Ultrafast manipulation of electron spin coherence

A technique is developed with the potential for coherent all-optical contro l over electron spins in semiconductors on femtosecond time scales. The exp eriments show that optical "tipping" pulses can enact substantial rotations of electron spins through a mechanism dependent on the optical Stark effec t. These rotations were measured as changes in the amplitude of spin preces sion after optical excitation in a transverse magnetic field and approach p i /2 radians. A prototype sequence of two tipping pulses indicates that the rotation is reversible, a result that establishes the coherent nature of t he tipping process.