Analysis and design of core-shell upconverting nanostructures

Core-shell upconverting nanostructures (CSUNs) improve upconversion efficiency by blocking midgap recombination. Consecutive 3-level systems absorb strongly but preserve long carrier lifetime in their final states. Germanium has a nearly-direct bandgap ideal for a CSUN core: strong optical absorption yet slow recombination. Electrons in the indirect conduction band valley absorb a second photon to escape the shell and reach the host, unable to return. Our model indicates that free carrier absorption (FCA) from L to continuum is the limiting step. Surface plasmon resonances (SPR) in Ge-AlGaAs may selectively enhance this absorption. CSUNs would help preserve current matching at dawn and dusk.