Active Suppression of Quantum Dephasing in Resonantly Driven Ensembles.

We have used quantum control to suppress the impact of random atom positions on coherent population transfer within atom pairs, enabling the observation of dipole-dipole driven Rabi oscillations in a Rydberg gas with hundreds of atoms. The method exploits the reduced coupling-strength sensitivity of the off-resonant Rabi frequency, and coherently amplifies the achievable population transfer in analogy to quasi-phase-matching in nonlinear optics. Simulations reproduce the experimental results and demonstrate the potential benefits of the technique to other many-body quantum control applications.

Angle-dependent strong-field ionization of halomethanes.

We study, experimentally and theoretically, the ionization probability of singly halogenated methane molecules, CH3Cl and CH3Br, in intense linearly polarized 800 nm laser pulses as a function of the angle between the molecular axis and the laser polarization. Experimentally, the molecules are exposed to two laser pulses with a relative time delay. The first, weaker pulse induces a nuclear rotational wave packet within the molecules, which are then ionized by the second, stronger pulse. The angle-dependent ionization yields are extracted from fits of the measured delay-dependent ionization signal to a superposition of moments of the rotational wave packet's angular distribution. Angle-dependent strong-field ionization (SFI) yields are also calculated using time-dependent density functional theory. Good agreement between measurements and theory is obtained. Interestingly, we find a marked difference between the angle-dependence of the ionization yields for these two halomethane species despite the similar structure of their highest occupied molecular orbitals. Calculations reveal that these differences are a result of multichannel (CH3Cl) vs single-channel (CH3Br) ionization and of increased hole localization on Br vs Cl. By adding calculations for CH3F, we can discern clear trends in the ionization dynamics with increasing halogen mass. These results are illustrative, as chemical functionalization and molecular alignment are likely to be important parameters for initiating and controlling charge migration dynamics via SFI.

Identifying Spatial Variation Along the HIV Care Continuum: The Role of Distance to Care on Retention and Viral Suppression.

Distance to HIV care may be associated with retention in care (RIC) and viral suppression (VS). RIC (≥ 2 HIV visits or labs ≥ 90 days apart in 12 months), prescribed antiretroviral therapy (ART), VS (< 200 copies/mL at last visit) and distance to care were estimated among 3623 DC Cohort participants receiving HIV care in 13 outpatient clinics in Washington, DC in 2015. Logistic regression models and geospatial statistics were computed. RIC was 73%; 97% were on ART, among whom 77% had VS. ZIP code-level clusters of low RIC and high VS were found in Northwest DC, and low VS in Southeast DC. Those traveling ≥ 5 miles had 30% lower RIC (adjusted odds ratio (aOR) 0.71, 95% CI 0.58, 0.86) and lower VS (OR 0.70, 95% CI 0.52, 0.94). Geospatial clustering of RIC and VS was observed, and distance may be a barrier to optimal HIV care outcomes.

High-energy electron emission from metallic nano-tips driven by intense single-cycle terahertz pulses.

Electrons ejected from atoms and subsequently driven to high energies in strong laser fields enable techniques from attosecond pulse generation to imaging with rescattered electrons. Analogous processes govern strong-field electron emission from nanostructures, where long wavelength radiation and large local field enhancements hold the promise for producing electrons with substantially higher energies, allowing for higher resolution time-resolved imaging. Here we report on the use of single-cycle terahertz pulses to drive electron emission from unbiased nano-tips. Energies exceeding 5 keV are observed, substantially greater than previously attained at higher drive frequencies. Despite large differences in the magnitude of the respective local fields, we find that the maximum electron energies are only weakly dependent on the tip radius, for 10 nm

Terahertz-induced field-free orientation of rotationally excited molecules.

We have used picosecond THz pulses to induce transient field-free orientation of OCS molecules. Coherent optical Raman excitation prepares the molecules in rotational superposition states prior to THz irradiation, substantially enhancing the degree of orientation. The time-dependent alignment and orientation are characterized via Coulomb explosion in an intense probe laser. The degree of OCS orientation is an order of magnitude larger than previously observed following THz irradiation and is achieved with a significantly smaller THz field.The field-free orientation level is comparable to that generated using pulsed, two-color laser fields but is obtained with negligible target ionization.

Ionization of excited atoms by intense single-cycle THz pulses.

We have employed intense, single-cycle THz pulses to explore strong-field ionization of low-lying Na Rydberg states in the low-frequency limit. At the largest fields used, F≃430 kV/cm, electrons with energies up to 60 eV are created. The field ionization threshold is greater than expected for adiabatic "over-the-barrier" ionization and is found to scale as n-3. In addition, for a given field amplitude, higher energy electrons are produced during the ionization of the most tightly bound states. These observations can be attributed to the suppression of scattering from the nonhydrogenic ion core, the long times required for Rydberg electrons to escape over the barrier in the field-dressed Coulomb potential, and the failure, in the single-cycle limit, of the standard prediction for electron energy transfer in an oscillating field. The latter, in particular, holds important implications for future strong-field experiments involving the interaction of ground-state atoms and molecules with true single-cycle laser fields.

Time-dependent electron interactions in double Rydberg wave packets.

We investigate the time-dependent evolution of a nonstationary three-body Coulomb system at energies just below the threshold for three-body breakup. Experimentally, short-pulse lasers excite two electrons in Ba to radially localized Rydberg wave packets with well-defined energy and angular momentum. Time-dependent interactions between the two electrons are probed using half-cycle electric field pulses. The measurements indicate that substantial energy exchange between the two electrons is nearly immediate upon the launch of the second wave packet. Fully quantum and classical calculations support this observation, predicting extremely rapid autoionization under the experimental conditions. The calculations also show very fast angular momentum exchange and sensitivity to the relative binding energies of the two electrons.

Probing electronic coherence in a gas of dipole-dipole coupled Rydberg atoms.

We demonstrate a novel time-domain method to probe electronic coherence in ensembles of cold Rydberg atoms coupled via nearly resonant dipole-dipole interactions. Short laser pulses create coherent superpositions of few-electron eigenstates which evolve under the influence of pulsed electric fields. The pulses steer the dynamics, enhancing the probability for finding atoms in np, rather than initially excited ns states. The enhancement reflects the underlying electronic coherence which persists for >10 µs, 2 orders of magnitude longer than previously measured dephasing times in the same system. Simulations suggest that atom motion is responsible for the eventual decoherence.

Phase-dependent electron-ion recombination in a microwave field.

Using picosecond laser photoionization of Li in a microwave field we have observed phase-dependent reccombination of the photoelectrons with their parent Li+ ions. Recombination occurs at phases of the microwave field such that energy is removed from the photoelectron in the first microwave cycle after excitation, and there are two maxima in the recombination in each microwave cycle. These observations are consistent with observations made using an attosecond pulse train phase locked to an infrared pulse and with the "simpleman's" model, modified to account for the fact that the photoelectrons are produced in a Coulomb potential.

Directional emission of multiply charged ions during dissociative ionization in asymmetric two-color laser fields.

Intense, asymmetric 1ω+2ω laser fields are used to affect the directional ejection of multiply charged ion fragments from a variety of molecules, including N2, O2, CO, HBr, and CO2. By tuning the relative phase, ϕ, between the two fields, we observe large forward-backward dissociation asymmetries. The largest asymmetries are obtained at the same values of ϕ for all species, suggesting a common dynamical mechanism. Following an independent phase calibration, the sign of the asymmetry appears to be opposite that expected from the standard enhanced ionization model.

Extracting the polarizability anisotropy from the transient alignment of HBr.

We use 40 fs, 780 nm laser pulses to transiently align HBr molecules. We study the temporal dynamics of the resultant rotational wavepacket to gain insight into the electronic properties of the molecule. We show that the HBr polarization anisotropy can be extracted by comparing the time dependence of the HBr alignment with both the analogous alignment behavior of N(2) and the predictions of a rigid-rotor model.

Multiphoton assisted recombination.

We have observed multiphoton assisted recombination in the presence of a 38.8 GHz microwave field. Stimulated emission of up to ten microwave photons results in energy transfer from continuum electrons, enabling recombination. The maximum electron energy loss is far greater than the 2Up predicted by the standard "simpleman's" model. The data are well reproduced by both an approximate analytic expression and numerical simulations in which the combined Coulomb and radiation fields are taken into account.

Preserving coherence in Rydberg quantum bits.

The effectiveness of decoherence suppression schemes is explored using quantum bits (qubits) stored in Li np Rydberg states. Following laser excitation, pulsed electric fields coherently control the electronic spin-orbit coupling, facilitating qubit creation, manipulation, and measurement. Spin-orbit coupling creates an approximate decoherence-free subspace for extending qubit storage times. However, sequences of fast NOT operations are found to be substantially more effective for preserving coherence.

Ionization of atoms by the spatial gradient of the pondermotive potential in a focused laser beam.

Ionization of atoms by the spatial gradient of the pondermotive potential in a focused laser beam is investigated. Rydberg ions, formed during the interaction of noble gas atoms with an intense laser pulse, are used to probe the gradient field. Rydberg ion species with higher ionization potentials are produced at locations where the gradient field is largest. The measured Rydberg ion yields differ dramatically from estimates that ignore gradient-field ionization, but are in good agreement with predictions that include the effect.

Probing two-electron dynamics of an atom.

Coherent short-pulse laser excitation has been used to control the approximate energy and relative proximity of two valence electrons within the same alkaline-earth atom, thereby providing insight into the dynamical evolution of a three-body Coulomb system. Our time-domain experiments enable direct experimental study of the electron dynamics at the classical limit of a two-electron atom. As an example, we look at the mechanism of autoionization for one two-electron configuration class and find that the doubly excited atom decays through a single violent electron-electron collision rather than a gradual exchange of energy between the electrons.

Time-domain investigation of electron recapture via post-collision interaction in a double photoionization continuum.

Picosecond laser pulses have been used to sequentially photoionize both valence electrons from neutral Ba atoms, producing two radially localized continuum wave packets. The Coulomb interaction between the two outgoing electrons can result in the recapture of one of the electrons by the parent ion. The energy distribution of Rydberg ions formed via this "post-collision" interaction is measured as a function of the delay between the ionizing laser pulses. The experimental data are in agreement with the results of both a quantum sudden approximation and a classical simulation.

Centrifugal electron-ion recombination.

Transient electric-field pulses have been used to stimulate electron/ion recombination in a low density plasma in the presence of a static electric field. The measured recombination rates exhibit a strong dependence on the relative orientation of the pulsed and static fields. For weak pulses, the recombination rate is significantly higher for orthogonal as opposed to parallel or antiparallel field configurations. The enhanced recombination rate is attributed to the dynamic stabilization of high-m Rydberg levels that are populated during the pulse. Classical simulations confirm the importance of angular momentum rather than energy transfer.

Ratiometric comparison of intense field ionization of atoms and diatomic molecules.

Intense-laser ionization rates for rare gas atoms and diatomic molecules have been precisely compared by making simultaneous measurements of ionization yield vs laser intensity for mixed atomic and molecular targets. At a given laser intensity, the N (2) and F (2) ionization yields are slightly greater than that of Ar. Conversely, comparison of O (2) and S (2) with Xe indicates significant ionization suppression in these molecules. Recent molecular ionization models that successfully describe ionization suppression in O (2) and its absence in N (2) fail to explain our observations in F (2) and S (2).