Excitonic electroabsorption sampling: A new approach to femtosecond optoelectronics

The quantum-well exciton has been combined with a very-high-speed geometry for electrical signal propagation in the form of an ultrathin substrate coplanar stripline, resulting in a novel approach to femtosecond optoelectronics. The basic aspects of this technique as well as its limits and some applications are discussed. A 10-μm gold coplanar well layer grown on a thin AlGaAs stop etch layer on a GaAs substrate is shown. The back of the sample is etched away, leaving a free-standing film approximately 1-μm thick, in a 1- × 2-mm area. Electrical pulses can be generated in a standard manner, as in an Auston switch. However, the detection method is different from all other approaches. A probe beam is focused through the stripline and measures the exciton absorption at a particular time delay. The excitonic response to an applied field is essentially instantaneous. Thus, the microscopic electrical signal (actual field in the quantum well) can be internally probed with a time resolution given by the laser pulse. Some recent experimental results are reported.