  We developed a technique that enables to replace a metallic waveguide
cladding with a low-index (n $\sim$ 1.4) material - CaF2 or BaF2 - that in
addition is transparent from the mid-IR up to the visible range: elevated
confinement is preserved while introducing an optical entryway through the
substrate. Replacing the metallic backplane also allows double-side patterning
of the active region. Using this approach, we demonstrate strong light-matter
coupling between an intersubband transition (lambda $\sim$ 10 microns) and a
dispersive resonator, at 300 K and at 78 K. Finally, we evaluate the potential
of this approach as a platform for waveguiding in the mid-IR spectral range,
with numerical simulations that reveal losses in the 1-10 cm$^{-1}$ range.
