Understanding the Mechanisms Behind Optical Gain in Bulk Solution Processible Semiconductors

Colloidal quantum dots (QDs) are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers. This is due to their appealing processing conditions, compared to e.g. epitaxy, resulting in lowering cost. They can also be patterned and their optical properties can be tuned. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although impressive milestones were achieved, there is to date no single material that can provide the demanding combination of gain metrics to be truly competitive with existing epitaxial growth approaches. In this talk, we take a look at material properties of CdS/Se nanocrystals in the regime of vanishing quantum confinement, so-called 'bulk nanocrystals'. We show that these unique materials display disruptive optical gain metrics in the green optical region. Indeed, while showing similar gain thresholds compared to state-of-the-art QD materials, the gain window (440-600 nm, 640- 750 nm), amplitude (up to 50.000/cm) and gain lifetime (up to 3 ns) vastly outpace other solution processible materials. These results, while very impressive, are also puzzling. In the solution processible community a material system without quantum confinement does not exactly inspire confidence to have good emission metrics. We attempt to explain the physics behind these huge gain coefficients, by using a bulk semiconductor model which includes a strong band-gap renormalization effect, and argue why going to a bulk semiconductor can be advantageous compared to confined systems for making integrated lasers.