Abstract

The exciton in quantum wells is a sensitive detector of electric fields. This field response has been used recently to detect ultrafast electrical transients in ultra-thin quantum well coplanar striplines with time resolution as fast at 180 fs [1]. In these experiments, it is found that the field in the gap is not uniform [2]. In such as case, it is interesting to use the spatial resolution of the excitonic electroabsorption effect to obtain space-time resolved electrical transient information. In the experiments, we use an infrared CPM dye laser operating at 820 nm wavelength [3], which is coincident with the exciton energy. The pump beam is focused into a 10 micron coplanar stripline which is patterned on the sample. The sample consists of a 0.4 micron thick layer of GaAs-AlGaAs quantum wells and a 0.35 micron thick AlGaAs stop etch layer. The substrate is etched out, leaving a 0.75 micron thick free-standing film. This structure has interesting high frequency propagation characteristics such as propagation speed of 82 % the speed of light, and low dielectric loss, radiation losses and modal dispersion [1,4]. The pump beam creates an ultrafast electrical transient which propagates in the coplanar stripline (Figure 1, inset). We probe the signal by focusing a delayed probe pulse through the stripline further down the line at the desired distance. In the present case, we investigate the spatial distribution of the field transient, as viewed by the quantum well excitons, which give a microscopic reading of the conditions in the structure. Figure 1 shows a result of focusing the probe beam to a 1 micron diameter spot and scanning the time delay at various positions in the gap.

© 1991 Optical Society of America