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 (10-10 m) and the wavelengths (10-10 m) of electromagnetic radiation relevant to their low-energy excitations. Here we introduce ferroelectric semiconductor NbOI 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 NbOI 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 NbOI 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.