Observation of a nonlinear mode in a cylindrical Fabry–Perot cavity

We report what is believed to be the first observation of a nonlinear mode in a cylindrical nonlinear Fabry–Perot cavity. The field enhancement from cavity buildup, as well as the large ${\chi }^{\left(3\right)}$ optical nonlinearity that is due to resonantly excited ${ }^{85}\mathrm{Rb}$ vapor, allows the nonlinear mode to form at low incident optical powers of less than 1 mW. The mode is observed to occur for both self-focusing and self-defocusing nonlinearity.