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Phenomena of free-electron X-ray radiation are treated almost exclusively with classical electrodynamics, despite the intrinsic interaction being that of quantum electrodynamics. The lack of quantumness arises from the vast disparity between the electron energy and the much smaller photon energy, resulting in a small cross-section that makes quantum effects negligible. Here we identify a fundamentally distinct phenomenon of electron radiation that bypasses this energy disparity, and thus displays extremely strong quantum features. This phenomenon arises when free-electron transverse scattering occurs during the radiation process, creating entanglement between each transversely recoiled electron and the photons it emitted. This phenomenon profoundly modifies the characteristics of free-electron radiation mediated by crystals, compared to conventional classical analysis and even previous quantum analysis. We also analyze conditions to detect this phenomenon using low-emittance electron beams and high-resolution X-ray spectrometers. These quantum radiation features could guide the development of compact coherent X-ray sources facilitated by nanophotonics and quantum optics. Free-electron X-ray radiation is typically treated classically. Here, the authors identify unprecedented quantum features of X-ray emission from crystals, due to transversely recoiled electron-photon entanglement, enabling precisely tuned emission via quantum control of free electrons. [ABSTRACT FROM AUTHOR]

Terahertz (THz) radiation has broad applications ranging from medical imaging to spectroscopy. One viable source of high‐intensity THz radiation is the Smith–Purcell (SP) effect, which involves charge carriers moving over a periodic surface. Conventional SP emitters use electron beams to generate charge carriers, necessitating bulky electron acceleration stages. Here, a compact design for generating THz SP radiation using mobile charge carriers within 2D materials is proposed. This circumvents the beam alignment and beam divergence challenge, allowing for a reduction in the electron‐grating separation from tens of nm to 5 nm or less, leading to more efficient near‐field excitation and a potentially chip‐level THz source. In such a configuration, it is shown that the optimal electron velocity and the corresponding maximum radiation intensity can be predicted from the electron‐grating separation. The numerical demonstration shows that hot electrons can excite SP radiation in graphene on a silicon grating, and the radiation intensity can be increased by graphene surface plasmons. This study can be extended to a broad variety of charge carriers in 2D materials, thus allowing for compact, tunable, and low‐cost THz sources. [ABSTRACT FROM AUTHOR]

We present the design and performance of a compact ultrafast electron diffraction instrument. The diffractometer provides a means of examining time-resolved ultrafast dynamical properties of solids. The system's utilization is discussed in terms of instrument parameters and diffraction data from selected condensed matter samples. The difractometer's performance is highlighted in terms of detection sensitivity, instrumental temporal resolution, and the electron beam transverse coherence length. Following specific details of the construction, we present a practical discussion of parameters such as repetition rate and provide advice on general construction approaches for laboratory-based, keV ultrafast electron diffractometers. In addition, design guidance for constructing a compact electron gun source that is well-suited for studying diffraction from hard condensed matter is given. A unique data acquisition scheme, utilizing high laser repetition rates, is presented. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) materials have attracted much interest for their myriad unique electronic, mechanical, and thermal properties. In particular, they are promising candidates for monochromatic, table‐top X‐ray sources. This work reveals that the versatility of the table‐top vdW X‐ray source goes beyond what has been demonstrated so far. By introducing a tilt angle between the vdW structure and the incident electron beam, it is theoretically and experimentally shown that the accessible photon energy range is more than doubled. This allows for greater versatility in real‐time tuning of the vdW X‐ray source. Furthermore, this work shows that the accessible photon energy range is maximized by simultaneously controlling both the electron energy and the vdW structure tilt. These results will pave the way for highly tunable, compact X‐ray sources, with potential applications including hyperspectral X‐ray fluoroscopy and X‐ray quantum optics. [ABSTRACT FROM AUTHOR]

The interaction of electrons with strong electromagnetic fields is fundamental to the ability to design high‐quality radiation sources. At the core of all such sources is a tradeoff between compactness and higher output radiation intensities. Conventional photonic devices are limited in size by their operating wavelength, which helps compactness at the cost of a small interaction area. Here, plasmonic modes supported by multilayer graphene metamaterials are shown to provide a larger interaction area with the electron beam, while also tapping into the extreme confinement of graphene plasmons to generate high‐frequency photons with relatively low‐energy electrons available from tabletop sources. For 5 MeV electrons, a metamaterial of 50 layers and length 50 µm, and a beam current of 1.7 µA, it is, for instance, possible to generate X‐rays of intensity 1.5 × 107 photons sr−1 s−1 1%BW, 580 times more than for a single‐layer design. The frequency of the driving laser dynamically tunes the photon emission spectrum. This work demonstrates a unique free‐electron light source, wherein the electron mean free path in a given material is longer than the device length, relaxing the requirements of complex electron beam systems and potentially paving the way to high‐yield, compact, and tunable X‐ray sources. [ABSTRACT FROM AUTHOR]