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

Abstract

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 × 1013 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.

Reexamination of the Giant Oscillator Strength Effect in CdSe Nanoplatelets

“…Unifying these findings of room temperature photoluminescence lifetimes, transient absorption studies, and OSE measurements is an important distinction between NPLs and quantum dots: due to their large lateral size, NPLs can potentially support multiple isolated excitons which collectively explain the large NPL absorption. 14,15,22,48 (Unlike quantum dots, absorption of one photon does not necessarily result in a half-filled conduction band level. 15 ) The remainder of this work seeks to examine the absorptive measures of oscillator strength at lower temperature to look for manifestations of the GOST effect reported on the basis of photoluminescence lifetimes or transient four-wave mixing at cryogenic temperatures.…”

“…Transient four-wave mixing experiments and optical gain measurements also support this finding. However, room temperature radiative lifetime and other measurements of exciton coherent motion imply that exciton center-of-mass motion is localized. ,− Instead of large areas of coherent exciton motion (i.e., without scattering) and large oscillator strength, room temperature data are consistent with large extinction coefficients but composed of NPLs that can individually support many band edge excitonic transitions.…”

2D II–VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

“…Mott transitions are driven by the screening of electrons and holes in an exciton at high carrier densities, which can be achieved by the generation of many excitons within a single NPL . When large number of excitons are generated, one report on the transient absorption of NPLs by Tomar et al shows the formation of an optical gain band on the red and blue sides of the first excitonic peak, although this was not found in other studies. , The blue gain band was attributed to an incomplete Mott transition involving an electron–hole plasma. However, even highly excited NPLs are dominated by excitonic species rather than free carriers.…”

“…NPL absorption-temperature-dependence of (a) the absorption of 4 ML CdSe NPLs. Data adapted from multiple literature sources. , (b) Infrared absorption spectra for 3 ML HgTe NPLs between 10 and 300 K. The inset is the experimental temperature dependence of the bandgap variation, represented as a function of HgTe NC and NPL E G confinement energy (at T = 10 K). The green dotted line is from the (3 + 1) kp model.…”

“…shows the formation of an optical gain band on the red and blue sides of the first excitonic peak, 247 although this was not found in other studies. 221,262 The blue gain band was attributed to an incomplete Mott transition involving an electron−hole plasma. However, even highly excited NPLs are dominated by excitonic species rather than free carriers.…”