Figure 14.

(a) The cg-QD laser prepared by depositing a cg-QD film on top of the second-order DFB grating. In this device, optical feedback is due to in-plane 2nd-order Bragg diffraction. The 1st-order Bragg scattering leads to surface emission whose direction is normal (or near-normal) to the device plane. The device is immersed into neat THF or a solution of LiEt₃BH in THF. In the latter case, the dots become negatively charged under exposure to incident light. The degree of charging can be controlled by varying the amount of LiEt₃BH. (b) The surface-emitted light intensity as a function of per-pulse pump fluence for varied degrees of charging (excitation by 190 fs pulses at 3.6 eV). For neutral cg-QDs, the lasing threshold (w_th,las) is 9 μJ cm^–2. Increasing the amount of reductant lowers w_th, las until it reaches ∼2 μJ cm^–2. This corresponds to the subsingle-exciton regime for which ⟨N_th,las⟩ = 0.31. (c) The spectra of surface emitted light for the neutral (black) and the charge (red) sample. The neutral sample does not show lasing at ⟨N⟩ = 1. However, after QDs are charged (⟨n_e⟩ = 3.1), the device exhibits single-mode, narrow-band lasing at a subsingle-exciton pump level for which ⟨N⟩ = 0.6. All panels adapted with permission from ref (61). Copyright 2019, The American Association for the Advancement of Science.