Ultrafast Control of the Optical Transition in Type-II Colloidal Quantum Wells

Manipulating the optical transition in semiconductors at ultrashort timescales is of both fundamental interest and central importance for emerging photonic applications. Traditionally, this manipulation is realized by electrostatic gating via Stark effects or band-gap renormalizations. Here, we report an ultrafast and all-optical route to engineer an indirect transition in core- crown colloidal quantum wells (CQWs), namely, CdSe/CdTe, with a type-II band alignment. Following the intense laser pulse excitation, the indirect band transition energy exhibits a pronounced blueshift-redshift crossover on the picosecond timescale, stemming from the formation and dissipation of the transient electric field (E-field) that forms upon photoexcitation to compensate for the driving force provided by the band offsets. Both the energy shift and dynamics of the transient E-field can be modulated optically by tuning the laser pulse excitation fluence. Our finding demonstrates a strong analogy between the type-II heterojunction and a p-n junction with respect to carrier equilibrium processes, which holds promise to facilitate the integration of CQWs within optical switching networks.