![]() ![]() The on-chip silicon-based topological photonic crystal (PC) was successfully fabricated with a small size of only 4.5 μm × 22 μm. Here, the topological photonic states are realized through constructing topological “Chern insulators” without the need of a magnetic field. ![]() Topological rainbow can separate and distribute different wavelengths of topological photonic states into different positions, and the light can be slowed and trapped through controlling the group velocities of the topological photonic states 31, 32, 33. In this work, we have constructed a topological rainbow device based on a synthetic dimension, which provides a method that is universally applicable for all optical lattice types, symmetries, materials, dimensions, and wavelength range. These challenges have restricted the development and applications of topological rainbow and various topological nanophotonic devices, including topological router, topological temporary storage, and many other on-chip integrated topological nanophotonic devices. Moreover, no effective method has been found to give direct measurement of topological photonic devices with multiple wavelengths at the nanoscale to date. Multi-frequency (or multi-wavelength) devices are essential components of nanophotonic chips for large information capacity applications, among which, topological rainbow, as a basic multi-wavelength topological photonic device, can separate and distribute different wavelengths of topological photonic states into different positions but related topological devices have not yet been fully explored. In topological photonics research, much attention has been paid to engineer the bulk band dispersion to realize topological states in order to achieve unidirectional propagation, topological laser, high-order topology, etc. Synthetic dimension provides a new degree of freedom which enables us to construct all dielectric on-chip topological nanophotonic components, breaking the restrictions of magnetic materials 27. The introduction of synthetic dimensions has brought opportunities and insight into topological photonics where the topology goes beyond the geometric dimensions 17, 18, 19, 20, 21, 22, 23, 24, 25, 26. However, topological nanophotonic devices 9, 10, 11, 12, 13, 14, 15, 16 are difficult to realize because of complexity of fabrication, challenges of measurement in the nano-scale, and the fact that magnetic response is inherently weak for natural materials in the visible and near infra-red range. Photonic devices become more robust against disorder and immune to scattering due to topological protection 7, 8, 9. In the past few years, topological photonics have made great progress from physical concept to new applications 1, 2, 3, 4, 5, 6. This work builds a bridge between silicon chip technologies and topological photonics. The topological rainbow based on synthetic dimension have no restrictions for optical lattice types, symmetries, materials, wavelength band, and is easy for on-chip integration. ![]() A homemade scattering scanning near-field optical microscope with high resolution is introduced to directly measure the topological rainbow effect of the silicon-based photonic chip. The topological rainbow can separate, slow, and trap topological photonic states of different frequencies into different positions. Here we give an experimental report of an on-chip nanophotonic topological rainbow realized by employing a translational deformation freedom as a synthetic dimension. Topological photonics provides a robust platform for next-generation nanophotonic chips. Multiple frequency on-chip nanophotonic devices are highly desirable for density integration, but such devices are more susceptible to structural imperfection because of their nano-scale. The era of Big Data requires nanophotonic chips to have large information processing capacity.
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