  Non-Hermitian physics has greatly enriched our understanding of
nonequilibrium phenomena and uncovered novel effects such as the non-Hermitian
skin effect (NHSE) that has profoundly revolutionized the field. NHSE is
typically predicted in systems with nonreciprocal couplings which, however, are
difficult to realize in experiments. Without nonreciprocal couplings, the NHSE
can also emerge in systems with coexisting gauge fields and loss or gain (e.g.,
in Floquet non-Hermitian systems). However, such Floquet NHSE remains largely
unexplored in experiments. Here, we realize the Floquet NHSEs in periodically
modulated optical waveguides integrated on a silicon photonics platform. By
engineering the artificial gauge fields induced by the periodical modulation,
we observe various Floquet NHSEs and unveil their rich topological transitions.
Remarkably, we discover the transitions between the normal unipolar NHSEs and
an unconventional bipolar NHSE which is accompanied by the directional reversal
of the NHSEs. The underlying physics is revealed by the band winding in complex
quasienergy space which undergoes a topology change from isolated loops with
the same winding to linked loops with opposite windings. Our work unfolds a new
route toward Floquet NHSEs originating from the interplay between gauge fields
and dissipation effects and offers fundamentally new ways for steering light
and other waves.
