Chirality is the property of an object that cannot be superimposed with its mirror image by any translation or rotation. This property plays a crucial role in the biological activities of molecules and in the optical and magnetic properties of materials. This review will show recent advances in the study of ultrafast chiral processes occurring in molecular systems and materials investigated by high-order harmonic generation (HHG) spectroscopy, both from a theoretical and experimental point of view. HHG is a highly nonlinear optical process providing coherent XUV radiation with attosecond duration. The chemical physics community, in its broadest sense, has developed an interest in HHG spectroscopy, given the increasing number of studies of complex molecules, including those of biological relevance, chemical reactions, and characterization of the structure of solids by means of an ultrafast probe. Indeed, HHG is a powerful tool to capture the photoinduced electron dynamics in their natural timescale, also enabling the investigation of transient chirality, which is so far mostly unexplored. The study of chirality on the ultrafast timescale represents an opportunity to bridge the gap between atomic physics, chemical physics, and materials science in their traditional definitions and gain insight into fundamental chiral processes at the electron level.

Abstract. Black carbon (BC) is a key atmospheric forcer due to its interaction with solar radiation and clouds. However, accurately quantifying and understanding the impact of atmospheric aging on BC properties and radiative forcing remains a major challenge. To address this, the AIDA (Atmospheric Interactions and Dynamics in the Atmosphere) aRCtic Transport Experiment (ARCTEx) project simulated BC aging under quasi-realistic Arctic conditions in the AIDA chamber. Four distinct scenarios were simulated based on reanalysis data, representing summer and winter conditions at both low and high altitudes, to capture the variability in BC aging processes in the presence of nitrate and organic matter precursors during Arctic transport. In the first part of the paper, we define the meteorological conditions characterizing northward transport under different scenarios and describe the technical solutions to simulate 5 d transport in the AIDA chamber. In the second part of the work, we assess the evolution of fundamental properties, including density, morphology, and mixing state, as observed during the aging process. The ARCTEx project demonstrates that large facilities such as AIDA can successfully reproduce environmental conditions, enabling a gradual aging process that closely follows the natural timescales observed in the atmosphere. Our experiments revealed that temperature strongly influences the aging timescale and the evolution of BC's diameter, effective density, and coating thickness. Low-altitude scenarios exhibited rapid aging, resulting in fully coated, compact BC particles within 39–98 h, corresponding to 50 and 80° N, respectively. In contrast, high-altitude transport was characterized by slow aging, with limited coating and compaction, even after 115 h of simulation. These findings provide valuable insights into the temporal evolution of BC properties during Arctic transport. In forthcoming work, we will report the implications of this evolution for climate-relevant properties such as light absorption and activation as cloud droplets and ice crystals. Together, these studies aim to enhance the representation of BC aging in climate models, reducing uncertainties in Arctic radiative-forcing estimates.

ABSTRACT When a star is torn apart by the tidal forces of a supermassive black hole (a so-called tidal disruption events, TDE) a transient accretion episode is initiated and a hot, often X-ray bright, accretion disc is formed. Like any accretion flow this disc is turbulent, and therefore the emission from its surface will vary stochastically. As the disc has a finite mass supply (i.e. at most the initial mass of the disrupted star) the disc will also undergo long-time-scale evolution, as this material is lost into the black hole. In this paper, we combine theoretical models for this long time evolution of the disc with models for the stochastic variability of turbulent accretion flows which are correlated on short (orbital) time-scales. This new framework allows us to demonstrate that (i) dimming events should be more prevalent than brightening events in long-term TDE X-ray light curves (i.e. their log-luminosity distribution should be asymmetric with a skew towards lower luminosities), (ii) TDE X-ray light curves should follow a near- (but formally sub-)linear correlation between their root mean square variability and the mean flux, (iii) the fractional variability observed on short time-scales across an X-ray observing band should increase with observing energy, and (iv) TDEs offer a unique probe of the physics of disc turbulence, owing to their clean spectra and natural evolutionary time-scales. We confirm predictions (i) and (ii) with an analysis of the long time-scale variability of two observed TDEs, and show strong support for prediction (iii) using the intra-observation variability of the same two sources.

Abstract In this work, laser-induced high-order harmonic (HH) generation in monolayer hexagonal boron nitride (hBN) is investigated using the semiconductor Bloch equation in the form of the density matrix, combined with the tight-binding model, employing an accelerated Bloch-like basis in the Wannier gauge. Using a linear-polarized 3 µm femtosecond pulse, the crystal orientation dependence of HH spectra is investigated. The carrier-envelope (CE) phase dependence of high-harmonic generation (HHG) spectra in hBN reveals that the minimum period is 2π rad, compared to π rad in the case of HHG in systems with inversion symmetry. Furthermore, we investigate the ellipticity dependence of HHG in monolayer hBN. The parallel and perpendicular emission of HHG to the major axis of elliptically polarized light is affected differently by the ellipticity. By utilizing the strong ellipticity dependence of HHG in the plateau region of parallel emission, we propose a polarization gate (PG) scheme composed of left- and right-circularly polarized laser pulses. Time–frequency analysis reveals that HHG under PG irradiation is predominantly confined to within half an optical cycle of the driving field, enabling the generation of extreme ultraviolet radiation with a natural attosecond timescale.

Abstract In the past few decades, the development of ultrafast lasers has revolutionized our ability to gain insight into light–matter interactions. The emergence of few-cycle light sources operating from the visible to the mid-infrared spectral range—as well as attosecond extreme ultraviolet and X-ray technologies—provide the possibility to directly observe and control ultrafast electron dynamics in matter on their natural timescale; however, the temporal characterization of few-femtosecond sources in the deep ultraviolet (4–6 eV, 300–200 nm) and the vacuum ultraviolet (VUV; 6–12 eV, 200–100 nm) spectral regions is challenging. Here we fully characterize the temporal shape of microjoule-energy VUV pulses tuned between 160 and 190 nm generated via resonant dispersive wave emission during soliton self-compression in a capillary using frequency-resolved optical gating based on two-photon photoionization in noble gases. The in situ measurements reveal that in most of the cases the pulses are shorter than 3 fs. These findings pave the way toward investigating ultrafast electron dynamics and valence excitation of a large class of atoms and molecules with a time-resolution that has been hitherto inaccessible when using VUV pulses.

Using gravitational waves to probe the geometry of the ringing remnant black hole formed in a binary black hole coalescence is a well-established way to test Einstein's theory of general relativity. However, doing so requires knowledge of when the predictions of black hole perturbation theory, i.e., quasinormal modes (QNMs), are a valid description of the emitted gravitational wave as well as what the amplitudes of these excitations are. In this work, we develop an algorithm to systematically extract QNMs from the ringdown of black hole merger simulations. Our algorithm improves upon previous ones in three ways: it fits over the two-sphere, enabling a complete model of the strain; it performs a reverse search in time for QNMs using a more robust nonlinear least squares routine called varpro; and it checks the variance of QNM amplitudes, which we refer to as ``stability,'' over an interval matching the natural timescale of each QNM. Using this algorithm, we not only demonstrate the stability of a multitude of QNMs and their overtones across the parameter space of quasicircular, nonprecessing binary black holes, but we also identify new quadratic QNMs that may be detectable in the near future using ground-based interferometers. Furthermore, we provide evidence which suggests that the source of remnant black hole perturbations is roughly independent of the overtone index in a given angular harmonic across binary parameter space, at least for overtones with $n\ensuremath{\lesssim}2$. This finding may hint at the spatiotemporal structure of ringdown perturbations in black hole coalescences, as well as the regime of validity of perturbation theory in the ringdown of these events.

Abstract We are interested in improving validity results for the nonlinear Schrödinger (NLS) approximation beyond the natural time scale for completely integrable systems. As a first step, we consider this approximation for the Korteweg–de Vries equation with initial conditions for which the scattering data contain no eigenvalues. By performing a linear Schrödinger approximation for the scattering data, the error made by this approximation has only to be estimated for a purely linear problem which gives estimates beyond the natural NLS time scale. The inverse scattering transform allows us to transfer these estimates to the original variables.

Abstract Permutation Entropy and statistiCal Complexity Analysis for astRophYsics (PECCARY) is a computationally inexpensive, statistical method by which any time series can be characterized as predominantly regular, complex, or stochastic. Elements of the PECCARY method have been used in a variety of physical, biological, economic, and mathematical scenarios, but have not yet gained traction in the astrophysical community. This study introduces the PECCARY technique with the specific aims to motivate its use in and optimize it for the analysis of astrophysical orbital systems. PECCARY works by decomposing a time-dependent measure, such as the x-coordinate or orbital angular momentum time series, into ordinal patterns. Due to its unique approach and statistical nature, PECCARY is well suited for detecting preferred and forbidden patterns (a signature of chaos), even when the chaotic behavior is short-lived or when working with a relatively short-duration time series or small sets of time-series data. A variety of examples are used to demonstrate the capabilities of PECCARY. These include mathematical examples (sine waves, varieties of noise, well-known chaotic functions), a double pendulum system, and astrophysical tracer particle simulations with potentials of varying intricacies. Since the adopted timescale used to diagnose a given time series can affect the outcome, a method is presented to identify an ideal sampling scheme, constrained by the overall duration and the natural timescale of the system. The accompanying PECCARY Python package and its usage are discussed.

Understanding how biomarkers change in relation to disease pathogenesis is a key area in biomedical research. We propose a semiparametric joint model to analyze the temporal evolution of biomarkers prior to the onset of disease. The model allows for a flexible biomarker trajectory that depends on two time scales: a natural time scale such as age and time to disease onset. In practice, the natural time scale often differs from time-on-study, leading to analytical challenges such as left-truncation bias. We introduce a profile kernel estimating equationapproach to estimate regression coefficients and unspecified baseline mean trajectory functions. We establish the large-sample properties of the proposed estimators and conduct simulation studies to evaluate their finite-sample performance. Our method is applied to investigate brain biomarker trajectories before the onset of preclinical Alzheimer's disease. We observed a decline in cortical thickness prior to disease onset across brain regions, with APOE4 carriers showing lower levels compared to non-carriers.

Attosecond microscopy aims to record electron movement on its natural length and time scale. It is a gateway to understanding the interaction of matter and light, the coupling between excitations in solids, and the resulting energy flow and decoherence behavior, but it demands simultaneous temporal and spatial resolution. Modern science has conquered these scales independently, with ultrafast light sources providing sub-femtosecond pulses and advanced microscopes achieving sub-nanometer resolving power. In this perspective, we inspect the challenges raised by combining extreme temporal and spatial resolution and then highlight how upcoming experimental techniques overcome them to realize laboratory-scale attosecond microscopes. Referencing proof-of-principle experiments, we delineate the techniques’ strengths and their applicability to observing various ultrafast phenomena, materials, and sample geometries.

Abstract The characterization of in-liquid discharges is known to be a challenging feat due to their stochastic nature and nanosecond time scale evolution. In this study, the time-resolved electron density (n e) of a spark discharge in water is analyzed by coupling optical emission spectroscopy (OES) measurements with a Bayesian model. It is first highlighted that a single Voigt profile cannot adequately describe the time-averaged Hα line profile; this is due to the significant time evolution of the discharge properties. To overcome this limitation, a model describing the temporal evolution of the line emission intensity and shape is developed and used to fit the time-integrated spectrum. The unknown parameters in the model are determined using the Dynesty python package, according to the Bayesian nested sampling method. With such model, the simulated and measured spectrum of the Hα transition agree very well. Over the range of experimental conditions investigated, it is found that the electron density rapidly reaches ∼ 2 × 10 25 m − 3 and then decreases exponentially with a decay time of ∼ 238 ns . These values are consistent with those determined using time-resolved measurements and analysis of the Hα and O I line broadenings. Overall, this study shows that time-resolved plasma properties can be obtained from time-integrated OES data by applying a Bayesian-based modeling approach. Further studies are needed to expand the scope of the developed model and determine plasma properties over a broad range of conditions.

Photonic solutions are potentially highly competitive for energy-efficient neuromorphic computing. However, a combination of specialized nanostructures is needed to implement all neuro-biological functionality. Here, we show that donor-acceptor Stenhouse adduct dyes integrated with III-V semiconductor nano-optoelectronics have combined excellent functionality for bio-inspired neural networks. The dye acts as synaptic weights in the optical interconnects, while the nano-optoelectronics provide neuron reception, interpretation and emission of light signals. These dyes can reversibly switch from absorbing to non-absorbing states, using specific wavelength ranges. Together, they show robust and predictable switching, low energy thermal reset and a memory dynamic range from days to sub-seconds that allows both short- and long-term memory operation at natural timescales. Furthermore, as the dyes do not need electrical connections, on-chip integration is simple. We illustrate the functionality using individual nanowire photodiodes as well as arrays. Based on the experimental performance metrics, our on-chip solution is capable of operating an anatomically validated model of the insect brain navigation complex.

The evolution mechanisms of electrons in isolated atoms, molecules and complex systems on a natural time-scale have long been a fundamental question in atomic and molecular physics, with significant implications for the applications of quantum materials. Over the past two decades, the development of attosecond light pulses and attosecond metrology has opened new opportunities for investigating the electronic dynamics, while also posing new challenges. Traditional detection techniques, such as time-of-flight and velocity map imaging spectrometers, can be used to study the attosecond scattering phase shifts in the photoemission and ionization processes with extremely high temporal and energy resolution. However, the limitations in multi-particle coincidence detection and three-dimensional momentum correlation limit the deeper exploration of many-body correlations and non-adiabatic ultrafast dynamics involving electron-nuclear coupling. To enable multidimensional and real-time observation of the three-dimensional momenta of both electrons and ions during photoionization, the attosecond interferometry has been integrated into electron-ion coincidence systems. In this study, we utilize an attosecond coincidence interferometer that combines an attosecond pump-infrared femtosecond probe scheme with cold target recoil ion momentum spectroscopy. The apparatus enables attosecond-time-resolved momentum imaging of all charged fragments in atomic and molecular systems, thereby providing deeper insights into the dynamics of photoionization. We also highlight the recent groundbreaking applications and advances of attosecond coincidence interferometer in studying photoionization dynamics in atoms, molecules, and more complex systems.

Many chirality-sensitive light–matter interactions are governed by chiral electron dynamics. Therefore, the development of advanced technologies making use of chiral phenomena would critically benefit from measuring and controlling chiral electron dynamics on their natural attosecond timescales. Such endeavours have so far been hampered by the lack of characterized circularly polarized attosecond pulses, an obstacle that has recently been overcome1,2. Here we introduce chiroptical spectroscopy with attosecond pulses and demonstrate attosecond coherent control over photoelectron circular dichroism (PECD)3,4, as well as the measurement of chiral asymmetries in the forward–backward and angle-resolved photoionization delays of chiral molecules. We show that co-rotating attosecond and near-infrared (IR) pulses can nearly double the PECD and even change its sign compared with single-photon ionization. We demonstrate that chiral photoionization delays depend on both polar and azimuthal angles of photoemission in the light-propagation frame, requiring 3D momentum resolution. We measure forward–backward chiral-sensitive delays of up to 60 as and polar-angle-resolved photoionization delays of up to 240 as, which include an asymmetry of about 60 as originating from chirality in the continuum–continuum transitions. Attosecond chiroptical spectroscopy opens the door to quantitatively understanding and controlling the dynamics of chiral molecules on the electronic timescale.

Zika virus is spread to human populations primarily by Aedes aegypti mosquitoes, and Zika virus disease has been linked to a number of developmental abnormalities and miscarriages, generally coinciding with infection during early pregnancy. In this paper, we propose a new mathematical model for the transmission of Zika and study a range of control strategies to reduce the incidence of affected pregnancies in an outbreak. While most infectious disease models primarily focus on measures of the spread of the disease, our model is formulated to estimate the number of affected pregnancies throughout the simulated outbreak. Thus the effectiveness of control measures and parameter sensitivity analysis is done with respect to this metric. In addition to traditional intervention strategies, we consider the introduction of Wolbachia-infected mosquitoes into the native population. Our results suggest a threshold parameter for Wolbachia as an effective control measure, and show the natural time scale needed for Wolbachia-infected mosquitoes to effectively replace the native population. Additionally, we consider the possibility of a Zika vaccine, both to avoid an outbreak through herd immunity and as a control measure administered during an active outbreak. With emerging data on persistence of Zika virus in semen, the proposed compartmental model also includes a component of post-infectious males, which introduces a longer time scale for sexual transmission than the primary route. While the overall role of sexual transmission of Zika in an outbreak scenario is small compared with the dominant human-vector route, this model predicts conditions under which subpopulations may make this secondary route more significant.

Resolving laser-driven electron dynamics on their natural time and length scales is essential for understanding and controlling light-induced phenomena. Capabilities to reveal these dynamics are limited by challenges in interpreting wave mixing of a driving and a probe pulse, low energy resolution at ultrashort time scales and a lack of atomic-scale resolution by standard spectroscopic techniques. Here, we demonstrate how ultrafast x-ray diffraction can access fundamental information on laser-driven electronic motion in solids. We propose a method based on subcycle-resolved x-ray-optical wave mixing that allows for a straightforward reconstruction of key properties of strong-field-induced electron dynamics with atomic spatial resolution. Namely, this technique provides both phases and amplitudes of the spatial Fourier transform of optically-induced charge distributions, their temporal behavior, and the direction of the instantaneous microscopic optically-induced electron current flow. It captures the rich microscopic structures and symmetry features of laser-driven electronic charge and current density distributions.

Over the past few years, many scholars began to study averaging principles for fractional stochastic differential equations since they can provide an approximate analytical method to reduce such systems. However, in the most previous studies, there is a misunderstanding of the standard form of fractional stochastic differential equations, which consequently causes the wrong estimation of the convergence rate. In this note, we take fractional stochastic differential equations with Lévy noise as an example to clarify these two issues. The corrections herein have no effect on the main proofs except the two points mentioned above. The innovation of this paper lies in three aspects: (i) the standard form of the fractional stochastic differential equations is derived under natural time scale; (ii) it is first proved that the convergence interval and rate are related to the fractional order; and (iii) the presented results contain and improve some well known research achievements.

Wait time to stochastic self-focusing

Reduced probability densities of long-lived metastable states as those of distributed thermal systems: Possible experimental implications for supercooled fluids

Advances in coherent light sources and development of pump-probe techniques in recent decades have opened the way to study electronic motion in its natural time scale. When an ultrashort laser pulse interacts with a molecular target, a coherent superposition of electronic states is created and the triggered electron dynamics is coupled to the nuclear motion. A natural and computationally efficient choice to simulate this correlated dynamics is a trajectory-based method where the quantum-mechanical electronic evolution is coupled to a classical-like nuclear dynamics. These methods must approximate the initial correlated electron-nuclear state by associating an initial electronic wavefunction to each classical trajectory in the ensemble. Different possibilities exist that reproduce the initial populations of the exact molecular wavefunction when represented in a basis. We show that different choices yield different dynamics and explore the effect of this choice in Ehrenfest, surface hopping, and exact-factorization-based coupled-trajectory schemes in a one-dimensional two-electronic-state model system that can be solved numerically exactly. This work aims to clarify the problems that standard trajectory-based techniques might have when a coherent superposition of electronic states is created to initialize the dynamics, to discuss what properties and observables are affected by different choices of electronic initial conditions and to point out the importance of quantum-momentum-induced electronic transitions in coupled-trajectory schemes.