ULTRASENSITIVE ELECTROABSORPTION DUE TO WANNIER-STARK LOCALIZATION IN EXTREMELY SHALLOW SUPERLATTICES AT 77-K

We have observed Wannier-Stark localization in extremely shallow superlattices (SL) at 77 K, which results in a quasi-blue shift of the absorption edge at very small electric fields. The formation of a Stark ladder is also clearly shown. The blue shift results in a change in absorption coefficient from 1.7-mu-m-1 (at 0.2 V/mu-m) to 0.98-mu-m-1 (at 0.6 V/mu-m) for a GaAs/Al0.07Ga0.93As SL; for a GaAs/Al0.02Ga0.98As SL we obtain a change from 2.1-mu-m-1 (at 1.1 V/mu-m) to 0.9-mu-m-1 (at 1.5 V/mu-m).