Attosecond transient absorption spectra near the energies of autoionizing states are analyzed in terms of the photon coupling mechanisms to other states. In a recent experiment, the autoionization lifetimes of highly excited states of xenon were determined and compared to a simple expression based on a model of how quantum coherence determines the decay of a metastable state in the transient absorption spectrum. Here it is shown that this procedure for extracting lifetimes is more general and can be used in cases involving either resonant or nonresonant coupling of the attosecond-probed autoionizing state to either continua or discrete states by a time-delayed near infrared (NIR) pulse. The fits of theoretically simulated absorption signals for the 6p resonance in xenon (lifetime = 21.1 fs) to this expression yield the correct decay constant for all the coupling mechanisms considered, properly recovering the time signature of twice the autoionization lifetime due to the coherent nature of the transient absorption experiment. To distinguish between these two coupling cases, the characteristic dependencies of the transient absorption signals on both the photon energy and time delay are investigated. Additional oscillations versus delay-time in the measured spectrum are shown and quantum beat analysis is used to pinpoint the major photon-coupling mechanism induced by the NIR pulse in the current xenon experiment: the NIR pulse resonantly couples the attosecond-probed state, 6p, to an intermediate 8s (at 22.563 eV), and this 8s state is also coupled to a neighboring state (at 20.808 eV).
We propose and study the manipulation of the macroscopic transient absorption of an ensemble of open two-level systems via temporal engineering. The key idea is to impose an ultrashort temporal gate on the polarization decay of the system by transient absorption spectroscopy, thus confining its free evolution and the natural reshaping of the excitation pulse. The numerical and analytical results demonstrate that even at moderate optical depths, the resonant absorption of light can be reduced or significantly enhanced by more than 5 orders of magnitude relative to that without laser manipulation. The achievement of the quasicomplete extinction of light at the resonant frequency, here referred to as resonant perfect absorption, arises from the full destructive interference between the excitation pulse and its subpulses developed and tailored during propagation, and is revealed to be connected with the formation of zero-area pulses in the time domain.
Tunneling plays a central role in the interaction of matter with intense laser pulses, and also in time-resolved measurements on the attosecond timescale. A strong laser field influences the binding potential of an electron in an atom so strongly, that a potential barrier is created which enables the electron to be liberated through tunneling. An important aspect of the tunneling is the geometry of the tunneling current flow. Here we provide experimental access to the tunneling geometry and provide a full understanding of the laser induced tunnel process in space and time. We perform laser tunnel ionization experiments using the attoclock technique, and present a correct tunneling geometry for helium and argon. In addition for argon the potential barrier is significantly modified by all the electrons remaining in the ion, and furthermore from a quantum state whose energy is Stark-shifted by the external field. The resulting modified potential geometry influences the dynamics of the liberated electron, changes important physical parameters and affects the interpretation of attosecond measurements. These effects will become even more pronounced for molecules and surfaces, which are more polarizable.
We obtain a probability distribution of Rydberg yield that shows close agreement with recent experimental results. Contrary to general expectations, we find that rescattering is not a significant mechanism in the creation of excited neutrals.
We present an ellipticity-resolved study of momentum distributions arising from strong-field ionization of helium. The influence of the ion potential on the departing electron is considered within a semiclassical model consisting of an initial tunneling step and subsequent classical propagation. We find that the momentum distribution can be explained by including the longitudinal momentum spread of the electron at the exit from the tunnel. Our combined experimental and theoretical study provides an estimate of this momentum spread.
Laser pulses with a duration of one femtosecond or shorter can be generated both in the visible-infrared (Vis-IR) and in the extreme UV, but the deep UV is a spectral region where such extremely short pulses have not yet been demonstrated. Here, a method for the synthesis of ultrashort pulses in the deep UV is demonstrated, which utilizes the temporal and spatial harmonics that are generated by two noncollinear Vis-IR pulses in a thin MgF2 plate. By controlling the groove-envelope phase of the Vis-IR pulses, spatial harmonics are concatenated to form deep UV waveforms with a duration of 1.5 fs.
In this paper we measured an "instantaneous" intensity independent tunneling delay time with an upper limit of 12 as [3]. Our experiments have given us direct access to the tunneling delay time with an unprecedented time accuracy of a few tens of attoseconds using attosecond angular streaking. Our results give strong indication that there is no real tunneling delay time and we expect that this will shed some light on the ongoing theoretical discussion on tunneling time and tunnel ionization in strong field physics.
It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 x 10(14) to 3.5 x 10(14) watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector.
