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Dissociation of the ethylene cation is a prototypical multistep pathway in which the exact mechanisms leading to internal energy conversions are not fully known. For example, it is still unclear how the energy is exactly redistributed among the internal modes and which step is rate-determining. Here we use few-femtosecond extreme-ultraviolet pulses of tunable energy to excite a different superposition of the four lowest states of C

This modern text provides detailed coverage of the important physical processes underpinning semiconductor devices. Advanced analysis of the optical properties of semiconductors without the requirement of complex mathematical formalism allows clear physical interpretation of all obtained results. The book describes fundamental aspects of solid-state physics and the quantum mechanics of electron-photon interactions, in addition to discussing in detail the photonic properties of bulk and quantum well semiconductors. The final six chapters focus on the physical properties of several widely-used photonic devices, including distributed feedback lasers, vertical-cavity surface-emitting lasers, quantum dot lasers, and quantum cascade lasers. This book is ideal for graduate students in physics and electrical engineering and a useful reference for optical scientists.

The first step to gain optical control over the ultrafast processes initiated by light in solids is a correct identification of the physical mechanisms at play. Among them, exciton formation has been identified as a crucial phenomenon which deeply affects the electro-optical properties of most semiconductors and insulators of technological interest. While recent experiments based on attosecond spectroscopy techniques have demonstrated the possibility to observe the early-stage exciton dynamics, the description of the underlying exciton properties remains non-trivial. In this work we propose a new method called extended Ptychographic Iterative engine for eXcitons (ePIX), capable of reconstructing the main physical properties which determine the evolution of the quasi-particle with no prior knowledge of the exact relaxation dynamics or the pump temporal characteristics. By demonstrating its accuracy even when the exciton dynamics is comparable to the pump pulse duration, ePIX is established as a powerful approach to widen our knowledge of solid-state physics.

We present a novel method, named STRIPE, for the complete temporal characterization of ultrashort light pulses. By applying STRIPE to both simulated and experimental data, we demonstrate its superior performances against other established reconstruction algorithms.

Excitons determine the ultrafast response of several technologically-relevant materials. Attosecond spectroscopy provides suitable time-resolution, but the access to the exciton response remains non-trivial. We propose a novel, ptychography-based approach to solve this task.

JPhys photonics 4, 034003 (2022). doi:10.1088/2515-7647/ac6ea5 special issue: "Focus on Nanophotonics and Biophotonics for Biomedical and Environmental Applications", The dynamics of biologically relevant molecules exposed to ionizing radiation contains many facets and spans several orders of magnitude in time and energy. In the extreme ultraviolet (XUV) spectral range, multi-electronic phenomena and bands of correlated states with inner-valence holes must be accounted for in addition to a plethora of vibrational modes and available dissociation channels. The ability to track changes in charge density and bond length during ultrafast reactions is an important endeavor toward more general abilities to simulate and control photochemical processes, possibly inspired by those that have evolved biologically. By using attosecond XUV pulses extending up to 35 eV and few-femtosecond near-infrared pulses, we have previously time-resolved correlated electronic dynamics and charge migration occurring in the biologically relevant molecule adenine after XUV-induced sudden ionization. Here, using additional experimental data, we comprehensively report on both electronic and vibrational dynamics of this nucleobase in an energy range little explored to date with high temporal resolution. The time-dependent yields of parent and fragment ions in the mass spectra are analyzed to extract exponential time constants and oscillation periods. Together with time-dependent density functional theory and ab-initio Green's function methods, we identify different vibrational and electronic processes. Beyond providing further insights into the XUV-induced dynamics of an important nucleobase, our work demonstrates that yields of specific dissociation outcomes can be influenced by sufficiently well-timed ultrashort pulses, therefore providing a new route for the control of the multi-electronic and dissociative dynamics of a DNA building block., Published by IOP Publishing, Bristol

Since the first demonstration of the generation of attosecond pulses (1 as = 10−18 s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron–nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

The interplay between intra-band motion and inter-band transitions in photoexcited germanium is investigated by attosecond transient reflection spectroscopy. Advanced theoretical simulations interpret the sub-femtosecond optical response as charge carrier dynamics in the band structure.

We present the experimental realization of spectrally tunable, ultrashort, quasi-monochromatic extreme ultraviolet (XUV) pulses generated at 100 kHz repetition rate in a user-oriented gas high harmonic generation (GHHG) beamline of the Extreme Light Infrastructure - Attosecond Light Pulse Source (ELI ALPS) facility. Versatile spectral and temporal shaping of the XUV pulses are accomplished with a double-grating, time-delay compensated monochromator accommodating the two composing stages in a novel, asymmetrical geometry. This configuration supports the achievement of high monochromatic XUV flux (2.8+/-0.9*1e10 photons/s at 39.7 eV selected with 700 meV FWHM bandwidth) combined with ultrashort pulse duration (4.0+/-0.2 fs using 12.1+/-0.6 fs driving pulses) and small spot size (sub-100 um). Focusability, spectral bandwidth, and overall photon flux of the produced radiation were investigated covering a wide range of instrumental configurations. Moreover, complete temporal (intensity and phase) characterization of the few-femtosecond monochromatic XUV pulses - a goal that is difficult to achieve by conventional reconstruction techniques - has been realized using ptychographic algorithm on experimentally recorded XUV-IR pump-probe traces. The presented results contribute to in-situ, time-resolved experiments accessing direct information on the electronic structure dynamics of novel target materials., Comment: 20 pages, 8 figures

Although attosecond transient absorption spectroscopy (ATAS) is a powerful experimental technique, it suffers from important limitations. The high absorption coefficient of solid state samples in the extreme-ultraviolet (XUV) spectral range and the relatively low photon flux of standard attosecond sources force the thickness of the samples to be below a few tens of nanometres. It is possible to overcome these drawbacks obtaining equivalent information by using a reflection geometry in the investigation of ultrafast carrier dynamics in solids [1] , [2] . Attosecond transient reflection spectroscopy (ATRS) investigates pump-induced variations in the reflectivity of a sample in the XUV spectral region on an attosecond timescale. We present a novel beamline suitable for performing ATRS experiments with attosecond temporal resolution [3] . Its double-foci configuration allows for a simultaneous characterization of the pump and probe pulses and for a precise calibration of the delay axis of the transient reflectivity measurement.

Excitons are electron-hole quasi-particles, which strongly influence the electro-optical properties of solids and hold the potential for a variety of applications [1] . Acquiring a detailed understanding of their dynamic nature is thus essential to promote their exploitation in advanced technological areas. In particular, the ultrafast processes unfolding at the femto- and attosecond domain are of primary relevance in view of the desired extension towards the petahertz regime. Here we applied attosecond transient reflectivity spectroscopy to study the ultrafast response of a MgF2 single crystal and report the first observation of sub-fs core-exciton dynamics [2] . Combined with simulations, our results allowed us to disentangle the dual atomic-solid nature of this fundamental quasi-particle.

The use of few-femtosecond, extreme ultraviolet (XUV) pulses, produced by high-order harmonic generation, in combination with few-femtosecond infrared (IR) pulses in pump-probe experiments has great potential to disclose ultrafast dynamics in molecules, nanostructures and solids. A crucial prerequisite is a reliable characterization of the temporal properties of the XUV and IR pulses. Several techniques have been developed. The majority of them applies phase reconstruction algorithms to a photoelectron spectrogram obtained by ionizing an atomic target in a pump-probe fashion. If the ionizing radiation is a single harmonic, all the information is encoded in a two-color two-photon signal called sideband (SB). In this work, we present a simplified model to interpret the time-frequency mapping of the SB signal and we show that the temporal dispersion of the pulses directly maps onto the shape of its spectrogram. Finally, we derive an analytical solution, which allows us to propose a novel procedure to estimate the second-order dispersion of the XUV and IR pulses in real time and with no need for iterative algorithms.

The electronic energy levels of cyclo(glycine–phenylalanine), cyclo(tryptophan–tyrosine) and cyclo(tryptophan–tryptophan) dipeptides are investigated with a joint experimental and theoretical approach. Experimentally, valence photoelectron spectra in the gas phase are measured using VUV radiation. Theoretically, we first obtain low-energy conformers through an automated conformer–rotamer ensemble sampling scheme based on tight-binding simulations. Then, different first principles computational schemes are considered to simulate the spectra: Hartree–Fock (HF), density functional theory (DFT) within the B3LYP approximation, the quasi-particle GW correction, and the quantumchemistry CCSD method. Theory allows assignment of the main features of the spectra. A discussion on the role of electronic correlation is provided, by comparing computationally cheaper DFT scheme (and GW) results with the accurate CCSD method.

Excitons characterize the ultrafast response of many materials of technological interest. While the development of attosecond science has unlocked the possibility of performing experiments with a suitable timeresolution, the access to the exciton properties remains a non-trivial step. We propose therefore a novel approach to disclose the physical properties behind the ultrafast exciton dynamics based on a phase-retrieval method.

The electro-optical properties of most semiconductors and insulators of technological interest are dominated by the presence of electron-hole quasi-particles, called excitons. The manipulation of excitons in dielectrics has recently received great attention, with possible applications in different fields including optoelectronics and photonics. Here, we apply attosecond transient reflection spectroscopy in a sequential two-foci geometry and observe sub-femtosecond dynamics of a core-level exciton in bulk MgF2 single crystals. Furthermore, we access absolute phase delays, which allow for an unambiguous comparison with theoretical calculations. Our results show that excitons surprisingly exhibit a dual atomic- and solid-like character, which manifests itself on different time scales. While the former is responsible for a femtosecond optical Stark effect, the latter dominates the attosecond excitonic response. Further theoretical investigation reveals a link with the exciton sub-femtosecond nanometric motion and allows us to envision a new route to control exciton dynamics in the close-to-petahertz regime., The capability to follow electron motion in solids is necessary to explore the ultimate speed limits of optical charge manipulation and signal processing in optoelectronic devices. Here, the authors reveal the sub-femtosecond dynamics of core excitons in MgF2 and find the dual atomic-solid nature of the exciton quasi-particle to deeply affect its ultrafast dynamics.

Frequency-resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB) is a well-known technique for the complete temporal characterization of ultrashort extreme ultraviolet (XUV) pulses, with durations down to a few tens of attoseconds. Recently, this technique was extended to few-femtosecond XUV pulses, produced by high-order harmonic generation (HHG) in gases, thanks to the implementation of a robust iterative algorithm: the extended ptychographic iterative engine (ePIE). We demonstrate, by using numerical simulations, that the ptychographic reconstruction technique is characterized by an excellent degree of convergence and robustness. We analyse the effects on pulse reconstruction of various experimental imperfections, namely, the jitter of the relative temporal delay between the XUV pulse and a suitably delayed infrared (IR) pulse and the noise of the measured FROG-CRAB spectrograms. We also show that the ePIE approach is particularly suitable for the reconstruction of incomplete FROG-CRAB spectrograms (i.e., spectrograms with a reduced number of measured time delays) and of spectrograms acquired with a reduced spectral resolution, particularly when relatively high-intensity IR pulses are employed.

The journal of physical chemistry letters 11(3), 891 - 899 (2020). doi:10.1021/acs.jpclett.9b03517, Extreme-ultraviolet-induced charge migration in biorelevant molecules is a fundamental step in the complex path leading to photodamage. In this work we propose a simple interpretation of the charge migration recently observed in an attosecond pump–probe experiment on the amino acid tryptophan. We find that the decay of the prominent low-frequency spectral structure with increasing pump–probe delay is due to a quantum beating between two geometrically distinct, almost degenerate charge oscillations. Quantum beating is ubiquitous in these systems, and at least on the few-to-tens of femtosecond time scales, it may dominate over decoherence the line intensities of time-resolved spectra. We also address the experimentally observed phase shift in the charge oscillations of two different amino acids, tryptophan and phenylalanine. Our results indicate that a beyond mean-field treatment of the electron dynamics is necessary to reproduce the correct behavior., Published by ACS, Washington, DC

Super-excited states of CO2 are studied by time-resolved photoelectron spectroscopy exploiting sub-15 fs vacuum-ultraviolet pulses. Comparison with quantum mechanical simulations allows us to identify the ultrafast mechanisms which dictate the super-excited state lifetime and coherence.

Sub-cycle MgF2 core-exciton dynamics was studied by attosecond reflection spectroscopy. We found that the interplay between the crystal and the core-exciton strongly alters the ultrafast material response, introducing a delay as big as 1 femtosecond.

The capability to follow the ultrafast internal relaxation dynamics which take place in molecules few femtoseconds after light excitation is of paramount importance to study the photostability and photochemistry of relevant molecular systems [1,2]. The high number of internal degrees of freedom which may play an active role makes the study of these dynamics a true challenge also in relatively small prototype molecules like ethylene. The existence of many different relaxation non-adiabatic paths leading to the same final observable can prohibit a clear experimental observation and makes difficult the theoretical modelling.

Generation of few-femtosecond (fs) pulses in the ultraviolet (UV) is important in ultrafast spectroscopy since several bio-relevant molecules, including DNA bases, possess electronic transitions in this spectral region [1]. Exploiting nonlinear processes driven by intense laser pulses has led to several advances to meet this challenge. We developed a source delivering 1.9 fs (2.2 cycles at 255 nm) deep-UV pulses with 150 nJ of energy on target [2].

Attosecond pump–probe spectroscopy is a unique tool for the direct observation of the light-activated electronic motion in molecules and it offers the possibility to capture the first instants of a chemical reaction. Recently, advances in attosecond technology allowed the charge migration processes to be revealed in biochemically relevant molecules. Although this purely electronic process might be key for a future chemistry at the electron time scale, the influence of this ultrafast charge flow on the reactivity of a molecule is still debated. In this work, we exploit extreme ultraviolet attosecond pulses to activate charge migration in two aromatic amino acids, namely phenylalanine and tryptophan. Advanced numerical calculations are performed to interpret the experimental data and to discuss the effects of the nuclear dynamics on the activated quantum coherences. By comparing the experimental results obtained in the two molecules, we show that the presence of different functional groups strongly affects the fragmentation pathways, as well as the charge rearrangement. The observed charge dynamics indeed present peculiar aspects, including characteristic periodicities and decoherence times. Numerical results indicate that, even for a very large molecule such as tryptophan, the quantum coherences can survive the nuclear dynamics for several femtoseconds. These results open new and important perspectives for a deeper understanding of the photo-induced charge dynamics, as a promising tool to control the reactivity of bio-relevant molecules via photo-excitation. This article is part of the theme issue ‘Measurement of ultrafast electronic and structural dynamics with X-rays’.

We review the capability of lasers to generate light pulses of incredibly short duration, from a few femtoseconds (1 fs = 10-15 s) in the visible down to a few tens of attoseconds (1 as = 10-18 s) in the extreme-ultraviolet, reaching peak powers up to several petawatts (1 PW = 1015 W).

A time resolved setup combining sub-2fs UV pump pulses with attosecond XUV probe pulses is presented. This scheme will allow the UV photo-induced electron dynamics in bio-chemically relevant molecules to be investigated with unprecedented time resolution.

Nature photonics 13(2), 91 - 95 (2019). doi:10.1038/s41566-018-0308-z, A key challenge in attosecond science is the temporal characterization of attosecond pulses that are essential for understanding the evolution of electronic wavefunctions in atoms, molecules and solids. Current characterization methods, based on nonlinear light–matter interactions, are limited in terms of stability and waveform complexity. Here, we experimentally demonstrate a conceptually new linear and all-optical pulse characterization method, inspired by double-blind holography. Holography is realized by measuring the extreme ultraviolet (XUV) spectra of two unknown attosecond signals and their interference. Assuming a finite pulse duration constraint, we reconstruct the missing spectral phases and characterize the unknown signals in both isolated pulse and double pulse scenarios. This method can be implemented in a wide range of experimental realizations, enabling the study of complex electron dynamics via a single-shot and linear measurement., Published by Nature Publ. Group, London [u.a.]

XXI International Conference on Ultrafast Phenomena 2018, UP 2018, Hamburg, Germany, 15 Jul 2018 - 20 Jul 2018; The European physical journal / Web of Conferences 205, 02009 (2019). doi:10.1051/epjconf/201920502009, Extreme-ultraviolet pulses as short as 5 fs, produced by high- order harmonic generation and selected by a time-delay compensated monochromator, were fully reconstructed thanks to an advanced ptychographic technique, while most commonly used reconstruction methods fail. These pulses constitute the shortest extreme ultraviolet pulses ever achieved via a time-delay compensated monochromator., Published by EDP Sciences, Les Ulis

A precise temporal characterization of the pulses involved in pump-probe experiments is crucial for a proper investigation of the ultrafast dynamics in several physical systems. Indeed, it is required for the assessment of the dynamical properties under examination with sufficient temporal resolution. In the fewfs/attosecond domain, typical reconstruction procedures require time-consuming interative methods, which are also sensitive to the experimental noise and to the distortion of the measurement. We developed an approach, called Simplified Trace Reconstruction In the Perturbative regimE (STRIPE), which allows us for a precise characterization of the infrared (IR) and extreme ultraviolet (XUV) pulses, used in a pump-probe experiment. Our method is not based on a phase retrival algorithm, and for this it is typically much faster than the other ones currently known. Moreover, it allows for easily including in the reconstruction the experimental non-idealities that may affect the measurement, like possible distortion due to the measurement procedure itself.

We generate attosecond pulse train (APT) in argon driven by the high repetition rate (HR) laser of the extreme light infrastructure-attosecond light pulse source (ELI-ALPS), providing 100 kHz, 80 W, 1030 nm, 40 fs pulses from a fiber chirped-pulse amplification (fiber-CPA) laser system. Under the current operating conditions of the high harmonic generation beamline (HR-GHHG), we observed the average pulse duration to be 395 as measured using the technique of reconstruction of attosecond beating by interference of two-photon transitions. The beamline uses an annular-shape laser beam so that the main part of the driving laser co-propagating with the APT can be eliminated by reflection on a holey mirror. An additional 100 nm aluminum foil is used to filter out the remaining laser and the low order harmonics, allowing 2 pJ APT with a bandwidth from 25 eV to 50 eV to be transported to the target position where the APT interacts with matter. The implementation of the HR-GHHG beamline in ELI-ALPS delivering attosecond pulse trains at 100 kHz paves the way for time-resolved experiments in the infrastructure, especially those that involve rare events and coincidence analysis, both of which need high statistics.

We present an innovative beamline for extreme ultraviolet (XUV)-infrared (IR) pump-probe reflection spectroscopy in solids with attosecond temporal resolution. The setup uses an actively stabilized interferometer, where attosecond pulse trains or isolated attosecond pulses are produced by high-order harmonic generation in gases. After collinear recombination, the attosecond XUV pulses and the femtosecond IR pulses are focused twice in sequence by toroidal mirrors, giving two spatially separated interaction regions. In the first region, the combination of a gas target with a time-of-flight spectrometer allows for attosecond photoelectron spectroscopy experiments. In the second focal region, an XUV reflectometer is used for attosecond transient reflection spectroscopy (ATRS) experiments. Since the two measurements can be performed simultaneously, precise pump-probe delay calibration can be achieved, thus opening the possibility for a new class of attosecond experiments on solids. Successful operation of the beamline is demonstrated by the generation and characterization of isolated attosecond pulses, the measurement of the absolute reflectivity of SiO2, and by performing simultaneous photoemission/ATRS in Ge., 18 pages, 9 figures

Hydrogen migration in the glycine cation has been investigated using a combination of a short train of attosecond extreme ultraviolet pulses with few-optical-cycle near-infrared pulses. The yield of the photofragments produced has been measured as a function of pump–probe delay. These time-dependent measurements reveal the presence of a hydrogen migration process occurring in 48 fs. Previous mass spectrometric experiments and theoretical calculations have allowed us to identify the conformations and cation states involved in the process induced by the broad band extreme ultraviolet radiation.

Optics express 26(6), 6771 - 6784 (2018). doi:10.1364/OE.26.006771, Sub-10-fs pulses tunable in the extreme-ultraviolet (XUV) spectral region are particularly important in many research fields: from atomic and molecular spectroscopy to the study of relaxation processes in solids and transition phase processes, from holography to free-electron laser injection. A crucial prerequisite for all applications is the accurate measurement of the temporal characteristics of these pulses. To fulfill this purpose, many phase retrieval algorithms have been successfully applied to reconstruct XUV attosecond pulses. Nevertheless, their extension to XUV femtosecond pulses is not trivial and has never been investigated/reported so far. We demonstrate that ultrashort XUV pulses, produced by high-order harmonic generation, spectrally filtered by a time-delay compensated monochromator, can be fully characterized, in terms of temporal intensity and phase, by employing the ptychographic reconstruction technique while other common reconstruction algorithms fail. This allows us to report on the generation and complete temporal characterization of XUV pulses with duration down to 5 fs, which constitute the shortest XUV pulse ever achieved via a time-delay compensated monochromator., Published by OSA, Washington, DC

Nature Communications 9(1), 302 (2018). doi:10.1038/s41467-017-02702-x, Coherent diffractive imaging of individual free nanoparticles has opened routes for the in situ analysis of their transient structural, optical, and electronic properties. So far, single-shot single-particle diffraction was assumed to be feasible only at extreme ultraviolet and X-ray free-electron lasers, restricting this research field to large-scale facilities. Here we demonstrate single-shot imaging of isolated helium nanodroplets using extreme ultraviolet pulses from a femtosecond-laser-driven high harmonic source. We obtain bright wide-angle scattering patterns, that allow us to uniquely identify hitherto unresolved prolate shapes of superfluid helium droplets. Our results mark the advent of single-shot gas-phase nanoscopy with lab-based short-wavelength pulses and pave the way to ultrafast coherent diffractive imaging with phase-controlled multicolor fields and attosecond pulses., Published by Nature Publishing Group UK, [London]