Thouless pumping of solitons in a nonlocal medium

Abstract

Thouless pumping is a fundamental phenomenon recognized as being widespread across various areas of physics, with optics holding a particularly prominent role. Here, we study this effect for optical solitons in a medium where the refractive index is shaped by two slowly sliding sublattices and a nonlocal nonlinear response. The spectral bands of such a potential can exhibit nontrivial topology, and excitations occupying these bands can undergo quantized transport governed by the space -- time Chern indices of the linear spectrum. We find that nonlocality of the medium profoundly affects the dynamics of Thouless pumping. Thus, we show that broad, low-power fundamental solitons do not exhibit transport, as they excite only a small portion of the spectral band, while high-power solitons with broader spectral projections do demonstrate stable quantized transport. The transition point between these two principally different light propagation regimes strongly depends on the degree of nonlocality of the nonlinear response and shifts to larger powers with increasing nonlocality. Notably, even a moderate level of nonlocality is sufficient to prevent the breakdown of topological transport at high powers commonly observed in local Kerr media. Beyond fundamental solitons, we also demonstrate that multipole solitons, such as dipole and tripole states can be pumped stably. This is the first time such complex soliton states have been shown to undergo Thouless pumping. While fundamental solitons require only exceeding a power threshold, multipoles exhibit stable transport only within a finite power window. This window is broader for dipoles than for tripoles and expands with increasing nonlocality, revealing a trade-off between structural complexity and stability.