| Solid-state single-quantum emitters become one of the workhorses of the emerging field of quantum nano-photonics in the past a few years.Among those Solid-state singlequantum emitters,colloidal quantum dots(QDs)have attracted much attention due to their strong broadband absorption,tunable narrow-band emission,solution processability,and perfect compatibility with micro/nano photonic structures.In this thesis,we use spectroscopy to investigate the complex and abundant light absorption and emission properties of single-particle CdSe/CdS core-shell colloidal QDs.The blinking behavior and the biexciton quantum efficiency of single QD emission are controlled at nanoscale resorting to metal nano-antenna effect,and the versatile functionality of single QDs acting as active near-field scanning probe are explored for novel near-field imaging applications.Unlike single atoms and single molecules,multi-carrier processes like multi-photon absorption,multi-exciton emission and conversion to charged states are common for QDs under the injection of light or current due to their higher electronic density of states.In this part,we accurately resolve the charging status,precisely assessing the absorption cross section,and determining the absolute emission quantum yield of monoexciton and biexciton states for neutral,positively charged,and negatively charged single QDs.We uncover very different photon statistics of the three charge states in single QDs and unambiguously identify their charge signs together with the information on their photoluminescence decay dynamics.We then show their distinct photoluminescence(PL)saturation behaviors and evaluate the absolute values of absorption cross sections and quantum efficiencies of monoexcitons and biexcitons.We demonstrate that the addition of an extra hole or electron in a QD not only changes its emission properties but also varies its absorption cross section.In the study of single QD emissions mediated by metal nano-antenna effect,we effectively suppress QD blinking and consequently boost the biexciton quantum efficiency by tuning the resonance peak of nano-antennae(i.e.gold nano-particles,GNPs)to match the single QD emission spectra.Though experimental supporting for the explanation is still lacking,single QD fluorescence blinking can indeed be suppressed by nanoscale optical fields.Here we use the nanoscale optical fields produced by GNPs to optimize the single QDs emission properties.Firstly,the resonance peaks of spherical GNPs are tuned to match the single QD emission spectra by laser melting and shaping techniques.Next the single QDs are brought to interact with the nanoscale optical fields produced by the manipulated GNPs,and the single QD PL saturation curves are measured by pulse excitation,along with the observation on the changes in biexciton quantum efficiency.Gradually changing the interaction distance between the single QDs and nanoscale optical fields,and recording the changes in single QD PL intensity and lifetime in the meanwhile,the QD blinking process is directly observed experimentally.Benefitting from the small size and luminescence capability,single-quantum emitters,which act as active near-field probes,become a research hotspot in the field of scanning near-field optical microscopy(SNOM),for which the current problems confronted are the over-large size of solid supporter or the instability in the emission of single quantum emitters.In this section,we adhere a 6 nm size emission-stable colloidal QD to a tapered optical fiber probe by nano-manipulation,and formed an active near field probe.This probe is used to the near-field imaging of single GNPs based on fluorescence lifetime imaging microscopy(FLIM),of which the resolution of imaging is analyzed.To further verify the imaging capabilities of the probe,a GNP dimer structure prepared based on nanomanipulation is used for near-field optical imaging,and the same area is imaged by atomic force microscopy(AFM)to verify the reliability of near-field optical imaging results. |
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