Light Dynamically Twists an Electron’s Wave Function

EPFL physicists have been able to create with light ultrafast electron vortex beams, with significant implications for fundamental physics, quantum computing, future data-storage, and even certain medical treatments. (Source. EPFL / E. Marushchenko, Ella Maru Studio)

In Jules Verne’s famous classic “Twenty Thousand Leagues Under the Sea”, the iconic submarine Nautilus disappears into the Moskens­traumen, a massive whirlpool off the coast of Norway. In space, stars spiral around black holes; on Earth, swirling cyclones, tornadoes, and dust-devils rip across the land. All these phenomena have a particular shape in common: the vortex. From galaxies to stirring milk into coffee, vortices appear every­where in nature – even in the subatomic world, when a stream of elementary particles or energy can spiral around a fixed axis like the tip of a cork-screw.

When particles move like this, they form vortex beams. These beams are very interes­ting because they imply that the particle has a well-defined orbital angular momentum, which describes the rotation of a particle around a fixed point. What this means is that vortex beams can give us new ways of inter­acting with matter, e.g. enhanced sensi­tivity to magnetic fields in sensors, or generate new absorption channels for the inter­action between radiation and tissue in medical treatments (e.g. radio­therapy). But vortex beams also enable new channels in basic interactions among elementary particles, promising new insights into the inner structure of particles such as neutrons, protons or ions.

The strange thing about matter is that, along with its particle nature, it also has a wave-nature. This means that we can make massive particles form vortex beams by simply modulating their wave function. This can be done with a passive phase mask, which can be thought of as a standing obstacle in the sea. When waves at sea crash into it, their “wave-ness” shifts and they form whirlpools. So far, physicists have been using the passive phase mask method to make vortex beams of electrons and neutrons. But now, scientists from the lab of Fabrizio Carbone at EPFL are challenging this idea. Demon­strating for the first time that it is possible to use light to dynami­cally twist an individual electron’s wave function, the researchers were able to generate an ultrashort vortex electron beam and actively switching its vorti­city on the atto­second timescale.

To do this, the team exploited one of the funda­mental rules governing the inter­action of particles on the nanoscale level: energy and momentum conser­vation. What this means is that the sum of the energies, masses and velocities of two particles before and after their collision must be the same. Such a constraint is responsible for an electron to gain orbital angular momentum during its inter­action with an ad hoc prepared light field, i.e. a chiral plasmon. In experi­mental terms, the scientists fired circularly polarized, ultrashort laser pulses through a nano-hole fabricated onto a metallic film. This induced a strong, localized electro­magnetic field – the chiral plasmon, and indi­vidual electrons were made to interact with it.

The scientists used an ultrafast trans­mission electron micro­scope to monitor the resulting phase profiles of the electrons. What they discovered was that, during the interaction of the electrons with the field, the wave function of the electrons took on a chiral modulation; a right- or left-handed movement whose handiness can be actively controlled by adjusting the polari­zation of the laser pulses. “There are many practical applications from these experi­ments,” says Fabrizio Carbone. “Ultrafast vortex electron beams can be used to encode and manipulate quantum information; the electrons’ orbital angular momentum can be trans­ferred to the spins of magnetic materials to control the topological charge in new devices for data storage. But even more intri­guingly, using light to dynamically twist matter waves offers a new perspective in shaping protons or ion beams such as those used in medical therapy, possibly enabling new radiation-matter inter­action mechanisms that can be very useful for selective tissue ablation techniques.” (Source: EPFL)

Reference: G. M. Vanacore et al.: Ultrafast generation and control of an electron vortex beam via chiral plasmonic near fields, Nat. Mat., online 6 May 2019; DOI: 10.1038/s41563-019-0336-1

Link: Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES), École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

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