Optical excitation of nonlinear spin waves

We demonstrate a technique for exciting spin waves in an ultracold gas of Rb-87 atoms on the basis of tunable ac Stark potentials. This technique allows us to excite normal modes of spin waves with arbitrary amplitudes in the trapped gas, including dipole, quadrupole, octupole and hexadecapole modes. These modes exhibit strong nonlinearities, which manifest as amplitude dependence of the excitation frequencies and departure from sinusoidal behavior. Our results are in good agreement with a full treatment of a quantum Boltzmann transport equation.