Optoelectronic sampling based on the electrooptic effect has become an important technique for the measurement of electrical signals with the highest time resolution, currently at 300 fs.1 We present results using a new technique for femtosecond electrical pulse measurement excitonic electroabsorption sampling (EES). We have previously shown that excitons exhibit a femtosecond electroabsorption response. However, the device which was used did not facilitate propagation studies over macroscopic distances.2 In our new embodiment, a coplanar stripline is fabricated on a GaAs multiple quantum well structure (Fig. 1). Applied voltages cause field ionization of the excitons.3 The detection sensitivity is ~1%/V in a 10-μm structure. We etch the GaAs substrate down to an AlGaAs stop-etch layer In a 1- X 2-mm area and leave the stripline free-standing on the 2.7-μm thick film, thus obtaining an extremely low dispersion structure. We use an IR dye laser (805 nm) which produces femtosecond pulses of 0.15- nJ energy at an 82-MHz repetition rate. Figure 2 shows the transmission change spectrum which is induced by a 0-5-V modulation as a function of the dc bias offset. The limiting time resolution of this detection scheme is determined by the bandwidth of the net positive transmission change region. At high dc offsets this region is broad enough to support modulation of even 50-fs optical pulses, whereas at low bias levels the time resolution would be limited to a few hundred femtoseconds. We expect that a time resolution of 100 fs or less may be possible with this technique,4 noting that electroabsorption is a purely electronic phenomenon, with no ionic lattice contribution such as that of LiTaO3.
© 1989 Optical Society of AmericaPDF Article