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Coherent control of optical processes through interfering interactions

Yi-Yian Yin, Rana Shehadeh, Edward R. Grant, Daniel S. Elliott

Published Dec 1, 1995• 0 citations
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Abstract

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.

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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.
Coherent control of interfering one- and two-photon processes has for decades been the subject of research to achieve the redirection of photocurrent. The present study develops two-pathway coherent control of ground-state helium atom above-threshold photoionization for energies up to the N=2N=2N=2 threshold, based on a multichannel quantum defect and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}$, the reduced electric field strength $\ensuremath{\chi}={\mathcal{E}}_{\ensuremath{\omega}}^{2}/{\mathcal{E}}_{2\ensuremath{\omega}}$, and the final state energy $\ensuremath{\epsilon}$. A small energy change near a resonance is shown to flip the emission direction of photoelectrons with high efficiency, through an example where 9090%90 of photoelectrons whose energy is near the 2p21Se2{p}^{2}{\phantom{\rule{4pt}{0ex}}}^{1}{S}^{e}2p21Se resonance flip their emission direction. However, the large fraction of photoelectrons ionized at the intermediate state energy, which are not influenced by the optical control, make this control scheme challenging to realize experimentally.
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.
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.
A coupled equations approach based on a general artificial channel method is used to demonstrate two color laser coherent control of photopredissociation and photoionization in Cl2. Photopredissociation of the 1Πg states of Cl2 is suppressed by a two plus four photon interference process 2ω2=4ω4, whereas the photoionization is controlled by interference between a six photon transition and a four photon transition 6ω4=2ω2+2ω4 using the same 1Πg photopredissociation resonances as intermediates. Photopredissociation angular distributions illustrate the suppression of the corresponding transition in selective rotational states.
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 investigate the coherent control of the photoelectron angular distribution in bichromatic atomic ionization. Neon is selected as target since it is one of the most popular systems in current gas-phase experiments with free-electron lasers (FELSs). In particular, we tackle practical questions, such as the role of the fine-structure splitting, the pulse length, and the intensity. Time-dependent and stationary perturbation theory are employed, and we also solve the time-dependent Schr\"odinger equation in a single-active electron model. We consider neon ionized by a FEL pulse whose fundamental frequency is in resonance with either $2p\text{\ensuremath{-}}3s$ or $2p\text{\ensuremath{-}}4s$ excitation. The contribution of the nonresonant two-photon process and its potential constructive or destructive role for quantum coherent control is investigated.