Figure 22.

Three different optical gain regimes in colloidal NPLs. In smaller-area NPLs, optical gain is controlled by “state filling” as in 0D nanocrystals. In this case, optical gain threshold, ⟨N_th,gain⟩, corresponds to excitation of approximately 1 exciton per NPL on average, that is, ⟨N_th,gain⟩ ≈ 1. In larger-area NPLs, whose lateral dimensions are greater than the size of the 2D exciton (2a_2D), the optical gain is controlled by effects of “space filling”. In this regime, the maximal excitonic occupancy of an NPL is defined by its area and is approximately equal to A_NPL/(πa²_2D). The gain threshold in this case is within the range 1 < ⟨N_th,gain⟩ < A_NPL/(πa²_2D). At high excitation levels, when the excitonic gas undergoes a Mott transition to a dense e-h plasma, the gain threshold shifts to ⟨N_th,gain⟩ > A_NPL/(πa²_2D). In this case, optical gain occurs due to stimulated emission from the degenerate e-h plasma. Center graph reproduced with permission from ref (148). Copyright 2015 American Chemical Society.