LOW-VOLTAGE, LOW-CHIRP, ABSORPTIVELY BISTABLE TRANSMISSION MODULATORSUSING TYPE-IIA AND TYPE-IIB IN0.3GA0.7AS AL0.33GA0.67AS/IN0.15GA0.85AS ASYMMETRIC COUPLED QUANTUM-WELLS/

Coupled InGaAs quantum-well systems which use field-induced spatial se paration of electron and hole states to modulate the magnitude of exci ton optical absorption, and hence transmission have been theoretically analyzed and experimentally demonstrated. The samples consisted of p- i-n diodes with an active region of 20 coupled wells, each coupled wel l containing a 50 angstrom In0.3Ga0.7As well and a 30 angstrom In0.15G a0.85As well separated by a 10 angstrom Al0.33Ga0.67As barrier. One st ructure was grown with the thinner well on the n-type side of each cou pled quantum well while in the other sample the thinner well was orien ted toward the p-type side. By applying bias to the structures, either the lowest electron or hole states effectively switch wells, thereby enhancing certain exciton resonances and quenching others. The two dev ices grown, despite their similar structure, operate through the field -induced switching of opposite carrier types. Because this method of m odulation does not require excitons to Stark shift, the device can pro duce large absorption/transmission changes with zero refractive index change under bias. These first nonoptimized samples produce changes in absorption per applied bias three times larger than single-well syste ms. In addition, optical bistability is realizable in these structures . In addition to their presently displayed use, the coupled quantum-we ll structure has numerous applications for waveguide or Fabry-Perot op tical modulator systems.