Mechanical synthesis of a Hall effect for light

MechSE associate professor Gaurav Bahl, graduate student Soonwook Kim, and post-docs Dongyu Benjamin Sohn and Christopher Peterson recently published in APL Photonics, a journal produced by the American Institute of Physics. Entitled “On-chip optical non-reciprocity through a synthetic Hall effect for photons,” their study investigates the use of an acoustically-modulated photonic resonator chain to produce a synthetic Hall effect.

“We used mechanical actuation of a system of optical resonators to create a new effect that has not previously been observed for any electromagnetic wave,” Bahl said. “It’s a very nice cross-disciplinary result.”

The Hall effect is a well-known physical phenomenon that appears when charged particles experience certain conditions. Any type of charged particle (e.g. electrons in a current) moving through a material experiences a transverse force when a magnetic field is applied.  This force causes the particles to drift to the sides of the material, leading to a buildup of positive charges opposite a buildup of negative charges that creates a measurable voltage known as the Hall voltage. If an electric field is then applied in the transverse direction, the flow of the original charged particles can be obstructed unidirectionally, which is significant for creating devices in which currents can only flow in one direction.

As photons are not charged particles, light cannot experience the Hall effect. Bahl’s team fabricated a chain of nanophotonic resonators and used piezoelectric actuators to apply local mechanical modulations to the chain. When calibrated correctly, the modulation was shown to simultaneously produce a synthetic electric field and synthetic magnetic field that could act on light, analogous to how real electric and magnetic fields produce a Hall effect on charged particles.

The team found that the resulting synthetic Hall effect could be used to achieve unidirectional propagation for light.

“Our team was the first to combine synthetic magnetic fields with synthetic electric fields within the same structure, and we showed that some big technical limitations of previous work could be overcome with this combination,” said Bahl. This accomplishment is significant for computation and signal processing technologies, which traditionally rely on electric and magnetic behaviors in materials.

“Presently, unidirectional optical devices are almost exclusively built using specialized garnet materials. The synthetic Hall effect that we have shown can also produce similar unidirectional behaviors, but can be implemented with nearly any optical material,” he said. “Further development of this technology could help develop completely new ways with which to influence quantized particles of sound (phonons) and light (photons).”

While the team has previously published similar research using microwave resonators, this study marks their successful application of the synthetic Hall effect for telecom-wavelength optical signals.

“This effect could be very useful in optical communication networks and for quantum technologies, especially where non-reciprocal devices like isolators are critically needed,” Bahl said.

The ongoing work, which has been funded through grants from NSF, DARPA, and the Office of Naval Research, will next investigate larger 1D and 2D resonator networks. “With even more degrees of freedom, we should be able to produce, in a designer synthetic space, more complex phenomena that are very tough to realize in a physical space,” Bahl said.