Rapid and robust control of single quantum dots

We study the triplet-singlet relaxation in two-electron semiconductor quantum dots. Both single dots and vertically coupled double dots are discussed. In our work, the electron-electron Coulomb interaction, which plays an important role in the electronic structure, is included. The spin mixing is caused by spin-orbit coupling which is the key to the triplet-singlet relaxation. We show that the selection rule widely used in the literature is incorrect unless near the crossing and/or anticrossing point in single quantum dots. The triplet-singlet relaxation in double quantum dots can be markedly changed by varying barrier height, interdot distance, external magnetic field, and dot size.

Summary form only given. Direct observations of the dynamics of single excitonic quantum dots require the use of transient optical spectroscopy. Measurements of the lifetimes of single QD states, Rabi oscillations, biexciton formation and decay, and the development of quantum logic gates are examples of dynamical effects in quantum dots most easily studied through transient nonlinear spectroscopy, but are extremely challenging at the single dot level. We report here the first observation of transient differential transmission from single quantum dots as well as evidence of Rabi oscillations in single quantum dots.Under investigation are quantum dots naturally formed by interface fluctuations in a 42 /spl Aring/ GaAs/Al/sub 0.3/Ga/sub 0.7/As quantum well. Excitons in isolated QD's are probed through 500 nm apertures in an Al mask laid directly onto the sample's surface.

Summary form only given. The excitonic optical response of single quantum dots (QD) has recently been isolated by using high spatial resolution techniques. Photoluminescence (PL) magneto-optical studies of these confined systems have shown interesting physical phenomena such as Zeeman splittings, diamagnetic shifts and the Overhauser effect. Coherent nonlinear optical spectroscopy provides a powerful additional tool for the study of the interaction between magnetic substrates, as has been demonstrated for a single quantum dot. By directly probing the quantum dot resonances under the effect of a magnetic field we can obtain information about spin relaxation times and the presence of dark states. In this paper we show the effect of a magnetic field on single quantum dot resonances (i.e. Zeeman splitting and diamagnetic shift) and report the nonlinear optical response as a function of magnetic field.

We report on the magneto-optical study of spin polarized energetic fine structures for exciton complex in single CdSe quantum dot (QD) by using micro- photoluminescence (micro-PL) spectroscopy. The zero-field splitting of exciton luminescence peak arisen from the anisotropic exchange interaction of carriers in the QDs was observed. The g-factors for exciton and negatively-charged exciton, i.e. trion in a single QD were determined by fitting the magnetic field dependence of the corresponding PL peaks. By exciting the single QD with circularly polarized light of σ- and σ+ polarization, the spin-up and spin-down trions were selectively generated. The ratio, τ/τsf, of the exciton lifetime and the time constants for the spin-flipping process of trion in a single QD was estimated to be 0.13, which implies a long spin-lifetime in single CdSe QD.

The observation of photoluminescence spectra of self-assembled single InGaAs quantum dots at room temperature was performed under weak excitation conditions using a near-field scanning optical microscope. Operation in illumination-collection mode with a highly sensitive double-tapered optical fibre probe enabled detection of weak photoluminescence signals at room temperature with high efficiency and high spatial resolution. Each single quantum dot was imaged with a spatial resolution of about 250 nm, which corresponded to a quarter of the wavelength of the photoluminescence from quantum dots. The photoluminescence yields of individual quantum dots were widely distributed and were found to decrease with photoluminescence energy. This result serves as a clue to be pursued for better understanding of the thermal excitation of the carrier from confined states in quantum dots.

The photoluminescence correlation from a single CdSe nanocrystal under pulsed excitation is studied, and a single photon is realized at wavelength 655 nm at room temperature. The single colloidal CdSe quantum dot is prepared on a SiO2/silicon surface by a drop-and-drag technique. The long-term stability of the single-photon source is investigated; it is found that the antibunching effect weakens with excitation time, and the reason for the weakening is attributed to photobleaching. The lifetimes of photoluminescence from a single quantum dot are analyzed at different excitation times. By analyzing the probability distribution of on and off times of photoluminescence, the Auger assisted tunneling and Auger assisted photobleaching models are applied to explain the antibunching phenomenon.

STM-photocurrent-spectroscopy on single self-assembled InGaAs quantum dots

We report on measurements of hole transport through a double quantum dot structure formed by trench isolation from a SiGe:Si heterostructure. A period change in the Coulomb oscillations is observed upon changing a gate bias, which is attributed to the lowering of one of the tunnel barriers, effectively changing the device to a single quantum dot. Accompanying the period change is a significant change in the level of noise associated with the oscillations. This is explained by a carrier energy filtering effect in the double dot compared with the single dot, caused by the slight difference in energy level spacing in each quantum dot.

A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.

We have grown InP quantum dots in GaInP by gas-source molecular-beam epitaxy on sharp tips etched out of a GaAs substrate. The growth parameters were optimized to produce single quantum dots on the tips, predominantly at the edges. Intensity correlation measurements of the fluorescence from single dots on these tips reveal antibunched photon emision. We propose to use these tip structures with stable single photon emitters at their apex as active probes for further controlled experiments in quantum and nano-optics.

Single molecule time-resolved fluorescence spectroscopy of CdSe/ZnS core-shell quantum dots (QDs) localized near a rough gold thin film demonstrates significant enhancement of multiphoton emission while at the same time showing a decrease in single photon emission. A rigorous analysis of time-resolved photon correlation spectroscopy and fluorescence lifetime data on single quantum dots at room temperature reveals an increase in radiative recombination rate of multiexcitons that is much higher than expected and, perhaps more significantly, is not correlated with concomitant increases in single exciton recombination rates. We believe that these results confirm a stronger coupling of multiexcitons to plasmon modes via a coupling to plasmon multipole modes.

In recent years interest arose into the study of transport through quantum dots. The interest is driven by two reasons. First, small devices like quantum dots and charging effects [1] In such devices open the possibility for future applications in electronics as transistors, memory cells, sensors etc. Small devices are especially interesting in high frequency applications due to their small capacitances. Although different schemes for applications have been proposed, none of it is under development or even in production and it is not clear what will be of real use in the end. The second reason for the interest into these systems is a basic physics point-of-view. These systems are extremely interesting since single quantum dots can be seen as a sort of artificial atom. In this manner-of-speaking coupled quantum dots can be called artificial molecules. These quantum dots are similar to atoms or molecules since a certain number of electrons is confined in a potential. But, in contrast to real atoms, in these artificial systems the number of electrons, the size of the system and the strength of the confinement potential can easily be changed. Therefore, in these quantum dots parameters, such as the ratio between cyclotron frequency and confinement energy or coupling strength between two quantum dots etc, can be easily varied, which is not achievable in real atoms or molecules.

A system of quantum dots based on Al x In1−xAs/Al y Ga1−yAs solid solutions is investigated. The use of Al x In1−xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al x In1−xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al x In1−xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al x In1−xAs quantum dots.

We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/μm2 is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.

We demonstrate optical single-electron spin initialization, storage, and readout in a single self-assembled InGaAs quantum dot using a spin memory device. Single-electron spin relaxation is monitored over time scales exceeding $\ensuremath{\ge}30\text{ }\ensuremath{\mu}\text{s}$, defined only by extrinsic experimental parameters such as the optical detection efficiency. The selective generation of a single electron in the dot is performed by resonant optical excitation and subsequent partial exciton ionization; the hole is removed from the dot while the electron remains stored. When subject to a magnetic field applied in Faraday geometry, we show how the spin of the electron can be prepared with a well-defined spin projection relative to the light propagation direction simply by controlling the voltage applied to the gate electrode. The spin is stored then in the dot before being read out using an optical implementation of spin to charge conversion, whereby the spin projection of the electron is mapped onto a more robust variable, the charge state of the dot. After spin to charge conversion, we show how the charge occupancy can be repeatedly and nonperturbatively measured by pumping a luminescence recycling transition. The approach is shown to provide a readout signal ${10}^{4}$ times stronger per spin when compared to previous methods. In combination with spin manipulation using the optical Stark effect or microwaves, our approach provides an ideal basis for probing spin coherence in single self-assembled quantum dots over long-time scales and the development of methods for coherent spin control.