  Nanomaterials capable of confining light are desirable for enhancing
spectroscopies such as Raman scattering, infrared absorption, and nonlinear
optical processes. Plasmonic superlattices have shown the ability to host
collective resonances in the mid-infrared, but require stringent fabrication
processes to create well-ordered structures. Here, we demonstrate how
short-range-ordered Au nanoparticle multilayers on a mirror, self-assembled by
a sub-nm molecular spacer, support collective plasmon-polariton resonances in
the visible and infrared, continuously tunable beyond 11 $\mu$m by simply
varying the nanoparticle size and number of layers. The resulting
molecule-plasmon system approaches vibrational strong coupling, and displays
giant Fano dip strengths, SEIRA enhancement factors ~10$^6$, light-matter
coupling strengths g~100 cm$^{-1}$, Purcell factors ~10$^6$, and mode volume
compression factors ~10$^8$. The collective plasmon-polariton mode is highly
robust to nanoparticle vacancy disorder and is sustained by the consistent gap
size defined by the molecular spacer. Structural disorder efficiently couples
light into the gaps between the multilayers and mirror, enabling Raman and
infrared sensing of sub-picolitre sample volumes.
