Attosecond Precision Research Articles

Attosecond emission delay from atoms and molecules using multi-dimensional XUV interferometry.

Abstract Multi-dimensional interferometry of high harmonic generation is
demonstrated using an inline Gouy interferometer. The rich data sets acquired with this technique, coupled with its inherent stability, enable robust phase analysis to be performed, allowing the delay in harmonic emission from a range of atomic and molecular gases to be measured relative to harmonic emission from argon with single-digit attosecond precision. Delays of −78 ± 6 as, −25 ± 6 as, 30 ± 3 as and 60 ± 3 as relative to emission from argon were measured for Xe, Kr, CO2 and N2, respectively. The scheme can easily be incorporated in other high harmonic experiments, such as tomographic imaging of molecular orbitals, and provides an experimentally simple route towards probing ultrafast dynamics in molecular systems.

Dual-oscillator infrared electro-optic sampling with attosecond precision

Electro-optic sampling of infrared electric fields has set sensitivity and dynamic-range records in broadband molecular vibrational spectroscopy. Yet, in these works, the 1-second-scale single-trace acquisition time leads to intra-scan noise accumulation and restricts the throughput in measurements of multiple samples and of dynamic processes. We present a dual-laser-oscillator approach capturing 2800 mid-infrared waveforms per second by scanning the relative delay between the sampled waveform and the gate pulses using a modulated repetition-frequency lock. The new technique of electro-optic delay tracking (EODT) provides delay calibration with down to few-attosecond precision and provides a general route to high-precision dual-oscillator spectroscopy with picosecond delay ranges. Our work has immediate applications in, e.g., precision electric-field metrology and high-speed biosensing.

Controlling electron motion with attosecond precision by a shaped femtosecond intense laser pulse

Controlling electron motion with attosecond precision by a shaped femtosecond intense laser pulse

Tracing attosecond electron emission from a nanometric metal tip.

Solids exposed to intense electric fields release electrons through tunnelling. This fundamental quantum process lies at the heart of various applications, ranging from high brightness electron sources in d.c. operation1,2 to petahertz vacuum electronics in laser-driven operation3-8. In the latter process, the electron wavepacket undergoes semiclassical dynamics9,10 in the strong oscillating laser field, similar to strong-field and attosecond physics in the gas phase11,12. There, the subcycle electron dynamics has been determined with a stunning precision of tens of attoseconds13-15, but at solids the quantum dynamics including the emission time window has so far not been measured. Here we show that two-colour modulation spectroscopy of backscattering electrons16 uncovers the suboptical-cycle strong-field emission dynamics from nanostructures, with attosecond precision. In our experiment, photoelectron spectra of electrons emitted from a sharp metallic tip are measured as function of the relative phase between the two colours. Projecting the solution of the time-dependent Schrödinger equation onto classical trajectories relates phase-dependent signatures in the spectra to the emission dynamics and yields an emission duration of 710 ± 30 attoseconds by matching the quantum model to the experiment. Our results open the door to the quantitative timing and precise active control of strong-field photoemission from solid state and other systems and have direct ramifications for diverse fields such as ultrafast electron sources17, quantum degeneracy studies and sub-Poissonian electron beams18-21, nanoplasmonics22 and petahertz electronics23.

Ultrafast energy exchange between two single Rydberg atoms on a nanosecond timescale

Rydberg atoms, with their enormous electronic orbitals, exhibit dipole–dipole interactions reaching the gigahertz range at a distance of a micrometre, making them a prominent contender for realizing ultrafast quantum operations. However, such strong interactions between two single atoms have so far never been harnessed due to the stringent requirements on the fluctuation of the atom positions and the necessary excitation strength. Here we introduce novel techniques to explore this regime. First, we trap and cool atoms to the motional quantum ground state of holographic optical tweezers, which allows control of the inter-atomic distance down to 1.5 μm with a quantum-limited precision of 30 nm. We then use ultrashort laser pulses to excite a pair of these nearby atoms to a Rydberg state simultaneously, far beyond the Rydberg blockade regime, and perform Ramsey interferometry with attosecond precision. This allows us to induce and track an ultrafast interaction-driven energy exchange completed on nanosecond timescales—two orders of magnitude faster than in any other Rydberg experiments in the tweezers platform so far. This ultrafast coherent dynamics gives rise to a conditional phase, which is the key resource for a quantum gate, opening the path for quantum simulation and computation operating at the speed limit set by dipole–dipole interactions with this ultrafast Rydberg platform.

Attosecond investigation of extreme-ultraviolet multi-photon multi-electron ionization

Multi-electron dynamics in atoms and molecules very often occur on sub- to few-femtosecond time scales. The available intensities of extreme-ultraviolet (XUV) attosecond pulses have previously allowed the time-resolved investigation of two-photon, two-electron interactions. Here we study double and triple ionization of argon atoms involving the absorption of up to five XUV photons using a pair of intense attosecond pulse trains (APTs). By varying the time delay between the two APTs with attosecond precision and the spatial overlap with nanometer precision, we obtain information on complex nonlinear multi-photon ionization pathways. Our experimental and numerical results show that A r 2 + is predominantly formed by a sequential two-photon process, whereas the delay dependence of the A r 3 + ion yield exhibits clear signatures of the involvement of a simultaneous two-photon absorption process. Our experiment suggests that it is possible to investigate multi-electron dynamics using attosecond pulses for both pumping and probing the dynamics.

Subcycle terahertz field waveforms clocked by attosecond high-harmonic pulses from relativistic laser plasmas

A high-intensity ultrashort laser pulse interacting with a thin plasma target is shown to couple to plasma electrons, driving electron oscillations within the plasma and making these electrons bounce back and forth between plasma boundaries. Each time these recirculating electrons traverse the plasma boundary, they emit bright subcycle terahertz (THz) field waveforms via laser-driven coherent transition radiation. As a concurrent process, laser-driven electrons near the front surface of the plasma target are accelerated to relativistic velocities to emit high-order harmonics (HHs), giving rise to attosecond pulses of vacuum-ultraviolet radiation. These attosecond pulses are shown to provide a high-precision clock for subcycle THz field waveforms. We demonstrate that the delay time between HH pulses and THz waveforms can be tuned with an attosecond precision by varying the thickness of the plasma target, thus opening an avenue toward HH-pump–THz-probe studies of ultrafast processes on the attosecond time scale with table-top laser sources.

Ultrafast microscopy of a twisted plasmonic spin skyrmion

We report a transient plasmonic spin skyrmion topological quasiparticle within surface plasmon polariton vortices, which is described by analytical modeling and imaging of its formation by ultrafast interferometric time-resolved photoemission electron microscopy. Our model finds a twisted skyrmion spin texture on the vacuum side of a metal/vacuum interface and its integral opposite counterpart in the metal side. The skyrmion pair forming a hedgehog texture is associated with co-gyrating anti-parallel electric and magnetic fields, which form intense pseudoscalar E·B focus that breaks the local time-reversal symmetry and can drive magnetoelectric responses of interest to the axion physics. Through nonlinear two-photon photoemission, we record attosecond precision images of the plasmonic vectorial vortex field evolution with nanometer spatial and femtosecond temporal (nanofemto) resolution, from which we derive the twisted plasmonic spin skyrmion topological textures, their boundary, and topological charges; the modeling and experimental measurements establish a quantized integer photonic topological charge that is stable over the optical generation pulse envelope.

Table-top interferometry on extreme time and wavelength scales.

Short-pulse metrology and dynamic studies in the extreme ultraviolet (XUV) spectral range greatly benefit from interferometric measurements. In this contribution a Michelson-type all-reflective split-and-delay autocorrelator operating in a quasi amplitude splitting mode is presented. The autocorrelator works under a grazing incidence angle in a broad spectral range (10 nm - 1 μm) providing collinear propagation of both pulse replicas and thus a constant phase difference across the beam profile. The compact instrument allows for XUV pulse autocorrelation measurements in the time domain with a single-digit attosecond precision and a useful scan length of about 1 ps enabling a decent resolution of E/ΔE = 2000 at 26.6 eV. Its performance for selected spectroscopic applications requiring moderate resolution at short wavelengths is demonstrated by characterizing a sharp electronic transition at 26.6 eV in Ar gas. The absorption of the 11th harmonic of a frequency-doubled Yb-fiber laser leads to the well-known 3s3p64p1P1 Fano resonance of Ar atoms. We benchmark our time-domain interferometry results with a high-resolution XUV grating spectrometer and find an excellent agreement. The common-path interferometer opens up new opportunities for short-wavelength femtosecond and attosecond pulse metrology and dynamic studies on extreme time scales in various research fields.

Realization of a Continuously Phase-Locked Few-Cycle Deep-UV/XUV Pump-Probe Beamline with Attosecond Precision for Ultrafast Spectroscopy

Chemical and physical processes in molecules can be controlled through the manipulation of quantum interferences between rotational, vibrational, and electronic degrees of freedom. Most of the past efforts have been focused on the control of nuclear dynamics. Even though electronic coherence and its coupling to nuclear degrees of freedom may profoundly affect the outcome of these processes, electron dynamics have received less attention. Proper investigation of electron dynamics in materials demands ultrafast sources in the visible, ultraviolet (UV), and extreme ultraviolet (XUV) spectral region. For this purpose, a few-cycle deep-UV and XUV beamlines have been constructed for studying ultrafast electron dynamics in molecules. To ensure the required high temporal resolution on the attosecond time scale, vibration isolation from environmental mechanical noise and active stabilization have been implemented to achieve attosecond timing control between pump and probe pulses with excellent stability. This is achieved with an actively phase-stabilized double-layer Mach-Zehnder interferometer system capable of continuous time-delay scans over a range of 200 fs with a root-mean-square timing jitter of only 13 as over a few seconds and ~80 as of peak-to-peak drift over several hours.

Ultra-rapid electro-optic sampling of octave-spanning mid-infrared waveforms.

We demonstrate ultra-rapid electro-optic sampling (EOS) of octave-spanning mid-infrared pulses centered at 9 μm, implemented by mechanically scanning a mirror with a sonotrode resonating at 19 kHz (forward and backward acquisition at 38 kHz). The instrument records the infrared waveform with a spectral intensity dynamic range of 1.6 × 105 for a single scan over a 1.6-ps delay range, acquired within 26 μs. The purely reflective nature of the delay scanning technique is compatible with broad optical bandwidths, short pulse durations (16 fs, centered at 1030 nm) and high average powers (Watt-level). Interferometric tracking of the sonotrode motion in combination with a predictor-corrector algorithm allows for delay-axis determination with down to single-digit attosecond precision. Ultra-rapid mid-infrared EOS will advance applications such as molecular fingerprinting of static samples as well as tracking of biological processes and chemical reactions and is likely to find new fields of application such as infrared-spectroscopic flow cytometry.

Attosecond intra-valence band dynamics and resonant-photoemission delays in W(110)

Time-resolved photoelectron spectroscopy with attosecond precision provides new insights into the photoelectric effect and gives information about the timing of photoemission from different electronic states within the electronic band structure of solids. Electron transport, scattering phenomena and electron-electron correlation effects can be observed on attosecond time scales by timing photoemission from valence band states against that from core states. However, accessing intraband effects was so far particularly challenging due to the simultaneous requirements on energy, momentum and time resolution. Here we report on an experiment utilizing intracavity generated attosecond pulse trains to meet these demands at high flux and high photon energies to measure intraband delays between sp- and d-band states in the valence band photoemission from tungsten and investigate final-state effects in resonant photoemission.

Coherent scattering of an optically modulated electron beam by atoms

Recent technological advances allowed the coherent optical manipulation of high-energy electron wavepackets with attosecond precision. Here we theoretically investigate the collision of optically-modulated pulsed electron beams with atomic targets and reveal a quantum interference associated with different momentum components of the incident broadband electron pulse, which coherently modulates both the elastic and inelastic scattering cross sections. We show that the quantum interference has a high spatial sensitivity at the level of Angstroms, offering potential applications in high-resolution ultrafast electron microscopy. Our findings are rationalized by a simple model.

Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction

Ubiquitous to most molecular scattering methods is the challenge to retrieve bond distance and angle from the scattering signals since this requires convergence of pattern matching algorithms or fitting methods. This problem is typically exacerbated when imaging larger molecules or for dynamic systems with little a priori knowledge. Here, we employ laser-induced electron diffraction (LIED) which is a powerful means to determine the precise atomic configuration of an isolated gas-phase molecule with picometre spatial and attosecond temporal precision. We introduce a simple molecular retrieval method, which is based only on the identification of critical points in the oscillating molecular interference scattering signal that is extracted directly from the laboratory-frame photoelectron spectrum. The method is compared with a Fourier-based retrieval method, and we show that both methods correctly retrieve the asymmetrically stretched and bent field-dressed configuration of the asymmetric top molecule carbonyl sulfide (OCS), which is confirmed by our quantum-classical calculations.

Direct measurement of Coulomb-laser coupling

The Coulomb interaction between a photoelectron and its parent ion plays an important role in a large range of light-matter interactions. In this paper we obtain a direct insight into the Coulomb interaction and resolve, for the first time, the phase accumulated by the laser-driven electron as it interacts with the Coulomb potential. Applying extreme-ultraviolet interferometry enables us to resolve this phase with attosecond precision over a large energy range. Our findings identify a strong laser-Coulomb coupling, going beyond the standard recollision picture within the strong-field framework. Transformation of the results to the time domain reveals Coulomb-induced delays of the electrons along their trajectories, which vary by tens of attoseconds with the laser field intensity.

Coherent control of boosted terahertz radiation from air plasma pumped by a femtosecond three-color sawtooth field

We report on the coherent control of boosted ultrashort terahertz pulses emitted from air plasma pumped by a femtosecond sawtooth pulse composed of three optical harmonic fields. With an inline optical setup, the relative phases between the three optical components of the pump pulse can be varied independently with attosecond precision without the complexity of external phase stabilization. We observe that the amplitude of the terahertz emission pumped by the phase-optimized sawtooth wave is enhanced by 1.8 times compared to the widely used two-color scheme. Moreover, by manipulating the relative phases between the three colors at attosecond precision, coherent control of the azimuthal angle, ellipticity, and polarity of terahertz radiation is achieved. A local current model reproduces the coherent control observations well.

Electron correlation effects in attosecond photoionization of CO2

A technique for measuring photoionization time delays with attosecond precision is combined with calculations of photoionization matrix elements to demonstrate how multi-electron dynamics affect photoionization time delays in carbon dioxide. Electron correlation is observed to affect the time delays through two mechanisms: autoionization of molecular Rydberg states and accelerated escape from a continuum shape resonance.

Spatial molecular interferometry via multidimensional high-harmonic spectroscopy

Interferometry is a basic tool to resolve coherent properties in a wide range of light or matter wave phenomena. In the strong-field regime, interferometry serves as a fundamental building block in revealing ultrafast electron dynamics. In this work we manipulate strong-field-driven electron trajectories and probe the coherence of a molecular wavefunction by inducing an interferometer on a microscopic level. The two arms of the interferometer are controlled by a two-colour field, while the interference pattern is read via advanced, three-dimensional high-harmonic spectroscopy. This scheme recovers the spectral phase information associated with the structure of molecular orbitals, as well as the spatial properties of the interaction itself. Zooming into one of the most fundamental strong-field phenomena—field-induced tunnel ionization—we reconstruct the angle at which the electronic wavefunction tunnels through the barrier and follow its evolution with attosecond precision. A single-molecule attosecond interferometry that can retrieve the spectral phase information associated with the structure of molecular orbitals, as well as the phase accumulated by an electron as it tunnels out, is demonstrated.

Optical field tuning of localized plasmon modes in Ag microcrystals at the nanofemto scale.

Nanoscale plasmonic field enhancement at sub-wavelength metallic particles is crucial for surface sensitive spectroscopy, ultrafast microscopy, and nanoscale energy transduction. Here, we demonstrate control of the spatial distribution of localized surface plasmon modes at sub-optical-wavelength crystalline silver (Ag) micropyramids grown on a Si(001) surface. We employ multiphoton photoemission electron microscopy (mP-PEEM) to image how the plasmonic field distributions vary with the photon energy, light polarization, and phase in coherent two-pulse excitation. For photon energy hυ > 2.0 eV, the mP-PEEM images show single photoemission locus, which splits into a dipolar pattern that straddles the Ag crystal at a lower energy. We attribute the variation to the migration of plasmon resonances from the Ag/vacuum to the Ag/Si interfaces by choice of the photon energy. Furthermore, the dipolar response of the Ag/Si interface follows the polarization state of light: for linearly polarized excitations, the plasmon dipole follows the in-plane electric field vector, while for circularly polarized excitations, it tilts in the direction of the handedness due to the conversion of spin angular momentum of light into orbital angular momentum of the plasmons excited in the sample. Finally, we show the coherent control of the spatial plasmon distribution by exciting the sample with two identical circularly polarized light pulses with delay defined with attosecond precision. The near field distribution wobbles at the pyramid base as the pump-probe delay is advanced due to interferences among the contributing fields. We illustrate how the frequency, polarization, and pulse structure can be used to design and control plasmon fields on the nanofemto scale for applications in chemistry and physics.

极低时间抖动的飞秒激光技术（特邀）

The time jitter of a femtosecond laser is the short-term deviation of the optical pulse position relative to its ideal equally spaced pulse position. Femtosecond lasers emit uniformly spaced ultrashort pulse train. The quantum-noise-limited timing jitter can be as low as few tens of attoseconds in millisecond time scale. This unique property and its advanced applications constitute a new branch of ultrafast research, Attosecond precision ultrafast photonics. In this paper, the recent advances in femtosecond laser timing jitter research, high-precision timing jitter characterization methods, and the ultralow timing jitter that can be achieved by different kinds of femtosecond laser sources were reviewed. Finally, the application of low-jitter femtosecond lasers in the fields of synchronization of large-scale scientific instruments, high-speed analog-to-digital conversion, absolute ranging technology and coherent beam combination are introduced.