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

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.

Using integrated superconducting single photon detectors, we probe ultra-slow exciton capture and relaxation dynamics in single self-assembled InGaAs quantum dots embedded in a GaAs ridge waveguide. Time-resolved luminescence measurements performed with on- and off-chip detection reveal a continuous decrease in the carrier relaxation time from 1.22 ± 0.07 ns to 0.10 ± 0.07 ns upon increasing the number of non-resonantly injected carriers. By comparing off-chip time-resolved spectroscopy with spectrally integrated on-chip measurements, we identify the observed dynamics in the rise time (τr) as arising from a relaxation bottleneck at low excitation levels. From the comparison with the temporal dynamics of the single exciton transition with the on-chip emission signal, we conclude that the relaxation bottleneck is circumvented by the presence of charge carriers occupying states in the bulk material and the two-dimensional wetting layer continuum. A characteristic τr ∝ P−2∕3 power law dependence is observed suggesting Auger-type scattering between carriers trapped in the quantum dot and the two-dimensional wetting layer continuum which circumvents the phonon relaxation bottleneck.

We present results on the charge dependence of the radiative recombination lifetime, $\ensuremath{\tau}$, and the emission energy of excitons confined to single self-assembled InGaAs quantum dots. There are significant dot-to-dot fluctuations in the lifetimes for a particular emission energy. To reach general conclusions, we present the statistical behavior by analyzing data recorded on a large number of individual quantum dots. Exciton charge is controlled with extremely high fidelity through an $n$-type field effect structure, which provides access to the neutral exciton (${X}^{0}$), the biexciton $(2{X}^{0})$, and the positively $({X}^{1+})$ and negatively $({X}^{1\ensuremath{-}})$ charged excitons. We find significant differences in the recombination lifetime of each exciton such that, on average, $\ensuremath{\tau}({X}^{1\ensuremath{-}})/\ensuremath{\tau}({X}^{0})=1.25$, $\ensuremath{\tau}({X}^{1+})/\ensuremath{\tau}({X}^{0})=1.58$, and $\ensuremath{\tau}(2{X}^{0})/\ensuremath{\tau}({X}^{0})=0.65$. We attribute the change in lifetime to significant changes in the single particle hole wave function on charging the dot, an effect more pronounced on charging ${X}^{0}$ with a single hole than with a single electron. We verify this interpretation by recasting the experimental data on exciton energies in terms of Coulomb energies. We directly show that the electron-hole Coulomb energy is charge dependent, reducing in value by 5%--10% in the presence of an additional electron, and that the electron-electron and hole-hole Coulomb energies are almost equal.

We demonstrate full charge control, narrow optical linewidths, and optical spin pumping on single self-assembled InGaAs quantum dots embedded in a $162.5\,\text{nm}$ thin diode structure. The quantum dots are just $88\,\text{nm}$ from the top GaAs surface. We design and realize a p-i-n-i-n diode that allows single-electron charging of the quantum dots at close-to-zero applied bias. In operation, the current flow through the device is extremely small resulting in low noise. In resonance fluorescence, we measure optical linewidths below $2\,\mu\text{eV}$, just a factor of two above the transform limit. Clear optical spin pumping is observed in a magnetic field of $0.5\,\text{T}$ in the Faraday geometry. We present this design as ideal for securing the advantages of self-assembled quantum dots -- highly coherent single photon generation, ultra-fast optical spin manipulation -- in the thin diodes required in quantum nano-photonics and nano-phononics applications.

Magneto-optics on single quantum dots

We demonstrate the monolithic integration of an on-demand quantum emitter in the form of a single self-assembled InGaAs quantum dot with an air clad, free standing directional coupler acting as a beam-splitter for anti-bunched light.

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.

We investigate the nuclear spin polarization in single self-assembled InAlAs quantum dots under magnetic fields that are sufficiently weak to make the impacts of the quadrupole interaction apparent. The nuclear spin depolarization rate decreases as the magnitude of the applied longitudinal magnetic field increases. This trend can be explained by the dissolution of quadrupolar mixing that is coupled to the temporal fluctuations of the hyperfine interaction. In addition, the optically injected electron spin generates a considerable Overhauser field under a wide range of experimental conditions. These findings are important for nuclear spin engineering and electron spin manipulation in self-assembled III–V semiconductor 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.

Introduction The advanced growth of semiconductor quantum dots (QDs) with high optical quality is one key to realizing novel devices in various research disciplines related to modern semiconductor technologies. Despite recent progress in the top-down lithographic fabrication of single semiconductor QD-like emitters [43], bottom-up fabrication methods are commonly applied for the realization of high quality light emitting QDs [7]. The exploitation of high-density QD arrays as an active material in laser diodes [17] and vertical cavity surface emitting lasers (VCSEL) resulted in a new class of devices featuring lower lasing thresholds and improved device performance (such as temperature stability) in comparison to devices with higher dimensional gain [2, 3]. Spintronics and quantum information processing are intensively studied fields in order to provide complementary or entirely novel routes to a future information technology [12, 5]. Single self-assembled quantum dots (QDs) grown in low-density arrays are promising candidates for realizing functional building blocks in these research fields as they allow to confine single charge carriers or spin carriers while providing a solid-state platform capable of electrical injection or manipulation. Extensive research has resulted in the realization of various devices relying on few or single QDs, for instance quantum light emitters such as single photon sources [23, 36], sources of pairs of entangled photons [1, 45], and fewto single-QD lasers [38]. Moreover, many milestone experiments have been performed with In(Ga)As QDs grown on GaAs, for instance the observation of the Purcell effect [10] or reaching the strong QD exciton–photon coupling regime [26], demonstration of the indistinguishability of photons emitted by a QD [29], or complete control of a single spin by ultrafast optical pulses [25].

The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules, quantum dots and plasmonic nanoparticles. Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive, yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics. In ensemble measurements, such control is often achieved by resorting to a closed-loop optimization strategy, where the spectral phase of a broadband laser field is iteratively optimized. This scheme needs long measurement times and strong signals to converge to the optimal solution. This requirement is in conflict with the nature of single emitters whose signals are weak and unstable. Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature, using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser. Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps, even when operating at the few-photon level. Full optimization of the single dot luminescence is obtained within ~100 trials, with a typical integration time of 100 ms per trial. These times are faster than the typical photobleaching times in single molecules at room temperature. Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules, thus extending the available experimental toolbox to the active control of nanoscale coherences.

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.

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 authors employ a focused laser beam both as a probe and as a local heat source to tune in situ, over a broad range and with resolution-limited accuracy, the quantized energy states of single optical microcavities and self-assembled quantum dots (QDs). The approach is demonstrated by bringing an optical mode of a microdisk into resonance with the emission of a single QD and by tuning spatially separated QDs in mutual resonance. This processing method may be used, e.g., to fabricate arrays of perfectly resonant QDs.