THz Pulses from the Creation of Polarized Electron-Hole Pairs in Biased Quantum Wells

The generation of THz pulses on the surface of semiconductors has long been believed to be caused by transient transport currents parallel to the surface depletion field of the material after excitation with an ultrashort laser pulse. [1] Recently, an additional explanation of those experiments was given, in which optical transitions in the depletion field create electrons and holes in states where they are already polarized. [2] This creates a polarization P that grows with the integrated pulse energy, and hence radiates an electrical transient according to E~∂2P/∂t2. The final proof of the importance of this process however, can only be given in a sample where transport current is inhibited!