Extremely low-intensity optical nonlinearity in asymmetric coupled quantum wells

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Abstract
We have observed an intensity‐dependent shift in the exciton absorption features for a system of coupled quantum wells in which a thin barrier separates two wells of unequal widths. We found that for a fixed external bias on the sample, the normal quantum Stark shift of the excitons in the wells was suppressed when light of 1 μW/cm2 intensity was used to measure the absorption. The shift became systematically larger as the light intensity was decreased to as low as 100 pW/cm2. This extremely low‐level nonlinearity may be due to internal fields that arise when free electrons and holes are separated within the coupled‐well system.