Quantum coherence in superlattices under electric fields

Experimental demonstration of the formation of the Stark ladder and a direct observation of the coherence of the superlattice electronic wave functions have been obtained using optical spectroscopy in GaAs/GaAlAs superlattices under electric fields. The band structure of a periodic system subject to an electric field is broken into discrete states, forming the so-called Stark ladder, which had hitherto not been observed and whose very existence in solids was somewhat controversial. The observation is a direct consequence of the formation of Stark ladders in the conduction and valence bands, and the field-induced structures correspond to interband transitions between Stark levels. The spatial extent of the observed Stark ladder gives a measure of the quantum coherence of the superlattice wave functions. The tunability provided by the electric field has been exploited to induce doubly resonant Raman scattering when the energy spacing between two states of the Stark ladder coincides with the energy of optical phonons.