The generation of electrical currents in semiconductor structures by optical excitation alone has been achieved at PTB. The procedure applied had so far only been realized at a few laboratories in the world. Special feature of the procedure is that it does not require an electrical field to accelerate the charge carriers. In a sense this corresponds to creating a current flow without a voltage source. Such procedures can become important in the future, among other things for high-frequency signal processing.
For 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 hundred femtoseconds in length are based on a combination of electronic and optical procedures that do not allow a variation. PTB has now succeeded in producing ultrashort current pulses by means of a solely optical method. With this method it is, in principle, possible to modify the shape of these current pulses.
At PTB special semiconductor nanostructures were produced and excited with short optical pulses to create the purely optically generated currents. Certain symmetry conditions had to be taken into account for the excitation process. By exploiting non-linear optical processes an electrical current is created in the semiconductor. In the 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 pulse and radiated electromagnetic pulses are merely a few 100 fs in duration. Such short pulses contain frequency components of several Terahertz which is why they are usually called Terahertz pulses. The temporal shape of the emitted Terahertz pulses is measured with electro-optic methods.
Further experiments will investigate the coup-ling of the optically generated current pulses into planar waveguides. Such waveguides are important for the characterization of high-frequency components.