Community Research and Development Information Service - CORDIS

Periodic Report Summary 1 - BSICS (Beam Shaping in Complex Systems)

The objective of the BSiCS project is to devise new ways of shaping light and matter to control light-matter interactions in complex and novel structures. We present new prospects of shaping wavepackets of photons and electrons. The research delves into fundamental aspects of electrodynamics, electromagnetism and quantum mechanics to suggest new kinds of basic interactions with both fundamental and applied implications.
Owing to the combination of modern fabrication tools and advancement in electron beam sources, the last few years have seen vast interest in making (holographic-like) nanoscale masks that directly shape the Schrödinger wavefunction of electrons in electron microscopes. This adds to the already well-developed techniques of shaping electromagnetic wavepackets and beams. In the last few years we have shown that these concepts can be taken beyond optics and into other fields (in particular, electron beam physics and relativistic quantum mechanics). There, we found intriguing effects that borrow from the analogy between optics and electron beam physics, as well as completely new effects that are unique to wavepackets of matter. Below are a few examples from the work published as part of the Marie Curie project.

Self-accelerating wavepackets of the Dirac equation: We found new families of spinor wavepacket solutions of the Dirac equation that have several surprising properties, e.g., accumulating Aharonov-Bohm phase without any electromagnetic potential. This gave us the tools to explore fundamental questions like how the lifetime of a decaying particle can be extended by wave shaping.

Electron beam shaping preventing electron-electron repulsion: Here we show how proper wavefunction shaping can break the resolution limit of electron microscopy by preventing the inherent nonlinear repulsive interaction between multiple electrons.

Čerenkov radiation from charge carriers: In a conceptual breakthrough that is now more than 80 years old, Čerenkov showed how charged particles emit shockwaves of light when moving faster than the phase velocity of light in a medium. The requirement for relativistic particles, however, makes Čerenkov emission inaccessible to most nanoscale electronic and photonic devices. In a recent project, we have shown that graphene plasmons provide the means to overcome this limitation through their low phase velocity and high field confinement. The interaction between graphene plasmons and the charge carriers flowing inside graphene presents a highly efficient 2D Čerenkov emission, giving a versatile, tunable, and ultrafast conversion mechanism from electrical signal to plasmonic excitation. What led to this work was our previous paper deriving the Čerenkov effect from a QED formalism and considering the implications of shaping the wavefunction of the charged particle.
These projects are only two examples of a new field of research we are developing: creating phenomena of electron beam physics “on-chip”.
Let me explain what I mean by “on-chip”: Modern condensed matter materials, like graphene, allow the speed of light (e.g., graphene plasmons) to be several orders of magnitude slower than c. It means that charge carriers flowing inside conductors are “relativistic” compared to the effective speed of light even though their velocity (e.g., Fermi velocity) is far below the speed of light in vacuum. This results in strong electron-photon interaction, opening up regimes of physics usually accessible only in large accelerator facilities. Examples include the inverse-Compton, Čerenkov, Smith-Purcell, and transition radiation, which we have been exploring recently, as well as other interactions that my future group will explore. Applications range from new concepts for light-emitting devices to sources of terahertz radiation.

Enabling forbidden transitions with 2D plasmonics: In the past year, with an MIT undergrad I mentored, we have revisited the theory of light-matter interaction in graphene and other metallic monolayers. Owing to the unique properties of light in these systems, and in particular to the confinement of plasmons almost to the atomic scale, we have discovered that optical transitions involving a spin flip can suddenly occur, violating one of the most fundamental rules of light-matter interaction. More generally, the formalism we developed, treating plasmonics physics with QED tools, reveals the occurrence of additional conventionally forbidden processes, which under some realistic conditions can be made dominant. For example, we have discovered that the rate of two-plasmon spontaneous emission, which is generally considered to be negligible, can compete with the rate of one-plasmon spontaneous emission, signaling the onset of the ultra-strong light-matter coupling regime in these special materials. Moreover, in principle, it is possible to observe atomic transitions whose free-space lifetimes are on the order of the age of the universe, which now, thanks to the physics of graphene, can be made as short as 1 ns (>20 orders of magnitude). Our findings enable new platforms for spectroscopy, sensing, and broadband light generation, as well as a new source of entangled photons.

Compact x-ray source: Modern sources of short-wavelength radiation such as the free electron laser and the synchrotron are based on the free electron-photon scattering in the Compton/Thomson effect. We have recently developed a new electron-plasmon scattering theory and use it to propose a highly directional, tunable and monochromatic radiation source based on free electrons interacting with graphene plasmons. The key property that differentiates this effect from conventional electron-photon interaction is the high momentum of the strongly confined graphene plasmons that take the role of the photon in the conventional theories. It enables the generation of high frequency radiation from relatively low energy electrons, bypassing the need for lengthy acceleration of the electrons (e.g., 20 keV photons generated from RF electron guns). 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 diagnostic tools in medicine, engineering and the natural sciences.

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