The topological edge state (TES) in a one-dimensional optical lattice has exhibited robust field localization or waveguiding against the structural perturbations that would give rise to fault-tolerant photonic integrations. However, the zero mode as a kind of TES usually deviates from the exact zero-energy state in a finite Hermitian lattice due to the coupling between these edge states, which inevitably weaken the topological protection. Here, we first show such a breakup of zero modes in finite Su-Schriffer-Heeger optical lattices and then reveal their recovery by introducing non-Hermitian degeneracies with parity-time ($PT$) symmetry. We carry out experiments in a finite silicon waveguide lattice, where a passive-$PT$ symmetry was implemented with carefully controlled lossy silicon waveguides. The experimental results are fully compatible with the theoretical prediction. Our results show that the topological property of an open system can be tuned by non-Hermitian lattice engineering, which offers a route to enhance the topological protection in a finite system.

Tomography is an informative imaging modality that is usually implemented by mechanical scanning, owing to the limited depth-of-field (DOF) in conventional systems. However, recent imaging systems are working towards more compact and stable architectures; therefore, developing nonmotion tomography is highly desirable. Here, we propose a metalens-based spectral imaging system with an aplanatic GaN metalens (NA = 0.78), in which large chromatic dispersion is used to access spectral focus tuning and optical zooming in the visible spectrum. After the function of wavelength-switched tomography was confirmed on cascaded samples, this aplanatic metalens is utilized to image microscopic frog egg cells and shows excellent tomographic images with distinct DOF features of the cell membrane and nucleus. Our approach makes good use of the large diffractive dispersion of the metalens and develops a new imaging technique that advances recent informative optical devices.

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 has been predicted in systems with nonreciprocal couplings which, however, are challenging 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 photonic platform. By engineering the artificial gauge fields induced by the periodical modulation, we observe various Floquet NHSE phases and unveil their rich topological transitions. Remarkably, we discover the transitions between the unipolar NHSE phases and an unconventional bipolar NHSE phase, 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 thus offers fundamentally new ways for steering light and other waves.

Coupling among closely packed waveguides is a common optical phenomenon, and plays an important role in optical routing and integration. Unfortunately, this coupling property is usually sensitive to the working wavelength and structure features that hinder the broadband and robust functions. Here, we report a new strategy utilizing an artificial gauge field (AGF) to engineer the coupling dispersion and realize a dispersionless coupling among waveguides with periodically bending modulation. The AGF-induced dispersionless coupling is experimentally verified in a silicon waveguide platform, which already has well-established broadband and robust routing functions (directional coupling and splitting), suggesting potential applications in integrated photonics. As examples, we further demonstrate a three-level-cascaded AGF waveguide network to route broadband light to desired ports with an overwhelming advantage over the conventional ones in comparison. Our method provides a new route of coupling dispersion control by AGF and benefits applications that fundamentally rely on waveguide coupling.