LOW-VOLTAGE, LOW-CHIRP, ABSORPTIVELY BISTABLE TRANSMISSION MODULATORS USING 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 separation of electron and hole states to modulate the magnitude of exciton 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 well containing a 50 angstrom In0.3Ga0.7As well and a 30 angstrom In0.15Ga0.85As well separated by a 10 angstrom Al0.33Ga0.67As barrier. One structure was grown with the thinner well on the n-type side of each coupled quantum well while in the other sample the thinner well was oriented 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 devices grown, despite their similar structure, operate through the field-induced switching of opposite carrier types. Because this method of modulation does not require excitons to Stark shift, the device can produce 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 systems. In addition, optical bistability is realizable in these structures. In addition to their presently displayed use, the coupled quantum-well structure has numerous applications for waveguide or Fabry-Perot optical modulator systems.