New laser technique could give electron microscopes a boost

18 Jan 2022 Hamish Johnston

Best of the bunch: the new technique can be used to split the electron beam into a train of phase-coherent pulses. (Courtesy: Shutterstock/Anteromite)

A new technique allowing lasers to manipulate the energy and phase of electrons in electron microscopes has been unveiled by researchers in Germany and Switzerland. The technique opens up new potential applications in electron spectroscopy and could be used in the future to generate electron-photon entanglement.

Developed in the 1930s, electron microscopy is arguably the most important technology we have for studying matter at the atomic scale. This is because the wavelength of the electrons used is much shorter than that of visible light – and even typical X-rays. However, whereas advances in photonics such as cavity quantum electrodynamics have allowed the control of light at extremely high precision, manipulating electron beams has remained significantly more challenging.

One opportunity lies in utilizing the coupling between electrons and photons, allowing an optical laser to modulate an electron beam. Unfortunately, this coupling is relatively weak, and has therefore required high-power lasers, which are expensive and cannot be operated in the continuous-wave regime as they would damage sensitive equipment.

Subtle solution

In the new research, scientists at the Max Planck Institute for Multidisciplinary Sciences and Georg-August University, both in Göttingen, and the Swiss Federal Institute of Technology in Lausanne arrived at a subtle solution using integrated photonics. They coupled an optical fibre to a silicon nitride cavity with a quality factor of about a million on a silicon chip. When passed down the optical fibre, a low-power continuous-wave laser beam resonantly pumps the cavity, amplifying milliwatts into thousands of watts, all at a very well-defined frequency.

The researchers then passed the beam of an electron microscope close to the cavity. They found that, at specific resonant frequencies, the electrons coupled to the cavity’s evanescent field, which is a non-propagating component of the electromagnetic field found only in the immediate vicinity of a source. This caused a series of sidebands to appear around the central electron energy peak. “The electron beam behaves in such a way that it depends on this microelectronvolt-resolution optical excitation wavelength,” explains Claus Ropers at Göttingen, who co-led the research: “There may be future possibilities not only to characterize these cavities but to translate this to other forms of electron spectroscopy.”

One example is electron energy loss spectroscopy, in which a beam of electrons with a narrow range of energies is passed through a material and the energy of the scattered electrons is measured and used to infer the material’s electronic energy levels. By using the cavity to select for specific excitation frequencies, it should be possible to control the excitation energy – and therefore to infer the loss after scattering – much more precisely than before.

Advanced spectroscopies

Other possibilities are more complex: the broadening in the energy spectrum causes the electron beam to split into a train of phase-coherent pulses, and the researchers hope to use these to study time-dependent interactions. “More complicated forms of spectroscopy have been developed in optics that allow for much deeper microscopic insights into dynamical processes if you go from regular spectroscopy into coherent spectroscopy,” says Ropers. “The long-term goal is to transfer the schemes that are very advanced in optical spectroscopy over to electron spectroscopy.”

The researchers are also working on applications beyond spectroscopy: “This is the most efficient, cleanest, most controlled way an electron beam has ever interfaced with photonics,” says co-leader Tobias Kippenberg at Lausanne. They believe this could be used to create quantum entanglement between electrons and photons. “Right now, there are many, many photons in the cavity, so the final state of the cavity is not really different if one electron has picked up one photon,” Ropers explains; “But let’s say you don’t drive this with a laser, but with a single photon source, then this cavity can lead to an entanglement between the state of the cavity and the state of the electron.”

Ultrafast laser physicist Martin Kozák of Charles University in Prague says that the work’s key result is the strong enhancement of the coupling strength between incident photons and electrons, which allows scientists to modulate the wave function of the electron beam using a continuous wave laser of only 1 mW power. This is made possible by the extraordinarily high quality factor of the optical cavity. “[There is research] already demonstrating the inelastic interactions of electrons with optical fields, but the combination of low power requirements with fibre coupling allows this device to be installed in any electron microscope” says Kozák. “This might seem like a technical advance but the continuous phase-modulated electron beams open up a whole new field of quantum optics with free electrons.”

The research is described in Nature.

from physicsworld.com     12/2/2022