Exfoliation and stacking of two-dimensional (2D) van der Waals (vdW) 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 sample sizes (10−6 – 10−5 m) and wavelengths (10−4 – 10−3 m) of electromagnetic radiation relevant to their low-energy excitations. Here, we introduce ferroelectric semiconductor NbOI2 as a 2D vdW material capable of operating as a vdW terahertz (THz) emitter. We demonstrate intense and broadband THz generation from NbOI2 with optical rectification efficiency over one-order-of-magnitude higher than that of ZnTe, the current standard THz emitter. Moreover, this NbOI2 THz emitter can be integrated into vdW heterostructures to enable on-chip near-field THz spectroscopy of a target vdW material and device. Our approach provides a general spectroscopic tool for 2D vdW materials and quantum matter.
