Electrons move in solids at very high speed — traversing atomic layers and interfaces within tens to hundreds of attoseconds (an attosecond is a billionth of a billionth of a second). These astonishingly brief travel times will ultimately limit the speed of the electronics of the future. Physicists have now experimentally probed such electron dynamics in real time. The cover illustrates the first attosecond spectroscopic measurement in a solid, revealing a 110-attosecond difference in the travel time of two different types of electrons following photoexcitation in a tungsten crystal. The ability to time electrons moving in solids over merely a few interatomic distances makes it possible to probe the solid-state electronic processes occurring at the ultimate speed limit and thus helps to advance technologies such as computation, data storage and photovoltaics, which all rely on exquisite control of electron transport in ever smaller structures of solid matter. When exposing a tungsten crystal to intense light, the travel times of emitted electrons differ by 110 attoseconds, depending on whether they were originally tightly bound to one atom in the crystal or delocalized over many atoms. This ability to directly probe fundamental aspects of solid-state electron dynamics could aid the further development of modern technologies such as electronics, information processing and photovoltaics. Comprehensive knowledge of the dynamic behaviour of electrons in condensed-matter systems is pertinent to the development of many modern technologies, such as semiconductor and molecular electronics, optoelectronics, information processing and photovoltaics. Yet it remains challenging to probe electronic processes, many of which take place in the attosecond (1 as = 10-18 s) regime. In contrast, atomic motion occurs on the femtosecond (1 fs = 10-15 s) timescale and has been mapped in solids in real time1,2 using femtosecond X-ray sources3. Here we extend the attosecond techniques4,5 previously used to study isolated atoms in the gas phase to observe electron motion in condensed-matter systems and on surfaces in real time. We demonstrate our ability to obtain direct time-domain access to charge dynamics with attosecond resolution by probing photoelectron emission from single-crystal tungsten. Our data reveal a delay of approximately 100 attoseconds between the emission of photoelectrons that originate from localized core states of the metal, and those that are freed from delocalized conduction-band states. These results illustrate that attosecond metrology constitutes a powerful tool for exploring not only gas-phase systems, but also fundamental electronic processes occurring on the attosecond timescale in condensed-matter systems and on surfaces.

Attosecond spectroscopy in condensed matter

In this article, we review progress in the development of high peak-power ultrafast lasers, and discuss in detail the design issues which determine the performance of these systems. Presently, lasers capable of generating terawatt peak powers with unprecedented short pulse duration can now be built on a single optical table in a small-scale laboratory, while large-scale lasers can generate peak power of over a petawatt. This progress is made possible by the use of the chirped-pulse amplification technique, combined with the use of broad-bandwidth laser materials such as Ti:sapphire, and the development of techniques for generating and propagating very short (10–30 fs) duration light pulses. We also briefly summarize some of the new scientific advances made possible by this technology, such as the generation of coherent femtosecond x-ray pulses, and the generation of MeV-energy electron beams and high-energy ions.

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High power ultrafast lasers

Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals

We present the advances in x-ray femtosecond pulse generation and the most recent discoveries in the field of ultrashort (femtosecond) x-ray science. Nowadays x-rays show their potential not only when it comes to resolving atomic spatial scale but also the inherent temporal scale of quantum dynamics in atoms, molecules and solids. We discuss ultrafast x-ray sources that are currently used to generate femtosecond duration pulses of soft and hard x-ray radiation. Several techniques of x-ray pulse characterization are presented along with a method to control the shape of coherent soft x-rays. A large number of experiments using femtosecond x-ray pulses have been conducted recently and we review some of them. The field of ultrafast x-ray science draws its strength from the large variety of different sources of femtosecond duration x-ray pulses that are complementary rather than competing.

https://iopscience.iop.org/article/10.1088/0034-4885/69/2/R04/pdf

Femtosecond x-ray science

Angle-resolved photoemission spectroscopy (ARPES) has emerged as a leading experimental probe for studying the complex phenomena in quantum materials, a subject of increasing importance The power of this technique stems from the directness and the richness of the momentum-resolved information it can provide, such as band dispersion, Fermi surface topology, and electron self-energy Over the past decade, the significantly improved energy and momentum resolution and carefully matched experiments have turned this technique into a sophisticated tool in characterizing the electronic structure of complex materials This revolution is mostly evident in the study of cuprate high-temperature superconductors More recently, this technique has played a critical role in advancing our understanding of the newly discovered iron-based superconductors and topological insulators In this paper we review some of the recent ARPES results and discuss the future perspective in this rapidly developing field

Angle-Resolved Photoemission Studies of Quantum Materials

Soft x‐ray photons, generated by the interaction of intense 610 nm subpicosecond laser pulses with rare gases, are used to carry out tunable angle‐resolved photoemission on a number of systems. When the intensity at the focus of the input radiation exceeds ∼1014 W/cm2 significant quantities of harmonic photons at odd multiples of the input 610 nm (2.04 eV) light are produced. To date, harmonics from the 5th (10.2 eV) to the 25th (51 eV) have been generated with fluxes sufficient for generating photoemission spectra. The experimental apparatus for harmonic generation and several experiments demonstrating the applications of this light are described.

Tunable photoemission with harmonics of subpicosecond lasers

The first direct observation of the As derived empty surface state on the Ge(111):As 1×1 surface is reported. The state is first populated with a subpicosecond pulse of 2.03 eV light and then probed with equally short pulses of light at 10, 14, 18, and 22 eV which photoemit electrons from the sample. The surface band gap at 300 K is determined to be 0.83±0.04 eV and compared well with a many-body calculation for this system. Photoexcited electrons are observed to rapidly scatter into the empty As derived state and are trapped for times in excess of 200 ps. The long lifetime is due to a paucity of electrically active defect states within the surface band gap

D. R. Peale

Papers

Tunable photoemission with harmonics of subpicosecond lasers

Antibonding state on the Ge(111):As surface: Spectroscopy and dynamics.

One-dimensional image state on stepped Cu(100).