Mid-infrared to ultraviolet efficient multiphoton frequency upconversion in NbOI₂ crystals

Nonlinear frequency upconversion of mid-infrared (MIR) light is of particular importance for infrared imaging, detection and spectroscopy. However, achieving efficient and broadband MIR upconversion is hindered by phase-matching constraints and limited nonlinear coefficients of optical materials. Here, we demonstrate phase-matching-free, broadband and efficient multiphoton frequency upconversion in NbOI₂. We observe generation of harmonics from the second to the 11th order, with upconverted signals spanning from the MIR to the ultraviolet. We also achieve nonlinear frequency mixing in the broad spectral window of 1500-5000 nm, including degenerate four-wave mixing, sum-frequency mixing and sum-frequency generation with an efficiency up to 6×10^-⁴ W^-¹, surpassing those of leading metasurface-based platforms. Finally, we demonstrate broadband MIR upconversion imaging by direct detection from a conventional silicon camera. Our results establish NbOI₂ as a promising platform for MIR nanophotonics, enabling advanced applications in ultrabroadband imaging, sensing and spectral conversion.