  Strong coupling between various kinds of material excitations and optical
modes has recently shown potential to modify chemical reaction rates in both
excited and ground states. The ground-state modification in chemical reaction
rates has usually been reported by coupling a vibrational mode of an organic
molecule to the vacuum field of an external optical cavity, such as a planar
Fabry-P\'erot microcavity made of two metallic mirrors. However, using an
external cavity to form polaritonic states might: (i) limit the scope of
possible applications of such systems, and (ii) be unnecessary. Here we
highlight the possibility of using optical modes sustained by materials
themselves to self-couple to their own electronic or vibrational resonances. By
tracing the roots of the corresponding dispersion relations in the complex
frequency plane, we show that electronic and vibrational polaritons are natural
eigenstates of bulk and nanostructured resonant materials that require no
external cavity. Several concrete examples, such as a slab of excitonic
material and a spherical water droplet in vacuum are shown to reach the regime
of such cavity-free self-strong coupling. The abundance of cavity-free
polaritons in simple and natural structures questions their relevance and
potential practical importance for the emerging field of polaritonic chemistry,
exciton transport, and modified material properties.
