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Physical Review A

Coherent control of extreme uv absorption and photoemission by the simultaneous irradiation of ultrashort extreme uv and laser pulses

Kenichi L. Ishikawa, Katsumi Midorikawa

Published Feb 5, 2002• 14 citations
Publisher

Abstract

We present a theoretical study on the response of a hydrogen atom to the simultaneous irradiation of ultrashort extreme ultraviolet (xuv) and laser pulses. The calculation is based on the time-dependent Schr\"odinger equation. The xuv photon energy is chosen to induce the $1s\ensuremath{-}2p$ transition. Our results indicate that we can transiently reduce the xuv absorption and change the atomic state within a few femtoseconds by injecting a femtosecond laser pulse. The laser-induced photoemission is affected by the addition of an xuv pulse and its yield depends much on the temporal overlap of the two pulses.

DOI
10.1103/physreva.65.031403

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We present a theoretical study on the response of a hydrogen atom to the simultaneous irradiation of ultrashort extreme ultraviolet (xuv) and laser pulses. The calculation is based on the time-dependent Schr"odinger equation. The xuv photon energy is chosen to induce the 1s\ensuremath−2p1s\ensuremath{-}2p1s\ensuremath−2p transition. Our results indicate that we can transiently reduce the xuv absorption and change the atomic state within a few femtoseconds by injecting a femtosecond laser pulse. The laser-induced photoemission is affected by the addition of an xuv pulse and its yield depends much on the temporal overlap of the two pulses.
We present a theoretical study of transient absorption and reshaping of extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately strong infrared (IR) laser field. We formulate the atomic response using both the frequency-dependent absorption cross section and a time-frequency approach based on the time-dependent dipole induced by the light fields. The latter approach can be used in cases when an ultrafast dressing pulse induces transient effects, and/or when the atom exchanges energy with multiple frequency components of the XUV field. We first characterize the dressed atom response by calculating the frequency-dependent absorption cross section for XUV energies between 20 and 24 eV for several dressing wavelengths between 400 and 2000 nm and intensities up to 1012{}^{12}12 W/cm2{}^{2}2. We find that for dressing wavelengths near 1600 nm, there is an Autler-Townes splitting of the $1s\ensuremath{\rightarrow}2p$ transition that can potentially lead to transparency for absorption of XUV light tuned to this transition. We study the effect of this XUV transparency in a macroscopic helium gas by incorporating the time-frequency approach into a solution of the coupled Maxwell-Schr\"odinger equations. We find rich temporal reshaping dynamics when a 61-fs XUV pulse resonant with the $1s\ensuremath{\rightarrow}2p$ transition propagates through a helium gas dressed by an 11-fs, 1600-nm laser pulse.
We present a theoretical study of the transient absorption of an ultrafast, extreme ultraviolet (XUV) pulse in the presence of an infrared (IR) control pulse in a macroscopic three-level helium gas. We concentrate on cases in which the IR pulse is shorter than the XUV pulse and induces transient transparency and amplification of the XUV light via coherent control. We give a time-dependent picture of the absorption process through the time dependence of the populations and relative coherences of the three states. We show that the XUV absorption can be dynamically changed into emission via phase control of the XUV-resonant 2p2p2p state through IR-induced coupling between the 2p2p2p and the 2s2s2s states. This can lead to compression and amplification of the XUV pulse when propagating through a laser-dressed macroscopic medium. We discuss the influence of atomic decoherence times on the XUV reshaping and show that fast decoherence means that the laser dressing acts more like a passive transmission window and less like a dynamical redistribution of energy.
We present a theoretical study of transient absorption spectra of laser-dressed hydrogen atoms, based on numerical solutions of the time-dependent Schr\"odinger equation. The timing of absorption is controlled by the delay between an extreme ultra violet (XUV) pulse and an infrared (IR) laser field. The XUV pulse is isolated and several hundred attoseconds in duration, which acts as a pump to drive the ground-state electron to excited ppp states. The subsequent interaction with the IR field produces dressed states, which manifest as sidebands between the 1s-npnpnp absorption spectra separated by one IR-photon energy. We demonstrate that the population of dressed states is maximized when the timing of the XUV pulse coincides with the zero crossing of the IR field, and that their energies can be manipulated in a subcycle time scale by adding a chirp to the IR field. An alternative perspective to the problem is to think of the XUV pulse as a probe to detect the dynamical ac Stark shifts. Our results indicate that the accidental degeneracy of the hydrogen excited states is removed while they are dressed by the IR field, leading to large ac Stark shifts. Furthermore, we observe the Autler-Townes doublets for the n=2n=2n=2 and 3 levels using the 656 nm dressing field, but their separation does not agree with the prediction by the conventional three-level model that neglects the dynamical ac Stark shifts.
We make a theoretical investigation of photoionization of an H2{\mathrm{H}}_{2}H2​ molecule, induced by the irradiation of an ultrashort extreme ultraviolet (XUV) laser pulse. We consider a system composed of a photoelectron ejected from H2{\mathrm{H}}_{2}H2​ and the resultant H2+{{\mathrm{H}}_{2}}^{+}H2​+ as a bipartite system. In order to clarify how the interparticle correlations among two electrons and two protons in H2{\mathrm{H}}_{2}H2​ are reflected to the bipartite system, we examine the entanglement between the photoelectron and the vibrational states of H2+{{\mathrm{H}}_{2}}^{+}H2​+ as well as the coherence in the vibrational states of H2+{{\mathrm{H}}_{2}}^{+}H2​+ by simulating the photoionization process of one-dimensional H2{\mathrm{H}}_{2}H2​. In the simulation, we solve a time-dependent Schr\"odinger equation using a symmetry-adapted grid method. On the basis of the simulations with ten different sets of three parameters characterizing an ultrashort XUV laser pulse, i.e., the pulse duration, the wavelength, and the peak intensity, we show that the extent of the entanglement depends sensitively on the coherence in the vibrational states of H2+{{\mathrm{H}}_{2}}^{+}H2​+.
Atoms irradiated with combined femtosecond laser and extreme ultraviolet (XUV) fields ionize through multiphoton processes, even when the energy of the XUV photon is below the ionization potential. However, in the presence of two different XUV photons and an intense laser field, it is possible to induce full electromagnetic transparency. Taking helium as an example, the laser field modifies its electronic structure, while the presence of two different XUV photons and the laser field leads to two distinct ionization pathways that can interfere destructively. This work demonstrates a new approach for coherent control in a regime of highly excited states and strong optical fields.
We show experimentally that spectral phase manipulation of ultrashort extreme-uv light pulses can induce and control coherent transient excitation of the He(1s3p)\mathrm{He}(1s3p)He(1s3p) excited state by the nonresonant components of the broadband extreme-ultraviolet light. The spectral phase manipulation of the 15th harmonic of an intense 805nm805\phantom{\rule{0.3em}{0ex}}\mathrm{nm}805nm, 80fs80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}80fs pulse is achieved by propagation of the euv light through a variable optical density of He gas. The acquired spectral phase due to the dispersive interaction of the off-resonance components in the euv pulse with the He(1s3p)\mathrm{He}(1s3p)He(1s3p) resonance enhances and modifies the transient excitation. The temporal evolution of the coherently prepared transient He(1s3p)\mathrm{He}(1s3p)He(1s3p) amplitude is probed by ionization to the continuum with a 400nm400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}400nm, 80fs80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}80fs pulse.