Spontaneous optical skyrmion generation by frequency doubling in underdense plasmas

Optical skyrmions have been widely explored in recent years. Among them, Stokes skyrmions require sophisticated wave engineering or photonic devices for their generation. We show that Stokes skyrmions can emerge spontaneously in second-order harmonic generation in underdense plasmas driven by optical vortices. The nonlinear response produces a structured frequency-doubled field whose polarization texture maps the Poincaré sphere. When plasma inhomogeneities are taken into account, the electron density gradient deforms the skyrmionic texture, enabling topological diagnosis of plasma density.