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Physical Review A

One- and two-photon phase-sensitive coherent control of total ionization yields in the presence of static electric fields

Agisilaos Bolovinos, S. Cohen, Ioannis Liontos

Published Feb 14, 2008• 11 citations
Publisher

Abstract

We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied, $s$, $p$ and $d$ character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth $V$ as a function of the static field strength $F$ for otherwise identical experimental conditions. The $V(F)$ curve exhibits a maximum (typically $\ensuremath{\sim}65%--85%$) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent $s$, $p$ and $d$ character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.

DOI
10.1103/physreva.77.023413

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We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied, sss, ppp and ddd character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth VVV as a function of the static field strength FFF for otherwise identical experimental conditions. The V(F)V(F)V(F) curve exhibits a maximum (typically \ensuremath∼65\ensuremath{\sim}65%--85%\ensuremath∼65) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent sss, ppp and ddd character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.
Phase sensitive coherent control on atomic Rydberg states and in the presence of a static electric field is theoretically studied using Harmin's frame transformation theory. It is shown that under the presence of the static field, the total excitation rate of an atomic target by a bichromatic laser field (consisting of a fundamental radiation frequency and of its second harmonic) depends on all the elements of the density-of-states matrix dictating the Stark effect. Some of these off-diagonal matrix elements are inaccessible by any other photoexcitation scheme and affect non-trivially the optimum achievable rate modulation contrast. This fact leads to important qualitative and quantitative differences with respect to static-field-free coherent control. The obtained formal expressions for the phase-controlled excitation rate are applied to the lithium target atom. It is demonstrated that the electric field strength can be used as an additional, externally adjustable, control and manipulation parameter. Finally, it is shown that above the saddle point energy, where quasi-discrete and continuum states coexist, the lineshapes of individual Fano-like resonances can be quite efficiently modified by applying this scheme.
We demonstrate that a weak, static electric field enables nearly complete coherent phase control of the total ionization yield in a two-color ionization process using fundamental and second-harmonic radiation. The static electric field induces a dipole-forbidden resonance in the two-photon transition amplitude so that the final photoelectron states are identical to those in a single photon transition. We demonstrate also a phase controllable circular dichroism effect in the total photoelectron yield. Experimental realization of this process using alkali metal atoms is discussed.
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.
Synopsis We present single-photon ionization Stark spectra of Sr as a function of static electric field strength and just above the saddle-point-energy. The spectra exhibit a significant number of avoided crossings between Stark resonances, superimposed on an ionization background due to the continuum excitation. The latter may be avoided by employing single- and two-photon ionization in conjunction with phase sensitive coherent control techniques. This scheme will lead to the isolation of the resonances in differential ionization cross section measurements.
We report the observation of coherent control of the ionization rate in four-photon resonant, five-photon ionization schemes in Kr and Xe. The intermediate resonance is excited via different interfering channels involving four-photon absorption of the fundamental laser frequency ( ionization) as well as one third harmonic and one fundamental frequency photon absorption ( excitation), a particular aspect of the scheme being that interference occurs at a virtual level. By varying the relative phase of the two electromagnetic fields, a large modulation depth has been observed which is due to the chosen non-linear excitation scheme. In the Xe excitation experiment the interference between the different channels could be additionally probed through the modulation of the third-harmonic signal.
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.