Giant second-harmonic generation in ferroelectric NbOI2

Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase-matching conditions requiring thicknesses the order of hundreds wavelengths, and disadvantaged short interaction depth nanometre-scale materials weak photon–photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant second-harmonic generation with conversion efficiencies are orders magnitude higher than commonly reported crystals. The response scales layer thickness strain- electrical-tunable; a record >0.2% absolute SHG efficiency an effective susceptibility $$\chi _{\mathrm{eff}}^{(2)}$$ 10?9 m V?1 demonstrated at average pump intensity 8 kW cm–2. Due to interplay between anisotropic polarization excitonic resonance NbOI2, spatial profile polarized can be tuned excitation wavelength. Our results represent new paradigm for ultrathin, efficient components. Strained flakes 20 nm bulk-like 1.1 × fundamental wavelength 1,050 nm.