Polariton formalism is applied for studying the propagation of a probe field of light in a cloud of cold atoms influenced by two control laser beams of larger intensity. The laser beams couple resonantly three hyperfine atomic ground states to a common excited state thus forming a tripod configuration of the atomic energy levels involved. The first control beam can have an optical vortex with the intensity of the beam going to zero at the vortex core. The second control beam without a vortex ensures the loseless (adiabatic) propagation of the probe beam at a vortex core of the first control laser. We investigate the storage of the probe pulse into atomic coherences by switching off the control beams, as well as its subsequent retrieval by switching the control beams on. The optical vortex is transferred from the control to the probe fields during the storage or retrieval of the probe field. We analyze conditions for the vortex to be transferred efficiently to the regenerated probe beam and discuss possibilities of experimental implementation of the proposed scheme using atoms like rubidium or sodium.

We consider ultracold atoms in a two-dimensional optical lattice of the dice geometry in a tightbinding regime. The atoms experience a laser-assisted tunneling between the nearest neighbor sites of the dice lattice accompanied by the momentum recoil. This allows one to engineer staggered synthetic magnetic fluxes over plaquettes, and thus pave a way towards the realization of topologically nontrivial band structures. In such a lattice the real-valued next-neighbor transitions are not needed to reach a topological regime. Yet, such transitions can increase a variety of the obtained topological phases. The dice lattice represents a triangular Bravais lattice with a three-site basis consisting of a hub site connected to two rim sites. As a consequence, the dice lattice supports three energy bands. From this point of view, our model can be interpreted as a generalization of the paradigmatic Haldane model which is reproduced if one of the two rim sub-lattices is eliminated. We demonstrate that the proposed upgrade of the Haldane model creates a significant added value, including an easy access to topological semimetal phases relying only on the nearest neighbor coupling, as well as enhanced topological band structures featuring Chern numbers higher than one leading to physics beyond the usual quantum Hall effect. The numerical investigation is supported and complemented by an analytical scheme based on the study of singularities in the Berry connection. arXiv:1501.00425v2 [cond-mat.quant-gas] 22 Sep 2015

Abstract. We consider propagation, storing and retrieval of slow light (probe beam) in a resonant atomic medium illuminated by two control laser beams of larger intensity. The probe and two control beams act on atoms in a tripod configuration of the light-matter coupling. The first control beam is allowed to have an orbital angular momentum (OAM). Application of the second vortex-free control laser ensures the adiabatic (lossles) propagation of the probe beam at the vortex core where the intensity of the first control laser goes to zero. Storing and release of the probe beam is accomplished by switching off and on the control laser beams leading to the transfer of the optical vortex from the first control beam to the regenerated probe field. A part of the stored probe beam remains frozen in the medium in the form of atomic spin excitations, the number of which increases with increasing the intensity of the second control laser. We analyse such losses in the regenerated probe beam and provide conditions for the optical vortex of the control beam to be transferred efficiently to the restored probe beam.

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 1016 to 5 × 1018 cm−3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 μm to 6 μm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 × 105 cm/s for the untreated surface. At even higher excitations, in the 1019–3 × 1020 cm−3 density range, D increase from 5 to 20 cm2/s due to carrier degeneracy was observed.

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