We have recently reported the successful fabrication of bright single-photon sources based on Ag-embedded nanocone structures that incorporate InAs quantum dots. The source had a photon collection efficiency as high as 24.6%. Here we show the results
of various types of photonic characterizations of the Ag-embedded nanocone structures that confirm their versatility as regards a broad range of quantum optical applications. We measure the first-order autocorrelation function to evaluate the coherence time of emitted photons, and the second-order correlation function, which reveals the strong suppression of multiple photon generation. The high indistinguishability of emitted photons is shown by the Hong-Ou-Mandel-type two-photon interference. With quasi-resonant excitation, coherent population flopping is demonstrated through Rabi oscillations. Extremely high single-photon purity with a $g^{(2)}$(0) value of 0.008 is achieved with $pi$-pulse quasi-resonant excitation.
Interference of a single photon generated from a single quantum dot is observed between two photon polarization modes. Each emitted single photon has two orthogonal polarization modes associated with the solid-state single photon source, in which two
non-degenerate neutral exciton states are involved. The interference between the two modes takes place only under the condition that the emitted photon is free from which-mode information.
High degree of preservation of spin states during energy relaxation processes mediated by optical phonons is demonstrated in a single quantum dot. Optical-phonon resonance and relevant suppression of spin relaxation are clearly identified as dip stru
ctures in photoluminescence excitation spectra probed by the positive trion emission. The absence of continuum states makes this observation possible under the cross-circularly polarized detection with respect to a circularly polarized pumping. Consequently, distinguishably high degree of circular polarization up to ~0.85 is achieved without applying external magnetic field at the optical-phonon resonance. Rate equation analysis reveals that the spin-flip probability during energy relaxation is restricted to less than 7.5%. It is also indicated that the spin flip time of the positive trion ground state is extended by more than 3 times compared with that of neutral exciton ground state. This corresponds to the spin flip time longer than 11 ns for the positive trion ground state. The influence of nuclear polarization to the present measurements is also discussed.
