  Alkali-filled hollow-core fibres are a promising medium for investigating
light-matter interactions, especially at the single-photon level, due to the
tight confinement of light and high optical depths achievable by light-induced
atomic desorption. However, until now these large optical depths could only be
generated for seconds at most once per day, severely limiting the practicality
of the technology. Here we report the generation of highest observed transient
($>10^5$ for up to a minute) and highest observed persistent ($>2000$ for
hours) optical depths of alkali vapours in a light-guiding geometry to date,
using a caesium-filled Kagom\'e-type hollow-core photonic crystal fibre. Our
results pave the way to light-matter interaction experiments in confined
geometries requiring long operation times and large atomic number densities,
such as generation of single-photon-level nonlinearities and development of
single photon quantum memories.
