Universal and giant second-harmonic generation interference beyond magnetism

Conventional second-harmonic generation (SHG) interference, such as nonlinear magneto-optical effects, typically requires magnetism and strong spin-orbit coupling, limiting its occurrence to rare materials, narrow frequency ranges, and restrictive tunability. Challenging this assumption, we demonstrate through symmetry analysis and first-principles calculations that SHG interference can arise purely from crystal symmetry without magnetism or spin-orbit coupling. We show a stable and giant crystal SHG interference occurs universally in ${C}_{3v}$-symmetric nonmagnetic materials within the broad nonresonant regime. The proposed interference mechanism transcends traditional limitations, offering a transformative potential for distinguishing twinned structures, enabling nonlinear optical diode effects, and advancing electric-field-controlled SHG technologies.