Alkali-filled hollow-core fibers are a promising medium for
investigating light–matter interactions, especially at the singlephoton
level, due to the tight confinement of light and high
optical depths achievable by light-induced atomic desorption
(LIAD). 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 the highest observed transient (> 105 for up to a
minute) and highest observed persistent (>2000 for hours)
optical depths of alkali vapors in a light-guiding geometry to
date, using a cesium-filled Kagomé-type hollow-core photonic
crystal fiber (HC-PCF). 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.