Polarization-Sensitive Third Harmonic Generation in resonant silicon nitride Metasurfaces for deep-UV Emission
We present a combined experimental and theoretical study of enhanced third-harmonic generation (THG) in silicon nitride metasurfaces. These structures exhibit strong resonant nonlinear responses, enabling up to two orders of magnitude enhancement in THG compared to a flat silicon nitride etalon, driven by strong electromagnetic field localization. We investigate two polarization-selective metasurface geometries supporting transverse electric (TE) and transverse magnetic (TM) resonances, implemented in fully planar architecture. When driven by ultrafast near-infrared laser pulses, these resonances confine optical energy at the nanoscale, enabling efficient frequency up-conversion from the visible to the ultraviolet (UV) and deep-UV spectral regions. Through spectral mapping of the nonlinear response under both TE and TM excitation, we quantify field confinement, extract the effective nonlinear enhancement, and characterize the spectral dependence of the third-harmonic generation efficiency. This two-dimensional periodic platform provides a flexible design toolbox for engineering polarization-dependent UV and deep-UV light sources. More broadly, our results demonstrate that silicon nitride, a CMOS-compatible dielectric, can support efficient nonlinear up-conversion deep into the UV. This finding shows that access to short-wavelength nonlinear photonics does not require complex materials or architectures, but can instead be achieved using widely available dielectrics through careful structural design.