arXiv subjects

Hashem Zoubi

Publications and source records attributed to Hashem Zoubi.

28 records · Page 2Linked to original sources

arXiv2011

Lifetime and Emission Characteristics of Electronic Excitations in 2D Optical Lattices

Collective electronic excitations "excitons" in planar optical lattices exhibit strong modifications of the radiative damping rate and directional emission pattern as compared to a single excited atom. Excitons for long wave numbers and polarizations orthogonal to the lattice plane exhibit superradiance with a very short life time and a tightly confined emission direction. For shorter wavelength and in plane polarization they can posses a long life time, which beyond a critical wave number tends to infinity. Those excitons thus become metastable and decoupled from the free radiation field. They can store a single energy quantum for a long time and transfer the excitation over long distances. In general the spatial, polarization and frequency dependence of the emission pattern can provide us with optical and electronic properties of optical lattices.

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arXiv2010

Hybrid Quantum System of a Nanofiber Mode Coupled to Two Chains of Optically Trapped Atoms

A tapered optical nanofiber simultaneously used to trap and optically interface of cold atoms through evanescent fields constitutes a new and well controllable hybrid quantum system. The atoms are trapped in two parallel 1D optical lattices generated by suitable far blue and red detuned evanescent field modes very close to opposite sides of the nanofiber surface. Collective electronic excitations (excitons) of each of the optical lattices are resonantly coupled to the second lattice forming symmetric and antisymmetric common excitons. In contrast to the inverse cube dependence of the individual atomic dipole-dipole interaction, we analytically find an exponentially decaying coupling strength with distance between the lattices. The resulting symmetric (bright) excitons strongly interact with the resonant nanofiber photons to form fiber polaritons, which can be observed through linear optical spectra. For large enough wave vectors the polariton decay rate to free space is strongly reduced, which should render this system ideal for the realization of long range quantum communication between atomic ensembles.

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arXiv2009

Collective Enhancement and Suppression of Excitation Decay in Optical Lattices

We calculate radiative lifetimes of collective electronic excitations of atoms in an infinite one dimensional lattice. The translational symmetry along the lattice restricts the photon wave vector component parallel to the lattice to the exciton wave number and thus the possible emission directions. The resulting radiation damping rate and emission pattern of the exciton strongly deviates from independent atom. For some wave numbers and polarizations the excitons superradiantly decay very fast, while other excitons show zero radiation damping rate and form propagating meta-stable excitations. Such states could be directly coupled via tailored evanescent fields from a nearby fiber.

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arXiv2009

Superradiant and Dark Exciton States in an Optical Lattice within a Cavity

We study ultracold atoms in a finite size one-dimensional optical lattice prepared in the Mott insulator phase and commonly coupled to a single cavity mode. Due to resonance dipole-dipole interactions among the atoms, electronic excitations delocalize and form {\it excitons}. These exciton modes are divided into two groups: antisymmetric modes which decouple from the cavity mode forming {\it dark states}, and symmetric modes significantly coupled to the cavity mode called {\it bright states}. In typical setups the lowest and most symmetric exciton is coupled to the cavity photons much stronger than the other bright states and dominates the optical properties response of the atoms ({\it superradiant state}). In the strong coupling regime this superradiant state is coherently mixed with the cavity photon to form a doublet of polariton states with the Rabi splitting.

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arXiv2009

Quantum Phases of Ultracold Bosonic Atoms in two Bands of an Optical-Lattice coupled by a Cavity Field

We study the quantum phase transitions between superfluid and Mott insulator states for ultracold bosons occupying two bands of an optical lattice. The two atomic states are resonantly coupled by a single cavity mode which mediates transitions between the two bosonic particle modes via absorption or emission of a cavity photon. This coupling between the bands shifts the appearance of the Mott insulator phase towards deeper optical lattice potentials and stronger on-site interaction strength, as atomic coherence can build up via photon assisted tunneling in both bands. Varying the intra and interband on-site interactions leads to several different atomic phase configurations. There are even parameter regions where a mean field approach predicts concurrence of a Mott insulator state in one band, while atoms in the second band stay superfluid.

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arXiv2008

Polarization-Mixing in Optical Lattices with Uniaxial Anisotropy

We study optical excitations of an ultracold gas in an optical lattice in the Mott insulator case and with anisotropic properties. The anisotropy is induced by an oriented transition dipole generated by optical pumping and external static fields, and is of the type of uniaxial crystals. Within a cavity, the electronic excitations and the cavity photons of both TE and TM polarizations are coherently mixed to form two polariton branches. Also a photonic branch appears which decouples from the electronic transitions. The photon polarization mixing can be observed by linear optical spectra. For example, for an incident field of TE polarization we obtain TE and TM transmitted and reflected fields. Also we calculate the phase shift in the TE and TM transmitted and reflected fields relative to the incident field phase, which lead to polarization rotations.

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arXiv2008

Coupling of Electronic and Motional Dynamics in a Cold Atom Optical Lattice

We study the coupling of internal electronic excitations to vibrational modes of the external motion of ultracold atoms in an optical lattice. For different ground and excited state potentials the on-site coupling of excitations and vibrations term renormalizes the effective electronic transition energy, which appears e.g. in clock transitions. In addition in the Mott state with filling factor one, the dipole-dipole coupling between neighbouring sites includes emission and absorption of vibrational quanta. Such processes create a significant mechanism for excitation of vibrations leading to motional heating of the lattice atoms by resonant light interaction. We calculate estimates of the corresponding parameters from a perturbation expansion in small atomic displacements.

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arXiv2007

Excitons and cavity polaritons for ultracold atoms in an optical lattice

We study the resonant electronic excitation dynamics for ultracold atoms trapped in a deep optical lattice prepared in a Mott insulator state. Excitons in these artificial crystals are similar to Frenkel excitons in Noble atom or molecular crystals. They appear when the atomic excited state line width is smaller than the exciton band width generated by dipole-dipole coupling. When the atoms are placed within a cavity the electronic excitations and the quantized cavity mode get coupled. In the collective strong coupling regime excitations form two branches of cavity polaritons with Rabi splitting larger than the atomic and the cavity line width. To demonstrate their properties we calculate the transmission, reflection, and absorption spectra for an incident weak probe field, which show resonances at the polariton frequencies.

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arXiv2007

Exciton-Polariton scattering for defect detection in cold atom Optical Lattices

We study the effect of defects in the Mott insulator phase of ultracold atoms in an optical lattice on the dynamics of resonant excitations. Defects, which can either be empty sites in a Mott insulator state with one atom per site or a singly occupied site for a filling factor two, change the dynamics of Frenkel excitons and cavity polaritons. While the vacancies in first case behave like hard sphere scatters for excitons, singly occupied sites in the latter case can lead to attractive or repulsive scattering potentials. We suggest cavity polaritons as observation tool of such defects, and show how the scattering can be controlled in changing the exciton-photon detuning. In the case of asymmetric optical lattice sites we present how the scattering effective potential can be detuned by the cavity photon polarization direction, with the possibility of a crossover from a repulsive into an attractive potential.

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arXiv2007

Bright and dark excitons in an atom--pair filled optical lattice within a cavity

We study electronic excitations of a degenerate gas of atoms trapped in pairs in an optical lattice. Local dipole-dipole interactions produce a long lived antisymmetric and a short lived symmetric superposition of individual atomic excitations as the lowest internal on-site excitations. Due to the much larger dipole moment the symmetric states couple efficiently to neighbouring lattice sites and can be well represented by Frenkel excitons, while the antisymmetric dark states stay localized. Within a cavity only symmetric states couple to cavity photons inducing long range interactions to form polaritons. We calculate their dispersion curves as well as cavity transmission and reflection spectra to observe them. For a lattice with aspherical sites bright and dark states get mixed and their relative excitation energies depend on photon polarizations. The system should allow to study new types of solid state phenomena in atom filled optical lattices.

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