  A novel regime of atom-cavity physics is explored, arising when large atom
samples dispersively interact with high-finesse optical cavities. A stable far
detuned optical lattice of several million rubidium atoms is formed inside an
optical ring resonator by coupling equal amounts of laser light to each
propagation direction of a longitudinal cavity mode. An adjacent longitudinal
mode, detunedby about 3 GHz, is used to perform probe transmission spectroscopy
of the system. The atom-cavity coupling for the lattice beams and the probe is
dispersive and dissipation results only from the finite photon-storage time.
The observation of two well-resolved normal modes demonstrates the regime of
strong cooperative coupling. The details of the normal mode spectrum reveal
mechanical effects associated with the retroaction of the probe upon the
optical lattice.
