Self-trapped exciton-phonon resonance enhanced nonlinear susceptibility in 2D KNbO2

Strong second harmonic generation (SHG) was observed when the two-photon energy matches to the self-trapped exciton resonance states in the lattice inversion symmetric system of two-dimensional (2D) KNbO2 crystals. It is ascribed to the symmetry broken induced by the bending distortion of O-Nb-O in individual octahedrons (low frequency T(2g )modes) that coupling to the two-photon frequency. The quantum confinement SHG response with more than one orders of enhancement is obtained due to the phonon-exciton-photon interaction with dimension reduction. In addition, the exciton-phonon interaction couples SHG photons to a non-equilibrium energy with a Stokes or anti-Stokes energy splitting of similar to 18-21 meV, which is corresponding to the Raman shift of T-2g modes.