Strong-field Regime Research Articles

Enantiosensitive steering of free-induction decay.

Chiral discrimination, a problem of vital importance, has recently become an emerging frontier in ultrafast physics, with remarkable progress achieved in multiphoton and strong-field regimes. Rydberg excitations, unavoidable in the strong-field regime and intentional for few-photon processes, arise in all these approaches. Here, we show how to harness this ubiquitous feature by introducing a new phenomenon, enantiosensitive free-induction decay, steered by a tricolor chiral field at a gentle intensity, structured in space and time. We demonstrate theoretically that an excited chiral molecule accumulates an enantiosensitive phase due to perturbative interactions with the tricolor chiral field, resulting in a spatial phase gradient steering the free-induction decay in opposite directions for opposite enantiomers. Our work introduces a general, extremely sensitive, all-optical enantiosensitive detection technique that avoids strong fields and takes full advantage of recent advances in structuring light.

Testing Horndeski Gravity from EHT Observational Results for Rotating Black Holes

The Event Horizon Telescope (EHT) collaboration recently unveiled the first image of the supermassive black hole M87*, which exhibited a ring of angular diameter θ d = 42 ± 3 μas, a circularity deviation of ΔC ≤ 0.1, and also inferred a black hole mass of M = (6.5 ± 0.7) × 109 M ⊙. This provides a new window onto tests of theories of gravity in the strong-field regime, including probes of violations of the no-hair theorem. It is widely believed that the Kerr metric describes astrophysical black holes, as encapsulated in the critical but untested no-hair theorem. Modeling Horndeski gravity black holes—with an additional hair parameter h besides the mass M and spin a of the Kerr black hole—as the supermassive black hole M87*, we observe that to be a viable astrophysical black hole candidate, the EHT result constrains the (a, h) parameter space. However, a systematic bias analysis indicates that rotating Horndeski black hole shadows may or may not capture Kerr black hole shadows, depending on the parameter values; the latter is the case over a substantial part of the constrained parameter space, allowing Horndeski gravity and general relativity to be distinguishable in the said space, and opening up the possibility of potential modifications to the Kerr metric.

Revealing the exotic structure of molecules in strong magnetic fields.

A novel implementation for the calculation of molecular gradients under strong magnetic fields is employed at the current-density functional theory level to optimize the geometries of molecular structures, which change significantly under these conditions. An analog of the ab initio random structure search is utilized to determine the ground-state equilibrium geometries for Hen and CHn systems at high magnetic field strengths, revealing the most stable structures to be those in high-spin states with a planar geometry aligned perpendicular to the field. The electron and current densities for these systems have also been investigated to develop an explanation of chemical bonding in the strong field regime, providing an insight into the exotic chemistry present in these extreme environments.

Self-Stopping of Light.

Here, we show that light can bring itself to a complete standstill (self-stop) via self-interaction mediated by the resonant nonlinearity in a fully homogeneous medium. An intense few-cycle pulse, entering the medium, may reshape to form a strongly coupled light-matter bundle, in which the energy is transferred from light to the medium and back periodically on the single-cycle scale. Such oscillating structure can decelerate, alter its propagation direction, and even completely stop, depending on the state of its internal degrees of freedom. This phenomenon is expected to occur in the few-cycle strong-field regime when the Rabi oscillation frequency becomes comparable with the frequency of the incoming light.

Constraining the Konoplya-Rezzolla-Zhidenko deformation parameters III: Limits from stellar-mass black holes using gravitational-wave observations

Gravitational-wave observations of binary black holes provide a suitable arena to test the fundamental nature of gravity in the strong-field regime. Using the data of the inspiral of 29 events detected by the LIGO-Virgo observatories, we perform a theory-agnostic test of the Kerr hypothesis. We compute the leading-order deviation to the gravitational waves emitted in the frequency domain and provide constraints on two deformation parameters (${\ensuremath{\delta}}_{1}$ and ${\ensuremath{\delta}}_{2}$) belonging to a general class of axisymmetric non-Kerr black hole spacetimes proposed by Konoplya, Rezzolla, and Zhidenko. Our study shows that all the analyzed events are consistent with the Kerr hypothesis. The LIGO-Virgo data provide stronger constraints on ${\ensuremath{\delta}}_{1}$ and ${\ensuremath{\delta}}_{2}$ than those obtained in our previous studies with x-ray data, while, on the other hand, they cannot constrain the other deformation parameters of the Konoplya-Rezzolla-Zhidenko metric (${\ensuremath{\delta}}_{3}$, ${\ensuremath{\delta}}_{4}$, ${\ensuremath{\delta}}_{5}$, and ${\ensuremath{\delta}}_{6}$).

Precision laser diagnostics for LUXE

Abstract Strong field QED is an active research frontier. The investigation of fundamental phenomena such as pair creation, photon-photon and photon-electron interactions in the nonlinear QED regime are a formidable challenge both experimentally and theoretically. Several experiments around the world are being planned or in preparation to probe this strong field regime. LUXE (Laser Und XFEL Experiment) is an experimental platform which envisages the collision of the high quality 16.5 GeV electron beam from the European XFEL accelerator with a 100 TW class high power laser. One of the unique features of LUXE is to measure the key observables such as pair rates (e + e −) with unprecedented accuracy in the characterization of both beams together with ample statistics. The state-of-art detector technologies for high energy particle/photon detection enable percent level precision. The state-of-art high power lasers offer high quality laser beams, however, the residual shot-to-shot fluctuations coupled with the large nonlinearity of the processes under investigation form a particular challenge. An uncertainty of 5% on the absolute laser intensity already leads to a very large ( about 40%) uncertainty in the pair rate. Hence it becomes essential to control the laser parameters precisely. To mitigate this issue a full suite of laser diagnostics is being currently developed at the JETI 40 laser in Jena with the aim of tagging the shot intensity to < 1%. In this presentation, details of the laser and the diagnostics suit for the single shot tagging of all the laser parameters will be presented. Moreover, results from an ongoing campaign to properly relay image the beam without significant distortion of the laser beam parameters for post-diagnosis will be discussed.

Theory and Phenomenology of a Four-Dimensional String–Corrected Black Hole

We construct an effective four-dimensional string-corrected black hole (4D SCBH) by rescaling the string coupling parameter in a D-dimensional Callan–Myers–Perry black hole. From the theoretical point of view, the most interesting findings are that the string corrections coincide with the so-called generalized uncertainty principle (GUP) corrections to black hole solutions, Bekenstein–Hawking entropy acquires logarithmic corrections, and that there exists a critical value of the coupling parameter for which the black hole temperature vanishes. We also find that, due to the string corrections, the nature of the central singularity may be altered from space-like to time-like singularity. In addition, we study the possibility of testing such a black hole with astrophysical observations. Since the dilaton field does not decouple from the metric, it is not a priori clear that the resulting 4D SCBH offers only small corrections to the Schwarzschild black hole. We used motion of the S2 star around the black hole at the center of our galaxy to constrain the parameters (the string coupling parameter and ADM mass) of the 4D SCBH. To test the weak gravity regime, we calculate the deflection angle in this geometry and apply it to gravitational lensing. To test the strong field regime, we calculate the black hole shadow radius. While we find that the observables change as we change the string coupling parameter, the magnitude of the change is too small to distinguish it from the Schwarzschild black hole. With the current precision, to the leading order terms, the 4D SCBH cannot be distinguished from the Schwarzschild black hole.

Recent advances in ultrafast plasmonics: from strong field physics to ultraprecision spectroscopy

Abstract Surface plasmons, the collective oscillation of electrons, enable the manipulation of optical fields with unprecedented spatial and time resolutions. They are the workhorse of a large set of applications, such as chemical/biological sensors or Raman scattering spectroscopy, to name only a few. In particular, the ultrafast optical response configures one of the most fundamental characteristics of surface plasmons. Thus, the rich physics about photon–electron interactions could be retrieved and studied in detail. The associated plasmon-enhanced electric fields, generated by focusing the surface plasmons far beyond the diffraction limit, allow reaching the strong field regime with relatively low input laser intensities. This is in clear contrast to conventional optical methods, where their intrinsic limitations demand the use of large and costly laser amplifiers, to attain high electric fields, able to manipulate the electron dynamics in the non-linear regime. Moreover, the coherent plasmonic field excited by the optical field inherits an ultrahigh precision that could be properly exploited in, for instance, ultraprecision spectroscopy. In this review, we summarize the research achievements and developments in ultrafast plasmonics over the last decade. We particularly emphasize the strong-field physics aspects and the ultraprecision spectroscopy using optical frequency combs.

Gravitational lensing by a black hole in effective loop quantum gravity

It is well known that general relativity is an effective theory of gravity at low energy scale, and actually quantum effects cannot be ignored in the strong-field regime. As a strong gravitational object, black hole plays a key role in testing the quantum effects of gravity in the strong-field regime. In this paper, we focus on black hole in effective loop quantum gravity and investigate what the influences are of the quantum effects on the weak and strong bending angles of light rays. We find that this black hole could be a Schwarzschild black hole, a regular black hole, a one-way traversable wormhole, or a two-way traversable wormhole for the different values of the quantum parameter, and the strong bending angle for this compact object exhibits two different divergent behaviors, i.e., the logarithmic divergence and non-logarithmic divergence. There are a series of relativistic images on both sides of the optical axis. Only the outermost one can be resolved as a single image, and all others are packed together at the limiting angular position. It is interesting to note that the angular separation between the outermost relativistic image and the others initially increases and then decreases as the quantum parameter increases, indicating that there is a maximum in the angular separation. The maximum is reached after the black hole becomes a wormhole, which can be taken as a signal for the formation of the wormhole. Moreover, the limiting angular position decreases as the quantum parameter increases but a little bounce occurs after the formation of the wormhole, and the relative magnification in magnitudes first decreases and then increases as the quantum parameter increases.

Constraints on charged black hole parameters using quasiperiodic oscillations data

Testing gravity theories and constraining their parameters using data from astrophysical observations of quasiperiodic oscillations (QPOs) is a powerful approach to study the features of black holes (BHs) and gravity theories in the strong field regime. To this end, we investigate the dynamics of the test particles in the vicinity of some charged BHs such as Reissner–Nordström (RN), regular Bardeen and Ayon–Beato–Garcia (ABG). In particular, we calculate harmonic oscillations and the innermost stable circular orbits (ISCOs) of test particles around these BHs. As an astrophysical application, we discuss the twin peak QPOs in relativistic precession (RP) model and give the possible values of the upper and lower frequencies of the QPOs at the distance from ISCO to infinity around the extremely charged BHs. Moreover, we also investigate the position of the orbit, where twin peak QPO may take place around central (charged) BH in the microquasar GRS 1915-105. We also study the dependence of the distance between the QPO position and ISCO from the BH charge. Using the upper and lower frequencies of the QPO, we obtain the relationship between the mass and charge of the central BH in the microquasar. Finally, we compare the effects of the spin of the Kerr BH and BH charges on the QPO in microquasar GRS 1915-105. We find that the BH charge can mimic the spin up to [Formula: see text] in the Bardeen model and up to [Formula: see text] in the case of special regular BH, admitting the same QPO frequencies.

Ultrafast plasmonic photoemission in the single-cycle and few-cycle regimes

Due to the highly increased interest in the development of state-of-the-art applications of photoemission in ultrafast electron microscopy, development of photocathodes and many more applications, a correct theoretical understanding of the underlying phenomena is needed. Within the framework of the single active electron approximation the most accurate results can be obtained by the direct solution of the time-dependent Schrödinger equation (TDSE). In this work, after a brief presentation of a numerically improved version of a mixed 1D-TDSE method, we investigated the characteristics of electron spectra obtained from the surface of metal nanoparticles irradiated with ultrashort laser pulses. During our investigation different decay lengths of the plasmonic-enhanced incident field in the vicinity of the metal were considered. Using the simulated spectra we managed to identify the behavior of the cutoff energy as a function of decay length in the strong-field, multiphoton and transition regimes.

On the attractive inverse-square potential in the induced electric dipole system under the influence of the harmonic oscillator

We obtain the analytical solutions to the Schrödinger equation for the attractive inverse-square potential in an induced electric dipole moment system under the influence of the harmonic oscillator. We show that bound states can exist when the electric field configuration brings a cut-off point that imposes a forbidden region for the neutral particle. Then, by dealing with s-waves, we obtain the energy eigenvalues in the strong electric field regime and for small values of the angular frequency of the harmonic oscillator. Further, we extend our discussion about the energy eigenvalues beyond the s-waves.

Constraints on Einstein-dilation-Gauss-Bonnet gravity from black hole-neutron star gravitational wave events

Recent gravitational wave observations allow us to probe gravity in the strong and dynamical field regime. In this paper, we focus on testing Einstein-dilation Gauss-Bonnet gravity which is motivated by string theory. In particular, we use two new neutron star black hole binaries (GW200105 and GW200115). We also consider GW190814 which is consistent with both a binary black hole and a neutron star black hole binary. Adopting the leading post-Newtonian correction and carrying out a Bayesian Markov-chain Monte Carlo analyses, we derive the 90\% credible upper bound on the coupling constant of the theory as $\sqrt{\alpha_{GB}} \lesssim 1.33\,\rm km$, whose consistency is checked with an independent Fisher analysis. This bound is stronger than the bound obtained in previous literature by combining selected binary black hole events in GWTC-1 and GWTC-2 catalogs. We also derive a combined bound of $\sqrt{\alpha_{GB}} \lesssim 1.18\,\rm km$ by stacking GW200105, GW200115, GW190814, and selected binary black hole events. In order to check the validity of the effect of higher post-Newtonian terms, we derive corrections to the waveform phase up to second post Newtonian order by mapping results in scalar-tensor theories to Einstein-dilation Gauss-Bonnet gravity. We find that such higher-order terms improve the bounds by $14.5\%$ for GW200105 and $6.9\%$ for GW200115 respectively.

Improving the accuracy of hard photon emission by sigmoid sampling of the quantum-electrodynamic table in particle-in-cell Monte Carlo simulations.

Research on laser-plasma interaction in the quantum-electrodynamic (QED) regime has been greatly advanced by particle-in-cell and Monte Carlo simulations (PIC-MC). While these simulations are widely used, we find that a noticeable numerical error arises due to inappropriate implementation of the quantum process accounting for hard photon emission and pair production in the PIC-MC codes. The error stems from the low resolution of the QED table used to sample photon energy, which is generated in the logarithmic scale and cannot resolve high energy photons. We propose a sampling method via sigmoid function that handles both the low energy and high energy end of the photon emission spectrum. It guarantees the accuracy of PIC-MC algorithms for hard photon radiation and other related processes in the strong-field QED regime.

Nonlinear Maxwell equations and strong-field electrodynamics

We investigate two Lagrangian models of nonlinear electrodynamics (NLED). These models lead to two different sets of nonlinear (NL) Maxwell equations. The first case deals with the well-known Heisenberg-Euler (HE) model of electromagnetic (EM) self-interactions in a vacuum where only the lowest orders in EM Lorentz invariants are considered. The second instance proposes an extension of the HE model. It consists of a NL Maxwell-Dirac spinor model where the EM field modifies the dynamics of the energy-momentum operator sector of the Dirac Lagrangian instead of its rest-mass term counterpart. This work complements our recent research on NL Dirac equations in the strong EM field regime.

Effective potential of scalar-tensor gravity with quartic self-interaction of scalar field

One-loop effective potential of scalar-tensor gravity with a quartic scalar field self-interaction is evaluated up to first post-Minkowskian order. The potential develops an instability in the strong field regime which is expected from an effective theory. Depending on model parameters the instability region can be exponentially far in a strong field region. Possible applications of the model for inflationary scenarios are highlighted. It is shown that the model can enter the slow-roll regime with a certain set of parameters.

Astrophysical constraints on compact objects in 4D Einstein-Gauss-Bonnet gravity

We study the properties of compact objects in a particular 4D Horndeski theory originating from higher dimensional Einstein-Gauss-Bonnet gravity. Remarkably, an exact vacuum solution is known. This compact object differs from general relativity mostly in the strong field regime. We discuss some properties of black holes in this framework and investigate in detail the properties of neutron stars, both static and in slow rotation. We find that for relatively modest deviations from general relativity, the secondary object in GW190814 is compatible with being a slowly-rotating neutron star, without resorting to very stiff or exotic equations of state. Remarkably, the equilibrium sequence of neutron stars matches asymptotically to the black hole limit, completetly closing the mass gap between neutron stars and black holes of same radius, although the stability of equilibrium solutions has yet to be determined. As a consequence, there exists a universal endpoint for the neutron star sequence, independent of the equation of state. In light of our results and of current observational constraints, we discuss specific constraints on the coupling constant that parametrizes deviations from general relativity in this theory.

Electron Dynamics and Correlations During High-Order Harmonic Generation in Be.

We investigate theoretically the high-order harmonic generation in beryllium atom irradiated by a short 1850 nm linearly polarized laser pulse in the intermediate strong-field ionization regime with the Keldysh parameter of 0.85. To this end, the respective time-dependent Schrödinger equation is solved by the time-dependent restricted-active-space configuration-interaction (TD-RASCI) method. By systematically increasing the active space of included configurations, we demonstrate an individual effect of different physical processes evoked by the pulse, which, all together, significantly enrich and extend the computed high-order harmonic generation spectrum.

Radiation reaction and limiting acceleration

We investigate the strong acceleration properties of the radiation reaction force and identify a new and promising limiting acceleration feature in the Eliezer-Ford-O'Connell model; in the strong field regime, for many field configurations, we find an upper limit to acceleration resulting in a bound to the rate of radiation emission. If this model applies, strongly accelerated particles are losing energy at a much slower pace than predicted by the usual radiation reaction benchmark, the Landau-Lifshitz equation, which certainly cannot be used in this regime. We explore examples involving various ``constant'' electromagnetic field configurations and study particle motion in a light plane wave as well as in a material medium.

Control of ultrafast photocurrent in twisted bilayer graphene by circularly polarized few-cycle lasers

We perform nonperturbative calculations of light field induced current in twisted bilayer graphene (tBLG) irradiated by a circularly polarized (CP) few-cycle laser pulse. The strong-field electron dynamics is simulated within a single-particle picture by two complementary approaches, including the velocity-gauge density-matrix equation and the length-gauge time-dependent Schr\"odinger equation, which both combine a tight-binding model for describing tBLG electronic structures. The two theoretical approaches yield the same result in the context of studying the carrier-envelope phase (CEP) and laser intensity dependent photocurrent. We show that the measured current exhibits a sinusoidal dependence on CEP of CP driving fields, with the sine phase determined by the electrode orientation with respect to the tBLG lattices. Moreover, it has been proven that an important reversal of the photocurrent direction occurs as the driving optical field strength increases to the strong-field regime, which cannot appear in the monolayer or conventional AA- and AB-stacked bilayer graphene. Based on the analysis of the conduction band population, we successfully identify that such current direction reversal in tBLG mainly originates from the interference of multichannel electron transition among two conduction bands and four valence bands closest to the Fermi level. Our results may pave the way toward investigation of nonlinear optical response in layered materials, and might provide insight for the design of future ultrafast optoelectronic devices.