We report an experiment in which an optical vortex is stored in a vapor of Rb atoms. Because of its 2pi phase twist, this mode, also known as the Laguerre-Gauss mode, is topologically stable and cannot unwind even under conditions of strong diffusion. For comparison, we stored a Gaussian beam with a dark center and a uniform phase. Contrary to the optical vortex, which stays stable for over 100 micros, the dark center in the retrieved flat-phased image was filled with light after a storage time as short as 10 micros. The experiment proves that higher electromagnetic modes can be converted into atomic coherences and that modes with phase singularities are robust to decoherence effects such as diffusion. This opens the possibility to more elaborate schemes for classical and quantum information storage in atomic vapors.

Reversible and coherent storage of light in atomic medium is a promising method with possible application in many fields. In this work, arbitrary two-dimensional images are slowed and stored in warm atomic vapor for up to 30 µs, utilizing electromagnetically induced transparency. Both the intensity and the phase patterns of the optical field are maintained. The main limitation on the storage resolution and duration is found to be the diffusion of atoms. A technique analogous to phase-shift lithography is employed to diminish the effect of diffusion on the visibility of the reconstructed image.PACS numbers: 42.50. Gy, 32.70.Jz When a resonant light pulse ("probe") impinges upon a gas of atoms, it is strongly absorbed, exciting the atoms to an upper state. However, if a second "pump" beam is present, which couples a second state to the same excited state, than the "probe" pulse will be able to pass the sample -a phenomenon known as electromagnetically induced transparency (EIT) [1]. The unique properties of EIT allow for a wide range of coherent light-matter phenomena, including non-linear optics [2], entanglement [3], generation of quantum pulses of light [4], and quantum communication [5]. In Ref.[6], the group velocity of the probe pulse, in a medium of ultra-cold atoms, was decreased to 17 m/s and similar results were achieved in warm vapor [7]. In Refs. [8,9,10], it was demonstrated that the probe pulse can be completely stopped, while it is contained inside the medium, by shutting off the pump. The pulse can be recovered by reopening the pump beam after a certain "storage duration". A prominent feature of this technique is the reversible and coherent storage of the information carried by the probe, in the atomic coherences.Here, we report a method for reversibly capturing complex three-dimensional light fields using EIT in atomic vapor [1,8]. The storage experiments discussed above have utilized a transverse Gaussian mode for both pump and probe beams. The current research is focused on slowing and storing information imprinted in the transverse plane of the probe beam ("images"), and on reducing the effects of atomic diffusion. In a previous work [11], the ability to slow images and delay them for several ns was demonstrated, using dispersion from fardetuned absorption lines. In the current work, we use EIT to slow images to a group velocity of several thousands m/s, achieving delays of several µs. We further use the unique properties of EIT to store the images in the atomic medium for a similar duration. The long slowing delays and storage durations are comparable with the typical diffusion time in which atoms cross the image. We demonstrate the deteriorating effect of diffusion by storing images of digits for different durations. Finally, we introduce a technique to diminish the effect of atomic diffusion by alternating the phase of neighboring features. This technique, which is the atomic analogue of the optical phase-shift lithography [12], is demonstrated by storing an image of three lines and study...

We analyze the operation of a free-electron laser in an open periodic structure. When operated as an amplifier, it is shown that exponential gain can develop in such a device. An analytic expression for this gain is presented, and it is shown that this gain decays exponentially with the beam height above the grating; the gain dependence on the beam thickness is also analyzed. As the energy extraction rate is much higher in an exponential gain regime, we also investigate the operation of an oscillator based on the same concept. In this case, the Smith-Purcell radiation "provides the startup" through a feedback system. The latter consists of several mirrors, which are placed in such a way that (1) only a wave with the desired frequency develops and, at the same time, (2) small deviations in the wave parameters due to the wave-beam interaction do not cause deAection of the wave from the feedback loop. The minimal current needed to sustain oscillations is given in an analytical form. Numerical calculations show that this current is more than three orders of magnitude lower than that necessary for an oscillator which operates in an algebraic gain regime.

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We experimentally demonstrate the manipulation of optical diffraction, utilizing the atomic thermal motion in a hot vapor medium of electromagnetically-induced transparency (EIT). By properly tuning the EIT parameters, the refraction induced by the atomic motion may completely counterbalance the paraxial free-space diffraction and by that eliminates the effect of diffraction for arbitrary images. By further manipulation, the diffraction can be doubled, biased asymmetrically to induced deflection, or even reversed. The latter allows an experimental implementation of an analogy to a negative-index lens.Any image, imprinted on a wave field and propagating in free space, undergoes a paraxial diffraction spreading and eventually blurs. In many disciplines, the possibility to reduce or manipulate the diffraction is explored, for purposes such as imaging, wave guiding, microlithography, and all-optical light processing. As was recently demonstrated, arbitrary images can be imprinted on light pulses which are dramatically slowed when traversing a medium of room-temperature atoms [ 1,2], via the process of electromagnetically induced transparency [ 3,4]. In addition to the regular free-space diffraction, the slow-light images undergo diffusion due to the thermal atomic motion [ 5,6]. Here we report an experimental demonstration of a novel technique to eliminate