Observation of Floquet erratic non-Hermitian skin effect in photonic mesh lattice

TL;DR

This study experimentally demonstrates the disorder-enabled, bulk form of the non-Hermitian skin effect in a driven photonic lattice, revealing a new type of localization driven by imaginary gauge fields and disorder.

Contribution

It introduces the observation of the erratic non-Hermitian skin effect in a programmable photonic system, highlighting a disorder-induced, bulk-localized phenomenon distinct from traditional boundary effects.

Findings

Disorder-driven topological transition between skin phases.

Boundary skin accumulation disappears at the transition point.

Erratic skin effect manifests as bulk-localized eigenstates with shared envelopes.

Abstract

In ordered, translationally invariant non-Hermitian systems, the skin effect is understood as a boundary phenomenon: nonreciprocal hopping drives an extensive accumulation of eigenstates towards the edges, whereas the periodic-boundary spectrum remains Bloch extended. Here we experimentally reveal the opposite limit -- a disorder-enabled, boundary-independent, and intrinsically bulk form of skin localization -- the recently predicted erratic non-Hermitian skin effect (ENHSE), realized in a driven photonic platform. Using a time-multiplexed photonic mesh lattice with programmable gain, loss, and phase modulation, we engineer spatially fluctuating imaginary gauge fields and realize a Floquet non-Hermitian lattice whose global reciprocity can be tuned independently of strong local nonreciprocity. We observe a disorder-driven non-Hermitian topological transition between two oppositely directed disordered skin phases through a critical point of global reciprocity. At this transition, boundary skin accumulation disappears, yet the wave dynamics self-organizes into bulk-localized patterns without any interface, providing direct evidence of ENHSE. The measured localization profiles agree with simulations and exhibit the defining feature that distinct eigenstates share a common bulk-localized envelope determined by the disordered imaginary gauge fields. By further introducing controllable on-site disorder, we reveal the competition between ENHSE and Anderson localization, and show how increasing scattering progressively suppresses erratic skin dynamics. Our results help establish ENHSE as a unique disorder-induced non-Hermitian phenomenon and open a route to engineering localization, transport, and topology beyond conventional Bloch and boundary-based paradigms.