Optical characterisation of two-dimensional WSe2 flakes for quantum light emission

Examensarbete för masterexamen
Frey, Joey
Two-dimensional (2D) materials are atomically thin materials that have received a lot of attention over the last decade due to their unique properties. Especially interesting for optics are semiconducting transition metal dichalcogenides, which show strong photoluminescence in monolayer form due to their direct band gap. Further, weak screening effects in 2D materials lead to strongly bound electron-hole pairs, called excitons. Recently, photoluminescence studies in WSe2 flakes noticed that localised emitters could be found at the edges of such flakes. The emitters were investigated with a Hanbury Brown & Twiss set-up which showed that they were single photon emitters: non-classical light sources only able to emit one photon at a time by radiative recombination of a localised exciton. Single photon emitters are very sought after in quantum technologies and can be used for flying qubits in quantum computing or for quantum cryptography. 2D materials show promise as novel materials enabling single photon emission because they are optically active on the surface and can be used with current on-chip technology compared to single photon emitters in quantum dots or diamond nitrogen vacancy centres. The goal of this work is to characterise photoluminescence emission in WSe2 flakes and to explore first steps towards engineering 2D-based quantum emitters in this material system. To this end, a micro-photoluminescence set-up is built and optimised to characterise non-resonant light emission from 2D materials and its time correlations. Large area excitation is used to generate a photoluminescence map which reveals localised emitters in strained mono- and fewlayer WSe2 flakes. Moreover, by taking temperature-dependent photoluminescence spectra we see defect emission emerging at low temperatures (<30K). The emission centers show sharp emission peaks and saturation behavior, which is indicative of single photon emission. From time-dependent photoluminescence measurements lifetimes of between 0.48-10.6ns are identified, which is consistent with those of localised excitons reported in other studies. The emitters show low spectral wandering, but change their position after cycling from 4 K to room temperature and back. This work examined the initial characterisation of non-classical light emission in WSe2 flakes. Future steps in the investigation of 2D quantum emitters could be focused on moiré based emitters or integration into photonic chips.
quantum optics , quantum technology , two dimensional materials , single photon sources
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