Dear Editor,

please find enclosed a manuscript on Operating a near-concentric cavity at the
last stable resonance.  While optical resonators are a staple tool in optical
and atomic physics, they usually are used far away from the geometrical
stability limit.

In this work, we explore how close to the limit of optical/geometrical
stability one can get, with the idea of obtaining the highest possible
electric field in the center of a cavity.  We find that we can reliably
keep an optical cavity at the last stable longitudinal resonance, which in our
case is about 200 nm away from the critical distance. As far as we are aware,
this is the first time such an operation is reported.

This manuscript covers mostly experimental work in classical optics, motivated
for the purpose of cavity quantum electrodynamics.  It is related to earlier
work in our group in this direction, PRA 96, 031802(R) (2017), and New Journal
of Physics 16, 103002 (2014), which deal more with the atom-light interaction
part.  The optical aspect covered in the work at hand marks an important step
towards making such near-concentric cavities useful for experiments, even
beyond the realm of traditional AMO physics.

Therefore, we feel that Physical Review A might be a suitable journal, and ask
you for your kind consideration of our manuscript as a regular article.

With Best Regards on behalf of all authors,

Christian Kurtsiefer