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Non-reciprocal light propagation is essential to control optical crosstalk and back-scatter in photonic systems. However, realizing high-fidelity non-reciprocity in low-loss integrated photonic circuits remains challenging. Here, we experimentally demonstrate a form of non-local acousto-optic light scattering to produce non-reciprocal single-sideband modulation and mode conversion in an integrated silicon photonic platform. In this system, a travelling-wave acoustic phonon driven by optical forces in a silicon waveguide spatiotemporally modulates light in a separate waveguide through linear interband Brillouin scattering. This process extends narrowband optomechanics-based schemes for non-reciprocity to travelling-wave physics, enabling large operation bandwidths of more than 125 GHz and up to 38 dB of non-reciprocal contrast between forward- and backward-propagating optical waves. The modulator operation wavelength is tunable over a 35-nm range by varying the optical drive wavelength. Such travelling-wave acousto-optic interactions provide a promising path toward the realization of broadband, low-loss isolators and circulators within integrated photonics.

The hallmark of topological insulators (TIs) is the scatter-free propagation of waves in topologically protected edge channels 1 . This transport is strictly chiral on the outer edge of the medium and therefore capable of bypassing sharp corners and imperfections, even in the presence of substantial disorder. In photonics, two-dimensional (2D) topological edge states have been demonstrated on several different platforms 2 – 4 and are emerging as a promising tool for robust lasers 5 , quantum devices 6 – 8 and other applications. More recently, 3D TIs were demonstrated in microwaves 9 and  acoustic waves 10 – 13 , where the topological protection in the latter  is induced by dislocations. However, at optical frequencies, 3D photonic TIs have so far remained out of experimental reach. Here we demonstrate a photonic TI with protected topological surface states in three dimensions. The topological protection is enabled by a screw dislocation. For this purpose, we use the concept of synthetic dimensions 14 – 17 in a 2D photonic waveguide array 18 by introducing a further modal dimension to transform the system into a 3D topological system. The lattice dislocation endows the system with edge states propagating along 3D trajectories, with topological protection akin to strong photonic TIs 19 , 20 . Our work paves the way for utilizing 3D topology in photonic science and technology. By introducing a further modal dimension to transform a two-dimensional photonic waveguide array, a photonic topological insulator with protected topological surface states in three dimensions, enabled by a screw dislocation, is demonstrated.

Topological insulators are materials that conduct on the surface and insulate in their interior due to non-trivial topology of the band structure. The edge states on the interface between topological (non-trivial) and conventional (trivial) insulators are topologically protected from scattering due to structural defects and disorders. Recently, it was shown that photonic crystals (PCs) can serve as a platform for realizing a scatter-free propagation of light waves. In conventional PCs, imperfections, structural disorders, and surface roughness lead to significant losses. The breakthrough in overcoming these problems is likely to come from the synergy of the topological PCs and silicon-based photonics technology that enables high integration density, lossless propagation, and immunity to fabrication imperfections. For many applications, reconfigurability and capability to control the propagation of these non-trivial photonic edge states is essential. One way to facilitate such dynamic control is to use liquid crystals (LCs), which allow to modify the refractive index with external electric field. Here, we demonstrate dynamic control of topological edge states by modifying the refractive index of a LC background medium. Background index is changed depending on the orientation of a LC, while preserving the topology of the system. This results in a change of the spectral position of the photonic bandgap and the topological edge states. The proposed concept might be implemented using conventional semiconductor technology, and can be used for robust energy transport in integrated photonic devices, all-optical circuity, and optical communication systems.

In the early days of quantum mechanics Kapitza and Dirac predicted that matter waves would scatter off the optical intensity grating formed by two counter-propagating light waves. This interaction, driven by the ponderomotive potential of the optical standing wave, was both studied theoretically and demonstrated experimentally for atoms and electrons. In the original version of the experiment, only the transverse momentum of particles was varied, but their energy and longitudinal momentum remained unchanged after the interaction. Here, we report on the generalization of the Kapitza-Dirac effect. We demonstrate that the energy of sub-relativistic electrons is strongly modulated on the few-femtosecond timescale via the interaction with a travelling wave created in vacuum by two colliding laser pulses at different frequencies. This effect extends the possibilities of temporal control of freely propagating particles with coherent light and can serve the attosecond ballistic bunching of electrons, or for the acceleration of neutral atoms or molecules by light.

The ionosphere's dynamic structure affects electromagnetic radiation by altering radio wave propagation, impacting daily communications. The characteristics of the ionosphere are primarily characterized by electron density parameters. This paper proposes a method to construct Three‐Dimensional (3‐D) electron density distributions with arbitrary spatiotemporal resolution in ISR observational regions. The method, termed Artificial Intelligence‐based data assimilation (AI‐Assim), integrates data assimilation directly into a neural network. It assimilates electron density from the IRI‐2020 model to fill ISR observation gaps. Experiments conducted using the Sanya Incoherent Scatter Radar (SYISR) in Hainan, China, successfully constructed a 3‐D electron density structure over the region, with a 0.2° latitude/longitude resolution and 1 km height resolution. The method's effectiveness was validated by calculating the mean square error and comparing the results with digisonde measurements. Plain Language Summary This study leverages the most powerful ionospheric observation tool, the ISR, to construct 3‐D electron density distributions with arbitrary spatial resolution. Relying solely on empirical models often leads to accuracy issues, while 3‐D electron density models based purely on observational methods typically suffer from low resolution. All observational methods encounter difficulties in achieving continuous, high spatial resolution monitoring of the entire sky, and ISR is one of the most effective techniques available. However, even with interpolation methods, the coverage area of ISR remains limited. Therefore, this study explores a method that uses the neural network to assimilate electron density values from the IRI‐2020 model, aiming to fill the gaps in ISR detection. By assimilating International Reference Ionosphere values to approximate observed values, the accuracy of the 3‐D electron density results is enhanced. Multiple iterations of AI‐ Assim enable the construction of 3‐D electron density distributions with arbitrary spatial resolution. Key Points We developed a method for constructing 3‐D electron density distributions with arbitrary spatiotemporal resolution at ISR stations The method termed AI‐Assim, continuously assimilating electron density from the IRI‐2020 model to fill ISR observation gaps Experiments using SYISR data achieved a 3‐D electron density model with 0.2° map resolution and 1 km height resolution

Pitch angle scattering of pickup ions (PUIs) in the outer heliosheath is studied using two-dimensional hybrid simulations, with the PUIs continuously injected at specified injection rates and pickup angles. The simulations reveal the growth of quasiparallel-propagating waves, quasi-antiparallel-propagating waves, and the oblique mirror mode. The excited waves scatter the PUIs and can reverse their parallel velocities with respect to the background magnetic field. The scattering time, defined as the time when the squared standard deviation of the pitch angles of the PUIs approaches thresholds of 0.2 or 0.4, increases as the PUI injection rate decreases, approximately following a power law. The scattering time estimated for a realistic injection rate in the outer heliosheath is about 0.5 yr at a 45° pickup angle. This scattering time is shorter than the average charge exchange time of about 2 yr for the PUIs to be converted into energetic neutral atoms (ENAs). This result conflicts with the spatial retention scenario of the secondary ENA mechanism, which assumes weak (or incomplete) scattering of PUIs in the off-ribbon directions to explain the ENA ribbon discovered by the Interstellar Boundary Explorer.

Accurate, precise, and computationally efficient removal of unwanted activity that exists as a combination of periodic, quasiperiodic, and nonperiodic systematic trends in time-series photometric data is a critical step in exoplanet transit analysis. Throughout the years, many different modeling methods have been used for this process, often called “detrending.” However, there is no community-wide consensus regarding the favored approach. In order to mitigate model dependency, we present an ensemble-based approach to detrending via a community of models and the democratic detrender: a modular and scalable open-source coding package that implements ensemble detrending. The democratic detrender allows users to select from a number of packaged detrending methods (including cosine filtering, Gaussian processes, and polynomial fits) or provide their own set of detrended light curves via methods of their choosing. It then combines the individually detrended light curves into a single method marginalized light curve. Additionally, the democratic detrender inflates each data point’s uncertainty based on the scatter between detrenders, thereby propagating model-selection uncertainty into the final light curve. This ensemble strategy does not guarantee improvement over the single best-performing detrending method, but it substantially reduces the risk of selecting a detrending solution that is poorly calibrated or overfit to noise.

A bstract We revisit the picture of jets propagating in the quark-gluon plasma. In addition to vacuum radiation, partons scatter on the medium constituents resulting in induced emissions. Analytical approaches to including these interactions have traditionally dealt separately with multiple, soft, or rare, hard scatterings. A full description has so far only been available using numerical methods. We achieve full analytical control of the relevant scales and map out the dominant physical processes in the full phase space. To this aim, we extend existing expansion schemes for the medium-induced spectrum to the Bethe-Heitler regime. This covers the whole phase space from early to late times, and from hard splittings to emissions below the thermal scale. Based on the separation of scales, a space-time picture naturally emerges: at early times, induced emissions start to build from rare scatterings with the medium. At a later stage, induced emissions due to multiple soft scatterings result in a turbulent cascade that rapidly degrades energy down to, and including, the Bethe-Heitler regime. We quantify the impact of such an improved picture, compared to the current state-of-the-art factorization that includes only soft scatterings, by both analytical and numerical methods for the medium-induced energy distribution function. Our work serves to improve our understanding of jet quenching from small to large systems and for future upgrades of Monte Carlo generators.

Parker Solar Probe (PSP) observations of a small dispersive event on 2022 February 27 and 28 indicate scatter-free propagation as the dominant transport mechanism between the low corona and greater than 35 solar radii. The event occurred during unique orbital conditions that prevailed along specific flux tubes that PSP encountered repeatedly between 25 and 35 Rs during outbound orbit 11. This segment of the PSP orbit exhibits almost stationary angular motion relative to the rotating solar surface, such that in the rotating frame, PSP’s motion is essentially radial. The time dispersion often observed in impulsive solar energetic particle (SEP) events continues in this case down to velocities including the core solar-wind ion velocities. Especially at the onset of this event, the 3He content is much larger than the usual SEP abundances seen in the energy range from ∼100 keV to several MeV for helium. Later in the event, iron is enhanced. The compositional signatures suggest this to be an example of an acceleration mechanism for generating the seed energetic particles required by shock (or compression) acceleration models in SEP events to account for the enrichment of various species above solar abundances in such events. A preliminary search of similar orbital conditions over the PSP mission has not revealed additional such events, although favorable conditions (isolated impulsive acceleration and well-ordered magnetic field connection with minimal magnetic field fluctuation) that would be required are infrequently realized, given the small fraction of the PSP trajectory that meets these observation conditions.

Locally generated fast magnetosonic (local MS) waves in the Martian ionosphere have been reported recently, which are different from the solar wind‐driven MS waves. However, their effects on particle scattering have not been reported yet. This study pioneers the investigation of electron, proton, and oxygen atomic ion scattering induced by local MS waves through quasi‐linear diffusion theory. The kinetic dispersion relations of local MS waves have been applied to compute diffusion coefficients. The results demonstrate that local MS waves can efficiently scatter suprathermal electrons via Landau resonance, especially for thermal electrons propagating at large pitch angles. For ion scattering, hot protons with small pitch angles undergo effective scattering through cyclotron resonance, while oxygen atomic ions at tens of eVs are scattered through high‐order cyclotron resonance. These findings reveal the potential role played by local MS waves in the evolution of Martian ionosphere.