Extreme Nonlinearities in All Dielectric Polaritonic Metasurfaces

We demonstrate an extremely nonlinear all-dielectric metasurface that employs intersubband polaritons to achieve a second-harmonic conversion coefficient of 5 x 10
^-4
W
^-1
and second-harmonic power conversion efficiency of 0.015% at nominal pump intensities of 11 kW/cm
²
. Furthermore, through heterostructure design, we achieve microscopic control of the nonlinearity and demonstrate suppression or enhancement of second-harmonic generation by controlling the polarity of the resonant χ
⁽²⁾
.