Attosecond Resolution Research Articles

Laser-induced electron diffraction: Imaging of a single gas-phase molecular structure with one of its own electrons.

Among the many methods to image molecular structure, laser-induced electron diffraction (LIED) can image a single gas-phase molecule by locating all of a molecule's atoms in space and time. The method is based on attosecond electron recollision driven by a laser field and can reach attosecond temporal resolution. Implementation with a mid-IR laser and cold-target recoil ion-momentum spectroscopy, single molecules are measured with picometer resolution due to the keV electron impact energy without ensemble averaging or the need for molecular orientation. Nowadays, the method has evolved to detect single complex and chiral molecular structures in 3D. The review will touch on the various methods to discuss the implementations of LIED toward single-molecule imaging and complement the discussions with noteworthy experimental findings in the field.

Unveiling out-of-loop attosecond timing jitter precision in Ti:sapphire mode-locked lasers with an optical heterodyne technique.

The out-of-loop timing jitter exhibited in free-running Ti:sapphire mode-locked lasers with attosecond resolution is demonstrated using an optical heterodyne technique. To assess the feasibility of the experiment and discrimination signal properties, numerical simulations were conducted for Ti:sapphire mode-locked lasers. For accurately characterizing the genuine phase noise exhibited by Ti:sapphire mode-locked lasers, out-of-loop measurements were conducted, and a straightforward yet improved optical heterodyne setup was employed, allowing simultaneous low-bandwidth locking and out-of-loop timing jitter measurements with two Ti:sapphire mode-locked lasers. The out-of-loop phase noise floor for a single mode-locked laser reaches -203.47 d B c/H z, assuming a 10 GHz carrier frequency. Additionally, the out-of-loop integrated timing jitter is 11.9 a s from 10 kHz to the Nyquist frequency (50 M H z).

Tailoring quantum trajectories for strong-field imaging

Strong-field imaging techniques such as laser-induced electron diffraction (LIED) provide unprecedented combined picometer spatial and attosecond temporal resolution by “self-imaging” a molecular target with its own rescattering electrons. Accessing the rich information contained in these experiments requires the ability to accurately manipulate the dynamics of these electrons—namely, their ionization amplitudes, and times of ionization and rescattering—with attosecond to femtosecond precision. The primary challenge is imposed by the multitude of quantum pathways of the photoelectron, reducing the effective measurement to a small range of energies and providing very limited spatial resolution. Here, we show how this ambiguity can be virtually eliminated by manipulating the rescattering pathways with a tailored laser field. Through combined experimental and theoretical approaches, a phase-controlled two-color laser waveform is shown to facilitate the selection of a specific quantum pathway, allowing a direct mapping between the electron’s final momentum and the rescattering time. Integrating attosecond control with Ångstrom-scale resolution could advance ultrafast imaging of field-induced quantum phenomena.

Attosecond photoionization delays in the vicinity of molecular Feshbach resonances

Temporal delays extracted from photoionization phases are currently determined with attosecond resolution by using interferometric methods. Such methods require special care when photoionization occurs near Feshbach resonances due to the interference between direct ionization and autoionization. Although theory can accurately handle these interferences in atoms, in molecules, it has to face an additional, so far insurmountable problem: Autoionization is slow, and nuclei move substantially while it happens, i.e., electronic and nuclear motions are coupled. Here, we present a theoretical framework to account for this effect and apply it to evaluate time-resolved and vibrationally resolved photoelectron spectra and photoionization phases of N2 irradiated by a combination of an extreme ultraviolet (XUV) attosecond pulse train and an infrared pulse. We show that Feshbach resonances lead to unusual non-Franck-Condon vibrational progressions and to ionization phases that strongly vary with photoelectron energy irrespective of the vibrational state of the remaining molecular cation.

Attosecond stable dispersion-free delay line for easy ultrafast metrology

We demonstrate a dispersion-free wavefront splitting attosecond resolved interferometric delay line for easy ultrafast metrology of broadband femtosecond pulses. Using a pair of knife-edge prisms, we symmetrically split and later recombine the two wavefronts with a few tens of attosecond resolution and stability and employ a single-pixel analysis of interference fringes with good contrast using a phone camera without any iris or nonlinear detector. Our all-reflective delay line is theoretically analyzed and experimentally validated by measuring 1st and 2nd order autocorrelations and the SHG-FROG trace of a NIR femtosecond pulse. Our setup is compact, offers attosecond stability with flexibility for independent beam-shaping of the two arms. Furthermore, we suggest that our compact and in-line setup can be employed for attosecond resolved pump-probe experiments of matter with few-cycle pulses.

Plasmon-induced excitation energy transfer in silver nanoparticle dimers: A real-time TDDFTB investigation.

Using real-time quantum dynamics calculations, we perform theoretical investigations of light-induced interactions and electronic excitation transfer in a silver nanoparticle dimer. Real-time time-dependent density functional tight-binding (RT-TDDFTB) calculations provide details of the quantum dynamical processes at an electronic/atomistic level with attosecond resolution. The computational efficiency of RT-TDDFTB allows us to examine electronic dynamics up to picosecond time scales. With time scales varying over six orders of magnitude, we provide insight into interactions between the nanoparticle and laser and between nanoparticles. Our results show that the coupling between nanoparticle monomers is dependent on the separation distance between the nanoparticles in the dimer. As the interparticle distance is varied, the dipole-dipole interactions and electronic excitation transfer mechanisms are markedly different. At large distances (from 50 to 20 Å), the energy transfer from NP1 to NP2 becomes more efficient as the interparticle distance decreases. The total dipole moment of the Ag14 nanoparticle dimer increases linearly at an interparticle distance of 20 Å and reaches its maximum after 1.2ps. The electronic excitation transfer is also the most efficient at 20 Å. At short distances, back-transfer effects reduce the ability of the dimer and NP1 to accept energy from the incident electric field. We attribute the distance-dependent features of the nanoparticle dimer to the beating between the laser acting on NP1 and the back transfer from NP2 to NP1.

Attosecond light field synthesis

The advancement of the ultrafast pulse shaping and waveform synthesis allowed to coherently control the atomic and electronic motions in matter. The temporal resolution of the waveform synthesis is inversely proportional to the broadening of its spectrum. Here, we demonstrate the light field synthesis of high-power waveforms spanning two optical octaves, from near-infrared to deep-ultraviolet with attosecond resolution. Moreover, we utilized the all-optical field sampling metrology for on-demand tailoring of light field waveforms to control the electron motion in matter. The demonstrated synthesis of the light field and the electron motion control pave the way for switching the photo-induced current signal in dielectric nanocircuit and establishing ultrafast photonics operating beyond the petahertz speed.

Dispersive Temporal Interferometry toward Single‐Shot Probing Ultrashort Time Signal with Attosecond Resolution

With the intensive research on ultrafast dynamics in physics and chemistry, the single‐shot measurement of ultrashort time change has become increasingly important. The most advanced oscilloscope with the time‐lens technique possesses subpicosecond resolution, while the interferometer can single‐shot probe time event within one optical cycle. However, the single‐shot measurement of time difference beyond optical cycle but less than time‐lens’ resolution is still inaccessible. Herein, a technique, the dispersive temporal interferometer (DTI), to break through the gap between interferometer and time lens on single‐shot time measurement, is reported. The concept is an analogy to the spatial interference: a pulse passes two paths to assemble into a pulse pair, and temporal interference arises after large dispersion, which can single‐shot record the imposed time change. Experimentally, it is measured that its resolution is sub‐20 as (0.004 optical cycle) over a range of larger than 2 ps (400 optical cycle), with a range‐to‐resolution ratio of 105 and sampling rate of 42.8 MHz. Moreover, a DTI‐based gyroscope with sensitivity of 348 as/(deg s−1) is fabricated to demonstrate its applications in optical sensors. This technique can precisely single‐shot monitor ultrashort time change in attosecond resolution and has a prosperous application prospect in capturing ultrafast process.

Phase Noise Measurements for L-Band Applications at Attosecond Resolution

This article summarizes theoretical and technical limitations when applying carrier-suppression interferometer (CSI) methods for precision phase-noise measurements at attosecond resolution. The CSI setup reported here outperforms conventional down-mixing methods by more than two orders of magnitude. For phase noise measurements at 1.3 GHz operation frequency, a detection floor of <inline-formula> <tex-math notation="LaTeX">$\mathscr {L}={-205}\,\,{\text {dBc/Hz}}$ </tex-math></inline-formula> and an excellent time resolution of 10.76 as within an offset frequency range from 40 Hz to 1 MHz have been achieved without the usage of cross correlation or power recycling techniques. Measurement setup resolution limits, calibration methods, and effects of transmission line group delays are reported. Phase shifters and attenuators in the CSI were identified as key components. Different phase shifter technologies based on passive switches, varactor diodes, and capacitors were investigated. The presented research can be used in future in combination with conventional receiver techniques and enhances the state of the art of phase noise measurements to attosecond resolution and above.

Direct Visualization of Deforming Atomic Wavefunction in Ultraintense High-Frequency Laser Pulses

Interaction of intense laser fields with atoms distorts the bound-state electron cloud. Tracing the temporal response of the electron cloud to the laser field is of fundamental importance for understanding the ultrafast dynamics of various nonlinear phenomena of matter, but it is particularly challenging. Here, we show that the ultrafast response of the atomic electron cloud to the intense high-frequency laser pulses can be probed with the attosecond time-resolved photoelectron holography. In this method, an infrared laser pulse is employed to trigger tunneling ionization of the deforming atom. The shape of the deforming electron cloud is encoded in the hologram of the photoelectron momentum distribution. As a demonstration, by solving the time-dependent Schrödinger equation, we show that the adiabatic deforming of the bound-state electron cloud, as well as the nonadiabatic transition among the distorted states, is successfully tracked with attosecond resolution. Our work films the formation process of the metastable Kramers-Henneberger states in the intense high-frequency laser pulses. This establishes a novel approach for time-resolved imaging of the ultrafast bound-state electron processes in intense laser fields.

Attosecond electron motion control in dielectric

Attosecond science capitalizes on the extreme nonlinearity of strong fields—driven by few-cycle pulses—to attain attosecond temporal resolution and give access to the electron motion dynamics of matter in real time. Here we measured the relative electronic delay response of a dielectric system triggered by a strong field of few-cycle pulses to be of the order of a few hundred attoseconds. Moreover, we exploited the electronic response following the strong driver field to demonstrate all-optical light-field-sampling methodology with attosecond resolution. This methodology provides a direct connection between the driver field and induced ultrafast dynamics in matter. Also, we demonstrate control of electron motion in a dielectric using synthesized light waveforms. This on-demand control of electron motion paves the way for establishing long-anticipated ultrafast switches and quantum electronics. This advancement promises to increase the limiting speed of data processing and information encoding to rates that exceed one petabit per second, opening a new realm of information technology.

Machine learning for laser-induced electron diffraction imaging of molecular structures

Ultrafast diffraction imaging is a powerful tool to retrieve the geometric structure of gas-phase molecules with combined picometre spatial and attosecond temporal resolution. However, structural retrieval becomes progressively difficult with increasing structural complexity, given that a global extremum must be found in a multi-dimensional solution space. Worse, pre-calculating many thousands of molecular configurations for all orientations becomes simply intractable. As a remedy, here, we propose a machine learning algorithm with a convolutional neural network which can be trained with a limited set of molecular configurations. We demonstrate structural retrieval of a complex and large molecule, Fenchone (C10H16O), from laser-induced electron diffraction (LIED) data without fitting algorithms or ab initio calculations. Retrieval of such a large molecular structure is not possible with other variants of LIED or ultrafast electron diffraction. Combining electron diffraction with machine learning presents new opportunities to image complex and larger molecules in static and time-resolved studies.

Publisher Correction: On-chip sampling of optical fields with attosecond resolution

Publisher Correction: On-chip sampling of optical fields with attosecond resolution

On-chip sampling of optical fields with attosecond resolution

Time-domain sampling of arbitrary electric fields with sub-cycle resolution enables a complete time-frequency analysis of a system's response to electromagnetic illumination. This provides access to dynamic information that is not provided by absorption spectra alone, and has recently been shown through measurements in the infrared that time-domain optical-field sampling offers significant improvements with regard to molecular sensitivity and limits of detection compared to traditional spectroscopic methods. Despite the many scientific and technological motivations, time-domain, optical-field sampling systems operating in the visible to near-infrared spectral regions are seldom accessible, requiring large driving pulse energies, and large laser amplifier systems, bulky apparatuses, and vacuum environments. Here, we demonstrate an all-on-chip, optoelectronic device capable of sampling arbitrary, low-energy, near-infrared waveforms under ambient conditions. Our solid-state integrated detector uses optical-field-driven electron emission from resonant nanoantennas to achieve petahertz-level switching speeds by generating on-chip attosecond electron bursts. These bursts are used to probe the electric field of weak optical transients. We demonstrated our devices by sampling the electric field of a ~5 fJ, broadband near-infrared ultrafast laser pulse using a ~50 pJ near-infrared driving pulse. Our sampling measurements recovered the weak optical transient as well as localized plasmonic dynamics of the emitting nanoantennas $in~situ$. This field-sampling device--with its compact footprint and low pulse-energy requirements--offers opportunities in a variety of applications, including: broadband time-domain spectroscopy in the molecular fingerprint region, time-domain analysis of nonlinear phenomena, and detailed studies of strong-field light-matter interactions.

Attosecond state-resolved carrier motion in quantum materials probed by soft x-ray XANES

Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, and TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 × 1021 cm−3. The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions.

Neighboring Atom Collisions in Solid-State High Harmonic Generation

High harmonic generation (HHG) from solids shows great application prospects in compact short-wavelength light sources and as a tool for imaging the dynamics in crystals with subnanometer spatial and attosecond temporal resolution. However, the underlying collision dynamics behind solid HHG is still intensively debated and no direct mapping relationship between the collision dynamics with band structure has been built. Here, we show that the electron and its associated hole can be elastically scattered by neighboring atoms when their wavelength approaches the atomic size. We reveal that the elastic scattering of electron/hole from neighboring atoms can dramatically influence the electron recombination with its left-behind hole, which turns out to be the fundamental reason for the anisotropic interband HHG observed recently in bulk crystals. Our findings link the electron/hole backward scattering with Van Hove singularities and forward scattering with critical lines in the band structure and thus build a clear mapping between the band structure and the harmonic spectrum. Our work provides a unifying picture for several seemingly unrelated experimental observations and theoretical predictions, including the anisotropic harmonic emission in MgO, the atomic-like recollision mechanism of solid HHG, and the delocalization of HHG in ZnO. This strongly improved understanding will pave the way for controlling the solid-state HHG and visualizing the structure-dependent electron dynamics in solids.

In-line ultra-thin attosecond delay line with direct absolute-zero delay reference and high stability.

We introduce an ultra-thin attosecond optical delay line based on controlled wavefront division of a femtosecond infrared pulse after transmission through a pair of micrometer-thin glass plates with negligible dispersion effects. The time delay between the two pulses is controlled by rotating one of the glass plates from absolute zero to several optical cycles, with 2.5 as to tens of attosecond resolution with 2 as stability, as determined by interferometric self-calibration. The performance of the delay line is validated by observing attosecond-resolved oscillations in the yield of high harmonics induced by time delayed infrared pulses, in agreement with a numerical simulation for a simple model atom. This approach can be extended in the future for performing XUV-IR attosecond pump-probe experiments.

All-optical attosecond time domain interferometry.

Interferometry, a key technique in modern precision measurements, has been used for length measurement in engineering metrology and astronomy. An analogous time-domain interferometric technique would represent a significant complement to spatial domain applications and require the manipulation of interference on extreme time and energy scales. Here, we report an all-optical interferometer using laser-driven high order harmonics as attosecond temporal slits. By controlling the phase of the temporal slits with an external field, a time domain interferometer that preserves both attosecond temporal resolution and hundreds of meV energy resolution is implemented. We apply this exceptional temporal resolution to reconstruct the waveform of an arbitrarily polarized optical pulse, and utilize the provided energy resolution to interrogate the abnormal character of the transition dipole near the Cooper minimum in argon. This novel attosecond interferometry paves the way for high precision measurements in the time-energy domain using all-optical approaches.

High-repetition-rate few-attosecond high-quality electron beams generated from crystals driven by intense X-ray laser

Advances in X-ray laser sources have paved the way to relativistic attosecond X-ray laser pulses and opened up the possibility of exploring high-energy-density physics with this technology. With particle-in-cell simulations, we investigate the interaction of realistic metal crystals with relativistic X-ray laser pulses of parameters that will be available in the near future. A wakefield of the order of TV/cm is excited in the crystal and accelerates trapped electrons stably even though the wakefield is locally modulated by the crystal lattice. Electron injection either occurs at the sharp crystal–vacuum boundary or is controlled by coating the crystal with a high-density film. High-repetition-rate attosecond (20 as) monoenergetic electron beams of energy 125 MeV, charge 100 fC, and emittance 1.6 × 10−9 m rad can be produced by shining MHz X-ray laser pulses of energy 2.1 mJ onto coated crystals several micrometers thick. Such a miniature crystal accelerator, which has high reproducibility and allows sufficient control of the parameters of the electron beams, greatly expands the applications of X-ray free electron lasers. For example, it could serve as an ideal electron source for ultrafast electron diffraction and ultrafast electron microscopy to achieve attosecond resolution.

Time-resolved recombination by attosecond-controlled high harmonic generation.

We theoretically investigate the coherent control of strong-field high-harmonic generation in the presence of an isolated attosecond pulse. It is found that the rapid modulation of the controlled signal exhibits interference fringe structures in the delay-dependent spectra. By comparing the classical trajectory model with quantum mechanical calculation, it is demonstrated that the fringes are resulted from the interference between the photon- and the tunnelling-initiated recombination pathways. The relative recombination times for the two paths are reconstructed from the interference fringes, which provides a novel scheme for optical observation of the interplay of the photionization and tunneling ionization electron dynamics in attosecond resolution.