XUV Pulse Duration Research Articles

Ultrafast extreme ultraviolet photoemission without space charge.

Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers >1011 photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of 58 × 100 μm2. From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon energy bandwidth of 93 fs and 65 meV, respectively. The high photocurrent, lack of strong space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced modifications of the photoelectron spectra at the 10−4 level, enabling time-resolved XUV photoemission experiments in a qualitatively new regime.

Angular streaking and sideband formation in rotating terahertz and far-infrared fields

Electron spectra in atomic ionization by extreme ultraviolet (XUV) or soft x-ray femtosecond pulses, in the presence of a strong circularly polarized far-infrared or terahertz (THz) field are theoretically investigated. When the XUV pulse duration is much shorter than the period of the THz field, the conditions for circular streaking are fulfilled. The peculiarities of the circular streaking are discussed. In the other extreme case when the duration of the XUV pulse is longer than the THz field period, an analog of the sidebands is formed. Special attention is paid to the intermediate case when the duration of the XUV pulse is similar to the THz period. In this case, a well-developed interference structure appears in the photoelectron spectra. A quasiclassical description of this interference structure is presented and compared with numerical calculations based on a quantum-mechanical approach.

A table-top monochromator for tunable femtosecond XUV pulses generated in a semi-infinite gas cell: Experiment and simulations.

We present a new design of a time-preserving extreme-ultraviolet (XUV) monochromator using a semi-infinite gas cell as a source. The performance of this beamline in the photon-energy range of 20 eV-42 eV has been characterized. We have measured the order-dependent XUV pulse durations as well as the flux and the spectral contrast. XUV pulse durations of ≤40 fs using 32 fs, 800 nm driving pulses were measured on the target. The spectral contrast was better than 100 over the entire energy range. A simple model based on the strong-field approximation is presented to estimate different contributions to the measured XUV pulse duration. On-axis phase-matching calculations are used to rationalize the variation of the photon flux with pressure and intensity.

Photoelectron sidebands induced by a chirped laser field for shot-by-shot temporal characterization of FEL pulses

We theoretically investigate the laser-assisted photoionization of He by an extreme ultra violet (XUV) pulse in the presence of a linearly chirped intense laser pulse by solving the time-dependent Schrödinger equation within the single-active-electron approximation. Analysis based on the time-dependent perturbation theory is also carried out to provide more physical insights. A new scheme is shown to be capable of extracting the arrival time of an XUV free-electron laser (FEL) pulse relative to an external laser pulse as well as the XUV pulse duration from the photoelectron sidebands resulting from XUV ionization in the presence of a chirped laser pulse. This scheme is independent of the energy fluctuation and the timing jittering of the FEL pulse. Therefore it can be implemented in a non-invasive way to characterize FEL pulses on a shot-by-shot basis in time-resolved spectroscopy.

HELIOS--A laboratory based on high-order harmonic generation of extreme ultraviolet photons for time-resolved spectroscopy.

In this paper, we present the HELIOS (High Energy Laser Induced Overtone Source) laboratory, an in-house high-order harmonic generation facility which generates extreme ultraviolet (XUV) photon pulses in the range of 15-70 eV with monochromatized XUV pulse lengths below 35 fs. HELIOS is a source for time-resolved pump-probe/two-color spectroscopy in the sub-50 fs range, which can be operated at 5 kHz or 10 kHz. An optical parametric amplifier is available for pump-probe experiments with wavelengths ranging from 240 nm to 20,000 nm. The produced XUV radiation is monochromatized by a grating in the so-called off-plane mount. Together with overall design parameters, first monochromatized spectra are shown with an intensity of 2 ⋅ 10(10) photons/s (at 5 kHz) in the 29th harmonic, after the monochromator. The XUV pulse duration is measured to be <25 fs after monochromatization.

Breakdown of the x-ray resonant magnetic scattering signal during intense pulses of extreme ultraviolet free-electron-laser radiation.

We present results of single-shot resonant magnetic scattering experiments of Co/Pt multilayer systems using 100 fs long ultraintense pulses from an extreme ultraviolet (XUV) free-electron laser. An x-ray-induced breakdown of the resonant magnetic scattering channel during the pulse duration is observed at fluences of 5 J/cm(2). Simultaneously, the speckle contrast of the high-fluence scattering pattern is significantly reduced. We performed simulations of the nonequilibrium evolution of the Co/Pt multilayer system during the XUV pulse duration. We find that the electronic state of the sample is strongly perturbed during the first few femtoseconds of exposure leading to an ultrafast quenching of the resonant magnetic scattering mechanism.

Photoionization of hydrogen by a chirped, short X-ray pulse in the presence of a laser field

The ionization of hydrogen by a chirped XUV pulse in the presence of a few cycle infrared laser pulse is investigated. It is found that the combined action of the chirped pulse and the laser field brings about asymmetries in the photoelectron momentum distribution that may be exploited for obtaining information on both the chirp and XUV pulse duration.

Asymmetries in the momentum distributions of electrons stripped by a XUV chirped pulse in the presence of a laser field

The ionization of hydrogen by a chirped XUV pulse in the presence of a few cycle infrared laser pulse has been investigated. The electron momentum distribution has been obtained by treating the interaction of the atom with the XUV radiation at the first order of the time-dependent perturbation theory and describing the emitted electron through the Coulomb-Volkov wavefunction. The results of the calculations agree with the ones found by solving numerically the time-dependent Schrödinger equation. It has been found that depending on the delay between the pulses the combined effect of the XUV chirp and of the steering action on the infrared field brings about asymmetries in the electron momentum distribution. These asymmetries may give information on both the chirp and the XUV pulse duration.

Isolated attosecond electron wave packet diffraction

Wave-particle duality is one of the most fundamental and mysterious natures of matters. Here, we present an interesting scheme of isolated electron wave packet diffraction with a few-cycle laser pulse and an extreme ultraviolet (XUV) pulse. The diffraction fringes are clearly present in the laser dressed XUV photoelectron spectra, strongly resembling the Airy diffraction pattern of optical waves. This phenomenon suggests a great potential of attosecond diffractometry. According to this scheme we also propose a simple method to determine the XUV pulse duration from the photoelectron spectra with a rather high resolution.

Extreme-ultraviolet pump–probe studies of one-femtosecond-scale electron dynamics

Pump–probe measurements are now an essential tool for investigating ultrafast dynamics in atoms and molecules. A lack of sources producing high-intensity attosecond pulses of extreme-ultraviolet (EUV) light has, however, hindered progress. Now, a technique that induces nonlinear processes with EUV light is demonstrated that could circumvent this problem. Ultrafast-dynamics studies and femtosecond-pulse metrology both rely on the nonlinear processes induced solely by an incident light pulse. Extending these approaches to the extreme-ultraviolet (XUV) spectral region would open up a new route to attosecond-scale dynamics. However, this has been hindered by the limited intensities available in coherent XUV continua. In the present work, we realized conditions at which simultaneous ejection of two bound electrons by two-XUV-photon absorption becomes more efficient than their removal one-by-one. In this regime we have succeeded in tracing atomic coherences evolving at the 1-fs scale with simultaneous determination of the average XUV-pulse duration. The rich and dense structure of the autoionizing manifold demonstrates the applicability of the approach to complex systems. This initiates the era of XUV-pump–XUV-probe experiments at the boundary between femto- and attosecond scales.

Temporal and spectral studies of high-order harmonics generated by polarization-modulated infrared fields

The temporal confinement of high harmonic generation (HHG) via modulation of the polarization of the fundamental pulse is studied in both temporal and spectral domains. In the temporal domain, a collinear cross-correlation setup using a $40\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ IR pump for the HHG and a $9\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ IR pulse to probe the generated emission is used to measure the XUV pulse duration. The observed temporal confinement is found to be consistent with theoretical predictions. An increased confinement is observed when a $9\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulse is used to generate the harmonics. An important spectral broadening, including a continuum background, is also measured. Theoretical calculations show that with $10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ driving pulses, either one or two main attosecond pulses are created depending on the value of the carrier envelope phase.

Time-resolved measurements of high order harmonics confined by polarization gating

We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration.

Quantum theory of attosecond XUV pulse measurement by laser dressed photoionization.

The first reported measurements of single attosecond pulses use laser dressed single-photon extreme ultraviolet (XUV) ionization of gas atoms. The determination of XUV pulse duration from the electron spectrum is based on a classical theory. Although classical models are known to give a qualitatively correct description of strong laser atom interaction, the validity must be scrutinized by a quantum-mechanical analysis. We establish a theoretical framework for the accurate temporal characterization of attosecond XUV pulses. Our analysis reveals an improved scheme that allows for direct experimental discrimination between single and multiple attosecond pulses.