Reducing the Optical Gain Threshold in Two-Dimensional CdSe Nanoplatelets by the Giant Oscillator Strength Transition Effect.

Two-dimensional CdSe nanoplatelets are promising lasing materials. Their large lateral areas reduce the optical gain threshold by increasing the oscillator strength and multiexciton lifetimes but also increase the gain threshold by requiring multiple band-edge excitons (>2) to reach the optical gain. We observe that the optical gain threshold of CdSe nanoplatelets at 4 K is ∼4-fold lower than that at room temperature. Transient absorption spectroscopy measurements indicate that the exciton center-of-mass coherent area is smaller than the lateral size at room temperature and extends to nearly the whole nanoplatelets at 4 K. This suggests that the reduction in the optical gain threshold at a low temperature can be attributed to exciton coherent area extension that reduces the saturation number of band-edge excitons to enable biexciton gain and increases the radiative decay rate, consistent with the giant oscillator strength transition effect. This work demonstrates a new direction for lowering the optical gain threshold of nanomaterials.