D.V. Regelman

Radiative lifetimes of single excitons in semiconductor quantum dots — manifestation of the spatial coherence effect

Using time correlated single photon counting combined with temperature dependent diffraction limited confocal photoluminescence spectroscopy we accurately determine, for the first time, the intrinsic radiative lifetime of single excitons confined within semiconductor quantum dots. Their lifetime is one (two) orders of magnitude longer than the intrinsic radiative lifetime of single excitons confined in semiconductor quantum wires (wells) of comparable confining dimensions. We quantitatively explain this long radiative time in terms of the reduced spatial coherence between the confined exciton dipole moment and the radiation electromagnetic field.

Spectroscopy of positively and negatively charged quantum dots: wave function extent of holes and electrons

Abstract We investigate semiconductor quantum dots by optically injecting a well controlled unequal number of electrons and holes into an isolated single dot. The injected carriers form charged complexes of many carriers in the dot. Radiative electron–hole pair recombination takes place after the charged complex relaxes to its ground state. We spectrally and temporally resolve the emission and show that it can be used to unambiguously determine the discrete charge states of the emitting quantum dot. In particular, we identify the emission from both negative and positive charge states of the same dot. We show that while negative charging results in red shifted emission energy, compared with a neutral dot, positive charging results in blue shifted emission energy. We explain this observation in terms of the better confined wave functions of the holes. Due to their smaller volume, the energy associated with hole–hole repulsion is larger than the combined energy associated with electron–hole attraction and electron–electron repulsion.

Dynamics of Excitons in Single Semiconductor Quantum Dots Probed by Time-Resolved Optical Spectroscopy

We resolve spatially, spectroscopically and temporally the photoluminescence emission from single self-assembled In(Ga)As/GaAs quantum dots, The temporal evolution of the emission spectrum after pulsed excitation is measured for various excitation intensities at various ambient temperatures. The evolution of the spectrum with the increase in both steady state and pulse excitation intensities is measured as well. A multi-exciton model is used for calculating the temporal and excitation intensity dependence of the measured spectra. The quantitative agreement between the measured and calculated spectra provides an unambiguous determination of the radiative lifetime of a single quantum dot exciton. This lifetime is 4-6 ns long and is temperature independent. The reduced spatial coherence between the confined exciton and the radiation electromagnetic field quantitatively explains this long radiative time.

Spectroscopy of single semiconductor quantum dots at negative, neutral, and positive charge states

We investigate semiconductor quantum dots by optically injecting a controlled unequal number of electrons and holes into an isolated single dot. The injected carriers form charged complexes of many carriers in the dot. Radiative electron-hole pair recombination takes place after the charged complex relaxes to its ground state. We resolve spectrally and temporally this emission and we show that while negative charging results in red shifted emission energy, compared with a neutral dot, positive charging results in blue shifted emission energy. We explain this observation in terms of the smaller volume of the hole wavefunctions compared with that of the electrons.

Tunable statistics of multicolor photons emitted from semiconducting quantum dots

We report the intensity auto- and cross-correlation functions of two consecutively emitted photons from an optically excited single semiconductor quantum dot. We show that a quantum dot is not only a source of non-classically correlated monochromatic photons but is also a source of multicolor photons with excitation power dependent correlation properties. We found that the emitted photon statistics is evolving, as the excitation power is increased, from a sub-Poissonian statistics, where the photons are temporally antibunched, to a super-Poissonian one, where they are temporally bunched.

Charging and switching the charge sign of single semiconductor quantum dots by all optical means

Summary form only given. Optical studies of semiconductor quantum dots (QDs) have been a subject of very intensive recent investigations. It has been experimentally and theoretically established that the number of carriers which occupy a photoexcited QD greatly affect its photoluminescence (PL) spectrum. In spite of its neutral nature, optical spectroscopy has very recently proved to be a useful means for investigating and preparing charged QD systems. We report here on continuous wave (cw) and pulsed optical PL spectroscopy of single self-assembled QDs (SAQDs) embedded within a mixed type quantum well (QW) structure. This specific design, which facilitates charge separation by optical means, is used here to tune the charge state of the QD under study. We compare the PL emission spectra for a single In(Ga)As/GaAs SAQD occupied with an increasing number of neutral multiexcitons, and the emission of the same type of dots embedded in a GaAs/AlAs mixed type structure. Using time-resolved spectroscopy and a comparison between cw and pulse-excited PL spectra we are able to accurately determine the charge state of the single QDs at a given excitation intensity level. We determine the collective carrier state from which each spectral line originates.