Photoelectron Streaking Research Articles

Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump-probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.

Retrieval of photoionization group delay in chirped-attosecond pulses photoelectron streaking experiments

Photoionization time delays have been investigated in many streaking experiments in which an extreme ultraviolet (XUV) attosecond pulse is used to ionize the target in the presence of a dressing infrared laser field. The discrepancies between the photoionization time delays thus experiment measured and those from many sophisticated theoretical simulations have generated a great deal of controversy in recent years. The difficulty of achieving an accuracy of the retrieved time delays comes from two facts: a so-called wavepacket approximation is introduced to construct the photoionization electron wavepacket, this approximation is invalid if atto-chirp of XUV is non-zero; the other one is that the lower sensitivity of the streaking spectra to the phase of the photoionization transition dipole. Here we present a time delay retrieval method born from our recently proposed ‘phase retrieval of broadband pulses auto correlation’ (PROBP-AC) technology to overcome above limitations. We carefully exam the validity of our method and make a few compare with some other common used retrieval codes, and the simulations demonstrate that more accurate results can be retrieved using PROBP AC. Based on the present method, the angular dependent photoionization time delays can also be retrieved. Our investigation casts doubts on the measured group time delays in previous streaking experiments. We also mention here that a single photoionization group time delay at the XUV peak energy is not enough to represent a complete photoemission process; instead, a fully characterization of the photoionization group time delay over the whole bandwidth of the wave packet is required.

Near-field imaging of dipole emission modulated by an optical grating

Attosecond measurements have been achieved in technically demanding pump-probe experiments by photoelectron streaking with stable infrared lasers and extreme-ultraviolet (XUV) instruments. Here, we demonstrate an efficient single-image all-optical measurement of an isolated attosecond pulse for its complete temporal characterization. We create the attosecond pulse with a 0.1-mJ, few-cycle, infrared pump beam and modulate it with an obliquely incident same-frequency weak beam. By refocusing the XUV beams, we obtain a spectrally resolved XUV image, showing the spectral phase of the attosecond pulse. Near-field imaging allows us to measure our pulse in 150 shots. This efficiency will be important for attosecond pulses in the water-window region. For complex systems, multi-electron dynamics is encoded in the temporal structure of attosecond pulses.

Laser streaking of dissociating D2+ in a strong midinfrared laser pulse

Laser streaking of photoelectrons has been a well-established technique to extract ultrafast information. In analogy to the streaking of photoelectrons, in this paper, we study the laser streaking of nuclei by numerically simulating the time-dependent Schr\"odinger equations of ${\mathrm{D}}_{2}^{+}$ in strong laser fields. We first demonstrate the nuclear streaking, where the dissociating wave packet excited by an isolated attosecond pulse and propagating on molecular potential surfaces is streaked by a strong midinfrared laser pulse. Subsequently, we use an attosecond pulse train composing two successive attosecond pulses to initiate the dissociation of ${\mathrm{D}}_{2}^{+}$. Depending on the time delay between the attosecond pulse train and the midinfrared laser pulse, the streaked nuclear wave packets show a novel delay-dependent interference pattern. By tracing the wave-packet evolution in the adiabatic presentation, the complicated dissociation pathways are revealed. Based on the streaked nuclear pattern, one may conceive a strategy to probe the real-time nuclear streaking process and extract instantaneous dynamics information experimentally.

Investigation of isolated attosecond pulse reconstruction from angular integrated photoelectron streaking spectra

We theoretically investigate the temporal characterization of isolated attosecond pulses from angular integrated photoelectron streaking spectra. In this work, our recently developed ‘phase retrieval of broadband pulses’ (PROBP) algorithm for attosecond pulse retrieval is modified so that photoelectrons emitted over a broad angular range can be used to retrieve the spectral phase of the broadband XUV to x-ray attosecond pulses. Compared to common retrieval methods that utilize photoelectrons only in the forward direction, the inclusion of electrons over a broad angular range has the advantage that stronger signals are analyzed to mitigate the effect of low statistics of photoelectrons for analysis. The present investigation also serves to test the commonly used approximation where the photoelectrons are collected over a finite angular range experimentally, but the theoretical analysis relies on electrons emitted only in the forward direction. Our results show that the extended PROBP method can be used to more accurately retrieve the spectral phase of an attosecond pulse from photoelectrons collected over a wider angular range.

Complete phase retrieval of photoelectron wavepackets

Coherent, broadband pulses of extreme ultraviolet light provide a new and exciting tool for exploring attosecond electron dynamics. Using photoelectron streaking, interferometric spectrograms can be generated that contain a wealth of information about the phase properties of the photoionization process. If properly retrieved, this phase information reveals attosecond dynamics during photoelectron emission such as multielectron dynamics and resonance processes. However, until now, the full retrieval of the continuous electron wavepacket phase from isolated attosecond pulses has remained challenging. Here, after elucidating key approximations and limitations that hinder one from extracting the coherent electron wavepacket dynamics using available retrieval algorithms, we present a new method called absolute complex dipole transition matrix element reconstruction (ACDC). We apply the ACDC method to experimental spectrograms to resolve the phase and group delay difference between photoelectrons emitted from Ne and Ar. Our results reveal subtle dynamics in this group delay difference of photoelectrons emitted form Ar. These group delay dynamics were not resolvable with prior methods that were only able to extract phase information at discrete energy levels, emphasizing the importance of a complete and continuous phase retrieval technique such as ACDC. Here we also make this new ACDC retrieval algorithm available with appropriate citation in return.

Polarization-sensitive reconstruction of transient local THz fields at dielectric interfaces

Intense THz fields can be used to excite specific low-frequency modes in condensed matter or drive confined currents in nano-structured arrays, paving the way for a new class of optoelectronic devices. The long wavelength enables spatially confined field enhancement due to interaction with metallic structures and interference effects at dielectric interfaces. We use THz photoelectron streaking for the polarization-sensitive reconstruction of all electric field components at dielectric and metallic interfaces. This is realized in a THz/x-ray pump–probe experiment where x-ray emitted photoelectrons probe the effective THz field in close surface proximity. We observe distinct differences in the THz response of a nano-structured Pt thin film and a Pt(111) bulk crystal, in particular for the parallel field component. Simulations of the nanoscale spatial field modulation at metallic islands on the thin film provide design parameters for different sensitivity regimes with respect to local fields.

Terahertz Oscilloscope for Recording Time Information of Ultrashort Electron Beams.

We propose and demonstrate a terahertz (THz) oscilloscope for recording time information of an ultrashort electron beam. By injecting a laser-driven THz pulse with circular polarization into a dielectric tube, the electron beam is swept helically such that the time information is uniformly encoded into the angular distribution that allows one to characterize both the temporal profile and timing jitter of an electron beam. The dynamic range of the measurement in such a configuration is significantly increased compared to deflection with a linearly polarized THz pulse. With this THz oscilloscope, nearly 50-fold longitudinal compression of a relativistic electron beam to about 15fs (rms) is directly visualized with its arrival time determined with 3fs accuracy. This technique bridges the gap between streaking of photoelectrons with optical lasers and deflection of relativistic electron beams with radio-frequency deflectors, and should have wide applications in many ultrashort electron-beam-based facilities.

Method for spectral phase retrieval of single attosecond pulses utilizing the autocorrelation of photoelectron streaking spectra

We propose an algorithm for retrieving the spectral phase of an isolated attosecond pulse based on the photoelectron spectra generated in atoms by the attosecond pulse in the presence of a time-delayed infrared laser. Instead of employing the whole set of the streaking spectra as in the previous phase retrieval algorithms, we calculate the autocorrelation (AC) of the streaking trace, and use it to extract the spectral phase. We illustrate that this method can be used to extract narrow- as well as broadband attosecond pulses. The method converges much faster and is more accurate. We also define two parameters, $V$ and $A$. By comparing the AC pattern and the $V$ and $A$ parameters from the experiment and from the retrieved pulse, this method provides a metric to evaluate the accuracy of the retrieved pulse.

Atomic scale electronic structure and response in attosecond photoemission delays: A case study using non-centrosymmetric BiTeCl

Attosecond time-resolved photoemission from the differently terminated BiTeCl surfaces yield a photoelectron streaking that cannot be explained by bulk propagation effects alone. Instead, the atomic scale electronic structure and dynamical screening for both surface terminations have to be taken into account.

Terahertz Streaking of Few-Femtosecond Relativistic Electron Beams

Streaking of photoelectrons with optical lasers has been widely used for temporal characterization of attosecond extreme ultraviolet pulses. Recently, this technique has been adapted to characterize femtosecond x-ray pulses in free-electron lasers with the streaking imprinted by farinfrared and Terahertz (THz) pulses. Here, we report successful implementation of THz streaking for time-stamping of an ultrashort relativistic electron beam of which the energy is several orders of magnitude higher than photoelectrons. Such ability is especially important for MeV ultrafast electron diffraction (UED) applications where electron beams with a few femtosecond pulse width may be obtained with longitudinal compression while the arrival time may fluctuate at a much larger time scale. Using this laser-driven THz streaking technique, the arrival time of an ultrashort electron beam with 6 fs (rms) pulse width has been determined with 1.5 fs (rms) accuracy. Furthermore, we have proposed and demonstrated a non-invasive method for correction of the timing jitter with femtosecond accuracy through measurement of the compressed beam energy, which may allow one to advance UED towards sub-10 fs frontier far beyond the ~100 fs (rms) jitter.

Polarization-sensitive pulse reconstruction by momentum-resolved photoelectron streaking.

The precise knowledge of the electric field in close proximity to metallic and dielectric surfaces is a prerequisite for pump-probe experiments aiming at the control of dynamic surface processes. We describe a model to reconstruct this electric field in immediate surface proximity from data taken in photoelectron THz-streaking experiments with an angle-resolved electron analyzer. Using Monte-Carlo simulations we are able to simulate streaking experiments on arbitrary surfaces with a variety of initial electron momentum distributions and to reconstruct the effective electric field at the surface. Our results validate the approach and suggest energy regimes for optimal pulse reconstruction.

Femtosecond profiling of shaped x-ray pulses

Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques.

A new method for accurate retrieval of atomic dipole phase or photoionization group delay in attosecond photoelectron streaking experiments

In recent years, attosecond streaking experiments have been used to extract the phase of photoionization dipole transition matrix element (or the photoionization group delay) in atoms, molecules and condensed materials. The most accurate retrieval method so far is based on the frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB). However, the FROG-CRAB employs a number of approximations that would cause errors in the retrieved results. In this article, we applied our recently proposed attosecond pulse characterization method phase retrieval of broadband pulses (PROBP) to the retrieval of photoionization group delay difference between Ne() and Ne() ionization channels, and between Ar() and Ne() channels. Our simulations demonstrate that more accurate results can be retrieved using PROBP than using FROG-CRAB, and cast doubts on group delays reported in previous streaking measurements.

Asymmetry in attosecond streaking from a degenerate state

By the numerical solution to the time-dependent Schr\"odinger equation, we observe a remarkable asymmetry in the photoelectron streaking spectra when the ${\mathrm{He}}^{+}$ ion is ionized by an attosecond pulse from the $2s$ state in the presence of a weak infrared pulse. An analytical theory based on the modified perturbation theory is developed to quantitatively account for the underlying mechanism. We find that a transition occurs between the degenerate $2s$ and $2p$ states under the influence of the infrared electric field before the arrival of the attosecond pulse. The observed asymmetry in the attosecond streaking is formed by the interference of photoelectrons ionized from the $2s$ and $2p$ states. Using the sensitivity of the asymmetry to the infrared (IR) electric field at the moment of attosecond photoionization, we show that the present finding provides a highly reliable way to fully characterize the IR waveform in the streaking experiment.

Phase space manipulation of free-electron pulses from metal nanotips using combined terahertz near fields and external biasing

We present a comprehensive experimental and numerical study of photoelectron streaking at metallic nanotips using single-cycle Terahertz (THz) transients and a static bias voltage as an external control parameter. Analyzing bias voltage dependent streaking spectrograms, we explore the THz-induced reshaping of photoelectron energy spectra, governed by the superimposed static field. Numerical simulations are employed to determine the local field strengths and spatial decay lengths of the field contributions, demonstrating electron trajectory control and the manipulation of the phase space distributions in confined fields with both dynamic and static components.

Retrieving plasmonic near-field information: A quantum-mechanical model for streaking photoelectron spectroscopy of gold nanospheres

Streaked photoemission from nanostructures is characterized by size- and material-dependent nanometer-scale variations of the induced nanoplasmonic response to the electronic field of the streaking pulse and thus holds promise of allowing photoelectron imaging with both subfemtosecond temporal and nanometer spatial resolution. In order to scrutinize the driven collective electronic dynamics in 10--200-nm-diameter gold nanospheres, we calculated the plasmonic field induced by streaking pulses in the infrared and visible spectral range and developed a quantum-mechanical model for streaked photoemission by extreme ultraviolet pulses. Our simulated photoelectron spectra reveal a significant amplitude enhancement and phase shift of the photoelectron streaking trace relative to calculations that exclude the induced plasmonic field. Both are most pronounced for streaking pulses tuned to the plasmon frequency and retrace the plasmonic electromagnetic field enhancement and phase shift near the nanosphere surface.

Attosecond photoelectron streaking with enhanced energy resolution for small-bandgap materials.

Attosecond photoelectron streaking spectroscopy allows time-resolved electron dynamics with a temporal resolution approaching the atomic unit of time. Studies have been performed in numerous systems, including atoms, molecules, and surfaces, and the quest for ever higher temporal resolution called for ever wider spectral extent of the attosecond pulses. For typical experiments relying on attosecond pulses with a duration of 200as, the time-bandwidth limitation for a Gaussian pulse implies a minimal spectral bandwidth larger than 9eV translating to a corresponding spread of the detected photoelectron kinetic energies. Here, by utilizing a specially tailored narrowband reflective XUV multilayer mirror, we explore experimentally the minimal spectral width compatible with attosecond time-resolved photoelectron spectroscopy while obtaining the highest possible spectral resolution. The validity of the concept is proven by recording attosecond electron streaking traces from the direct semiconductor gallium arsenide (GaAs), with a nominal bandgap of 1.42eV at room temperature, proving the potential of the approach for tracking charge dynamics also in these technologically highly relevant materials that previously have been inaccessible to attosecond science.

Monitoring Nonadiabatic Electron-Nuclear Dynamics in Molecules by Attosecond Streaking of Photoelectrons.

Streaking of photoelectrons has long been used for the temporal characterization of attosecond extreme ultraviolet pulses. When the time-resolved photoelectrons originate from a coherent superposition of electronic states, they carry additional phase information, which can be retrieved by the streaking technique. In this contribution we extend the streaking formalism to include coupled electron and nuclear dynamics in molecules as well as initial coherences. We demonstrate how streaked photoelectrons offer a novel tool for monitoring nonadiabatic dynamics as it occurs in the vicinity of conical intersections and avoided crossings. Streaking can provide high time resolution direct signatures of electronic coherences, which affect many primary photochemical and biological events.

Critical evaluation of attosecond time delays retrieved from photoelectron streaking measurements

A photoelectron streaking experiment which was conceived as a means to extract the electron wave packet of single-photon ionization has also been employed to retrieve time delays in the fundamental photoemission processes. The discrepancies between the time delays thus measured and those from many sophisticated theoretical calculations have generated a great deal of controversy in recent years. Here we present a careful examination of the methods that were used to retrieve the time delays and demonstrate the difficulty of achieving an accuracy of the retrieved time delays of a few to tens of attoseconds in typical streaking measurements. The difficulty owes more to the lower sensitivity of the streaking spectra to the phase of the photoionization transition dipole than to the spectral phase of the attosecond light pulse in the experiment. The retrieved time delay contains extra errors when the attochirp of the attosecond pulse is large so that the dipole phase becomes negligible compared to it.