Coherent interaction between free electrons and a photonic cavity.

Advances in the research of interactions between ultrafast free electrons and light have introduced a previously unknown kind of quantum matter, quantum free-electron wavepackets1-5. So far, studies of the interactions of cavity-confined light with quantum matter have focused on bound electron systems, such as atoms, quantum dots and quantum circuits, which are considerably limited by their fixed energy states, spectral range and selection rules. By contrast, quantum free-electron wavepackets have no such limits, but so far no experiment has shown the influence of a photonic cavity on quantum free-electron wavepackets. Here we develop a platform for multidimensional nanoscale imaging and spectroscopy of free-electron interactions with photonic cavities. We directly measure the cavity-photon lifetime via a coherent free-electron probe and observe an enhancement of more than an order of magnitude in the interaction strength relative to previous experiments of electron-photon interactions. Our free-electron probe resolves the spatiotemporal and energy-momentum information of the interaction. The quantum nature of the electrons is verified by spatially mapping Rabi oscillations of the electron spectrum. The interactions between free electrons and cavity photons could enable low-dose, ultrafast electron microscopy of soft matter or other beam-sensitive materials. Such interactions may also open paths towards using free electrons for quantum information processing and quantum sensing. Future studies could achieve free-electron strong coupling6,7, photon quantum state synthesis8 and quantum nonlinear phenomena such as cavity electro-optomechanics9.

Light emission by free electrons in photonic time-crystals.

Photonic time-crystals (PTCs) are spatially homogeneous media whose electromagnetic susceptibility varies periodically in time, causing temporal reflections and refractions for any wave propagating within the medium. The time-reflected and time-refracted waves interfere, giving rise to Floquet modes with momentum bands separated by momentum gaps (rather than energy bands and energy gaps, as in photonic crystals). Here, we present a study on the emission of radiation by free electrons in PTCs. We show that a free electron moving in a PTC spontaneously emits radiation, and when associated with momentum-gap modes, the electron emission process is exponentially amplified by the modulation of the refractive index. Moreover, under strong electron-photon coupling, the quantum formulation reveals that the spontaneous emission into the PTC bandgap experiences destructive quantum interference with the emission of the electron into the PTC band modes, leading to suppression of the interdependent emission. Free-electron physics in PTCs offers a platform for studying a plethora of exciting phenomena, such as radiating dipoles moving at relativistic speeds and highly efficient quantum interactions with free electrons.

Tunable photon-induced spatial modulation of free electrons.

Spatial modulation of electron beams is an essential tool for various applications such as nanolithography and imaging, yet its conventional implementations are severely limited and inherently non-tunable. Conversely, proposals of light-driven electron spatial modulation promise tunable electron wavefront shaping, for example, using the mechanism of photon-induced near-field electron microscopy. Here we present tunable photon-induced spatial modulation of electrons through their interaction with externally controlled surface plasmon polaritons (SPPs). Using recently developed methods of shaping SPP patterns, we demonstrate a dynamic control of the electron beam with a variety of electron distributions and verify their coherence through electron diffraction. Finally, the nonlinearity stemming from energy post-selection provides us with another avenue for controlling the electron shape, generating electron features far below the SPP wavelength. Our work paves the way to on-demand electron wavefront shaping at ultrafast timescales, with prospects for aberration correction, nanofabrication and material characterization.

Coherent radiation at visible wavelengths from sub-keV electron beams.

The Smith-Purcell effect allows for coherent free-electron-driven compact light sources over the entire electromagnetic spectrum. Intriguing interaction regimes, with prospects for quantum optical applications, are expected when the driving free electron enters the sub-keV range, though this has until now remained an experimental challenge. Here, we demonstrate the Smith-Purcell light emission from UV to visible using engineerable, fabricated gratings with periodicities as low as 19 nm and with electron energies as low as 300 eV. Our findings constitute a major step toward broadband, highly tunable, on-chip light sources, observation of quantum recoil effects, and tunable EUV and x ray sources from swift electrons.

All-optical spin valve effect in nonlinear optics.

More than three decades after the inception of electron spin-based information encoding inspired by nonlinear electro-optic devices, we present a complementary approach: nonlinear optical devices directly inspired by spintronics. We theoretically propose an all-optical spin-valve device and a spin-dependent beam splitter, where the optical pseudospin is a superposition of signal and idler beams undergoing a sum-frequency generation process inside a 2D nonlinear photonic crystal. We delve into the operation of these devices, examining key properties such as the transmission angle and splitting ratio, optically controlled by the pump beam. Our findings open new avenues for both classical and quantum optical information processing in the frequency domain.

Topological Transitions and Surface Umklapp Scattering in Weakly Modulated Periodic Metasurfaces.

Controlling and manipulating surface waves is highly beneficial for imaging applications, nanophotonic device design, and light-matter interactions. While deep-subwavelength structuring of the metal-dielectric interface can influence surface waves by forming strong effective anisotropy, it disregards important structural degrees of freedom such as the interplay between corrugation periodicity and depth and its effect on the beam transport. Here, we unlock these degrees of freedom, introducing weakly modulated metasurfaces, structured metal-dielectric surfaces beyond effective medium. We utilize groove-structuring with varying depths and periodicities to demonstrate control over the transport of surface waves, dominated by the depth-period interplay. We show unique backward focusing of surface waves driven by an umklapp process-momentum relaxation empowered by the periodic nature of the structure and discover a yet unexplored, dual-stage topological transition. Our findings can be applied to any type of guided wave, introducing a simple and versatile approach for controlling wave propagation in artificial media.

Cylindrical Metalens for Generation and Focusing of Free-Electron Radiation.

Metasurfaces constitute a powerful approach to generate and control light by engineering optical material properties at the subwavelength scale. Recently, this concept was applied to manipulate free-electron radiation phenomena, rendering versatile light sources with unique functionalities. In this Letter, we experimentally demonstrate spectral and angular control over coherent light emission by metasurfaces that interact with free-electrons under grazing incidence. Specifically, we study metalenses based on chirped metagratings that simultaneously emit and shape Smith-Purcell radiation in the visible and near-infrared spectral regime. In good agreement with theory, we observe the far-field signatures of strongly convergent and divergent cylindrical radiation wavefronts using in situ hyperspectral angle-resolved light detection in a scanning electron microscope. Furthermore, we theoretically explore simultaneous control over the polarization and wavefront of Smith-Purcell radiation via a split-ring-resonator metasurface, enabling tunable operation by spatially selective mode excitation at nanometer resolution. Our work highlights the potential of merging metasurfaces with free-electron excitations for versatile and highly tunable radiation sources in wide-ranging spectral regimes.

Nonlinear Forced Response of Plasmonic Nanostructures.

Incorporating optical surface waves in nonlinear processes unlocks unique and sensitive nonlinear interactions wherein highly confined surface states can be accessed and explored. Here, we unravel the rich physics of modal-nonmodal state pairs of short-range surface plasmons in thin metal films by leveraging "dark nonlinearity"-a nonradiating nonlinear source. We control and observe the nonlinear forced response of these modal-nonmodal pairs and present nonlinearly mediated direct access to nonmodal plasmons in a lossless regime. Our study can be generalized to other forms of surface waves or optical nonlinearities, toward on-chip nonlinearly controlled nanophotonic devices.

Spin-Orbit Interaction of Light in Plasmonic Lattices.

In the past decade, the spin-orbit interaction (SOI) of light has been a driving force in the design of metamaterials, metasurfaces, and schemes for light-matter interaction. A hallmark of the spin-orbit interaction of light is the spin-based plasmonic effect, converting spin angular momentum of propagating light to near-field orbital angular momentum. Although this effect has been thoroughly investigated in circular symmetry, it has yet to be characterized in a noncircular geometry, where whirling, periodic plasmonic fields are expected. Using phase-resolved near-field microscopy, we experimentally demonstrate the SOI of circularly polarized light in nanostructures possessing dihedral symmetry. We show how interaction with hexagonal slits results in four topologically different plasmonic lattices, controlled by engineered boundary conditions, and reveal a cyclic nature of the spin-based plasmonic effect which does not exist for circular symmetry. Finally, we calculate the optical forces generated by the plasmonic lattices, predicting that light with mere spin angular momentum can exert torque on a multitude of particles in an ordered fashion to form an optical nanomotor array. Our findings may be of use in both biology and chemistry, as a means for simultaneous trapping, manipulation, and excitation of multiple objects, controlled by the polarization of light.

Modeling the sensitivity dependence of silicon-photonics-based ultrasound detectors.

With recent advances in optical technology, interferometric sensing has grown into a highly versatile approach for ultrasound detection, with many interferometric detectors relying on optical waveguides to achieve high levels of sensitivity and miniaturization. In this Letter, we establish a practical model for assessing the sensitivity of silicon-photonics waveguides to acoustic waves. The analysis is performed for different polarizations, waveguide dimensions, and acoustic wave types. Our model was validated experimentally in the acoustic frequency band of 1-13 MHz by measuring the sensitivities of the two polarization modes in a silicon strip waveguide. Both the experimental results and theoretical prediction show that the transverse-magnetic polarization achieves a higher sensitivity and suppression of surface acoustic waves compared to the transverse-electric polarization for the geometries studied.

Quantum sensing of strongly coupled light-matter systems using free electrons.

Strong coupling in light-matter systems is a central concept in cavity quantum electrodynamics and is essential for many quantum technologies. Especially in the optical range, full control of highly connected multi-qubit systems necessitates quantum coherent probes with nanometric spatial resolution, which are currently inaccessible. Here, we propose the use of free electrons as high-resolution quantum sensors for strongly coupled light-matter systems. Shaping the free-electron wave packet enables the measurement of the quantum state of the entire hybrid systems. We specifically show how quantum interference of the free-electron wave packet gives rise to a quantum-enhanced sensing protocol for the position and dipole orientation of a subnanometer emitter inside a cavity. Our results showcase the great versatility and applicability of quantum interactions between free electrons and strongly coupled cavities, relying on the unique properties of free electrons as strongly interacting flying qubits with miniscule dimensions.

Free-electron Ramsey-type interferometry for enhanced amplitude and phase imaging of nearfields.

The complex range of interactions between electrons and electromagnetic fields gave rise to countless scientific and technological advances. A prime example is photon-induced nearfield electron microscopy (PINEM), enabling the detection of confined electric fields in illuminated nanostructures with unprecedented spatial resolution. However, PINEM is limited by its dependence on strong fields, making it unsuitable for sensitive samples, and its inability to resolve complex phasor information. Here, we leverage the nonlinear, overconstrained nature of PINEM to present an algorithmic microscopy approach, achieving far superior nearfield imaging capabilities. Our algorithm relies on free-electron Ramsey-type interferometry to produce orders-of-magnitude improvement in sensitivity and ambiguity-immune nearfield phase reconstruction, both of which are optimal when the electron exhibits a fully quantum behavior. Our results demonstrate the potential of combining algorithmic approaches with state-of-the-art modalities in electron microscopy and may lead to various applications from imaging sensitive biological samples to performing full-field tomography of confined light.

Emulating spin transport with nonlinear optics, from high-order skyrmions to the topological Hall effect.

Exploring material magnetization led to countless fundamental discoveries and applications, culminating in the field of spintronics. Recently, research effort in this field focused on magnetic skyrmions - topologically robust chiral magnetization textures, capable of storing information and routing spin currents via the topological Hall effect. In this article, we propose an optical system emulating any 2D spin transport phenomena with unprecedented controllability, by employing three-wave mixing in 3D nonlinear photonic crystals. Precise photonic crystal engineering, as well as active all-optical control, enable the realization of effective magnetization textures beyond the limits of thermodynamic stability in current materials. As a proof-of-concept, we theoretically design skyrmionic nonlinear photonic crystals with arbitrary topologies and propose an optical system exhibiting the topological Hall effect. Our work paves the way towards quantum spintronics simulations and novel optoelectronic applications inspired by spintronics, for both classical and quantum optical information processing.