Transitions Of Charged Excitons Research Articles

Highly nonlinear trion-polaritons in a monolayer semiconductor

Highly nonlinear optical materials with strong effective photon-photon interactions are required for ultrafast and quantum optical signal processing circuitry. Here we report strong Kerr-like nonlinearities by employing efficient optical transitions of charged excitons (trions) observed in semiconducting transition metal dichalcogenides (TMDCs). By hybridising trions in monolayer MoSe2 at low electron densities with a microcavity mode, we realise trion-polaritons exhibiting significant energy shifts at small photon fluxes due to phase space filling. We find the ratio of trion- to neutral exciton–polariton interaction strength is in the range from 10 to 100 in TMDC materials and that trion-polariton nonlinearity is comparable to that in other polariton systems. The results are in good agreement with a theory accounting for the composite nature of excitons and trions and deviation of their statistics from that of ideal bosons and fermions. Our findings open a way to scalable quantum optics applications with TMDCs.

Quadrature Demodulation of a Quantum Dot Optical Response to Faint Light Fields.

The amplitude and phase of a material's nonlinear optical response provide insight into the underlying electronic dynamics that determine its optical properties. Phase-sensitive nonlinear spectroscopy techniques are widely implemented to explore these dynamics through demodulation of the complex optical signal field into its quadrature components; however, complete reconstruction of the optical response requires measuring both the amplitude and phase of each quadrature, which is often lost in standard detection methods. Here, we implement a heterodyne-detection scheme to fully reconstruct the amplitude and phase response of spectral hole-burning from InAs/GaAs charged quantum dots. We observe an ultra-narrow absorption profile and a corresponding dispersive lineshape of the phase, which reflect the nanosecond optical coherence time of the charged exciton transition. Simultaneously, the measurements are sensitive to electron spin relaxation dynamics on a millisecond timescale, as this manifests as a magnetic-field dependent delay of the amplitude and phase modulation. Appreciable amplitude modulation depth and nonlinear phase shift up to ~0.09×π radians (16°) are demonstrated, providing new possibilities for quadrature modulation at faint photon levels with several independent control parameters, including photon number, modulation frequency, detuning, and externally applied fields.

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS2 MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.

Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition.

In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots.

Electronic Enhancement of the Exciton Coherence Time in Charged Quantum Dots.

Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information and photonics research. Nature sets the ultimate limit, however, given by the strength of the system's coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and enhance the optical coherence time of the charged exciton transition in quantum dots embedded in a photonic waveguide. By manipulating the electronic wave functions through an applied lateral electric field, we increase the coherence time from ∼1.4 to ∼2.7 ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to ∼6 nm, which leads to a 30% weaker transition dipole moment. The ability to electronically control the coherence time opens new avenues for quantum communication and novel coupling schemes between distant qubits.

Controlled tunneling-induced dephasing of Rabi rotations for high-fidelity hole spin initialization

We report the sub-picosecond initialization of a single heavy hole spin in a self-assembled quantum dot with >98.5 % fidelity and without external magnetic field. Using an optically adressable charge and spin storage device we tailor the relative electron and hole tunneling escape timescales from the dot and simultaneously achieve high-fidelity initialization, long hole storage times and high efficiency readout via a photocurrent signal. We measure electric field-dependent Rabi oscillations of the neutral and charged exciton transitions in the ultrafast tunneling regime and demonstrate that tunneling induced dephasing (TID) of excitonic Rabi rotations is the major source for the intensity damping of Rabi oscillations in the low Rabi frequency, low temperature regime. Our results are in very good quantitative agreement with quantum-optical simulations revealing that TID can be used to precisely measure tunneling escape times and extract changes in the Coulomb binding energies for different charge configurations of the quantum dot. Finally, we demonstrate that for sub-picosecond electron tunneling escape TID of a coherently driven exciton transition facilitates ultrafast hole spin initialization with near-unity fidelity.

Spectral diffusion of neutral and charged exciton transitions in single CdSe/ZnS nanocrystals due to quantum-confined Stark effect

Photoluminescence (PL) fluctuations of single semiconductor nanocrystals (NCs), such as PL blinking and spectral diffusion (SD), reflect the quantum nature of charges in the NCs. Through simultaneous measurements of PL spectra and lifetimes on single CdSe/ZnS NCs, PL of neutral excitons is found to exhibit a unique behavior of SD, which is accompanied by changes of radiative recombination lifetime. We find that the SD of neutral excitons originated from the quantum-confined Stark effect, which also affects the SD of charged excitons observed during PL blinking.

Optical in-plane anisotropy of ZnO/(Zn,Mg)O quantum wells grown ona-plane sapphire: Implications for optical spin control

Utilizing the anisotropic thermal expansion of $a$-plane sapphire, we study the role of anisotropic in-plane strain on ZnO quantum well structures. X-ray data reveal a deformation of the hexagonal unit cell. The symmetry reduction shows up in the optical spectra by distinct linear-polarization features, both for the neutral and charged exciton transition. We elaborate the excitonic fine structure behind this observation, accounting for spin-orbit coupling and electron-hole exchange in the presence of anisotropic strain. The consequences for optical spin control are discussed.

Sharp emission from single InAs quantum dots grown on vicinal GaAs surfaces

We report on optical studies of single InAs quantum dots grown on vicinal GaAs(001) surfaces. To ensure low quantum dot density and appropriate size, we deposit InAs layers 1.4 or 1.5 ML thick, thinner than the critical thickness for Stranski–Krastanov quantum dot formation. These dots show sharp and bright photoluminescence. Lifetime measurements reveal an exciton lifetime of 500 ps. Polarization measurements show an exciton fine structure splitting of 15 μeV and allow to identify the exciton and charged exciton transitions with linewidth as narrow as 23 μeV.

Second hidden triplet-singlet crossover of charged excitons inn-doped (Cd,Mn)Te/(Cd,Mg)Te in ultra-high magnetic fields

We studied charged excitons realized in dilute magnetic semiconductor single-quantum wells (Cd,Mn)Te/(Cd,Mg)Te in high magnetic fields by photoluminescence (PL) measurements. We assigned two PL peaks which appeared in high magnetic fields as the singlet and the triplet charged excitons, respectively. It was shown from our PL analysis that the second hidden crossover between the triplet and the singlet charged excitons took place above 60 T. The singlet charged exciton transition was found to be the lowest energy in a higher magnetic side. Linear increase in the conduction-electron $g$ factor with increasing magnetic field was deduced from the intensity ratio of two PL lines in magnetic fields up to 140 T.

Charge and spin readout scheme for single self-assembled quantum dots

We propose an all optical spin initialization and readout concept for single self-assembled quantum dots, and demonstrate its feasibility. Our approach is based on a gateable single dot photodiode structure that can be switched between charge and readout modes. After optical electron generation and storage, we propose to employ a spin-conditional absorption of a circularly polarized light pulse tuned to the single negatively charged exciton transition to convert the spin information of the resident electron to charge occupancy. Switching the device to the charge readout mode then allows us to probe the charge state of the quantum dot $(1e,2e)$ using nonresonant luminescence. The spin orientation of the resident electron is then reflected by the photoluminescence (PL) yield of doubly $({X}^{2\ensuremath{-}})$ and singly $({X}^{\ensuremath{-}1})$ charged transitions in the quantum dot. To verify the feasibility of this spin readout concept, we have applied time gated photoluminescence to confirm that selective optical charging and efficient nonperturbative measurement of the charge state can be performed on the same dot. The results show that, by switching the electric field in the vicinity of the quantum dot, the charging rate can be switched between a regime of efficient electron generation $(\ensuremath{\Gamma}⪢{10}^{6}\text{ }{\text{s}}^{\ensuremath{-}1}\text{ }{\text{W}}^{\ensuremath{-}1}\text{ }{\text{cm}}^{2})$ and a readout regime, where the charge occupancy and, therefore, the spin state of the dot can be tested via PL over millisecond timescales, without altering it. Our results show that such a quasicontinuous, nonperturbative readout of the charge state of the dot allows increasing the dark time available for undisturbed spin manipulation and storage into the millisecond range, while still providing sufficient signal for high fidelity readout. Consequently, our readout scheme would allow the investigation of spin relaxation and decoherence mechanisms over the long timescales, predicted by theory.

Spatially direct charged exciton photoluminescence in undoped ZnSe∕BeTe type-II quantum wells

Photoluminescence (PL) spectra occurred as a spatially direct optical transition inside of the ZnSe layer in undoped ZnSe∕BeTe∕ZnSe type-II quantum structures have been studied. We have found that the charged exciton transition was observed at the lower energy side of the exciton transition in the spatially direct PL. The formation of the charged exciton was attributed to the accumulated electrons in the ZnSe layer after the photoexcitation accompanied by the holes being escaped from this well and injected into the BeTe layer.

Internal Transitions of Charged Excitons in AlGaAs/GaAs Lateral Fluctuation Quantum Dots

Internal Transitions of Charged Excitons in AlGaAs/GaAs Lateral Fluctuation Quantum Dots

Spin dependent transitions of charged excitons in type-II quantum wells

We present results of the reflection, the absorption, and the photoluminescence measurements performed on ZnMgBeSe/ZnSe/BeTe quantum wells. In n-doped samples the reflection and the absorption spectra have shown a typical negatively charged exciton feature in spin-dependent polarization under magnetic field. The pronounced linear polarization of the photoluminescence indicates that photoluminescence transitions of an indirect type take place at respective interfaces of ZnSe/BeTe/ZnSe. A new type of charged excitons peculiar to the type-II quantum structures has been proposed.

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron–hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron–hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron–electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.

Spin coherence and formation dynamics of charged excitons inCdTe/Cd1−x−yMgxZnyTequantum wells

We report on a study of the spin coherence and on the formation dynamics of charged excitons in a p-doped ${\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mg}}_{x}{\mathrm{Zn}}_{y}\mathrm{Te}$ quantum well by time-resolved photoluminescence under strictly resonant excitation of either the neutral or the charged exciton transition. The analysis of the decay of the charged exciton photoluminescence polarization and of the oscillation of this polarization when a transverse magnetic field is applied allows us to conclude that the formation of a charged exciton via an exciton state does not affect either the electron spin orientation or its coherence. The radiative lifetime of the charged exciton is directly measured and is found to be \ensuremath{\sim}60 ps. Its formation time is determined from the experiments via a detailed model and is found to be \ensuremath{\sim}65 ps. We also obtain information on the excitonic spin-flip time (12 ps) and the single-carrier spin-flip times within the exciton (electron, 60 ps and hole, 20 ps).

Charged magnetoexcitons in two-dimensions: magnetic translations and families of dark states

We show that optical transitions of charged excitons in semiconductor heterostructures are governed in magnetic fields by a novel exact selection rule, a manifestation of magnetic translations. It is shown that the spin-triplet ground state of the quasi-two-dimensional charged exciton X--a bound state of two electrons and one hole-is optically inactive in photoluminescence at finite magnetic fields. Internal bound-to-bound X- triplet transition has a specific spectral position, below the electron cyclotron resonance, and is strictly prohibited in a translationally invariant system. These results allow one to discriminate between free and disorder-affected charged excitons.

Theory of Internal Transitions of Charged Excitons in Quantum Wells in Magnetic Fields

For charged semiconductor complexes in magnetic fields B, we discuss an exact classification of states, which is based on magnetic translations. In this scheme, in addition to the total orbital angular momentum projection Mz and electron and hole spins Se, Sh, a new exact quantum number appears. This oscillator quantum number, k, is related physically to the center of the cyclotron motion of the complex as a whole. In the dipole approximation k is strictly conserved in magneto-optical transitions. We discuss implications of this new exact selection rule for internal intraband magneto-optical transitions of charged excitons X– in quantum wells in B.

Internal transitions of two-dimensional charged magneto-excitons X −: theory and experiment

Internal spin-singlet and spin-triplet transitions of charged excitons X − in magnetic fields in quantum wells have been studied experimentally and theoretically. The allowed X − transitions are photoionizing and exhibit a characteristic double-peak structure, which reflects the rich structure of the magnetoexciton continua in higher Landau levels (LLs). We discuss a novel exact selection rule, a hidden manifestation of translational invariance, that governs transitions of charged mobile complexes in a magnetic field.