Purely optically generated current pulses

For the characterization of highest-frequency components it is desirable to produce ultrashort current pulses, the temporal form of which can be varied arbitrarily. Hitherto methods to produce current pulses of a few 100 fs in length are based on a combination of electronic and optical procedures that do not allow a variation. The Terahertz-Optics group investigates the generation of ultrashort current pulses by means of solely optical methods. With these methods it is, in principal, possible to modify the shape of these current pulses.

At PTB special semiconductor nanostructures were produced. These nanostructures are excited with short optical pulses taking certain symmetry conditions into account. By exploiting non-linear optical processes an electrical current is created in the semiconductor. In this process the charge carriers are not accelerated in an existing electric field as would be for a normal electrical current.

The pulses are measured via the simultaneously generated electromagnetic radiation: the pulses produce a polarization variation which acts as a source for electromagnetic radiation emitted into free space. Due to the ultrashort optical excitation the current pulses and radiated electromagnetic pulses are merely a few 100 fs in duration. Such short pulses contain frequency components of several THz which is why they are usually called THz pulses. The temporal shape of the emitted THz pulses is measured using electro-optic sampling methods.

A part of the experimental set-up. The propagation paths of the laser and THz pulses (red and blue lines, respectively) are marked.