Terahertz emission from giant optical rectification in a van der Waals material

The exfoliation and stacking of two-dimensional van der Waals crystals have created unprecedented opportunities in the discovery of quantum phases. A major obstacle to the advancement of this field is the limited spectroscopic access due to a mismatch in the sample sizes (10^-6-10^-5 m) and the wavelengths (10^-4-10^-3 m) of electromagnetic radiation relevant to their low-energy excitations. Here we introduce ferroelectric semiconductor NbOI₂ as a two-dimensional van der Waals material capable of operating as a van der Waals terahertz emitter. We demonstrate intense and broadband terahertz generation from NbOI₂ with an optical rectification efficiency that is more than one order of magnitude higher than that of ZnTe, the current standard terahertz emitter. Moreover, this NbOI₂ terahertz emitter can be integrated into van der Waals heterostructures to enable on-chip near-field terahertz spectroscopy of a target van der Waals material and device. Our approach provides a general spectroscopic tool for two-dimensional van der Waals materials and quantum matter.