Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. QWs have several advantages as gain media in semiconductor lasers, including tunable emission wavelengths and low threshold currents. So far, however, QWs have been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically in large volumes and at low cost. In these quantum-dot (QD) systems, however, carriers are confined in all three dimensions, and only a small number of exciton states exist at the optical bandgap energy. Since QDs cannot be packed together closer than their diameters, the maximum optical gain in a layer of QDs is limited. Moreover, initial demonstrations of optical gain from colloidal QDs involved high thresholds, which were attributed to rapid Auger processes. Attempts to reduce thresholds based on designing QDs to reduce the effects of Auger recombination have culminated in thresholds as low as ~26 µJ/cm2. In this case, however, the lower threshold comes at the expense of the maximum obtainable gain, because the QDs have think shells that reduce their packing density.
Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). The faces of these platelets are capped with organic ligands, and the platelets are typically surrounded by solvents or by air. This means that carrier confinement and exciton binding energies are much stronger in colloidal NPLs than in epitaxial QWs. The stronger confinement and binding energies, in turn, are likely to result in significantly different carrier dynamics.
We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials.
We therefore investigated amplified spontaneous emission (ASE) using close-packed films of NPLs. We observed thresholds as low as 6 µJ/cm2, more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is as high as 600 cm-1, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.