Optical characterisation of single quantum dots emitting at 1300nm

This thesis deals with the optical characterization of single quantum dot devices emitting at 1300nm. Thanks to the development and optimization of the growth technique we were able to achieve at the same time emission at 1300nm and ultra low QD densities. Our single QD devices present clear and reproducible spectral signatures in which we can identify exciton, biexciton and charged exciton transitions. Quasi-resonant excitation at 70K demonstrates background free single exciton transitions, which is very promising for the realization of a single photon device operating at temperatures in easy reach of thermoelectric coolers. A time-correlated single photon counting setup was built and used to measure the radiative lifetimes of single exciton transitions. These measurements also present new evidence on a background emission superposed to the narrow spectral transitions. Demonstration of single photon emission at these wavelengths required building a setup to measure the correlations between fiber-coupled single photons in a 300ps time window, emitted from a nano-device in free space at cryogenic temperatures, and with the capability of maintaining the optical alignment on a micrometer scale for several hours. With such a setup we have demonstrated that our QDs can generate single photon states at 1300nm. We used this single photon source to characterize novel detectors based on superconducting nanowires and measured for the first time the intensity correlation function at 1300nm on single photons from a QD. These detectors show at least 2 orders of magnitude improvement on the signal to noise ratio as compared to InGaAs APDs; this is very important since, for a QKD system, the detector noise, amongst others, determines the maximum distance over which a secure key can be exchanged. It should be noted that, due to the di±culties in these measurements, to date there has been only one other clear demonstration of single photon emission at 1300nm.

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