InP colloidal quantum dots have become widespread in luminescent colour conversion however further studies are necessary to understand the origin of the emitting state. Additionally, reports on optical gain are scarce and lack follow-up research. In this paper, we study the properties of InP/ZnSe and InP/ZnSe/ZnS quantum dots with Transient Absorption Spectroscopy. We propose a state-filling model were the band-edge bleach can be interpreted as the filling of the conduction and valence band edge states by delocalized holes and electrons. According to this interpretation, optical gain should be observed once the average exciton density is larger than
1. We explain this lower than expected threshold and the properties of the observed stimulated emission band by acknowledging that Stokes shift is the main spectral shift. The proposed exciton-phonon coupling leads to a unique mechanism where the stimulated emission band results from the counteraction between the absorption and photoluminescence bands. To fully take advantage of this mechanism, we propose that InP-based QDs with narrower emission lines and slower Auger recombination at higher pump intensities are needed.
Ellipsometry is an optical technique to characterize materials or phenomena that occurs at an interface or thin film between two different media. In this paper, we present an experimental low-cost version of a photometric ellipsometer, assembled with commonly found material at every Optics laboratory. The polarization parameters measurement was performed using a Thorlabs PAX5710 polarimeter. The uncertainty computed using the Guide to the Expression of Uncertainty in Measurement (GUM) procedures. With the assembled ellipsometer we were able to measure the thickness of a 10 nm thick SiO2 thin film deposited upon Si, and the complex refractive index of Gold and Tantalum samples. The SiO2 thickness we achieved had an experimental deviation of 4.5% with 2.00 nm uncertainty. The value complex refractive index of Gold and Tantalum measured agrees with the different values found in several references. The uncertainty values were found to be mostly limited by the polarimeter’s uncertainty.
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