Artificial Second-Order Nonlinear Optics in a Centrosymmetric Optical Material BiVO4: Breaking the Prerequisite for Nonlinear Optical Materials

Second-order nonlinear optics (NLO) is the foundation of frequency conversion for the generation of coherent light at frequencies where lasers have no emissions or operate poorly. The prerequisite for NLO materials is noncentrosymmetric symmetry that can generate an effectively non-counterbalanced spontaneous electronic polarization. Here, we propose that this material restriction can be broadened by controlling the electron distribution with a local internal electrostatic field (IEF), and we demonstrate artificially created and manipulated second harmonic generation (SHG) in a centrosymmetric optical material, a superimposed Co2+- and Mo6+-doped BiVO4 thin film with 2/m point group symmetry, where a homojunction producing tunable effective polarization is formed. The SHG was characterized and tuned by IEF. This work breaks the structural symmetry constraint on NLO materials. Besides, the phase-matching-like condition was realized for the further improvement of the efficient frequency conversion. Because polarization is also a prerequisite for many other functions besides SHG, we believe that this work should provide some inspiration for the further development of optoelectronic, photonic, and electronic materials.