Interplay between optical gain and photoinduced absorption in CdSe nanocrystals

A potential complication associated with the realization of the optical-gain regime in semiconductor nanocrystals (NCs) arises from the competing phenomenon of excited-state absorption. Here, we apply an ultrafast transient absorption technique to study the competition between optical gain and excited-state (photoinduced) absorption in CdSe NCs as a function of NC size, surface passivation, and solvent/matrix identity. We observe that for NCs prepared in such a commonly used solvent as hexane, the contribution from photoinduced absorption rapidly increases with decreasing NC radius, and it completely suppresses optical gain in NCs of small sizes. Further, the magnitude of photoinduced absorption is sensitive both to the type and the quality of surface passivation, as well as to the identity of the solvent/matrix material. These observations, along with a strong size dependence, indicate that photoinduced absorption interfering with optical gain is not intrinsic to CdSe NCs but, instead, results from the involvement of NC interface states/processes.