Spiral-chirped nonlinear photonics enabling broadband orbital angular momentum second harmonic generation

We demonstrate a nonlinear frequency conversion strategy that transforms a Gaussian fundamental beam (carrying no orbital angular momentum, OAM) into a second-harmonic (SH) vortex wave with engineered OAM across an ultrabroad bandwidth. By implementing spiral-chirped quasi-phase-matching (QPM) in a femtosecond-laser-poled Sr_0.61Ba_0.39Nb₂O₆ (SBN) crystal, our system simultaneously imposes an azimuthal phase modulation to generate OAM in the SH wave and a longitudinal period chirp to achieve spectral broadening. This dual modulation converts collimated Gaussian beams (1200-1300 nm) into OAM-entrained SH vortices, exhibiting a spectral acceptance bandwidth significantly surpassing conventional helical-phase-matched systems while maintaining stable efficiency. This controllable second-harmonic OAM generation method opens new possibilities for applications including tunable structured light sources, high-capacity optical imaging using OAM multiplexing, and quantum light generation with tailored spectral properties, pushing forward the field of nonlinear optics that combines spectral and spatial control.