Excitons In Single Quantum Dots Research Articles

Resonance-fluorescence spectral dynamics of an acoustically modulated quantum dot

Quantum technologies that rely on photonic qubits require a precise controllability of their properties. For this purpose hybrid approaches are particularly attractive because they offer a large flexibility to address different aspects of the photonic degrees of freedom. When combining photonics with other quantum platforms like phonons, quantum transducers have to be realized that convert between the mechanical and optical domain. Here, we realize this interface between phonons in the form of surface acoustic waves (SAWs) and single photons, mediated by a single semiconductor quantum dot exciton. In this combined theoretical and experimental study, we show that the different sidebands exhibit characteristic blinking dynamics that can be controlled by detuning the laser from the exciton transition. By developing analytical approximations we gain a better understanding of the involved internal dynamics. Our specific SAW approach allows us to reach the ideal frequency range of around 1 GHz that enables simultaneous temporal and spectral phonon sideband resolution close to the combined fundamental time-bandwidth limit.

Photoinduced Kerr rotation spectroscopy for microscopic spin systems using heterodyne detection.

We develop a transient photoinduced Kerr rotation spectroscopy technique using a heterodyne detection scheme to study spin dynamics of microscopic quantum states in solids, such as single quantum dots and spin helixes. The use of the heterodyne beat note signal generated by the interference of the frequency-shifted probe and reference pulses realizes the Kerr rotation measurements in combination with micro-spectroscopy, even when the probe pulse propagates collinearly with the strong pump pulse, which resonantly excites the probing state. In addition, the interference gives an optical amplification of the Kerr signal, which provides a clear observation of the photoinduced spin dynamics by the weak probe intensity. Here, we present results of Kerr rotation measurements for a single quantum dot exciton, which shows a maximum rotation angle of few µrad.

Optomechanical wave mixing by a single quantum dot

Wave mixing is an archetypical phenomenon in bosonic systems. In optomechanics, the bidirectional conversion between electromagnetic waves or photons at optical frequencies and elastic waves or phonons at radio frequencies is building on precisely this fundamental principle. Surface acoustic waves (SAWs) provide a versatile interconnect on a chip and thus enable the optomechanical control of remote systems. Here we report on the coherent nonlinear three-wave mixing between the coherent fields of two radio frequency SAWs and optical laser photons via the dipole transition of a single quantum dot exciton. In the resolved sideband regime, we demonstrate fundamental acoustic analogues of sum and difference frequency generation between the two SAWs and employ phase matching to deterministically enhance or suppress individual sidebands. This transfer between the acoustic and optical domains is described by theory that fully takes into account direct and virtual multiphonon processes. Finally, we show that the precision of the wave mixing is limited by the frequency accuracy of modern radio frequency electronics.

Spectroscopy of Single AlInAs Quantum Dots

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.

Path-dependent initialization of a single quantum dot exciton spin in a nanophotonic waveguide

We demonstrate a scheme for in-plane initialization and readout of a single exciton spin in an InGaAs quantum dot (QD) coupled to a GaAs nanobeam waveguide. The chiral coupling of the QD and the optical mode of the nanobeam enables spin initialization and readout fidelity approaching unity in magnetic field $B=1$ T and $\sim$0.9 without the field. We further show that this in-plane excitation scheme is independent of the incident excitation laser polarization and depends solely on the excitation direction. This scheme provides a robust in-plane spin excitation basis for a photon-mediated spin network for quantum information applications.

Coherent control of the dynamics of a single quantum-dot exciton qubit in a cavity

In this work we demonstrate theoretically how to use external laser field to control the population inversion of a single quantum dot exciton qubit in a nanocavity. We consider the Jaynes-Cummings model to describe the system, and the incoherent losses were take into account by using Lindblad operators. We have demonstrated how to prepare the initial state in a superposition of the exciton in the ground state and the cavity in a coherent state. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area and losses over the qubit dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the qubit against cavity loss.

Energy spectrum of confined positively charged excitons in single quantum dots

A theoretical model, which relates the binding energy of a positively charged exciton in a quantum dot with the confinement energy is presented. It is shown that the binding energy, defined as the energy difference between the corresponding charged and neutral complexes confined on the same excitonic shell strongly depends on the shell index. Moreover, it is shown that the ratio of the binding energy for positively charged excitons from the $p$- and $s$-shells of a dot depends mainly on the nearly perfect confinement in the dot, which is due to the "hidden symmetry" of the multi-electron-hole system. We applied the theory to the excitons confined to a single GaAlAs/AlAs quantum dots. The relevant binding energy was determined using the micro-photoluminescence and micro-photoluminescence excitation magneto-spectroscopy. We show that within our theory, the confinement energy determined using the ratio of the binding energy corresponds well to the actual confinement energy of the investigated dot.

Interfacing single photons and single quantum dots with photonic nanostructures

Photonic nanostructures provide means of tailoring the interaction between light and matter and the past decade has witnessed a tremendous experimental and theoretical progress in this subject. In particular, the combination with semiconductor quantum dots has proven successful. This manuscript reviews quantum optics with excitons in single quantum dots embedded in photonic nanostructures. The ability to engineer the light-matter interaction strength in integrated photonic nanostructures enables a range of fundamental quantum-electrodynamics experiments on, e.g., spontaneous-emission control, modified Lamb shifts, and enhanced dipole-dipole interaction. Furthermore, highly efficient single-photon sources and giant photon nonlinearities may be implemented with immediate applications for photonic quantum-information processing. The review summarizes the general theoretical framework of photon emission including the role of dephasing processes, and applies it to photonic nanostructures of current interest, such as photonic-crystal cavities and waveguides, dielectric nanowires, and plasmonic waveguides. The introduced concepts are generally applicable in quantum nanophotonics and apply to a large extent also to other quantum emitters, such as molecules, nitrogen vacancy ceters, or atoms. Finally, the progress and future prospects of applications in quantum-information processing are considered.

Strongly coupled slow-light polaritons in one-dimensional disordered localized states

Cavity quantum electrodynamics advances the coherent control of a single quantum emitter with a quantized radiation field mode, typically piecewise engineered for the highest finesse and confinement in the cavity field. This enables the possibility of strong coupling for chip-scale quantum processing, but till now is limited to few research groups that can achieve the precision and deterministic requirements for these polariton states. Here we observe for the first time coherent polariton states of strong coupled single quantum dot excitons in inherently disordered one-dimensional localized modes in slow-light photonic crystals. Large vacuum Rabi splittings up to 311 μeV are observed, one of the largest avoided crossings in the solid-state. Our tight-binding models with quantum impurities detail these strong localized polaritons, spanning different disorder strengths, complementary to model-extracted pure dephasing and incoherent pumping rates. Such disorder-induced slow-light polaritons provide a platform towards coherent control, collective interactions, and quantum information processing.

Double Magic Coincidence in an Optomechanical Laser Cavity

Vertical cavity surface emitting lasers (VCSELs) are stacks of tens of different semiconductor layers that constitute the basic components of the lasers: mirrors, optical cavity, and active material. Contrary to edgeemitting lasers, which use the cleaved faces of the semiconductor device as mirrors to confine the photon field laterally, VCSELs provide a laser emission along the stacking axis of the heterostructure. Thanks to this specific geometry that offers unique optical properties, VCSELs could be developed in an impressively wide research field, covering both applied physics and fundamental phenomena. These semiconductor lasers are good candidates for the realization of widely tunable solid state laser sources [1] and have applications in high-speed optical interconnects (in the datacom and telecom range of wavelengths) [2] and gas spectroscopy (2–3 micron range) [3]. And more than this, thanks to advances in semiconductor growth in the late nineties, VCSELs also emerged in the field of strong light-matter coupling, with the observation of 2D light-matter quasiparticles called exciton-polaritons that can form Bose-Einstein condensates [4] with superfluid properties [5]. More recently, these devices have been used to study the strong coupling between a single quantum dot exciton and a cavity mode in pillar geometries [6]. And despite the impressive number of publications on this kind of optical cavity, the VCSEL spring continues to flow.

Optical transition of the charged excitons in InAs single quantum dots

Quantum dot (QD) samples studied in the experiment are grown by molecular-beam epitaxy on semi-insulating GaAs substrates. The photoluminescences (PLs) of the excitons in a single QD are measured at 5 K. The PL spectra of the excitons, biexcitons and charged excitons are identified by measuring and analyzing both PL peaks of the circular and linear polarization and power-dependent PL properties. The charged exciton emissions can be tuned by applying a bias voltage, i.e., negatively charged excitons are changed into positively charged excitons by changing the voltage from 1.0 to -1.0 V. It is shown that the electron-spin will slowly relax compared with that of the hole when they relax from wetting layer into the QD.

Charged exciton creation with two-color optical excitation method and analysis of initialization process of electron spin qubit in quantum dots

An optical pumping method for creating charged excitons in semiconductor quantum dots is studied using a micro-photoluminescence (PL) technique with a two-color excitation method. This method employs two laser sources whose energies are in the resonant and non-resonant barrier excitation conditions, respectively; one laser creates excitons, the other creates electrons. The two-color excitation exploits an asymmetric energy band structure for conduction and valence bands, which induces negatively charged excitons in single quantum dots. The PL spectra of a single quantum dot clearly vary from exciton-originated PL to charged exciton-originated PL as the excitation conditions are changed. This excitation method is applied to the initialization process of an electron spin qubit without a magnetic field. Experimental results are analyzed theoretically based on the use of a density matrix method for the initialization. The analysis reveals that the electron spin in quantum dots is sufficiently polarized for subsequent quantum computation processes. These results provide a simple and effective way of implementing quantum computing with spin qubits.

Efficient quantum dot single photon extraction into an optical fiber using a nanophotonic directional coupler

We demonstrate a spectrally broadband and efficient technique for collecting emission from a single InAs quantum dot directly into a standard single mode optical fiber. In this approach, an optical fiber taper waveguide is placed in contact with a suspended GaAs nanophotonic waveguide with embedded quantum dots, forming a broadband directional coupler with standard optical fiber input and output. Efficient photoluminescence collection over a wavelength range of tens of nanometers is demonstrated, and a maximum collection efficiency of 6% (corresponding single photon rate of 3.0 MHz) into a single mode optical fiber is estimated for a single quantum dot exciton.

Random population model to explain the recombination dynamics in single InAs/GaAs quantum dots under selective optical pumping

We model the time-resolved and time-integrated photoluminescence of a single InAs/GaAs quantum dot (QD) using a random population description. We reproduce the joint power dependence of the single QD exciton complexes (neutral exciton, neutral biexciton and charged trions). We use the model to investigate the selective optical pumping phenomenon, a predominance of the negative trion observed when the optical excitation is resonant to a non-intentional impurity level. Our experiments and simulations determine that the negative charge confined in the QD after exciting resonance to the impurity level escapes in 10 ns.

Theory of quantum light emission from a strongly-coupled single quantum dot photonic-crystal cavity system

We present a rigorous medium-dependent theory for describing the quantum field emitted and detected from a single quantum dot exciton, strongly coupled to a planar photonic crystal nanocavity, from which the exact spectrum is derived. By using simple mode decomposition techniques, this exact spectrum is subsequently reduced to two separate user-friendly forms, in terms of the leaky cavity mode emission and the radiation mode emission. On application to study exciton-cavity coupling in the strong coupling regime, besides a pronounced modification of the usual vacuum Rabi spectral doublet, we predict several new effects associated with the leaky cavity mode emission, including the appearance of an off-resonance cavity mode and a loss-induced on-resonance spectral triplet. The cavity mode emission is shown to completely dominate the emitted spectrum, even for large cavity-exciton detunings, whereby the usual cavity-QED formulas developed for radiation-mode emission drastically fail. These predictions are in qualitative agreement with several "mystery observations" reported in recent experiments, and apply to a wide range of semiconductor cavities.

Single quantum dot controlled gain modulation in high‐Q micropillar lasers

AbstractWe present single quantum dot lasing effects in optically pumped high quality (Q) micropillar cavities. The micropillar cavities are realized by means of high resolution electron beam lithography and plasma etching on planar microcavity structures grown by molecular beam epitaxy. We observe lasing action for micropillars with a diameter as small as 1.0 μm containing on average less than 80 QDs in the active layer. Pronounced single quantum dot (QD) controlled lasing effects are observed by tuning a single QD exciton on resonance with the lasing mode. When lasing action is due to the contribution of an ensemble of QDs, photon autocorrelation studies of the emitted photons reveal a transition from thermal light to coherent light near threshold pump power. In contrast, single QD gain modulation which helps to lower the threshold pump power of a 1.6 μm micropillar by a factor of two, is reflected in non‐classical photon antibunching up to about two times the on‐resonance threshold power. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Broadening of single quantum dot exciton luminescence spectra due to interaction with randomly fluctuating environmental charges

We studied line broadening of single-dot exciton emissions in terms of fluctuating interaction between the quantum dot (QD) exciton and environmental charges. We examined two types of QD excitons: one accompanied by trapped charge whereby the exciton line redshifts under the trapped charge population and the other nearly free from trapped charges. To avoid population decay of exciton---which results from multiple-carrier capture into the QDs, thereby leading to line broadening---and to highlight the effect of the fluctuating charges, we selected QDs that reject the capture of carriers in the barrier material. Interferometric correlation measurements of the single-dot emissions uncover the importance of the trapped charges near the QDs: The fluctuation of the Coulomb field caused by the trapping or detrapping of charges in point defects changes the autocorrelation function into Gaussian decay. A model dealing with the dynamics of the charge occupation driven by the thermally activated carrier capture and photoinduced carrier emissions is constructed consistent with the experiment. Upon temperature rise, the rate of the phase decoherence prevails over that of the charge fluctuation, and the autocorrelation function thereby turns to exponential. In contrast, in the QD nearly free from the trapped nearby charges, the autocorrelation decay obeys a simple exponential function, which is consistent with the absence of the fluctuating charge perturbation. Under an intense excitation, another decoherence due to the exciton or biexciton transition opens. We finally discuss the autocorrelation function in terms of the zero-phonon line and the phonon sideband.

Tuning spin properties of excitons in single CdTe quantum dots by annealing

Using polarization-resolved photoluminescence imaging in external magnetic fields we measurestatistically significant distributions of exchange splitting, diamagnetic shift and effectiveg-factor of excitons in tens of single CdTe quantum dots. Comparison between the as-grownand annealed structures shows strong suppression of ensemble inhomogeneities, withthe average exchange splitting reduced by half and significant narrowing of theg-factor distribution. Remarkably, the average value of the excitonicg-factor remains unchanged. This unique ability to yield highly uniform quantum dotensembles without hampering the exciton Zeeman splitting makes annealing ahighly attractive means for tuning the spin properties of quantum dot excitons.

Interferometric spectroscopy for excitons in InP single quantum dots

Dynamical dephasing processes of an exciton and a charged exciton in single InP quantum dots were studied by using interferometric spectroscopy. Interferometric spectroscopy enabled us to observe with high sensitivity the dephasing of exciton or other exciton complexes in single quantum dots. In order to observe the dephasing of the exciton or exciton complexes, emitted single-photons generated from single InP quantum dots were detected through the Michelson interferometer. The contrast of the interferometric signal due to the exciton and the charged exciton shows non-exponential decay under band-to-band excitation for the GaInP matrix. The band-to-band excitation generates carriers trapped in the matrix and the trapped carriers modulate the energy of the quantum dots because of the quantum-confined Stark effect. Therefore the non-exponential decays are caused by energy fluctuation due to the trap carriers in the long timescale.

Coherent control of single quantum dot exciton embedded in a photodiode

We study the coherent properties of a two-level system based on the ground state neutral exciton of a single InGaAs self-assembled quantum dot (SAQD) embedded in an n-i-Schottky diode. By using a photocurrent technique, we observe Rabi oscillations of the exciton population for pulse-areas of up to 6π. We perform two pulse quantum interference experiments to deduce the decoherence rate.