Abstract

The transition of particles between electronic bands is one of the most discussed physical phenomena in the last century [1]. Band structures and occupancies are widely studied but observed mainly in the spectral domain. However, direct observations of the temporal evolution of transitions are very rare due to the need of a high time resolution in most quantum and solid state systems. We utilize the analogy of quantum mechanics and optics to map the time coordinate onto space, which enables convenient access to the spatial evolution. Here, we present a binary superlattice system, which consists of two quantum wells in the unit cell. This system is exposed to an external force (Fig. la), which is artificially applied by circular bending of the waveguides (Fig. lb). This structure enables to observe Bloch-Zener dynamics and tunneling between the subbands of the system. We fabricated the samples by femtosecond laser waveguide writing and used the recently developed fluorescence microscopy [2] to visualize the dynamics in these systems. In a first instance every waveguide is manufactured with equal refractive index increase, resulting in ordinary Bloch oscillations, which are observed by Gaussian excitation covering 9 waveguides (FWHM) (Fig. 1c upper graphs). The band occupancy was measured and a complete tunneling between the subbands occurs. Gradually a shift of the propagation constant of adjacent waveguides was generated by lowering the writing velocity of every second guide. Therefore the more general case of Bloch-Zener oscillations occur. The tunneling becomes more and more incomplete (Fig. 1c, lower graphs). Additionally, the double-periodicity of such systems was analyzed, by single site excitation, where a breathing modes appear [3]. The generated shift of the propagation constant yields a subperiodicity, which characteristic length is the half of the loch cycle. Finally, a possible application of highly dispersive beamsplitter is proposed.

© 2009 IEEE