Electron microscopy used to study nonlinear optics in photonic chip
Image credit: Yang et al. DOI: 10.1126/science.adk2489
A team of researchers at the École polytechnique fédérale de Lausanne (EPFL) and the Max Planck Institute for Multidisciplinary Sciences has investigated the interactions between light waves trapped in a chip and a beam of electrons from a transmission electron microscope (TEM), a type of microscope that uses electrons for imaging instead of light. The study, published in the journal Science, could offer new ways of probing nonlinear optics on a tiny scale, and of manipulating particles with photonic chips.
Nonlinear optics is integral to technological and scientific advances, from laser development and optical frequency metrology to gravitational wave astronomy and quantum information science. In addition, recent years have seen nonlinear optics applied in optical signal processing, telecommunications, sensing, spectroscopy, and light detection and ranging. All these applications involve the miniaturisation of devices that manipulate light in nonlinear ways on a small chip, enabling complex light interactions.
At the heart of the new study, which was led by Professor Tobias Kippenberg at EPFL and Professor Claus Ropers, director of the Max Planck Institute for Multidisciplinary Sciences, are “Kerr solitons”. These are waves of light that hold their shape and energy as they move through a material, like a perfectly formed surf wave travelling across the ocean. In particular, the researchers used “dissipative” Kerr solitons, which are stable, localised pulses of light that last tens of femtoseconds and form spontaneously in the microresonator. Dissipative Kerr solitons can also interact with electrons, which made them crucial for this study.
The scientists formed dissipative Kerr solitons inside a photonic microresonator, a tiny chip that traps and circulates light inside a reflective cavity, creating the perfect conditions for these waves. “We generated various nonlinear spatiotemporal light patterns in the microresonator driven by a continuous-wave laser,” explains EPFL researcher Yujia Yang. “These light patterns interacted with a beam of electrons passing by the photonic chip, and left fingerprints in the electron spectrum.”
Specifically, the approach demonstrated the coupling between free electrons and dissipative Kerr solitons, which allowed the researchers to probe soliton dynamics in the microresonator cavity and perform ultrafast modulation of electron beams.
“Our ability to generate dissipative Kerr solitons [DKS] in a TEM extends the use of microresonator-base frequency combs to unexplored territories,” says Kippenberg. “The electron-DKS interaction could enable high repetition-rate ultrafast electron microscopy and particle accelerators empowered by a small photonic chip.”
Ropers adds: “Our results show electron microscopy could be a powerful technique for probing nonlinear optical dynamics at the nanoscale. This technique is non-invasive and able to directly access the intracavity field, key to understanding nonlinear optical physics and developing nonlinear photonic devices.”
