Phase-matching-free second-harmonic generation in an ultrahigh-order standing-wave field

In nonlinear wavelength conversion processes, maintaining phase matching is crucial to ensuring the conservation of photon momentum. Consequently, the requirement for phase matching limits the effectiveness of all parametric nonlinear optical processes. Techniques such as quasi-phase-matching, birefringent phase matching, and higher-order-mode phase matching have been developed to address this limitation. However, these methods necessitate specific beam arrangements and precise dispersion engineering, and are typically narrowband. In this study, we demonstrate that a submillimeter metal-clad waveguide can bypass the phase-matching requirement for on-chip nonlinear wavelength conversion by exciting an ultrahigh-order standing-wave field at nonspecial matching incident angles. Additionally, efficient secondharmonic generation is observed within the submillimeter waveguide across a broad range of pump wavelengths (from visible to infrared). The results indicate that the ultrahigh-order standing-wave field mitigates traditional phase-matching constraints, facilitating nonlinear interactions and the miniaturization of nonlinear devices.