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Water-window X-rays are crucial in medical and biological applications, enabling the natural-contrast imaging of biological cells without external staining. However, water-window X-ray sources with bespoke photon energies—needed in high-contrast imaging—remain challenging to obtain, except at large synchrotron facilities. Here we address this challenge by demonstrating tabletop, water-window X-ray generation from free-electron-driven van der Waals materials, enabling the continuous tuning of photon energies across the entire water-window regime. Additionally, we present a truly predictive theoretical framework combining first-principles electromagnetism with Monte Carlo simulations to accurately predict the photon flux and brightness in absolute quantities. We obtain fundamental scaling laws for the tunable photon flux, matching the experimental results and providing a way to design powerful emitters based on free-electron-driven quantum materials. We show that we can potentially achieve photon fluxes needed for imaging and spectroscopy applications (over 108photons s–1on the sample—verified by our framework based on our experimentally achieved fluxes of about 103photons s–1using ~50 nA current). Importantly, our theory highlights the critical role played by the large mean free paths and interlayer atomic spacings unique to van der Waals structures, showing the latter’s advantages over other materials in generating water-window X-rays.

Optimizing the emission properties of materials in ultraviolet and deep blue (UV-DB) is interesting in the development of new scintillator devices for the detection of X-ray, $\gamma $ -ray, and radiation particles, as those materials can be strong candidates for high light yield and fast scintillators. While their intrinsic material properties are already well studied, photonic enhancement generated through optical confinement could significantly improve their emission characteristics; however, one needs to overcome the problem of relatively low refractive indices contrast resulting in poor confinement of UV-DB light. This motivates the search for resonator structures built from readily accessible materials that can boast strong confinement in this spectral regime. Here, we present such a structure, leveraging bound states in the continuum (BICs) to realize large-area confinement of UV-DB light with ultrahigh quality factors up to $Q~\sim ~10^{7}$ . These ultrahigh $Q$ -factors, in turn, result in strong enhancements in light emission via the Purcell effect. We demonstrate the operation of such a design by simulating the mode shape, $Q$ -factor, and emission behavior in organic, hybrid perovskite, and III–V scintillating materials. By tailoring the structure geometry, it can be robustly tuned to match the emission characteristics of chosen materials. We start with considering ideal infinite structure supporting perfect BIC; we extend our model on finite-sized structures, and we discuss the limitations associated with the self-absorption and thickness of the structure. Our findings pave the way to cost-effective and efficient designs for scintillators in the UV-DB regime.

The emission of light from charged particles underlies a wealth of scientific phenomena and technological applications. Classical theory determines the emitted photon energy by assuming an undeflected charged particle trajectory. In 1940, Ginzburg pointed out that this assumption breaks down in quantum electrodynamics, resulting in shifts—known as quantum recoil—in outgoing photon energies from their classically predicted values. Since then, quantum recoil in free-electron light-emission processes, including Cherenkov radiation and Smith–Purcell radiation, has been well-studied in theory, but an experimental demonstration has remained elusive. Here we present an experimental demonstration of quantum recoil, showing that this quantum electrodynamical effect is not only observable at room temperature but also robust in the presence of other electron-scattering mechanisms. By scattering free electrons off the periodic two-dimensional atomic sheets of van der Waals materials in a tabletop platform, we show that the X-ray photon energy is accurately predicted only by quantum recoil theory. We show that quantum recoil can be enormous, to the point that a classically predicted X-ray photon is emitted as an extremely low-energy photon. We envisage quantum recoil as a means of precision control over outgoing photon and electron spectra, and show that quantum recoil can be tailored through a host of parameters: the electron energy, the atomic composition and the tilt angle of the van der Waals material. Our results pave the way to tabletop, room-temperature platforms for harnessing and investigating quantum electrodynamical effects in electron–photon interactions.

Tunable sources of X-ray radiation are widely used for imaging and spectroscopy in fundamental science, medicine and industry. The growing demand for highly tunable, high-brightness laboratory-scale X-ray sources motivates research into new fundamental mechanisms of X-ray generation. Here, we demonstrate the ability of van der Waals materials to serve as a platform for tunable X-ray generation when irradiated by moderately relativistic electrons available, for example, from a transmission electron microscope. The radiation spectrum can be precisely controlled by tuning the acceleration voltage of the incident electrons, as well as by our proposed approach: adjusting the lattice structure of the van der Waals material. We present experimental results for both methods, observing the energy tunability of X-ray radiation from the van der Waals materials WSe2, CrPS4, MnPS3, FePS3,CoPS3and NiPS3. Our findings demonstrate the concept of material design at the atomic level, using van der Waals heterostructures and other atomic superlattices, for exploring novel phenomena of X-ray physics.

The vanishingly small response of matter to light above ultraviolet frequencies makes the manipulation of light emission at such frequencies challenging. As a result, state-of-the-art sources of high-frequency light are typically active, relying on strong external electromagnetic fields. Here, we present a fundamental mechanism of light emission that is fully passive, relying instead on vacuum fluctuations near nanophotonic structures. This mechanism can be used to generate light at any frequency, including high-frequency radiation such as X-rays. The proposed mechanism is equivalent to a quantum optical two-photon process, in which a free electron spontaneously emits a low-energy polariton and a high-energy photon simultaneously. Although two-photon processes are nominally weak, we find that the resulting X-ray radiation can be substantial. The strength of this process is related to the strong Casimir–Polder forces that atoms experience in the nanometre vicinity of materials, with the essential difference being that the fluctuating force here acts on a free electron, rather than a neutral, polarizable atom. The light emission can be shaped by controlling the nanophotonic geometry or the underlying material electromagnetic response at optical or infrared frequencies. Our results reveal ways of applying the tools of nanophotonics even at frequencies where materials have an insubstantial electromagnetic response. The process we study, when scaled up, may also enable new concepts for compact and tunable X-ray 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.

Rapid progress in nanofabrication methods has fuelled a quest for ultra-compact photonic integrated systems and nanoscale light sources. The prospect of small-footprint, high-quality emitters of short-wavelength radiation is especially exciting due to the importance of extreme-ultraviolet and X-ray radiation as research and diagnostic tools in medicine, engineering and the natural sciences. Here, we propose a highly directional, tunable and monochromatic radiation source based on electrons interacting with graphene plasmons. Our complementary analytical theory and ab initio simulations demonstrate that the high momentum of the strongly confined graphene plasmons enables the generation of high-frequency radiation from relatively low-energy electrons, bypassing the need for lengthy electron acceleration stages or extreme laser intensities. For instance, highly directional 20 keV photons could be generated in a table-top design using electrons from conventional radiofrequency electron guns. The conductive nature and high damage threshold of graphene make it especially suitable for this application. Our electron–plasmon scattering theory is readily extended to other systems in which free electrons interact with surface waves.

We derive an asymptotically accurate formula for the beam waist of ultrashort, tightly focused fundamental linearly polarized, radially polarized, and azimuthally polarized modes in free space. We compute the exact beam waist via numerical cubature to ascertain the accuracy with which our formula approximates the exact beam waist over a broad range of parameters of practical interest. Based on this, we describe a method of choosing parameters in the model given the beam waist and pulse duration of a laser pulse.

We propose a direct electron acceleration scheme that uses a two-color pulsed radially polarized laser beam. The two-color scheme achieves electron acceleration exceeding 90% of the theoretical energy gain limit, over twice of what is possible with a one-color pulsed beam of equal total energy and pulse duration. The scheme succeeds by exploiting the Gouy phase shift to cause an acceleration-favoring interference of fields only as the electron enters its effectively final accelerating cycle. Optimization conditions and power scaling characteristics are discussed.