Low-threshold near-infrared lasing at room temperature using low-toxicity Ag₂Se quantum dots

The development of colloidal near-infrared quantum dot (QD) lasers has been hindered by the high state degeneracy of lead salt QDs. Here, we show that this challenge can be addressed by utilizing orthorhombic Ag2Se QDs. We demonstrate that the lowest quantized states of Ag2Se QDs display a low, 2-fold degeneracy by employing femtosecond transient absorption (TA) spectroscopy. The optical gain threshold is evaluated to be 156 μJ cm-2, corresponding to ∼1.4 excitons per QD on average. Consequently, the amplified spontaneous emission (ASE) threshold of Ag2Se QD films is as low as 183 μJ cm-2. A large modal gain (∼470 cm-1) of the film is observed by variable stripe length (VSL) measurements. We leverage the low-threshold gain of the QDs to produce microlasers that exhibit single-mode near-infrared emission and a low threshold of 163 μJ cm-2 at room temperature. In addition, the cytotoxicity of Ag2Se QDs is remarkably negligible. Our work represents a significant step toward environmental-friendly near-infrared QD lasers.