THz emitters based on the photo-Dember effect

The photo-Dember effect is a source of pulsed THz emission following femtosecond pulsed optical excitation. The emission results from the ultrafast spatial separation of electron-hole pairs in carrier gradients due to their different diffusion coefficients. The associated time dependent polarization of the photo-carriers within the photo-Dember emitters is oriented perpendicular to the excited surface. THz emission under photo-excitation can also be due to the surface field effect or optical rectification. We review the studies that attempt to understand surge current and study the surface emission of different semiconductor materials. We also review the work that has been done on increasing the efficiency of THz surface emitters using plasmonic interactions and other resonant effects. We later focus on a new THz emission mechanism called lateral photo-Dember which proposes a scheme for generating strong carrier current parallel to the excited surface, using the Dember field as well as dipole quenching effect. Because the resulting currents are oriented parallel to the surface and the THz radiation is emitted collinearly to the optical excitation. Surprisingly, the lateral photo-Dember THz emitters provide a higher bandwidth than photoconductive emitters. The theory for the mechanism of emission is reviewed and the parameters that affect the performance of the lateral photo-Dember emitters, namely fluence and polarization, are summarized. Finally we review multiplexing geometries with periodically tailored photo-excited spatial carrier distributions that create phase coherent photo-Dember and surface field currents which enhance the THz emission. These multiple emitters can reach electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.