four-wave mixing in a hot atomic vapor. The conservation of orbital angular momentum in the four-wave mixing process is studied as well as the spatial distribution of the quantum correlations obtained with different configurations of orbital angular momentum. Intensity-difference squeezing of up to -6.7 dB is demonstrated with beams carrying orbital angular momentum. Delocalized spatial correlations between the twin beams are observed.     

Experimental study of vapor-cell magneto-optical traps for efficient trapping of radioactive atoms

We have studied magneto-optical traps (MOTs) for efficient on-line trapping of radioactive atoms. After discussing a model of the trapping process in a vapor cell and its efficiency, we present the results of detailed experimental studies on Rb MOTs. Three spherical cells of different sizes were used. These cells can be easily replaced, while keeping the rest of the apparatus unchanged: atomic sources, vacuum conditions, magnetic field gradients, sizes and power of the laser beams, detection system. By direct comparison, we find that the trapping efficiency only weakly depends on the MOT cell size. It is also found that the trapping efficiency of the MOT with the smallest cell, whose diameter is equal to the diameter of the trapping beams, is about 40% smaller than the efficiency of larger cells. Furthermore, we also demonstrate the importance of two factors: a long coated tube at the entrance of the MOT cell, used instead of a diaphragm; and the passivation with an alkali vapor of the coating on the cell walls, in order to minimize the losses of trappable atoms. These results guided us in the construction of an efficient large-diameter cell, which has been successfully employed for on-line trapping of Fr isotopes at INFN’s national laboratories in Legnaro, Italy.     

A high-resolution spatial heterodyning interference spectrometer

A novel spectrometer which has a resolution of 0.013 nm with high throughput is described. The system uses a Michelson interferometer in which the mirrors have been replaced with Littrow-mounted diffraction gratings. First-order diffracted beams returning from the gratings interfere at the interferometer output to produce a spatial interferogram which is heterodyned about an optical frequency determined by the setting angles of the gratings. The heterodyning process leads to increased resolution, but limits the free-spectral range of the instrument to, in our case, about 3 nm. This is sufficient for characterizing narrow-band sources such as laser diodes. We present the theory of operation of the instrument and some sample spectra obtained from a sodium vapor lamp, a HeNe discharge, and a 670 nm diode laser.     

Optimizing the capture process in optical traps

We present techniques for the most efficient capture of atoms in a modified vaporcell magneto-optical trap. We explain how changing the size and power of the trapping beams affects the capture rate of the trap. We calculate the requirement of wall coatings designed to minimize the interaction of an alkali vapor with the wall and explain how this affects trapping efficiency. Finally, we present measurements of the performance of a Dryfilm coated cell.     

Spectroscopic observation of the rotational Doppler effect

We report on the first spectroscopic observation of the rotational Doppler shift associated with light beams carrying orbital angular momentum. The effect is evidenced as the broadening of a Hanle electromagnetically induced transparency coherence resonance on Rb vapor when the two incident Laguerre-Gaussian laser beams have opposite topological charges. The observations closely agree with theoretical predictions.     

Spatial Thirring-type solitons via electromagnetically induced transparency

We show that the giant Kerr nonlinearity in the regime of electromagnetically induced transparency in vapor can give rise to the formation of Thirring-type spatial solitons, which are supported solely by cross-phase modulation that couples the two copropagating light beams.     

Generation of long plasma channels in air by focusing ultrashort laser pulses with an axicon

We show that by focusing ultrashort-pulsed laser beams in air with an axicon, relatively long plasma channels can be generated. The axicon generates Bessel-like beams, where the on-axis intensity stays high over distances much longer compared to focusing with conventional lenses. We developed a scheme to detect the presence of the plasma, based on its screening property. Using this scheme, we detected plasma channels longer than 1 m and 3.5 m generated by 8 mJ and 90 mJ input pulse energies, respectively. Our simulations show that axicon focusing can yield self-guided propagation with or without contribution of plasma, depending on the input pulse power.