Physical Review A

As early as 1979, Pierre Agostini discovered above-threshold ionization (ATI), that is, large kinetic energies of photoelectrons with peaks separated by the photon energy when an atom was illuminated by an intense laser pulse [Phys. Rev. Lett. 42, 1127 (1979)]. Investigating the emitted light in such a process, Anne L’Huillier discovered high-harmonic generation (HHG), that is the emission of light bursts at multiples of the photon frequency with a characteristic plateau of nearly constant yield over many orders of high harmonics [J. Phys. B 21, L31 (1988)].

Both phenomena originate in the same process, namely the recollision of the ionized electron with its parent ion, driven back to it by the intense electric field of the laser pulse. Gaining energy from the recollision leads to ATI; if the electron recombines with the ion, the energy set free is released in the form of HHG. This light-driven electron dynamics can be described in simple terms with the so-called three-step model [Phys. Rev. Lett. 71, 1994 (1993), Phys. Rev. A 49, 2117 (1994)], which has given rise to the strong-field approximation (SFA) [Phys. Rev. A 51, 1495 (1995)]. It is surprisingly accurate despite neglecting the Coulomb potential and arguably has paved the way for the development of the ideas which have led to the generation of attosecond pulses. The latter came in two distinct modes in the beginning. Ferenc Krausz realized that removing a few harmonics close to the end of the plateau produces isolated attosecond pulses [Nature 414, 509 (2001)]. Pierre Agostini generated pulse trains with individual components of attosecond duration [Science 292, 1689 (2001)]. The research field prior to 2009 was reviewed in RMP [Rev. Mod. Phys. 81, 163 (2009)].

Attosecond science brings new insights into the dynamics of ultrafast processes in matter. Almost from the beginning, there have been two strands of attosecond research: The first one has aimed at exploiting the obvious “timing” capability of ultrashort pulses, namely to start and stop a clock for timing ultrafast processes. Up until recently one could only generate a single attosecond pulse but not pairs of such pulses with variable delay as needed for a clock. Therefore, the second pulse has been replaced by an alternative indicator—the instantaneous amplitude of a (longer) femtosecond pulse which is encoded in the momentum of the electron ionized by the attopulse, a method called streaking [Phys. Rev. Lett. 88, 173903 (2002)]. The streaked electron momenta can be used to measure the attopulse. Since the femtosecond pulse is used to generate the attopulse, they are both phase locked, rendering this setup also convenient for a pump-probe streaking scheme. Equivalently, one can employ the sidebands [Phys. Rev. A 54, 721 (1996)] generated in a setup called, nowadays, RABBITT (“Reconstruction of Attosecond Beating By Interference of Two-photon Transitions”).

More recently, the so-called attoclock has been invented [Nat. Phys. 4, 565 (2008)] where the extreme nonlinearity of the ionization process in strong laser fields is exploited to start the clock with a known direction of the rotating polarization vector of the light at a given time. If the ionized electron momentum at the detector points into a different direction, the difference can be converted into a delay of typically some tens of attoseconds. Measuring time delays in all kinds of targets — from atoms to molecules, clusters, solids and even liquids — became a popular tool in several active lines of research on their own, also in photoionization by streaking or RABBITT. However, the interpretation of measured attosecond delays is a matter of current debate [Phys. Rev. Lett. 125, 113202 (2020), Phys. Rev. Lett. 129, 203201 (2022)].

Apart from “timing” setups, there is a second strand of research with attosecond pulses: transient absorption [Phys. Rev. A 83, 033405 (2011), Phys. Rev. Lett. 106, 123601 (2011), Phys. Rev. A 86, 063408 (2012)]. Due to their broadband nature (a 500-as pulse contains photons with energy bandwidth of about 3 eV), attosecond pulses excite a target at many frequencies simultaneously, giving rise to a coherent broadband response which contains ample information about the target. High-harmonic spectroscopy, the most generally applied technique in the context of attosecond science, promises new insights into the dynamics of ultrafast processes in matter, from atoms to molecules and, more recently, solids [Phys. Rev. Lett. 107, 167407 (2011), Phys. Rev. Lett. 115, 193603 (2015), Phys. Rev. A 91, 043839 (2015)].

Along with the development of HHG, and that means laboratory-based sources for attosecond pulses, free-electron lasers realized at large-scale research facilities with accelerators all over the world can in principle generate pairs and even triples of attosecond pulses using chicanes in the electron beams which generate later the corresponding light-pulse sequences. This synergy happens almost as a byproduct of the overarching XFEL research goal centered on the production of high-frequency pulses of unprecedented intensity [Phys. Rev. Lett. 120, 264801 (2018)]. Hence, lab-based and accelerator-based attosecond pulse sources are not only complementary in their characteristic operation (small university teams versus large consortia executing beamline campaigns) but also cover different spectral bands with a small overlap in the XUV frequency range.

Recently, quantum entanglement has been used to enhance attoscience [Phys. Rev. Lett. 128, 043201 (2022)]. New light sources will make it possible to generate entangled photons all the way to optical cat states which can open an entirely new field of entangled light-matter interaction [Phys. Rev. Lett. 128, 123603 (2022)].
Frequency comb pulses within optical cavities to amplify the light intensity are an intriguing technological development which can reach the attosecond domain in the future [Rev. Sci. Instrum. 93, 123303 (2022)].
Although attosecond technology is expensive and requires very special skills of a well-trained team, there are an increasing number of groups worldwide entering the field Physical Review X Collection on Ultrafast Imaging of Coherent Electron Dynamics. This fact and the at least “dual-purpose” usage of attosecond pulses, both to time ultrafast dynamics and to enable coherent broadband response, will, in the future, undoubtedly create fascinating applications reaching well beyond physics, from engineering to medicine.