Semiclassical motion of a multiband Bloch particle in a time-dependent field: Optical visualization

An optical visualization of the semiclassical dynamics of a multiband Bloch particle subjected to an external time-dependent force is experimentally reported using an array of tunneling-coupled lithium-niobate optical waveguides with a curved axis. It is shown that a Gaussian beam injected into the array breaks into wave packets belonging to different bands of the array which propagate as semiclassical Bloch particles in a time-dependent force simulated by the waveguide axis curvature. According to the semiclassical analysis and to numerical simulations of the scalar beam propagation equation, the paths followed by the wave packets is proven to depend on the initial momentum, which is controlled by varying the incidence angle of the Gaussian beam. In particular, we study in detail, both theoretically and experimentally, the wave packet refocusing dynamics which occurs in the presence of a semicycle sinusoidal force and reveal its deep difference with respect to Bloch oscillations in a dc field.