Femtosecond Excitonic Electroabsorption Sampling

Optoelectronic sampling based on the Pockels' effect1 has become an important technique for the measurement of electrical signals with the highest time resolution, currently at 300 fs. We present first results obtained 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 distances2. In our new embodiment, a coplanar stripline is fabricated on a GaAs multiple quantum well mesa ridge structure (Fig. 1). We thus obtain optical modulation by parallel-field electroabsorption, which is due to lifetime broadening by field ionization of the excitons3. The detection sensitivity is about 1%/volt in a 10 micron structure. We etch the GaAs substrate down to a 1 micron AlGaAs stop-etch layer in a 1×2 mm area and leave the stripline free-standing on the 1 micron thick film, thus obtaining an extremely low dispersion structure to test the EES concept. We use an infrared dye laser which produces femtosecond pulses at a wavelength of 805 nm4 at 82 MHZ repetition rate. The exciton energy is temperature-tuned to the laser with a Peletier device, in this case operating at about 5 degrees above ambient temperature. At 300 fs pulsewidth the laser spectrum is already comparable to the exciton linewidth, and we expect that shorter pulses will provide reduced sensitivity relative to the DC response. We expect that time resolution of 100 fs or less may be possible with this technique. We note that electroabsorption is a purely electronic phenomenon, with no ionic lattice contribution such as that of LiTaO3.