Gain roll-off in cadmium selenide colloidal quantum wells under intense optical excitation

Colloidal quantum wells, or nanoplatelets, show among the lowest thresholds for amplified spontaneous emission and lasing among solution-cast materials and among the highest modal gains of any known materials. Using solution measurements of colloidal quantum wells, this work shows that under photoexcitation, optical gain increases with pump fluence before rolling off due to broad photoinduced absorption at energies lower than the band gap. Despite the common occurrence of gain induced by an electron-hole plasma found in bulk materials and epitaxial quantum wells, under no measurement conditions was the excitonic absorption of the colloidal quantum wells extinguished and gain arising from a plasma observed. Instead, like gain, excitonic absorption reaches a minimum intensity near a photoinduced carrier sheet density of 2 x 10(13) cm(-2) above which the absorption peak begins to recover. To understand the origins of these saturation and reversal effects, measurements were performed with different excitation energies, which deposit differing amounts of excess energy above the band gap. Across many samples, it was consistently observed that less energetic excitation results in stronger excitonic bleaching and gain for a given carrier density. Transient and static optical measurements at elevated temperatures, as well as transient X-ray diffraction of the samples, suggest that the origin of gain saturation and reversal is a heating and disordering of the colloidal quantum wells which produces sub-gap photoinduced absorption.

Automated summary

Colloidal quantum wells, or nanoplatelets, exhibit low thresholds for amplified spontaneous emission and high modal gains. Optical gain in these quantum wells increases with pump fluence up to a certain point due to broad photoinduced absorption. Unlike bulk materials and epitaxial quantum wells, excitonic absorption in colloidal quantum wells is not extinguished and gain from plasma is not observed. The origin of gain saturation and reversal in these quantum wells is attributed to heating and disordering, leading to sub-gap photoinduced absorption. Transient and static optical measurements, as well as transient X-ray diffraction, support these findings.