selftrapped

Self-trapping refers to the localization of a particle or excitation due to interactions with its surrounding medium, producing a localized distortion and a bound state that lowers the system’s energy. The phenomenon is observed when the coupling between the carrier and the medium is strong enough to overcome the carrier’s tendency to delocalize.

In condensed matter physics, self-trapping often involves electrons or holes becoming localized as polarons. A polaron forms when lattice vibrations respond to the carrier, creating a distortion that travels with the carrier. Depending on the coupling strength and the spatial extent of the distortion, carriers may form small polarons, localized to a single site, or larger polarons with a more extended pattern. Self-trapped excitons can also occur, where an electron–hole pair distorts the lattice sufficiently to remain localized. Alkali halides and other crystals with strong electron–phonon coupling are common contexts for observing self-trapped excitons, which influence luminescence and transport properties.

In nonlinear optics, self-trapping describes the localization of light due to intensity-dependent changes in the medium’s refractive index, such as Kerr nonlinearity. Diffraction is balanced by nonlinear focusing, producing optical solitons or self-focused beams that propagate without spreading in certain media.

In ultracold atomic systems, macroscopic self-trapping can arise in Bose–Einstein condensates with repulsive interactions confined in double-well potentials or optical lattices, where nonlinearity prevents complete tunneling and the condensate becomes localized on one side.

Modeling approaches include polaron theories (such as the Holstein and Fröhlich models), the nonlinear Schrödinger equation, and discrete nonlinear Schrödinger equations. Self-trapping is not universal; it depends on coupling strength, dimensionality, temperature, and material or system specifics, and may be reversible or depend on external conditions.