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Influence of Dipole-Dipole Interaction on Electron Dynamics in a Cold Rydberg Gas

Tao Zhou

Published Apr 27, 2016• 0 citations
Publisher

Abstract

Electron dynamics in dipole-dipole (DD) coupled Rydberg atoms show some unique properties that do not exist in individual atoms.The goal of the research in this dissertation is to reveal and control the electron dynamics influenced by DD interaction in cold Rydberg gases.We have studied the decay of Rydberg excitations in a cold Rb gas and find no evidence for superradiance.The decay rates and population redistribution we observe are consistent with a model that considers only spontaneous emission from, and blackbody redistribution within, isolated atoms.Suppression of superradiant emission is likely due to variations in transition energies across the cold Rydberg atom sample.For initial s states, these variations are dominated by inhomogeneities in DD exchange interactions within the random ensemble.Such inhomogeneities will necessarily be present in any measurement involving a large number of atoms where the separation between atoms is not well defined.For initial p states, the suppression is likely due to a combination of DD exchange and electric field inhomogeneities.We have also explored the evolution of Rydberg wavepackets in the presence of strong dipole-dipole interactions in a frozen gas.The distribution of atom separations results in an inhomogeneity in the strength of the exchange coupling between neighboring atoms, causing a dephasing of the macroscopic coherence.We have also explored the mechanism of wavepacket coherence transfer from one Rydberg atom to a neighboring Rydberg atom via DD interactions utilizing the resonant DD coupling transition 25s33s↔24p34p.The phase-shift of the observed interference modulations in the 34p signal, relative to that in 25s, is a signature of wavepacket coherence transfer between atoms driven by electron correlations resulting from the controlled DD coupling between them.I would like to thank my advisor Professor Bob Jones for his guidance and help, not only in the detailed research performance but also in scientific ethics and methodology.He taught me how to operate most of the instruments as well as helped me fix failing apparatus.His rigorous, yet optimistic, working attitude always inspires me in my research.I don't remember how many times I have felt frustrated after getting nothing from a several days' experiment.At those times, I was always motivated and energetic from his guidance, constructive criticism and sincere encouragement, after even a short talk with him.The accomplishment of this dissertation involved a lot of his help, ranging from grammar checking, to language polishing, to error correction.I really feel grateful and fortunate to have worked with him for more than 5 years.Thanks also goes to my colleagues who have helped me in the research.Brian Richards worked with me a lot to fix failing instruments.He also programmed the data collection software.Sha Li taught me how to build setups to generate the THz radiation and build dye lasers.Kisra Nayomal Egodapitiya helped me adjust lasers and amplifiers many times.I also got invaluable inspiration from daily talks with them.

DOI
10.18130/v34k4x

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Electron dynamics in dipole-dipole (DD) coupled Rydberg atoms show some unique properties that do not exist in individual atoms.The goal of the research in this dissertation is to reveal and control the electron dynamics influenced by DD interaction in cold Rydberg gases.We have studied the decay of Rydberg excitations in a cold Rb gas and find no evidence for superradiance.The decay rates and population redistribution we observe are consistent with a model that considers only spontaneous emission from, and blackbody redistribution within, isolated atoms.Suppression of superradiant emission is likely due to variations in transition energies across the cold Rydberg atom sample.For initial s states, these variations are dominated by inhomogeneities in DD exchange interactions within the random ensemble.Such inhomogeneities will necessarily be present in any measurement involving a large number of atoms where the separation between atoms is not well defined.For initial p states, the suppression is likely due to a combination of DD exchange and electric field inhomogeneities.We have also explored the evolution of Rydberg wavepackets in the presence of strong dipole-dipole interactions in a frozen gas.The distribution of atom separations results in an inhomogeneity in the strength of the exchange coupling between neighboring atoms, causing a dephasing of the macroscopic coherence.We have also explored the mechanism of wavepacket coherence transfer from one Rydberg atom to a neighboring Rydberg atom via DD interactions utilizing the resonant DD coupling transition 25s33s↔24p34p.The phase-shift of the observed interference modulations in the 34p signal, relative to that in 25s, is a signature of wavepacket coherence transfer between atoms driven by electron correlations resulting from the controlled DD coupling between them.I would like to thank my advisor Professor Bob Jones for his guidance and help, not only in the detailed research performance but also in scientific ethics and methodology.He taught me how to operate most of the instruments as well as helped me fix failing apparatus.His rigorous, yet optimistic, working attitude always inspires me in my research.I don't remember how many times I have felt frustrated after getting nothing from a several days' experiment.At those times, I was always motivated and energetic from his guidance, constructive criticism and sincere encouragement, after even a short talk with him.The accomplishment of this dissertation involved a lot of his help, ranging from grammar checking, to language polishing, to error correction.I really feel grateful and fortunate to have worked with him for more than 5 years.Thanks also goes to my colleagues who have helped me in the research.Brian Richards worked with me a lot to fix failing instruments.He also programmed the data collection software.Sha Li taught me how to build setups to generate the THz radiation and build dye lasers.Kisra Nayomal Egodapitiya helped me adjust lasers and amplifiers many times.I also got invaluable inspiration from daily talks with them.
We have studied the decay of Rydberg excitations in a cold Rb gas. A 10 ns, pulsed-dye-amplified diode laser excites Rb atoms at $70\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a magneto-optical trap to nsnsns or npnpnp Rydberg states with principal quantum numbers $26\ensuremath{\le}n\ensuremath{\le}40$. Time-delayed state-selective field ionization is used to directly monitor the population in the initial and neighboring Rydberg levels. The measured time dependence of the Rydberg population is well described by numerical simulations which consider only spontaneous emission and population transfer by blackbody radiation. No evidence for collective decay is found at atom densities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This result is in contrast to a previous study [Wang et al., Phys. Rev. A 75, 033802 (2007)], in which superradiant decay was theoretically predicted and experimentally inferred for atom density and laser focal volume conditions very similar to those considered here. Suppression of collective emission is likely due to variations in transition energies within the atom sample, dominated by inhomogeneities in dipole-dipole exchange interactions for initial sss states, or by a combination of dipole-dipole and electric field inhomogeneities for the initial ppp states.
We have studied the evolution of Rydberg wave packets in the presence of interatomic dipole-dipole interactions in a frozen Rb gas. Rb atoms in a magneto-optical trap (MOT) are first laser excited to nsnsns Rydberg eigenstates. A picosecond THz pulse further excites them into coherent superposition states involving the initial-level and neighboring npnpnp states. A second, identical, time-delayed THz pulse probes the wave-packet dynamics. As the wave packets evolve they are influenced by dipole-dipole interactions, predominantly pairwise excitation-exchange processes of the form $|s\ensuremath{\rangle}|p\ensuremath{\rangle}\ensuremath{\leftrightarrow}|p\ensuremath{\rangle}|s\ensuremath{\rangle}$. The coherent electronic evolution of the ensemble dephases due to the variation in dipole-dipole coupling strength between atom pairs in the MOT. The experimental results are in good agreement with numerical calculations that simulate the interactions between nearest neighbors in a frozen gas.
The dipole-dipole interaction strongly couples ultracold highly excited atoms, known as Rydberg atoms, and is the dominant mechanism for energy exchange among such atoms on microsecond time scales. I present the results of experiments that investigate this interaction using rubidium atoms excited to Rydberg states with an all-diode laser system. In the first of these, high resolution dipole-dipole interaction spectra for different magnetic sublevels of the 32d5/2 state are collected and analyzed. In the second experiment, ultracold rubidium atoms are implusively excited to coherent superpositions of three closely spaced |mj|sublevels of the 32d5/2 state in a weak electric field. The magnitude of the field sets the energy level spacing between these states, and hence the frequency of the phase dependent oscillations resulting from interference in resonant energy exchange processes. In the third experiment, the dipole-dipole interaction induced resonant energy exchange between columns of ultracold rubidium Rydberg atoms is imaged using a new technique involving selective field ionization with a spatially sensitive ion detector.
When the dipole-dipole energy transfer, $\mathrm{Rb} {25s}_{1/2}+\mathrm{Rb} {33s}_{1/2}\ensuremath{\rightarrow}\mathrm{Rb} {24p}_{1/2}+\mathrm{Rb} {34p}_{3/2},$ resonant at fields of 3.0 and 3.4 V/cm, is observed in a cold Rydberg gas, the resonances are broader than expected for this dipole-dipole interaction alone. Using a variant of the Ramsey interference method, in which the system is detuned from the resonance, we observe a density-dependent dephasing which we attribute to the inhomogeneities in the always resonant dipole-dipole excitation exchange interactions $\mathrm{Rb} {25s}_{1/2}+\mathrm{Rb} {24p}_{1/2}\ensuremath{\rightarrow}\mathrm{Rb} {24p}_{1/2}+\mathrm{Rb} {25s}_{1/2}$ and $\mathrm{Rb} {33s}_{1/2}+\mathrm{Rb} {34p}_{3/2}\ensuremath{\rightarrow}\mathrm{Rb} {34p}_{3/2}+\mathrm{Rb} {33s}_{1/2}.$ Since the dephasing rate is comparable to the observed widths, it appears that all of these interactions contribute to the observed linewidths.
We have explored the dipole-dipole mediated, resonant energy transfer reaction, $32{p}_{3/2}+32{p}_{3/2}\ensuremath{\rightarrow}32s+33s$, in an ensemble of cold 85Rb^{85}\mathrm{Rb}85Rb Rydberg atoms. Stark tuning is employed to measure the population transfer probability as a function of the total electronic energy difference between the initial and final atom-pair states over a range of Rydberg densities, $2\ifmmode\times\else\texttimes\fi{}{10}^{8}\ensuremath{\le}\ensuremath{\rho}\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}{10}^{9} {\mathrm{cm}}^{\ensuremath{-}3}$. The observed line shapes provide information on the role of beyond nearest-neighbor interactions, the range of Rydberg atom separations, and the electric field inhomogeneity in the sample. The widths of the resonance line shapes increase approximately linearly with the Rydberg density and are only a factor of 2 larger than expected for two-body, nearest-neighbor interactions alone. These results are in agreement with the prediction [B. Sun and F. Robicheaux, Phys. Rev. A 78, 040701(R) (2008)] that beyond nearest-neighbor exchange interactions should not influence the population transfer process to the degree once thought. At low densities, Gaussian rather than Lorentzian line shapes are observed due to electric field inhomogeneities, allowing us to set an upper limit for the field variation across the Rydberg sample. At higher densities, non-Lorentzian, cusplike line shapes characterized by sharp central peaks and broad wings reflect the random distribution of interatomic distances within the magneto-optical trap (MOT). These line shapes are well reproduced by an analytic expression derived from a nearest-neighbor interaction model and may serve as a useful fingerprint for characterizing the position correlation function for atoms within the MOT.
I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. ii This thesis reports the first observation of the resonant electric dipole-dipole interaction between cold Rydberg atoms using microwave spectroscopy, the observation of the magnetic field suppression of resonant interactions, and the development of a unique technique for precise magnetic field measurements. ARydbergstate(46d 5/2) of laser cooled 85 Rb atoms has been optically excited. A fraction of these atoms has been transferred to another Rydberg state (47p 3/2 or 45f 5/2,7/2) to introduce resonant electric dipole-dipole interactions. The line broadening of the two-photon 46d 5/2 − 47d 5/2 microwave transition due to the interaction of 46d 5/2 with 47p 3/2 or 45f 5/2,7/2 atoms has been used as a probe of the interatomic interactions. This experiment has been repeated with a DC magnetic field applied. The application of a weak magnetic field ( ≤ 0.6 G) has reduced