Discrete Optical Solitons Observed In Warm Rubidium Vapor

Scientists from the Quantum Technologies Group at the Institute of Physics, Vjekoslav Vulić, Neven Šantić, and Damir Aumiler, in collaboration with Hrvoje Buljan from the Department of Physics at the Faculty of Science (PMF), have experimentally demonstrated for the first time the existence of discrete one-dimensional (1D) solitons in an optically induced photonic lattice in warm rubidium vapor. The work was published in Laser & Photonics Reviews. These results open new possibilities for research in nonlinear optics, photonic lattice physics, and complex quantum systems.

Photonic lattices are periodic structures through which light propagates in a way analogous to the motion of electrons in crystals. Conventional implementations of such lattices, for example in semiconductors, lithium niobate, glass, or optical waveguides, generally have fixed properties. In this work, the researchers used warm rubidium vapor as a dynamic and tunable medium in which the lattice is created by laser light. Two coupling laser beams intersect at a small angle inside a glass cell containing rubidium vapor, producing an interference pattern that spatially modulates the refractive index of the vapor. An additional probe laser beam, focused onto a single lattice site, then propagates through the lattice. At low intensity, the beam gradually spreads to neighboring sites. This phenomenon is known as discrete diffraction: because of the medium’s periodic structure, light propagation differs from diffraction in a homogeneous medium.

As the probe-beam intensity increases, the response of the atomic vapor becomes nonlinear, and the beam begins to self-focus. At sufficiently high power, this effect can counterbalance discrete diffraction. The researchers thus observed a discrete soliton – a localized beam of light whose transverse profile remains nearly unchanged as it propagates through the medium. In the experiment, the soliton formed at a probe-laser power of approximately 6 mW.

The physical mechanism relies on the interaction of rubidium atoms with two laser fields in a configuration known as the Λ-scheme, which involves three atomic energy levels. An important role is played by electromagnetically induced transparency under conditions of large detuning from single-photon resonance. To explain the observed phenomena, the authors developed a theoretical model that couples the paraxial wave equation, which describes laser-beam propagation, with optical Bloch equations describing the response of rubidium atoms. The model includes the full hyperfine structure of the atom, and the very good agreement between experimental and numerical results confirms the importance of this approach for describing nonlinear light propagation.

The particular significance of this experimental platform lies in the unique properties and flexibility of atomic vapor compared with solid-state materials. The properties of the optical lattice can be tuned by changing the frequencies and intensities of the coupling beams, as well as other experimental parameters. Because it allows coherent control of optical gain and loss, warm atomic vapor provides a highly tunable environment for studying non-Hermitian nonlinear dynamics, parity-time (PT) symmetry, dissipative solitons, and quantum fluids of light.

The results contribute to a better understanding of nonlinear light propagation in tunable photonic structures. In the longer term, they may also be relevant to the development of new approaches in quantum optics and atomic technologies, for example in research on quantum memories, atomic clocks, and chip-scale devices.