Several important proposals to use semiconductor quantum dots in quantum information technology rely on the control of the dark exciton ground states, such as dark exciton based qubits with a $μ$s life time. In this paper, we present an efficient way to occupy the dark exciton ground state by a single short laser pulse. The scheme is based on an optical excitation with a longitudinal field component featured by, e.g., radially polarized beams or certain Laguerre-Gauss or Bessel beams. Utilizing this component, we show within a configuration interaction approach that high-energy exciton states composed of light-hole excitons and higher dark heavy-hole excitons can be addressed. When the higher exciton relaxes, a dark exciton in its ground state is created.
In semiconductor quantum dots the coupling to the environment, i.e. to phonons, plays a crucial role for optical state preparation. We analyze the phonon impact on two methods for a direct optical preparation of the dark exciton, which is enabled by a tilted magnetic field: the excitation with a chirped laser pulse and the excitation with a detuned pulse. Our study reveals that for both methods phonons either do not impede the proposed mechanism or are even useful by widening the parameter range where a dark state preparation is possible due to phonon-assisted dark exciton preparation. In view…
We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes, and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton-polaritons and indirect excitons can be realized. Our protocol relies on the initial preparation of an exciton polariton condensate for the far bl…
Because of their weak coupling to the electromagnetic field dark excitons in semiconductor quantum dots possess extremely long lifetimes, which makes them attractive candidates for quantum information processing. On the other hand, preparation and manipulation of dark states is challenging, because commonly used optical excitation mechanisms are not applicable. We propose a new, efficient mechanism for the deterministic preparation of the dark exciton exploiting the application of a tilted magnetic field and the optical excitation with a chirped, i.e., frequency modulated, laser pulse.
It is demonstrated how the exciton and the biexciton state of a quantum dot can be prepared with high fidelity on a picosecond time-scale by driving the dot with a strong laser pulse that is tuned above the exciton resonance for exciton preparation and in resonance with the exciton transition for biexciton preparation. The proposed schemes make use of the phonon-induced relaxation towards photon dressed states in optically driven quantum dots and combine the simplicity of Rabi flopping with the robustness of adiabatic rapid passage schemes. Our protocols allow for an on-demand, fast and almost…
Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area ($\geq10π$) near resonant optical pulses, we experimentally demonstrate how phonon mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity $\geq 70\%$. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher fidelity preparation for large pulse areas …
For many applications of semiconductor quantum dots in quantum technology a well controlled state preparation of the quantum dot states is mandatory. Since quantum dots are embedded in the semiconductor matrix, the interaction with phonons plays often a major role in the preparation process. In this review, we discuss the influence of phonons on three basically different optical excitation schemes which can be used for the preparation of exciton, biexciton, and superposition states: a resonant excitation leading to Rabi rotations in the excitonic system, an excitation with chirped pulses explo…
The preparation of a coherent heavy-hole spin via ionization of a spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt geometry magnetic field is investigated. For a dot with a 17 ueV bright-exciton fine-structure splitting, the fidelity of the spin preparation is limited to 0.75, with optimum preparation occurring when the effective fine-structure of the bright-exciton matches the in-plane hole Zeeman energy. In principle, higher fidelities can be achieved by minimizing the bright-exciton fine-structure splitting.
The exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment are studied. The negligible (less than $0.2~μ$eV) value of the anisotropic exchange interaction in these QDs prevents a mixing of the excitonic basis states with pure spin and allows for the formation of spin polarized bright excitons for quasi-resonant circularly polarized excitation. In a longitudinal magnetic field, the recombination and spin dynamics of the excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade…
Optical probing of Wigner crystallization in monolayer WSe$_2$ via diffraction of longitudinal excitons
ITMO University（ITMO大学）；Moscow Institute of Physics and Technology (National Research University)（莫斯科物理科学与技术学院（国立研究大学））；Russian Quantum Center（俄罗斯量子中心）；Laboratory of Programmable Functional Materials, Center for Neurophysics and Neuromorphic Technologies（可编程功能材料实验室，神经物理与类脑技术中心）；Moscow Center for Advanced Studies（莫斯科高级研究中心）；Queen’s University（女王大学）
In monolayers of the semiconducting transition metal dichalcogenides, the electron-hole exchange interaction splits the exciton dispersion into a massive transverse branch, and a longitudinal branch that has very light or even zero mass depending on the form of screened Coulomb interaction. The group velocity of the longitudinal branch is sensitive to the strength of electron-hole exchange, which can be engineered through the dielectric environment. Here we show that dielectric patterning of the substrate can be exploited to realize waveguide of the exciton in the longitudinal branch in a homo…
We propose and demonstrate the sequential initialization, optical control, and read-out of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent read-out of the hole spin state.
We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS$_2$. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and $\mathbf k \cdot \mathbf p$ perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with av…
When a detuned and strong laser pulse acts on an optical transition, a Stark shift of the corresponding energies occurs. We analyze how this optical Stark effect can be used to prepare and control the dark exciton occupation in a semiconductor quantum dot. The coupling between the bright and dark exciton states is facilitated by an external magnetic field. Using sequences of laser pulses, we show how the dark exciton and different superposition states can be prepared. We give simple analytic formulas, which yield a good estimate for optimal preparation parameters. The preparation scheme is qui…
We present results on the rates of interlayer energy transfer between excitons in two-dimensional transition metal dichalcogenides (TMDs). We consider both radiative (mediated by real photons) and non-radiative (mediated by virtual photons) mechanisms of energy transfer using a unified Green's function approach that takes into account modification of the exciton energy dispersions as a result of interactions. The large optical oscillator strengths associated with excitons in TMDs result in very fast energy transfer rates. The energy transfer times depend on the exciton momentum, exciton linewi…
The effects resulting due to dressing of an exciton with phonons are analysed as the source of unavoidable decoherence of orbital degrees of freedom in quantum dots. The dressing with longitudinal optical phonons results in energetic shift of order of a few meV even of the ground state of exciton in a state-of-the-art InAs/GaAs dot and the mediating role of longitudinal acoustical phonons is essential in this process. The characteristic time needed for dressing of the exciton with optical phonons is of a picosecond order. That time can be regarded as the lowest limit for decoherence for optica…
The picosecond dynamics of excitons in colloidal CdSe nanorods are directly measured via their 1s to 2p-like internal transitions by ultra-broadband terahertz spectroscopy. Broadened absorption peaks from both the longitudinal and transverse states are observed at 8.5 and 11 THz, respectively. The onset of exciton-LO phonon coupling appears as a bleach in the optical conductivity spectra at the LO phonon energy for times > 1 ps after excitation. Simulations show a suppressed exciton temperature due to thermally excited hole states being rapidly captured onto ligands or unpassivated surface sta…
We have investigated the optical properties of a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires was fabricated by epitaxial growth and electron-beam patterning. The elongation of transverse exciton spin relaxation time of the exciton state with decreasing excitation power was observed by first-order photon correlation measurements. This behavior is well explained by the motional narrowing mechanism induced by Gaussian fluctuations of environmental charges in the InP nanowire. The longitudinal exciton spin relaxation time was evaluated by the degree o…
Wannier-Mott excitons in a semiconductor layered superlattice (SL) of period much smaller than the 2D exciton Bohr radius in the presence of a longitudinal external dc electric field directed parallel to the SL axis are investigated analytically. The exciton states and the optical absorption coefficient are derived in the tight-binding and adiabatic approximations. Strong and weak electric fields providing spatially localized and extended electron and hole states, respectively, are studied. The dependencies of the exciton states and the exciton absorption spectrum on the SL parameters and the …
Theoretical and numerical calculations of the optical absorption spectra of excitons interacting with longitudinal-optical phonons in quasi-2D polar semiconductors are presented. In II-VI semiconductor quantum wells, exciton binding energy can be tuned on- and off-resonance with the longitudinal-optical phonon energy by varying the quantum well width. A comprehensive picture of this tunning effect on the temperature-dependent exciton absorption spectrum is derived, using the exciton Green's function formalism at finite temperature. The effective exciton-phonon interaction is included in the Be…
We have found a novel mechanism of spectral broadening and dephasing in quantum dots (QDs) due to the coupling to longitudinal-optical (LO) phonons. In theory, this mechanism comes into play only if the complete manifold of exciton levels (including those in the wetting-layer continuum) is taken into account. We demonstrate this nontrivial dephasing in different types of QDs, using the exactly solvable quadratic coupling model, here generalized to an arbitrary number of excitonic states.
We report condensation of hexagonal whispering gallery modes (WGM) at room temperature in ZnO microwires that embody nearly perfect polygonal whispering gallery microresonators. The condensate regime is achieved in the UV spectral range only at energies below the first longitudinal optical (LO) phonon replica of the free ZnO A-exciton transition and at non-zero wave vectors. We demonstrate that the multimodality of the WGM system and the high population of free excitons and phonons with various momenta strongly enhance the probability of an interaction of quasiparticles of the cavity exciton-p…
We present an analytical investigation of the exciton optical absorption in a narrow armchair graphene nanoribbon (AGNR) in the presence of a longitudinal external electric field directed parallel to the ribbon axis. The two-body 2D Dirac equation for the massless electron and hole subject to the ribbon confinement, Coulomb interaction and electric field is employed. The ribbon confinement is assumed to be much stronger than the internal exciton electric field which in turn considerably exceeds the external electric field. This justifies an adiabatic approximation implying a slow longitudinal …
We study the relaxation of the exciton spin (longitudinal relaxation time $T_{1}$) in single asymmetrical quantum dots due to an interplay of the short--range exchange interaction and acoustic phonon deformation. The calculated relaxation rates are found to depend strongly on the dot size, magnetic field and temperature. For typical quantum dots and temperatures below 100 K, the zero--magnetic field relaxation times are long compared to the exciton lifetime, yet they are strongly reduced in high magnetic fields. We discuss explicitly quantum dots based on (In,Ga)As and (Cd,Zn)Se semiconductor …
We investigate theoretically the temporal behavior of a quantum dot under off-resonant optical excitation targeted at fast acoustic phonon-assisted state preparation. We demonstrate that in a preparation process driven by short laser pulses three processes can be identified: a dressing of the states during the switch on of the laser pulse, a subsequent phonon-induced relaxation and an undressing at the end of the pulse. By analyzing excitation scenarios with different pulse shapes we highlight the decisive impact of an adiabatic undressing on the final state in short pulse protocols. Furthermo…
Fermion-exciton condensates (FECs) -- computationally and theoretically predicted states that simultaneously exhibit the character of superconducting states and exciton condensates -- are novel quantum states whose properties may involve a hybridization of superconductivity and the dissipationless flow of energy. Here, we exploit prior investigations of superconducting states and exciton condensates on quantum devices to identify a tuneable quantum state preparation entangling the wave functions of the individual condensate states. Utilizing this state preparation, we prepare a variety of FEC …
We study the polarization optical properties of microcavities with embedded (110)-oriented quantum wells. The spin dynamics of exciton polaritons in such structures is governed by the interplay of the spin-orbit splitting of exciton states, which is odd in the in-plane momentum, and the longitudinal-transverse splitting of cavity modes, which is even in the momentum. We demonstrate the generation of polariton spin currents by linearly polarized optical pump and analyze the arising polariton spin textures in the cavity plane. Tuning the excitation spot size, which controls the polariton distrib…
Phonons are well known to be the main mechanism for the coupling between bright and dark excitons in nonmagnetic semiconductors. Here, we investigate diluted magnetic semiconductors where this process is in direct competition with the scattering at localized magnetic impurities. To this end, a recently developed quantum kinetic description of the exciton spin dynamics in diluted magnetic semiconductor quantum wells is extended by the scattering with longitudinal acoustic phonons. A strong phonon impact is found in the redistribution of exciton momenta on the exciton parabola that becomes espec…
We develop a model of the coherent exciton spin-valley dynamics in two-dimensional transition metal dichalcogenides under elastic strain. The strain splits the exciton radiative doublet in linearly polarized states. Consequently, it induces an effective magnetic field acting on the exciton pseudospin and causes its precession. As a result, under circularly polarized excitation, the circular polarization of excitons oscillates with time, also a time-oscillating linear polarization appears. We study competition of the strain-induced effective magnetic field with the field caused by the exciton l…
We propose an implementation of a CNOT quantum gate for quantum computing based on a patterned microcavity polariton system, which can be manufactured using the modern technological facilities. The qubits are encoded in the spin of polaritons. The structure consists of two wire cavities oriented at 45 degrees with a micropillar between them. The polariton spin rotates due to the Longitudinal-Transverse splitting between polarisation eigenstates in the wires. In the pillar, the optically generated circularly polarised polariton macrooccupied state plays the role of the control qubit. Because of…
This is a review of spin-dependent (polarization) properties of multicomponent exciton-polariton condensates in conditions when quasi-equilibrium mean-field Gross-Pitaevskii description can be applied. Mainly two-component (spin states $\pm1$) polariton condensates are addressed, but some properties of four-component exciton condensates, having both the bright (spin $\pm1$) and the dark (spin $\pm2$) components, are discussed. Change of polarization state of the condensate and phase transitions in applied Zeeman field are described. The properties of fractional vortices are given, in particula…
Luminescence properties of colloidal quantum dots have found applications in imaging, light-sources, and displays. Despite wide interest, several experimental observations from these quantum dots, such as the short lifetime on the scale of microseconds and a zero-longitudinal optical phonon line in low-temperature photoluminescence spectrum, remain unexplained by existing models. Here we propose a model including the effect of solid-state environment on luminescence by capturing coherent and incoherent interactions of band-edge exciton with phonon modes. Our model predicts the formation of dre…
Several theoretical predictions have claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch, with the longitudinal one, which is the upper branch, exhibiting an extraordinary strong dispersion in the meV range within the light cone. Historically, this was linked for semiconductor quantum wells to strong far-field optical dipole coupling, or strong electronic long-range exchange interactions, describing two sides of the same coin. Recently, experiments utilizing Fourier-space spectroscopy have shown that the exciton (exciton-polariton) dispersion can…
Magnetically doped semiconductors are well known for their giant Zeeman splittings which can reach several meV even in relatively small external magnetic fields. After preparing a nonequilibrium exciton distribution via optical excitation, the spin dynamics in diluted magnetic semiconductor quantum wells is typically governed by spin-flip scattering processes due to the exciton-impurity exchange interaction. Our theoretical calculations show that the giant Zeeman splitting in these materials in combination with the influence of longitudinal acoustic phonons lead to a quantum ratchet-type dynam…
We study theoretically the contribution of quantum effects to the exciton diffusion coefficient in atomically thin crystals. It is related to the weak localization caused by the interference of excitonic wavefunctions on the trajectories with closed loops. Due to a weak inelasticity of the exciton-phonon interaction the effect is present even if the excitons are scattered by long-wavelength acoustic phonons. We consider exciton interaction with longitudinal acoustic phonons with linear dispersion and with flexural phonons with quadratic dispersion. We identify the regimes where the weak locali…
We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking InGaAs quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few Fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we ini…
We investigate theoretically the influence of an external magnetic field on the spin dynamics of excitons in diluted magnetic semiconductor quantum wells. To this end, we apply a quantum kinetic theory beyond the Markov approximation which reveals that non-Markovian effects can significantly influence the exciton spin dynamics. If the magnetic field is oriented parallel to the growth direction of the well, the Markovian spin-transfer rate decreases monotonically with increasing field as predicted by Fermi's golden rule. The quantum kinetic theory follows this result qualitatively but predicts …
We present a comprehensive theoretical investigation of the photo-generated excitons in transition-metal dichalcogenide monolayers (TMD-ML's) by Laguerre-Gaussian beams, a celebrated kind of twisted lights (TL's) carrying quantized orbital angular momenta (OAM). We show that the photo-excitation of TL incident to a TMD-ML leads to the formation of spatially localized exciton wave packets, constituted by the superposition of finite-momentum exciton states determined by the intriguing interplay between the multiple degrees of freedom of the optical and excitonic subsystems. Consequently, the TL-…
We report on the resonant optical pumping of the |\pm1> spin states of a single Mn dopant in an InAs/GaAs quantum dot embedded itself in a charge tuneable device. The experiment relies on a "W" scheme of transitions reached when a suitable longitudinal magnetic field is applied. The optical pumping is achieved via the resonant excitation of the central Λ system at the neutral exciton X0 energy. For a specific gate voltage, the red-shifted photoluminescence of the charged exciton X- is observed, which allows non-destructive readout of the spin polarization. An arbitrary spin preparation in the …
Quantum fluctuations of the electromagnetic field are known to produce the atomic Lamb shift. We here reveal their iconic signature in semiconductor physics, through the blue-shift they produce to optically bright excitons, thus lifting the energy of these excitons above their dark counterparts. The electromagnetic field here acts in its full complexity: in addition to the longitudinal part via interband \textit{virtual Coulomb} processes, the transverse part -- which has been missed up to now -- also acts via resonant and nonresonant \textit{virtual photons}. These two parts beautifully combi…
We study the potential for controlled transfer of population to the biexciton state of a semiconductor quantum dot coupled with a metal nanoparticle, under the influence of an electromagnetic pulse with hyperbolic secant shape, and derive analytical solutions of the density matrix equations, for both zero and nonzero biexciton energy shift. These solutions lead to efficient transfer to the biexciton state, for various interparticle distances, including relatively small values. In certain cases, when the distance between the two particles is small, the transfer of population is strongly modifie…
The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi-flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust preparation of a quantum state using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept throu…
Recently, longitudinal acoustic phonons have been identified as the main source of the intensity damping observed in Rabi rotation measurements of the ground-state exciton of a single InAs/GaAs quantum dot. Here we report experiments of intensity damped Rabi rotations in the case of detuned laser pulses, the results have implications for the coherent optical control of both excitons and spins using detuned laser pulses.
We demonstrate charge tuning in strain free GaAs/AlGaAs quantum dots (QDs) grown by droplet epitaxy on a GaAs(111)A substrate. Application of a bias voltage allows the controlled charging of the QDs from $-3|e|$ to $+2|e|$. The resulting changes in QD emission energy and exciton fine-structure are recorded in micro-photoluminescence experiments at T=4K. We uncover the existence of excited valence and conduction states, in addition to the s-shell-like ground state. We record a second series of emission lines about 25meV above the charged exciton emission coming from excited charged excitons. Fo…
We study theoretically the fluctuation properties of acoustic phonons created in a semiconductor quantum dot after ultrafast optical excitation. An excitation with a single ultrafast pulse creates an exciton confined to the quantum dot, which is coupled to longitudinal acoustic phonons. This leads to the formation of a polaron in the quantum dot accompanied by the emission of a phonon wave packet. We show that the fluctuations of the lattice displacement associated with the wave packet after a single laser pulse excitation in resonance with the exciton transition are always larger than their r…
We present a first comprehensive study on deterministic spin preparation employing excited state resonances of droplet etched GaAs quantum dots. This achievement facilitates future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions the properties of different quantum dot energy levels, i.e. shells, are revealed. The innovative use of polarization resolved excitation and detection in quasi-resonant excitation spectroscopy facilitates determination of $85$ $\%$ max…
Optical transitions in a semiconductor quantum dot are theoretically investigated, with emphasis on the coupling to longitudinal optical phonons, and including excitonic effects. When limiting to a finite number of $m$ electron and $n$ hole levels in the dot, the model can be solved exactly within numerical accuracy. Crucial for this to work is the absence of dispersion of the phonons. A suitable orthogonalization procedure leaves only $m(m+1)/2+n(n+1)/2-2$ phonon modes to be coupled to the electronic system. We calculate the linear optical polarization following a delta pulse excitation, and …
We study a large ensemble of nuclear spins interacting with a single electron spin in a quantum dot under optical excitation and photon detection. When a pair of applied laser fields satisfy two-photon resonance between the two ground electronic spin states, detection of light scattering from the intermediate exciton state acts as a weak quantum measurement of the effective magnetic (Overhauser) field due to the nuclear spins. If the spin were driven into a coherent population trapping state where no light scattering takes place, then the nuclear state would be projected into an eigenstate of …
In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly non-monotonous, sign changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of …
We present a theoretical study of interacting electron-hole pairs located on a magnetized surface of a strong topological insulator (TI). The excitonic energy levels and the optical absorption on such surface display unique and potentially measurable features such as (i) an enhanced binding energy for excitons whose total angular momentum is aligned with the magnetic exchange field, (ii) a stark dependence of the optical absorption on the direction of the magnetic exchange field as well as on the chirality of the incident light, and (iii) a tunable center-of-mass motion of spinful excitons ind…
Time-resolved Faraday rotation is used to study both transverse and longitudinal spin relaxation in chemically-synthesized CdSe nanocrystals (NCs) 22-80 Angstroms in diameter. The precession of optically-injected spins in a transverse magnetic field occurs at distinct frequencies whose assignment to electron and exciton spins is developed through systematic studies of the size-dependence and theoretical calculations. It is shown that the transverse spin lifetime is limited by inhomogeneous dephasing to a degree that cannot be accounted for by the NC size distribution alone. Longitudinal spin r…
Exciton spin and related optical polarization in self-assembled InAs/In$_{0.53}$Ga$_{0.23}$Al$_{0.24}$As/InP(001) quantum dashes emitting at 1.55 μm are investigated by means of polarization- and time-resolved photoluminescence, as well as photoluminescence excitation spectroscopy, at cryogenic temperature. We investigate the influence of highly non-resonant and quasi-resonant optical spin pumping conditions on spin polarization and spin memory of the quantum dash ground state. We show that a spin pumping scheme, utilizing the longitudinal-optical-phonon-mediated coherent scattering process, c…
