Bloch oscillations
Bloch oscillations is referred to as the oscillation of charge carriers in solid state under the effect of a static electric field.
Cause of these oscillations is the relationship between the effective mass of a carrier, and the dispersion relation in a periodic potential.
The mass of charge carriers in crystalline solids is substantially dependent on the periodic properties of the grid. And direction ( anisotropy ) and amount (dispersion ) of the load carrier relative to the speed of the crystal lattice have a bearing on the effective mass. For simple approximation of an anisotropic solid it is inversely proportional to the curvature (ie, the second " derivative ") of the dispersion curve:
The effective mass can assume arbitrary real values , especially negative. An electric field externally applied constant now leads to an acceleration of the charge carriers
In this case, the pulse such that the effective mass is negative, performs a further action of force is not to further acceleration, but to decelerate, followed by an acceleration in the opposite direction. Since the dispersion relation symmetric with respect to positive and negative pulses, a turning point is reached at a negative pulse then again, so there is an oscillation instead. The oscillation frequency
Is proportional to the strength of the applied external field and the grating period d of the solid. This oscillation of an electric charge causes electromagnetic radiation which is in principle be measured.
In natural solids is due to the relatively small grating periods even at very high electric fields is not sufficiently high for the charge carriers can perform within scattering and tunneling times complete oscillations. The experimental proof of Bloch oscillations could therefore not be provided since its theoretical prediction by Leo Esaki in 1970, long time. Only the progress in semiconductor technology over the past years and decades, enabled by use of artificial semiconductor ( so-called semiconductor superlattice ) the fabrication of structures with sufficiently large superlattice periods. In such structures, the period of the oscillations is less than the scattering times of electrons so that scattering in the time multiple oscillations can be observed in a time-resolved experiment. The observation of Bloch oscillations in superlattices succeeded for the first time at temperatures close to absolute zero (Jochen Feldmann, 1992; Karl Leo, 1992). An important milestone was the observation of coherent terahertz radiation from Bloch oscillations ( Hartmut Roskos, 1993). At room temperature, Bloch oscillations have also been demonstrated experimentally ( Dekorsy Thomas, 1995).
Another system in which Bloch oscillations can be relatively easy to observe, are optical lattice of neutral atoms
Find potential application Bloch oscillations in electronic components for the generation of terahertz radiation. However, it is still not possible to realize an electrical component that emits due to Bloch oscillations terahertz radiation.