Measuring and Modelling the Build-up of Optical Gain in Semiconductor Quantum Dots
jaco geuchies a, gianluca grimaldi a, nicholas kirkwood a, ward van der stam a, arjan houtepen a
a Delft University of Technology, The Netherlands, Julianalaan, 136, Delft, Netherlands
Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2018 (NFM18)
S5 Charge Carrier Dynamics at the Nanoscale
Torremolinos, Spain, 2018 October 22nd - 26th
Organizers: David Egger, Arjan Houtepen and Freddy Rabouw
Poster, jaco geuchies, 322
Publication date: 6th July 2018

Colloidal semiconductor quantum dots are attractive materials for realizing solution processable lasers[1]. However, light amplification and optical gain in these materials is relatively hard to achieve, due to the non-unity degeneracy of the electron and hole states, which implies that multiexcitons are necessary to reach the lasing regime[2]. A better understanding of the relationship between carrier population and optical gain will give solid handholds to improve these materials for applications in low-threshold gain media.

 

Here, we present a model which is able to describe the build-up of optical gain in semiconductor quantum dots, and we validate the model with femto-second transient absorption spectroscopy (fs-TA). We perform fluence-dependent fs-TA measurements on CdSe/CdS/ZnS nanocrystals, which serves as a well-established platform test the feasibility of our model. The model itself is build up out of three parts: (1) a transition-counting scheme, which explicitly considers the relative weights of the electron and hole contributions to the band-edge absorption bleach, also when the number of excitons exceeds the degeneracy of the band edge states, (2) Poissonian excitation statistics, and (3) a coupled differential equation scheme which takes into account Auger recombination during the cooling of hot carriers. The only experimental inputs are the absorption cross-section of the quantum dots (to obtain the average exciton occupancy per nanocrystal) and the biexciton lifetime, which both are readily recovered from the fs-TA measurements. The experimental data is accurately reproduced with our model, which demonstrates the feasibility of the model.

 

The unique approach of modelling combined with quantitative fs-TA allows us to create a complete description of the build-up of optical gain in quantum dot materials. Furthermore, the presented model can also be used to determine the band-edge degeneracies for less well-established quantum dot systems (e.g. InP and CuInS). The experiments presented here open up pathways for more quantitative studies of optical gain in semiconductor quantum dots and help rationalize design rules for novel low threshold gain materials.

AJH, JJG and WvdS gratefully acknowledge financial support from the European Research Council Horizon 2020 ERC Grant Agreement No. 678004 (Doping on Demand). GG acknowledges financial support from STW (Project No. 13903, Stable and Non-Toxic Nanocrystal Solar Cells).

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