  Second-order nonlinear optics is the base for a large variety of devices
aimed at the active manipulation of light. However, physical principles
restrict its occurrence to non-centrosymmetric, anisotropic matter. This
significantly limits the number of base materials exhibiting nonlinear optics.
Here, we show that embedding chromophores in an array of conical channels 13 nm
across in monolithic silica results in mesoscopic anisotropic matter and thus
in a hybrid material showing second-harmonic generation (SHG). This non-linear
optics is compared to the one achieved in corona-poled polymer films containing
the identical chromophores. It originates in confinement-induced orientational
order of the elongated guest molecules in the nanochannels. This leads to a
non-centrosymmetric dipolar order and hence to a non-linear light-matter
interaction on the sub-wavelength, single-pore scale. Our study demonstrates
that the advent of large-scale, self-organised nanoporosity in monolithic
solids along with confinement-controllable orientational order of chromophores
at the single-pore scale provides a reliable and accessible tool to design
materials with a nonlinear meta-optics.
