3D ultra-broadband optically dispersive microregions in lithium niobate.

3D in-substrate integration of optical functionalities fully utilizes the vertical dimension of space and is valuable for advancing next-generation integrated optoelectronics. However, as a key optical effect, optical dispersion remains unavailable to be tailored at the microscale in 3D. We introduce artificial dispersive microregions in lithium niobate crystals to engineer free-space ultra-broadband optical dispersion. The microregions are formed by ultrafast laser-induced sub-wavelength phase-transition nanostripes, which modulate the crystal's birefringence to establish localized frequency-dependent interference of ordinary and extraordinary light. This approach operates across an ultra-broad wavelength range (>1300 nm) within an exceptionally compact volume (50 × 10 × 6 µm³), and allows for precise, on-demand dispersion control in 3D space. The dispersive microregions exhibit viewing-angle independence, stability to harsh conditions (600 °C high temperature, contamination, corrosion, and mechanical damage), and wide applicability across various birefringent crystals. We demonstrate the versatility of our method in developing broadband on-chip micro-spectrometers and applications of spectral imaging, information recording, and encryption.