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Nature Communications

Sensitivity of nonlinear photoionization to resonance substructure in collective excitation

Tommaso Mazza, Antonia Karamatskou, Markus Ilchen, S. Bakhtiarzadeh, A. J. Rafipoor, Patrick O’Keeffe, Thomas J. Kelly, Nichola Walsh, John Costello, Michael Meyer, Robin Santra

Published Apr 9, 2015• 32 citations
Publisher

Abstract

Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources.

DOI
10.1038/ncomms7799

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Interfacing Rydberg atoms with light and observing their interaction driven dynamics

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This thesis investigates new phenomena that arise when light is strongly coupled to an interacting atomic gas. For this, a new apparatus has been built which allows to create ultracold and dense atomic samples and to detect individual atoms in high-lying atomic states, so-called Rydberg atoms. Strong light-matter coupling is achieved using the collective coupling of the atomic cloud to the light field under conditions of electromagnetically induced transparency (EIT). In experiments on EIT involving non-interacting Rydberg states, we characterize the light-matter coupling and demonstrate the first combined optical and matter based probing of EIT. By coupling to strongly-interacting Rydberg states, we investigate the effect of interactions which we observe as a strong nonlinear optical \nresponse of the atomic gas as well as in the emergence of strong correlations between the hybrid quasiparticles associated with the strong light-matter coupling. We employ the nonlinear response of the atomic cloud to image Rydberg atoms immersed in the atomic cloud. In a theoretical proposal, we show that this novel imaging technique allows to investigate many-body Rydberg states with single particle sensitivity. Using the proposed imaging method, we demonstrate imaging of small numbers of Rydberg atoms with high time-resolution in single shot experiments. In experiments exploiting the dipolar exchange interaction between Rydberg atoms, we employ the new imaging technique to follow dipole-mediated transport of Rydberg excitations through the cloud. The transport dynamics is determined by the continuous spatial projection of the electronic quantum state under observation and features an emergent spatial scale of micrometer size induced by Rydberg-Rydberg interactions.

•Jan 1, 2014•0 citations
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources.
We study the emergence of a collective optical response of a cold and dense 87Rb^{87}\mathrm{Rb}87Rb atomic cloud to a near-resonant low-intensity light when the atom number is gradually increased. Experimental observations are compared with microscopic stochastic simulations of recurrent scattering processes between the atoms that incorporate the atomic multilevel structure and the optical measurement setup. We analyze the optical response of an inhomogeneously broadened gas and find that the experimental observations of the resonance line shifts and the total collected scattered light intensity in cold atom clouds substantially deviate from those of thermal atomic ensembles, indicating strong light-induced resonant dipole-dipole interactions between the atoms. At high densities, the simulations also predict a significantly slower decay of light-induced excitations in cold than in thermal atom clouds. The role of dipole-dipole interactions is discussed in terms of resonant coupling examples and the collective radiative excitation eigenmodes of the system.
The interaction between atomic transition dipoles and photons leads to the formation of many-body states with collective dissipation and long-ranged interactions. Here, we put forward and explore a scenario in which a dense atomic gas---where the separation of the atoms is comparable to the transition wavelength---is weakly excited by an off-resonant laser field. We develop the theory for describing such a dressed many-body ensemble and show that collective excitations are responsible for the emergence of many-body interactions, i.e., effective potentials that cannot be represented as a sum of binary terms. We illustrate how collective effects may be probed experimentally through microwave spectroscopy. We analyze time-dependent line shifts, which are sensitive to the phase pattern of the dressing laser, and show that the strong interactions lead to a dramatic slowdown of the relaxation dynamics. Our study provides a so far little explored perspective on dense atomic ensembles interacting with light and promotes this platform as a setting for the exploration of dissipative nonequilibrium many-body physics.
We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms,includinglaser-inducedphase(LIP),time-varying(AC)Starkshift,quantumpathinterference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which accoun tfor the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.
We analyze the temporal response of the fluorescence light that is emitted from a dense gas of cold atoms driven by a laser. When the average interatomic distance is comparable to the wavelength of the photons scattered by the atoms, the system exhibits strong dipolar interactions and collective dissipation. We solve the exact dynamics of small systems with different geometries and show how these collective features are manifest in the scattered light properties such as the photon emission rate, the power spectrum and the second-order correlation function. By calculating these quantities beyond the weak (linear) driving limit, we make progress in understanding the signatures of collective behavior in these many-body systems. Furthermore, we shed light on the role of disorder and averaging on the resonance fluorescence, of direct relevance for recent experimental efforts that aim at the exploration of many-body effects in dipole–dipole interacting gases of atoms.
The experimental discovery of dilute ultracold quantum gases in 1995 has instigated a broad interest in ultracold atoms and molecules, and paved the way for extensive studies of a wide range of both experimental and theoretical topics. Various theories were developed to describe Bose-Einstein condensates (BECs) for different ranges of temperature and interaction strengths. This thesis focuses on studying the mean-field theory of the Gross-Pitaevskii (GP) equation which provides a good description for the macroscopic wave function when both quantum and thermal fluctuations are negligible. This means that the range of validity is restricted to small interaction strengths at zero temperature. Within the GP theory we study analytically and numerically the nonlinear dynamics of BECs which are induced by a harmonic modulation of either the interaction strength or the harmonic trapping potential. At first a detailed numerical investigation reveals that solving the partial differential equation of Gross and Pitaevskii can be reduced to solving ordinary differential equations for the condensate widths within a Gaussian variational approach. Despite this tremendous simplification the prominent nonlinearity of the underlying GP equation is inherited by the variational equations, which thus allow to describe a rich variety of nonlinear phenomena in different experimental setups. In order to describe them analytically we transfer the Poincar\'e-Lindstedt method, which represents a well-established tool in the field of nonlinear dynamics, to the Gaussian variational approach. Modulation of some of the parameters of a BEC can give rise to prominent nonlinear features, such as shifts in the frequencies of collective oscillation modes, higher harmonics generation, and resonant mode coupling. As a first application we consider a recent Bose-Einstein condensate of Li-7, which has been excited experimentally by a harmonic modulation of the atomic s-wave scattering length with the help of a Feshbach resonance. Combining an analytical perturbative approach with extensive numerical simulations, we analyze the emerging nonlinear dynamics. In particular, we present the resulting shift of collective excitation spectra close to parametric resonances where the driving frequency coincides with a collective mode frequency. Another application of experimental importance is to study geometric resonances in BECs with two-body and three-body contact interactions, where changing the geometry of the trapping potential leads to commensurate collective excitation frequencies with emerging nonlinear effects such as frequency shifts and resonant mode coupling. In this context we also show that a small repulsive three-body interaction is able to extend the stability region of the condensate for an attractive two-body interaction as it increases the critical number of atoms in the trap. Apart from a periodic modulation of a system parameter the dipole mode frequency can be excited by introducing an abrupt change in the potential, where the center of mass oscillates according to the Kohn theorem back and forth in the trapping potential with the natural trap frequency irrespective of both the strength and the type of the two-particle interaction. To this end, we study the collective excitation modes of a Bose-Einstein condensate in a harmonic Ioffe- Pritchard in the vicinity of a Feshbach resonance for experimentally realistic parameters of a Rb-85 BEC, where the dipole mode is excited in z-direction. A linearization of the underlying equations of motions shows that the dipole mode frequency decreases when the bias magnetic field approaches the Feshbach resonance, so the Kohn theorem is violated. Finally within a variational approach we discuss the physical properties of a BEC in an axially-symmetric harmonic trap with a single vortex in the center. At first we analyze the equilibrium configuration and determine the vortex size as well as the Thomas- Fermi radii of the condensate in the Thomas-Fermi limit. Then we calculate the critical rotation frequency for the emergence of the vortex and compare our findings with the literature. Finally, we investigate how the presence of the vortex changes the collective excitation frequencies and we discuss the free expansion of the BEC. All these results indicate how nonlinear dynamics properties could be made clearly observable in future experiments on the basis of our results.
This thesis investigates new phenomena that arise when light is strongly coupled to an interacting atomic gas. For this, a new apparatus has been built which allows to create ultracold and dense atomic samples and to detect individual atoms in high-lying atomic states, so-called Rydberg atoms. Strong light-matter coupling is achieved using the collective coupling of the atomic cloud to the light field under conditions of electromagnetically induced transparency (EIT). In experiments on EIT involving non-interacting Rydberg states, we characterize the light-matter coupling and demonstrate the first combined optical and matter based probing of EIT. By coupling to strongly-interacting Rydberg states, we investigate the effect of interactions which we observe as a strong nonlinear optical \nresponse of the atomic gas as well as in the emergence of strong correlations between the hybrid quasiparticles associated with the strong light-matter coupling. We employ the nonlinear response of the atomic cloud to image Rydberg atoms immersed in the atomic cloud. In a theoretical proposal, we show that this novel imaging technique allows to investigate many-body Rydberg states with single particle sensitivity. Using the proposed imaging method, we demonstrate imaging of small numbers of Rydberg atoms with high time-resolution in single shot experiments. In experiments exploiting the dipolar exchange interaction between Rydberg atoms, we employ the new imaging technique to follow dipole-mediated transport of Rydberg excitations through the cloud. The transport dynamics is determined by the continuous spatial projection of the electronic quantum state under observation and features an emergent spatial scale of micrometer size induced by Rydberg-Rydberg interactions.