Optical Resonators Probed with Ion Beams

Ion beams can subtly measure and adjust the performance of microscale optical resonators, a finding that could help refine production of such cavities for applications in sensing and quantum computing.

Researchers at the National Institute of Standards and Technology (NIST) used beams of lithium ions with a radius of approximately 50 nm to induce picometer-scale deformations on the surfaces of silicon microresonators. These deformation were detected through shifts in optical resonance wavelengths.

"We map out the oscillation pattern by focusing the ion beam on different locations on the resonator's surface and noting the change in the vibration," said physicist Vladimir Aksyuk. "If the vibration at that spot is strong, the vibration is strongly affected. If the vibration there is weak, then the perturbation has no effect."

The focused ion-beam tool developed at the National Institute of Standards and Technology can inject ions into microscale resonators, creating tiny bulges that affect the structures' optical properties. Courtesy of NIST.

The ion beams essentially give researchers the ability to edit the tone of a resonator and makes it possible to tune two resonators to have the same vibrational qualities — a feat now impossible to achieve during manufacturing, but necessary for applications such as quantum information processing with single photons.

Lithium ions are particularly well suited for the task because of their low mass, said Aksyuk.

Visualizing optical resonance patterns could help scientists perfect microresonator-based technologies like sensors for detecting biomolecules and single atoms.

Optical near-fields extending outside the resonators make them hypersensitive to changes in the environment. Any perturbation of a near field, such as by a stray molecule or atom, will affect the light inside the resonator in a detectable way, much in the same way that touching a ringing bell will change its tone or volume or silence it altogether.

At present, the vibrational profiles of microresonators are measured using needle-like near-field optical probes. The problem with using a probe is that it strongly disturbs the near-fields before it is able to get close enough to the surface to perform high-resolution imaging. Ion beams enable higher-resolution imaging without disturbing the near-fields.

Meanwhile, study of how lithium ions interact with microresonators' silicon lattices could also yield insights for the lithium-ion battery and semiconductor industries, the researcher said.

The research was published in Nature Photonics (doi: 10.1038/nphoton.2015.248).