Strong and Robust Second-harmonic Generation in Two-dimensional H-Phase CrO ₂

Two-dimensional (2D) monolayers provide dangling-bond-free, atomically flat optical media with strong light–matter interactions. Compared with transition-metal dichalcogenides, transition-metal oxides are expected to exhibit stronger chemical bonding and improved robustness, motivating the search for stable 2D oxides with enhanced nonlinear optical responses. Here, using first-principles calculations, we identify a dynamically and thermally stable 2H-phase CrO 2 monolayer and show that it is energetically more favorable than the 1T polymorph. The 2H-CrO 2 monolayer is an indirect-gap semiconductor with an HSE06 gap of 0.92 eV and exhibits a giant second-harmonic generation (SHG) response reaching 1297.91 pm/V, nearly four times that of monolayer MoS 2 . Notably, four strong SHG resonances span a broad 480–1500 nm spectral window. Under biaxial tensile strain up to 5%, the band gap is continuously reduced and the SHG response is substantially enhanced, peaking at 2273.1 pm/V near the long-wavelength visible/near-infrared region. Momentum-space-resolved analysis reveals symmetry- and locality-selected k -space hotspots underlying the strong SHG and its strain enhancement. These results establish 2H-CrO 2 as a promising candidate for broadband nonlinear photonics and flexible optoelectronics.