W.V. Schoenfeld

Photon correlation spectroscopy of a single quantum dot

It is by now widely accepted that various quantum dot ~QD! structures exhibit features in transport 1,2 or optical spectroscopy 3‐5 that indicate full three-dimensional confinement of carriers. Identification of QD’s as artificial atoms has been strengthened by the recent observation of strong photon antibunching in single-exciton emission, 6,7 which is the typical signature of an anharmonic quantum system: after a photon is emitted from a single two-level ~anharmonic! emitter, the system is necessarily in the radiatively inactive ground state and a second photon cannot be emitted immediately after the first one. Even though the coherence properties of QD single-exciton emission closely follow those of atoms, the overall spectral features of single QD’s are significantly more complicated. Since the size of QD’s is roughly two orders of magnitude larger than those of atoms, multiparticle excitations give rise to emission peaks with energies comparable to that of a single exciton. Of primary importance in QD spectroscopy is the biexciton state, which corresponds to a doubly-excited QD with completely filled lowest electronand hole-energy levels. When the biexciton state decays by radiative recombination, the final state is a single-exciton state and the generated photon is shifted as compared to the single exciton emission due to Coulomb interaction between the carriers. Biexciton emission in QD spectroscopy has been traditionally identified using the ~quadratic! pump-power dependence of the corresponding peak. In this paper, we demonstrate that photon-correlation measurements provide a powerful tool for characterizing the multiexciton spectral features of QD’s. Our measurements provide a strong support for the identification of a biexciton emission peak, by demonstrating its strong correlations with the subsequent single-exciton emission. We observe that biexciton intensity autocorrelation exhibits bunching together with antibunching or only antibunching under continuous-wave ~cw! excitation depending on the excitation level. In contrast, we find strong antibunching under pulsed excitation. The large difference between the levels of antibunching under continuous wave and pulsed excitations points out to the importance of excitation mechanism and the role of free carriers in QD physics. The lack of polarization correlation between biexciton and single-exciton emissions indicates that spin dephasing is likely to play a key role under nonresonant excitation. We also observe that a third emission peak in QD spectra exhibits strong correlations with both exciton and biexciton fluorescence: we argue that these correlation signatures suggest the identification of this additional line as a charged-exciton emission. Our self-assembled InAs QD’s were grown by molecularbeam epitaxy using the partially covered island technique. 8 Growth resulted in typically lens-shaped QD’s with a base diameter of 40‐50 nm and a height of 3 nm, having their single-excitonic emissions between 925 nm and 975 nm in the spectrum. In our sample, the QD’s were embedded in the center of a 200-nm-thick GaAs microdisk structure located above a 0.5-mm-thick Al0.65Ga0.35As post. The diameter of the disks was 5-mm and the average number of QD’s within the disks was less than one. Details of the microdisk processing can be found elsewhere. 9 Our experimental setup consisted of a combination of a low-temperature diffractionlimited scanning optical microscope and a Hanbury Brown

A quantum dot single-photon source

We demonstrate heralded single photon emission from a self-assembled InAs quantum dot (QD). Pulsed optical excitation (82MHz) together with Coulomb renormalization effects allows for the realization of regular single photon emission at the excitonic transiton (1X) with nearly 100 % efficiency. By temperature tuning, we are able to shift the 1X transition into resonance with a whispering gallery mode of a microdisk (Q ∼ 6500) and achieve turnstile operation of the coupled QD-cavity system. On resonance, the Purcell effect causes a reduction of the 1X transition lifetime leading to a reduced time jitter of the photon emission event and ensuring that photons are primarily emitted into a cavity mode.

Cavity-QED using a single InAs quantum dot and a high-Q whispering gallery mode

Summary form only given. Our samples were grown by molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate. The microdisks consist of a 5 /spl mu/m diameter disk and a pedestal area. The self-assembled InAs QDs were grown in the center of the 200 nm disk region. The QD density of the sample was /spl les/10/sup 8/ cm/sup -2/. The samples are mounted in a He gas flow cryostat (4-300 K). Optical pumping is performed with a continuous-wave Ti-sapphire laser operating at 760 nm, generating free electron-hole pairs in the GaAs layer. Excitation and collection was done through the same objective in normal direction. Due to the low QD density, in average less than one QD at a time was within the laser spot. The collected PL was dispersed by a 50 cm spectrometer and detected by a charge coupled device detector for measurements of optical spectra or by a Hanbury Brown and Twiss setup for photon correlation measurements (time resolution 420 ps).

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.

Heralded single photons from a single quantum dot

Summary form only given. A deterministic single photon source emitting triggered (heralded) photons on demand, also termed single photon turnstile device, is one of the key elements in the emerging field of quantum information science. We present a single photon source based on pulsed laser excitation of a single self-assembled InAs quantum dot (QD) embedded in a high quality (Q) factor microcavity structure. Regulation of the photon emission process of the QD exciton ground state (1X) transition is accomplished by a combination of two effects: Coulomb interactions creating an anharmonic multiexciton spectrum and a slow relaxation of highly excited QDs leading to vanishing re-excitation probability following the photon emission event at the 1X transition.

Pure luminescence transitions from a small InAs/GaAs quantum dot exhibiting a single electron level

Pure photoluminescence spectra originating from a single InAs/GaAs quantum dot, which is small enough to possess only one single-electron level, are demonstrated. A symmetric fine structure of the exciton and the biexciton is observed.

Quantum dots as a sensitive tool to monitor charge

The excitonic photoluminescence spectra reveal an abrupt change of the charge-state of single In(Ga)As/GaAs quantum dots, for a certain transformation when the excitation energy below the barrier band gap. This transform energy is dependent on the thickness of the underlying wetting layer and the crucial energy is associated with an optical transition involving an energy level of the wetting layer. The effect is proposed to be used as a tool, not only to monitor charge in quantum dots in order to study related phenomena, but also to gain new insight in the electronic structure of strained In(Ga)As/GaAs layers by using the quantum dots as charge-probes.

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.

Generation of Regulated Single Photons by Pulsed Excitation of a Single InAs Quantum Dot

We report a single-photon turnstile device based on pulsed excitation of a single self-assembled InAs quantum dot. This device can be used in quantum cryptography and future quantum information processing applications.