Observation of topological frequency combs
arxiv(2024)
摘要
On-chip generation of optical frequency combs using nonlinear ring resonators
has opened the route to numerous novel applications of combs that were
otherwise limited to mode-locked laser systems. Nevertheless, even after more
than a decade of development, on-chip nonlinear combs still predominantly rely
on the use of single-ring resonators. Recent theoretical investigations have
shown that generating combs in a topological array of resonators can provide a
new avenue to engineer comb spectra. Here, we experimentally demonstrate the
generation of such a novel class of frequency combs, topological frequency
combs, in a two-dimensional (2D) lattice of hundreds of nonlinear ring
resonators. Specifically, the lattice hosts topological edge states that
exhibit fabrication-robust linear dispersion and spatial confinement at the
boundary of the lattice. Upon optical pumping of the topological edge band,
these unique properties of the edge states lead to the generation of a nested
frequency comb that is spectrally confined within the edge bands across
≈40 longitudinal modes. Moreover, using spatial imaging of our
topological lattice, we confirm that light generated in the comb teeth is
indeed spatially confined at the lattice edge, characteristic of linear
topological systems. Our results bring together the fields of topological
photonics and optical frequency combs, providing an opportunity to explore the
interplay between topology and nonlinear systems in a platform compatible with
commercially available nanofabrication processes.
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