We have applied a tomographic imaging method to recreate the full three-dimensional distribution of photoelectrons produced in a strong-field photodetachment process. The method is general and can be applied to any laser polarization. This stands in contrast to traditional imaging inversion methods, such as Abel inversion, which require prior knowledge of the symmetries of the electron distribution and are therefore limited to experiments using linear or circular polarization. Our method is also useful in a situation where linear polarization is used, since it compensates for detector inhomogeneities by spreading the information on a larger detector surface. In addition, it facilitates a method to detect polarization defects. Measurements were made for photodetachment of ${\mathrm{Ag}}^{\ensuremath{-}}$ at laser wavelengths of 1310 and 2055 nm, and were found to agree well with simulations in the strong field approximation. The data in the 1310 nm case revealed an unexpected asymmetry in the plane in which the laser polarization axis is rotated. Using a quasistationary quasienergy state model, a residual elliptical polarization of $\ensuremath{\varepsilon}=0.21\ifmmode\pm\else\textpm\fi{}0.01$ consistently explains the observed asymmetry. We conclude that the method described in this paper has the potential to be applied in experiments where a more complete characterization of electron emission distributions is required.

THz microscopic imaging is used to extract the Gouy phase shift of the transverse and longitudinal field components of a tightly focused, radially polarized beam. We demonstrate that the applied THz time domain approach allows to observe directly the evolution of the geometric phase as the wave propagates through the focus. Our method yields a Gouy phase shift of 2 pi for the transverse component and of pi for the longitudinal component. In addition, we apply our method to the well-known case of a focused, linearly polarized beam and pinpoint a fundamental connection between the field components of tightly focused, radially and linearly polarized light. The applied procedure is universal and may even allow to determine the geometric phase of arbitrary shaped and polarized propagating waves.

Radially polarized beams represent an important member of the family of vector beams, in particular due to the possibility of using them to create strong and tightly focused longitudinal fields, a fundamental property that has been exploited by applications ranging from microscopy to particle acceleration. Since the properties of such a focused beam are intimately related to the Gouy phase shift, proper knowledge of its behavior is crucial. Terahertz microscopic imaging is used to extract the Gouy phase shift of the transverse and longitudinal field components of a tightly focused, radially polarized beam. Since the applied terahertz time-domain approach is capable of mapping the amplitude and phase of an electromagnetic wave in space, we are able to directly trace the evolution of the geometric phase as the wave propagates through the focus. We observe a Gouy phase shift of 2π for the transverse and of π for the longitudinal component. Our experimental procedure is universal and may be applied to determine the geometric phase of other vector beams, such as optical vortices, or even arbitrarily shaped and polarized propagating waves.

A near-resonant rf field pumping the hyperfine transition between the two ground states of a Lambda-shaped dark resonance leads to a resonance tripling, each component displaying electromagnetically induced transparency (EIT). We investigate the three resonances under high spectral and temporal resolution. The triplet formation is analogous to that of the Mollow triplet but distinct in that the role played by the spontaneous emission rate is now taken by the one-photon scattering rate of the optical Raman transition. Complex phase relations exist between the three em fields under EIT conditions. We explain our observations using numerical solutions of the quantum master equation as well as a simple analytical dressed-state model.

We investigate the Berry phase in an ensemble of thermal Rb-87 atoms which we prepare in a superposition state under conditions near and at electromagnetically induced transparency. The geometric phase is imprinted in the atoms by rotating the laboratory magnetic field. Phase-stabilized light fields permit us to monitor phase changes of the atomic sample in a Ramsey-type interferometer by instant probing of the absorptive response of the atoms as well as by monitoring the free-induction decay of the coherent superposition. The absolute sign of the phase is determined by reference to controllable phase shifts imposed by the experimenter. We prove that the geometric phase is independent of the rotational frequency of the magnetic field in the adiabatic regime, that the phase is additive in multiple rotations, and it is independent of the Lande factor of the atomic magnetic moment, as predicted in Berry's seminal paper. We show that the absolute sign of the phase encodes the sign of the observable angular momentum in relation to laboratory coordinates.

The equivalence of a complete characterization of linear momenta of fragments from a many-body fragmentation process and the spatial wave function of the many-body system is discussed. Our experiment on three-body dissociation of state selected H3 and D3 molecules into ground-state hydrogen atoms strongly suggests the existence of such a close relationship as it is also predicted by theory in the form of the imaging theorem. We conclude that prudent imaging of many-body fragmentation provides a laboratory view of the squared many-body wave function at a spatial scale of molecular dimensions at which fragments exit into the realm of independent free particles.

Three-particle dissociation of high-lying Rydberg states of D3 is induced by an external electric field. We observe that the momentum vector correlation map of the center-of-mass motion of the fragments converges near the ionization threshold to two distinct fragment configurations, the near linear geometry and the symmetric acute angle geometry. A comparison is made with the momentum vector correlation map recorded in dissociative recombination of D3+ with slow electrons and with the corresponding results for H3 where the acute angle geometry is conspicuously absent.

We report experiments on the control of the phase. in quantum mechanical superposition states which emerge in electromagnetically induced transparency, vertical bar psi > = (vertical bar 1 > + e(i eta)vertical bar 2 >)/root 2. We interpret our findings in terms of the measurement role that spontaneous emission and the light fields play in selecting the optically dark and bright superpositions. A phase-switching tool is introduced which enables the rapid measurement of the absolute depth of the dark-state minimum and its absolute position on the frequency scale without detuning the lasers. The phase-switching technique allows one to determine the relative phase eta in an ensemble of quantum mechanical superposition states of a Lambda system in a minimally invasive way. DOI: 10.1103/PhysRevA.87.013430

We discuss the dynamical behavior of the entanglement between the internal and the external degrees of freedom of a trapped atom in electromagnetically-induced transparency (EIT) laser cooling. It is shown that essential features of the intricate entanglement dynamics observed in full numerical simulations of the underlying quantum master equation can be understood in terms of a two-state model on the basis of Landau-Zener splittings in the atom-laser field Hamiltonian. An extension of this model to an effective non-Hermitian Hamiltonian is constructed which describes the decay of entanglement by spontaneous emission processes. We also discuss schemes for the control of entanglement and demonstrate that a permanent entanglement can be imprinted on trapped atoms through a rapid switch off of the driving fields. Finally, we point out fundamental distinctions between the entanglement created in EIT cooling and in the cooling scheme based on velocity-selective coherent population trapping.

For the example of the D-1 line of Rb-87 we analyze the experimental parameters that control the transient response of electromagnetically induced transparency. Quantum coherent free-induction decay is observed on time scales exceeding several milliseconds in a buffer-gas vapor cell. Numerical solutions of the quantum master equation and approximate analytical solutions are tested and absolute comparisons of the transient time scales, power broadening, resonance contrast, and frequency shifts are made. Two actively-phase-locked lasers are employed. The effects of laser phase noise that is not fully correlated are studied.

Anomalous photoelectron angular distributions are observed at certain wavelengths in strong-field ionization of H-2. We relate this feature to ac Stark shifts from bound-bound transitions in the Rydberg manifold of principal quantum number n = 3 and 4. A model of the multistate interaction supports this interpretation.

The correlation pattern in the center of mass motion of the three fragments from dissociation of well-defined Rydberg states of H-3 and D-3 is studied. Dissociation of the molecules is induced by an external electric field. Through a comparison with results obtained in radiative cascading we can show that the correlation pattern is that of the short-lived 2s (2)A(1)' electronic state, of which a tiny amplitude is admixed by the external electric field. A comparison of our results with the predictions by M. Lehner and M. Jungen [J. Phys. B 42, 065101 (2009)] and U. Galster [Phys. Rev. A 81, 032517 (2010)] for predissociation of the 2s (2)A(1)' state is made. We show that the experimental vector correlation maps are direct images of the spatial symmetry of the product of the vibrational wave function and spatial dependence of the nonadiabatic coupling operator.

We study the entanglement between the internal electronic and the external vibrational degrees of freedom of a trapped atom which is driven by two lasers into electromagnetically induced transparency. It is shown that basic features of the intricate entanglement dynamics can be traced to Landau-Zener splittings (avoided crossings) in the spectrum of the atom-laser field Hamiltonian. We further construct an effective Hamiltonian that describes the behavior of entanglement under dissipation induced by spontaneous emission processes. The proposed approach is applicable to a broad range of scenarios for the control of entanglement between electronic and translational degrees of freedom of trapped atoms through suitable laser fields.

Strong-field ionization of molecular hydrogen is studied at wavelengths ranging from 300 to 800 nm using pulses of 100-fs duration. We find that over this wide wavelength range, from nominally 4-photon to 11-photon ionization, resonance features dominate the ionization probability at intensities below 10(14) W/cm(2). Photoelectron momentum maps recorded by an imaging spectrometer are analyzed to identify the wavelength-dependent ionization pathways in single ionization of molecular hydrogen. A number of models, some empirical, which are appropriate for a quantitative interpretation of the spectra and the ionization yield are introduced. A near-absolute comparison of measured ionization yields at 398 nm is made with the predictions based on a numerical solution [Y.V. Vanne and A. Saenz, Phys. Rev. A 79, 023421 (2009)] of the time-dependent Schrodinger equation for two correlated electrons.

We report a terahertz near-field imaging approach providing spatially resolved measurements of amplitude, phase, and polarization of the electric field. Using this approach we extract the microscopic near-field signatures in plasmonic devices and planar metamaterials.

We show experimentally that an external electric field can be used to control the amplitudes of nonadiabatic paths taken by a dissociating molecule. In the example presented here, this control is achieved by Stark-field mixing in H(3) Rydberg states with different decay paths. The final state continuum is in each path formed by three-particle wave packets of slow neutral hydrogen atoms in their electronic ground state. Their momentum vector correlations show signs of interference, since the molecule can access the identical continuum via two distinctly different paths, involving different nonadiabatic coupling mechanisms. As an added feature a preferred alignment of the fragmentation plane in the laboratory frame emerges, corresponding to a selective dissociation of molecules oriented along the field direction.

We employ near- and far-field measurements of single-cycle THz pulses and numerical simulations to investigate the influence of diffraction in metamaterial arrays. We find that radiative coupling leads to substantial modifications of the spectral response.

Three-particle dissociation of high-lying Rydberg states of H-3, in the immediate vicinity of the ionization threshold, is induced by an external electric field. We observe that the momentum vector correlation map of the center-of-mass motion of the fragments changes rapidly with electronic energy. Near-and above-threshold substantial contributions of fragment orientations which are characteristic of near-linear geometry appear, i.e., two atoms with opposing momenta, the third atom being nearly at rest. This finding is similar to observations by Strasser et al. [Phys. Rev. A 66, 032719 (2002)], who studied dissociative recombination of cold H-3(+) ions with slow electrons. We discuss the likely dissociation paths responsible for the observed correlation.

Over the past decade, terahertz spectroscopy has evolved into a versatile tool for chemically selective sensing and imaging applications. In particular, the potential to coherently generate and detect short, and hence, broadband terahertz pulses led to the development of efficient and compact spectrometers for this interesting part of the electromagnetic spectrum, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Although early proof-of-principle demonstrations have shown the great potential of terahertz spectroscopy for sensing and imaging, the technology still often lacks the required sensitivity and suffers from its intrinsically poor spatial resolution. In this review we discuss the current potential of terahertz pulse spectroscopy and highlight recent technological advances geared towards both enhancing spectral sensitivity and increasing spatial resolution.