Optical Moiré Lattices Produce Solitons

Researchers from Jiao Tong University discovered that optical moiré lattices can produce solitons — self-trapped solitary waves — at extremely low power levels. The research establishes potential to explore nonlinear phenomena such as four-wave mixing and second-harmonic generation.

The research stems from an earlier discovery in Fangwei Ye’s lab, in which his team discovered a way to stop the spreading of light and localize a laser into a tight spot using moiré lattices. The current work is an extension of the discovery; it excites the light in moiré lattices into self-sustaining soliton pulses. Solitons retain their shape as they propagate over long distances, which makes them important to telecommunications as information carriers.

The solitons are able to fend off diffraction with the nonlinear effect, a self-reinforcing phenomenon whereby the medium through which light shines modifies the light’s behavior. In this case, the medium is a photorefractive strontium barium niobite crystal with nonlinear holographic properties.

The researchers imprinted a moiré pattern on the crustal by shining a light beam stenciled by two twisted lattice masks. Then they shined a second light beam and observed how the beam profile evolved as they changed the masks’ twist angle and the laser powers.

The moiré lattices produced solitons above a certain laser power threshold, depending on the twist angles in the patterning masks. Typically, nonlinearity is a weak phenomenon that only occurs at high laser powers. However, Ye and his team found that their power threshold is quite low — only nanowatts of power is required.

Key to the low power requirement is the flat energy band in the moiré lattice. The photons in optical moiré lattices are squeezed into a narrow range of energies at certain twist angles. That energy rate supports only certain self-trapped modes of light. The light diffraction is therefore much weaker in such flat energy bands, so only a small nonlinear effect is necessary to generate solitons.

“Thanks to the almost flat bands in moiré lattices,” Ye said, “this experiment brings the power threshold down to an extremely low level, representing a big step in soliton research.”

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-0679-9).