Free-electron crystals for enhanced X-ray radiation

Bremsstrahlung—the spontaneous emission of broadband radiation from free electrons that are deflected by atomic nuclei—contributes to the majority of X-rays emitted from X-ray tubes and used in applications ranging from medical imaging to semiconductor chip inspection.Here,we show that the bremsstrahlung intensity can be enhanced significantly—by more than three orders of magnitude—through shaping the electron wavefunction to periodically overlap with atoms in crystalline materials.Furthermore,we show how to shape the bremsstrahlung X-ray emission pattern into arbitrary angular emission profiles for purposes such as unidirectionality and multi-directionality.Importantly,we find that these enhancements and shaped emission profiles cannot be attributed solely to the spatial overlap between the electron probability distribution and the atomic centers,as predicted by the paraxial and nonrecoil theory for free electron light emission.Our work highlights an unprecedented regime of free electron light emission where electron waveshaping provides multi-dimensional control over practical radiation processes like bremsstrahlung.Our results pave the way towards greater versatility in table-top X-ray sources and improved fundamental understanding of quantum electron-light interactions.

Electron-beam-driven anomalous Doppler effects in Smith–Purcell radiation

The interaction between electrons and matter is an effective means of light emission,through mechanisms including Cherenkov radiation and Smith–Purcell radiation(SPR).In this study,we show that the superlight inverse Doppler effects can be realized in reverse Smith–Purcell radiation excited by a free electron beam with a homogeneous substrate.In particular,we find that two types of anomalous SPR exist in the homogenous substrate:special SPR and reverse SPR.Our results reveal that the electron velocity can be tuned to simultaneously excite different combinations of normal SPR,special SPR,and reverse SPR.The proposed manifold light radiation mechanism can offer greater versatility in controlling and shaping SPR.

Free-electron interactions with van der Waals heterostructures: a source of focused X-ray radiation

The science and technology of X-ray optics have come far,enabling the focusing of X-rays for applications in highresolution X-ray spectroscopy,imaging,and irradiation.In spite of this,many forms of tailoring waves that had substantial impact on applications in the optical regime have remained out of reach in the X-ray regime.This disparity fundamentally arises from the tendency of refractive indices of all materials to approach unity at high frequencies,making X-ray-optical components such as lenses and mirrors much harder to create and often less effcient.Here,we propose a new concept for X-ray focusing based on inducing a curved wavefront into the X-ray generation process,resulting in the intrinsic focusing of X-ray waves.This concept can be seen as effectively integrating the optics to be part of the emission mechanism,thus bypassing the effciency limits imposed by X-ray optical components,enabling the creation of nanobeams with nanoscale focal spot sizes and micrometer-scale focal lengths.Specifically,we implement this concept by designing aperiodic vdw heterostructures that shape X-rays when driven by free electrons.The parameters of the focused hotspot,such as lateral size and focal depth,are tunable as a function of an interlayer spacing chirp and electron energy.Looking forward,ongoing advances in the creation of many-layer vdw heterostructures open unprecedented horizons of focusing and arbitrary shaping of X-ray nanobeams.

Metasurface-based multi-harmonic freeelectron light source

Metasurfaces are subwavelength spatial variations in geometry and material where the structures are of negligible thickness compared to the wavelength of light and are optimized for far-field applications,such as controlling the wavefronts of electromagnetic waves.Here,we investigate the potential of the metasurface near-field profile,generated by an incident few-cycle pulse laser,to facilitate the generation of high-frequency light from free electrons.In particular,the metasurface near-field contains higher-order spatial harmonics that can be leveraged to generate multiple higher-harmonic X-ray frequency peaks.We show that the X-ray spectral profile can be arbitrarily shaped by controlling the metasurface geometry,the electron energy,and the incidence angle of the laser input.Using ab initio simulations,we predict bright and monoenergetic X-rays,achieving energies of 30 keV(with harmonics spaced by 3 keV)from 5-MeV electrons using 3.4-eV plasmon polaritons on a metasurface with a period of 85 nm.As an example,we present the design of a four-color X-ray source,a potential candidate for tabletop multicolor hard X-ray spectroscopy.Our developments could help pave the way for compact multi-harmonic sources of high-energy photons,which have potential applications in industry,medicine,and the fundamental sciences.

Surface Dyakonov-Cherenkov radiation

Recent advances in engineered material technologies(e.g.,photonic crystals,metamaterials,plasmonics,etc.)provide valuable tools to control Cherenkov radiation.In all these approaches,however,the particle velocity is a key parameter to affect Cherenkov radiation in the designed material,while the influence of the particle trajectory is generally negligible.Here,we report on surface Dyakonov-Cherenkov radiation,i.e.the emission of directional Dyakonov surface waves from a swift charged particle moving atop a birefringent crystal.This new type of Cherenkov radiation is highly susceptible to both the particle velocity and trajectory,e.g.we observe a sharp radiation enhancement when the particle trajectory falls in the vicinity of a particular direction.Moreover,close to the Cherenkov threshold,such a radiation enhancement can be orders of magnitude higher than that obtained in traditional Cherenkov detectors.These distinct properties allow us to determine simultaneously the magnitude and direction of particle velocities on a compact platform.The surface Dyakonov-Cherenkov radiation studied in this work not only adds a new degree of freedom for particle identification,but also provides an all-dielectric route to construct compact Cherenkov detectors with enhanced sensitivity.