Abstract

Excitonic electroabsorption in thin GaAs-AlGaAs free-standing films is a novel approach to femtosecond optoelectronics in which limits of femtosecond electrical pulse generation, propagation and detection can be investigated [1]. The use of ultrathin substrates as a propagation medium for femtosecond electrical signals appears to be a interesting new feature. As the substrate becomes thinner, the TE cutoff frequency increases, until the field lines ultimately propagate in air above and below the lines. This drastically reduces the modal dispersion and phonon losses. The ultimate limit for this technique is achieved when the substrate thickness is only one optical absorption length; for thinner substrates, no electroabsorption is expected. We have fabricated a sample which is only 0.75 microns thick, for which the calculated TE cutoff frequency is 30 THz, and the propagation speed is almost the speed of light. This sample is transparent in the visible spectral range. We investigate the excitonic nonlinear response to resonant excitation, then apply a DC field and measure the propagation of femtosecond electrical signals.

© 1990 Optical Society of America