Manipulation and enhancement of group delays based on topological transitions in black phosphorus metasurfaces

We theoretically investigate group delay in multilayer black phosphorus (BP) metasurfaces. For orthogonally stacked four-layer structures, enhanced group delay for the transmitted beam occurs near the topological transition frequencies, as derived by effective medium theory. Its magnitude depends linearly on the incident wave’s polarization angle, while its spectral position is tuned via the electron density on each layer. Exploiting slow-light effects, configuring the electron densities on BP metasurfaces enables seven-frequency asynchronous and four-frequency synchronous optical switching. Enhanced and polarization-tunable group delay is also observed for reflected beams near their transition frequencies. Increasing the layer count and tuning densities extends group delay to more frequency bands. These findings provide insights for the design of mid-infrared BP-based delay devices.