The potential of using interferences between transition amplitudes for the coherent control of photoionization rates and of asymmetric photoelectron angular distributions has been demonstrated previously. Current efforts in our laboratories are directed toward control of photoionization of NO, and on control of photoionization channels in atomic barium. This latter system is, of course, much simpler than a molecular system, and thus may provide better insight into the mechanisms involved. We will discuss our investigations of the effects of nonlinear coupling, absorption, and dispersion of the laser beams on their propagation through dense media, as observed via interference effects. Laser band structure effects and the effect of Doppler broadening will also be discussed.
The potential of using interferences between transition amplitudes for the coherent control of photoionization rates and of asymmetric photoelectron angular distributions has been demonstrated previously. Current efforts in our laboratories are directed toward control of photoionization of NO, and on control of photoionization channels in atomic barium. This latter system is, of course, much simpler than a molecular system, and thus may provide better insight into the mechanisms involved. We will discuss our investigations of the effects of nonlinear coupling, absorption, and dispersion of the laser beams on their propagation through dense media, as observed via interference effects. Laser band structure effects and the effect of Doppler broadening will also be discussed.
We present the results of our study on coherent control of photoionization of atomic barium. Our study focused on the understanding of the controllability, especially due to the effect of the coherent interaction between the atomic system and the laser field. The first half of the study investigates the mechanisms of the control behind the previously observed laser phase-insensitive product state control. The controllability of this excitation scheme, two-color two-photon resonantly enhanced excitation, was analyzed from two aspects, the role of ac Stark shift introduced by the strong laser field and the multi-pathway quantum mechanical interferences. We have analyzed the excitation scheme from the analysis of the photoelectron angular distribution measured using the excitation scheme and the monitoring of the intermediate state population. Analysis of the data as well as the numerical simulation showed clear understanding of the role of two mechanisms in the product state control reported. We also investigated the control of the phase lag during the product state control. We conducted the control of the phase lag in the study of asymmetric photoelectron angular distribution, which arises from the concurrent even-odd parity outgoing electron wave excitation. The phase lag was controlled in full range, 2π, and the results were analyzed in terms of the role of autoionizing resonance structures as well as the nature of outgoing electron waves at different locations of the autoionizing resonances.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
Prospects for the coherent control of optically-induced processes have attracted a great deal of interest of late. In this talk we will discuss our recent demonstration of the first example of two-pathway coherent control on a bound-free transition in a molecular system. In this demonstration, we controlled the asymmetry of the angular distribution of photoelectrons produced by photoionization of NO. Preparation of the NO in its A{sup 2}{Sigma}, state allowed isolation of a single rotational state, which was subsequently ionized via interfering one- and two-photon interactions. We discuss the role of intermediate-state resonances and laser mode quality on two-pathway control processes.
Preface. Part I: Atomic and Molecular systems. Scenarios in Coherent Control P. Brumer, M. Shapiro. Quantum Control in Atomic Systems D. Charalambidis, et al. Laser-phase-sensitive Coherent Control of Photoion Product States in 2-Photon vs. 2-Photon Ionization of Atomic Barium, F. Wang, et al. Coherent Phase control of Photoionization and Photodissociation R.J. Gordon, et al. Coherent Control of Atomic Photoionization and Autoionization P. Lambropoulos, T. Nakajima. Lasing Without Inversion via Interference of Double-Dark Resonances in Atomic and Quantum Well Systems M.O. Scully, et al. Part II: Coherent Control in Semiconductors. Coherence Control of Free Carriers in Bulk Semiconductors H.M. van Driel, et al. Coherent Control of 2D Excitons Probed by Time-Resolved Secondary Emission X. Marie, et al. Many-Body Effects in the Ultrafast Population Transfer in p-Doped Semiconductor Quantum Wells R. Binder, M. Lindberg. Coherent Control of Heavy-Light Hole and Phonon Quantum Beats T. Kuhn, et al. Coherent Control Schemes in Semiconductor Double Wells X. Hu, W. Poetz. Terahertz-Control of Charge-Carrier Wavepackets in Semiconductor Quantum Wells S. Hughes, D.S. Citrin. Microcavity Exciton-Polariton H. Cao. Part III: Coherent Phenomena and Related Topics. Coherent Control of Quantum Localization M. Holthaus. Tunneling of Low-Energetic Electrons in the Presence of Intense Laser Fields: The Formation of Dynamical Barrier States M. Wagner. Hopping Mechanism of Coherent Photovoltaic Effect and Photoinduced Polar Anisotropy in Glass E.M. Baskin, M.V. Entin. Heterodyning Scheme Employing Quantum Interference J.B. Khurgin, et al. Generation of Widely Tunable THz-Wave Using Nonlinear Optics K. Kawase, H.Ito. List of Participants.
Laser light has been used as a probe of atoms, molecules, and solids since the invention of the laser. The use of laser light in a more active role, to modify and process surfaces, and initiate chemical reactions, followed shortly thereafter. But usually it is the intensity and the directionality of the laser light that is employed, not necessarily its coherence, and not particularly the fact that it has a well-defined phase. 'Coherence control' can be broadly understood as the set of processes whereby light modifies matter in a way that is critically dependent on the incident light beams possessing well-defined phases. While in a laser matter is manipulated to produce light of the desired properties, in coherent control light is manipulated -- in particular, its phase and intensity is adjusted -- to produce a material response of the desired type. Of the various coherent control processes that are currently being investigated, some involve a transition in the material medium from an initial state to a final state by two or more possible processes. With each of these is associated a quantum mechanical amplitude, and hence the probability for the transition can show interference effects between the two amplitudes, just as in the familiar two-slit interference experiment the probability for the electron to be observed at a given position involves a probability that is the square of the sum of two amplitudes. In quantum interference control (QUIC), the relative phase of the two amplitudes is adjusted by adjusting the relative phase of two polarizations of a single beam, or the relative phase of two beams at different frequencies. It is this particular type of coherent control that is of interest in this communication.
Coherent effects are studied in a dense atomic vapor driven by laser fields. With\noptical properties dramatically modified by these effects, the medium can be used\nto manipulate some of the properties of laser field. Our experiments demonstrate\nthe coherent control over transmission, spatial distribution and noise feature of the\nlaser field interacting with coherent media. The results have potential applications\nin the field such as precision metrology, precision spectroscopy, optical imaging and\nlithography.\nWe develop an experiment to investigate the atomic excitation by few-cycle radio\nfrequency (RF) pulses interacting with Zeeman sublevels. The system provides the\nflexibility to fully control all parameters of RF pulses. Such a flexibility can not be\nachieved in optical domain. Based on this system, experiments can be conducted to\nsimulate processes in ultra-short laser physics. In particular, we study the carrier-envelope\neffect of few-cycle pulses and the strong off-resonant excitation by short\npulses.\nWe also discuss the selective reflection spectrum on a highly dense atomic vapor\nin which the dipole-dipole interaction can not be neglected. The spectrum broadening\ndue to dipole-dipole interaction is much broader than the Doppler broadening. Our\nexperiments show that the excitation by a pump laser can reduce the dipole-dipole\ninteraction, thus reduce the broadening and improve the spectral resolution. The\nexcitation dependence is studied at various atomic densities.