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Physical Review A

Coherent interaction of femtosecond extreme-uv light with He atoms

Daniel Strasser, Thomas Pfeifer, Brian J. Hom, Astrid M. Müller, Jürgen Plenge, Stephen R. Leone

Published Feb 24, 2006• 29 citations
Publisher

Abstract

We show experimentally that spectral phase manipulation of ultrashort extreme-uv light pulses can induce and control coherent transient excitation of the $\mathrm{He}(1s3p)$ excited state by the nonresonant components of the broadband extreme-ultraviolet light. The spectral phase manipulation of the 15th harmonic of an intense $805\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ 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 $\mathrm{He}(1s3p)$ resonance enhances and modifies the transient excitation. The temporal evolution of the coherently prepared transient $\mathrm{He}(1s3p)$ amplitude is probed by ionization to the continuum with a $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $80\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulse.

DOI
10.1103/physreva.73.021805

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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.
Shaped near-infrared (NIR) femtosecond pulses are used for the first time to control the generation of coherent deep-ultraviolet (UV) radiation in an atomic resonance-mediated (2+1) three-photon excitation. The broadband excitation coherently involves pathways that are on resonance with the intermediate resonance state as well as pathways that are near resonance with it. Experimental and theoretical results are presented for phase controlling the total emitted UV yield in atomic sodium. Depending on the NIR spectrum of the excitation pulse, the coherent UV emission is either predominantly due to a single excited real state that is accessed resonantly or due to a manifold of virtual states. The former leads to a narrowband UV emission, while the latter leads to a broadband UV radiation. Basic phase control is exercised in both cases, with excellent agreement between experiments and calculations. The tunability is over an order-of-magnitude UV-yield range.
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.
Abstract Ultrashort, coherent light pulses in the extreme ultraviolet (XUV) wavelength region are excellent tools for a wide range of experiments. These XUV pulses are in particular used to characterize electron dynamics. Additionally, the high photon energy enables quantum control of short-lived transitions in atoms. These transitions are of interest for ultrafast quantum control as the bandwidth of the control pulses needs to be less than the transition frequency. Coherent XUV pulses would be even more useful if the tools to control and manipulate them existed. Presently, however, no modulators exist for these frequencies, which significantly reduces the number of possible ultrafast optics experiments and applications in the XUV regime. This Ph.D. tutorial addresses this problem and focuses on the control of XUV light in time and space. To enable control of XUV light we have developed a method for opto-optical modulation (OOM), which modulates the phase of XUV light emitted from a gas of atoms. The atoms are resonantly excited with a coherent XUV pulse generated through high order harmonic generation and emit light after the excitation pulse passed. By shifting the resonance frequencies through the AC Stark shift with a non-resonant infrared control pulse, the phase of the emitted light, and therefore its wavefront, is controlled. With OOM the direction of XUV light emitted from noble gases has been controlled, and the temporal dynamics of the emission studied. By varying the delay between the excitation and the control pulse, the time of redirection was controlled, which enabled high signal to noise study of the temporal dynamics of the emission. Furthermore, a proof-of-principle experiment with two control pulses was performed, resulting in a shaped XUV pulse with controlled duration. The technique of OOM presented in this article is still at a very early stage and in order to obtain complete control of the amplitude and phase of the XUV pulses further development is necessary. Such development would present new opportunities for ultrafast XUV quantum control.
An optical coherent control scheme has been proposed and theoretically investigated where an extreme ultraviolet single attosecond pulse (SAP) propagates through dense helium gas dressed by a time-delayed femtosecond laser pulse. The laser pulse couples the 2s2p( 1 P) and 2s 2 ( 1 S) autoionizing states when the SAP excites the 2s2p state. After going through the gas, the spectral and temporal profiles of the SAP are strongly distorted. A narrowed but enhanced spike in the spectrum shows up for specific intensities and time delays of the laser, which exemplifies the control of a broadband photon wave packet by an ultrashort dressing field for the first time. We analyse the photon and electron dynamics and determine the dressing condition that maximizes this enhancement. The result demonstrates new possibilities of attosecond optical control.
Metastable states are important actors in the ionisation of atoms and molecules. Sub-femtosecond extreme ultraviolet pulses can coherently populate several transiently bound states at once, thus starting the attosecond clocks which are required to monitor and control ultrafast electronic evolution above the ionisation threshold. Here we illustrate, from a theoretical point of view, the effects coherent superpositions of 1 P o doubly excited states in the helium atom have on channel-resolved photoelectron spectra as well as on the transient absorption spectrum of the atom in the extreme ultraviolet region, when they are created by a single-attosecond pulse in the presence of a strong few-cycle near-infrared/visible pulse which acts as a probe. Interference fringes varying rapidly with the pump-probe time delay are visible in both photoelectron and transient absorption spectra. From such fringes, the wave packet itself can conceivably be reconstructed. Conversely, all observables are modulated by the characteristic beating periods of the wave packet, so that control of partial ionisation yields, branching ratios, and light absorption or amplification can be achieved.
An optical coherent control scheme has been proposed and theoretically investigated where an extreme ultraviolet single attosecond pulse (SAP) propagates through a dense helium gas dressed by a time-delayed femtosecond laser pulse. The laser pulse couples the 2s2p([superscript]1P) and 2s[superscript]2([superscript]1S) autoionizing states when the SAP excites the 2s2p state. After going through the gas, the spectral and temporal profiles of the SAP are strongly distorted. A narrowed but enhanced spike in the spectrum shows up for specific intensities and time delays of the laser, which exemplifies the control of a broadband photon wave packet by an ultrashort dressing field for the first time. We analyze the photon and electron dynamics and determine the dressing condition that maximizes this enhancement. The result demonstrates new possibilities of attosecond optical control.