Abstract
The general theory of dark solitons relies on repulsive interactions and, therefore, predicts the impossibility to form dark-soliton bound states. One important exception to this prediction is the observation of bound solitons in nonlocal nonlinear media. Here, we report that exciton-polariton superfluids can also sustain dark-soliton molecules, although the interactions are fully local. With a novel all-optical technique, we create two dark solitons that bind together to form an unconventional dark-soliton molecule. We demonstrate that the stability of this structure and the separation distance between two dark solitons is tightly connected to the driven-dissipative nature of the polariton fluid.
- Received 28 February 2020
- Revised 27 August 2020
- Accepted 28 September 2020
DOI:https://doi.org/10.1103/PhysRevX.10.041028
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Solitons are localized waves propagating with unchanged shape in a nonlinear medium. Since their discovery in 1834 by Scottish civil engineer Russell in the shallow waters of the Union Canal, solitons have been observed in a variety of systems such as plasmas, molecules, and quantum fluids. Solitons can be bright, with a localized bump, or dark, with a dip on a homogeneous background. In this work, we use a novel all-optical technique to imprint dark solitons in a type of quantum fluid and find that two dark solitons can bind together to form a “dark-soliton molecule.”
Bright solitons are known to attract or repel each other, but previous work has shown that dark solitons experience only mutual repulsive forces, except in media with nonlocal long-range interactions. Our work shows this is not always the case: Dark solitons can attract one another in media with local interactions as well.
In our experiments, the medium is a quantum fluid of polaritons, quasiparticles composed of a photon coupled to an electric dipole. We show that two dark solitons can be imprinted parallel to each other and bound to form a stable “molecule,” which propagates on the fluid over macroscopic distances. Remarkably, the distance between the solitons is independent of the fluid properties and is determined only by the intrinsic dissipation of the polariton system. This behavior is in sharp contrast with observations in other quantum fluids such as atomic Bose-Einstein condensates.
Our results are a significant advance in the understanding of nonlinear phenomena in out-of-equilibrium quantum fluids, and our all-optical technique opens a way to the systematic study of quantum turbulence.