Switching of photon-echo polarization and direction by coherent optical fields

We report a novel method for the observation of photon echoes and other coherent transient effects. In this method, the polarization, direction, and frequency of the polarized coherent radiation emitted by a system are switched by optical pulses. We apply this method to detect photon echoes generated on the $6{P}_{\frac{1}{2}}\ensuremath{-}7{S}_{\frac{1}{2}}$ transition in atomic thallium by two collinear and vertically polarized excitation pulses which are separated by 20 nsec. At the time of the echo, two orthogonally polarized probe pulses resonant with the $6{P}_{\frac{3}{2}}\ensuremath{-}7{S}_{\frac{1}{2}}$ transition of atomic thallium are applied to the sample to switch the photon echo from its original polarization and direction. A horizontally polarized photon echo with its propagation vector turned by about 10 mrad and its axis of polarization rotated by 90\ifmmode^\circ\else\textdegree\fi{} is detected. No electro-optical shutter is used in the experiment. The echo signal disappears either when one of the probe pulses is blocked or when both probe pulses are horizontally or vertically polarized. Echo polarization switching is also demonstrated using an unpolarized probe pulse applied prior to the echo formation time.