Nanocrystal shape and the mechanism of exciton spin relaxation

The rate of exciton spin relaxation (flips) between the bright exciton states (F = +/-1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals. That observation is explained by deriving an expression for the electronic coupling matrix element that mixes the bright exciton states.     

Spin Injection Processes in ZnSe-Based Double Quantum Dots of Diluted Magnetic Semiconductors

Spin injection processes in the double quantum dots of ZnSe-based diluted magnetic semiconductors are discussed. Double quantum dots are fabricated from ZnSe-based double quantum wells by electron beam lithography and wet etching. In these samples, the photo-excited carriers in the magnetic dots are injected into the non-magnetic dots. The circular polarization degrees of photoluminescence from the non-magnetic dots are measured by micro-photoluminescence measurement system under the magnetic field up to 5 T. The maximum spin polarization degrees of injected carriers determined from our experiment are 10% for double quantum wells and 15% for double quantum dots. The spin injection efficiency was estimated both from the observed circular polarization degree and the diffusion length of carriers. We concluded that the spin injection efficiency is increased in the double quantum dots.     

Ultrafast exciton fine structure relaxation dynamics in lead chalcogenide nanocrystals

The rates of fine structure relaxation in PbS, PbSe, and PbTe nanocrystals were measured on a femtosecond time scale as a function of temperature with no applied magnetic field by cross-polarized transient grating spectroscopy (CPTG) and circularly polarized pump-probe spectroscopy. The relaxation rates among exciton fine structure states follow trends with nanocrystal composition and size that are consistent with the expected influence of material dependent spin-orbit coupling, confinement enhanced electron-hole exchange interaction, and splitting between L valleys that are degenerate in the bulk. The size dependence of the fine structure relaxation rate is considerably different from what is observed for small CdSe nanocrystals, which appears to result from the unique material properties of the highly confined lead chalcogenide quantum dots. Modeling and qualitative considerations lead to conclusions about the fine structure of the lowest exciton absorption band, which has a potentially significant bearing on photophysical processes that make these materials attractive for practical purposes.     

Spin–Lattice Relaxation of Mn Ions in Nanostructures with Semiconductor Quantum Dots

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.     

Thermally Activated Photoluminescence in Lead Selenide Colloidal Quantum Dots

The thermal activation processes in PbSe colloidal quantum dots and their influence on the ground‐state exciton emission are discussed. Activation of a dark exciton occurs at 1.4–7 K, assisted by an acoustic phonon coupling. Activation of a bright exciton occurs at 100–200 K, which appears as a sudden change in the photoluminescence band intensity, energy, and full width at half maximum. This activation overcomes the dark–bright‐state splitting, when the activation temperature increases with the decrease of the dots' size. The dark exciton lifetime is found to be ≈6–12 µs at 1.4 K, while the bright exciton lifetime at 300 K evaluated as 450 ns varies slightly with the change in the size of the dots. In addition, the emission quantum yield of these dots, measured at a variety of temperatures when dissolved in various solvents, reveals information about the influence of the environment on the recombination processes.     

Spin-resolved Purcell effect in a quantum dot microcavity system

We demonstrate the spin selective coupling of the exciton state with cavity mode in a single quantum dot (QD)-micropillar cavity system. By tuning an external magnetic field, each spin polarized exciton state can be selectively coupled with the cavity mode due to the Zeeman effect. A significant enhancement of spontaneous emission rate of each spin state is achieved, giving rise to a tunable circular polarization degree from -90% to 93%. A four-level rate equation model is developed, and it agrees well with our experimental data. In addition, the coupling between photon mode and each exciton spin state is also achieved by varying temperature, demonstrating the full manipulation over the spin states in the QD-cavity system. Our results pave the way for the realization of future quantum light sources and the quantum information processing applications.     

Piezoelectric coupling effect on exciton–phonon scattering rates in CdSe quantum dots embedded in glass matrix

The scattering of excitons by acoustic phonons in nanostructures such as quantum dots generally controls relaxation process to the lowest energy states, and is a basis for understanding optical properties and coherence effects in these systems. In our work we have studied theoretically the scattering of excitons via acoustic phonons in a CdSe disk-shaped semiconductor quantum dot under an applied magnetic field. The scattering rate (SR) is calculated considering the exciton–phonon scattering by the piezoelectric coupling potential mechanism. We discuss the influence of the external applied magnetic field, and the quantum dot size on the SR. Our calculations show that the exciton–acoustic phonon scattering rate depends significantly on these parameters.     

Hole spin relaxation in Ge-Si core-shell nanowire qubits

Controlling decoherence is the biggest challenge in efforts to develop quantum information hardware. Single electron spins in gallium arsenide are a leading candidate among implementations of solid-state quantum bits, but their strong coupling to nuclear spins produces high decoherence rates. Group IV semiconductors, on the other hand, have relatively low nuclear spin densities, making them an attractive platform for spin quantum bits. However, device fabrication remains a challenge, particularly with respect to the control of materials and interfaces. Here, we demonstrate state preparation, pulsed gate control and charge-sensing spin readout of hole spins confined in a Ge-Si core-shell nanowire. With fast gating, we measure T(1) spin relaxation times of up to 0.6 ms in coupled quantum dots at zero magnetic field. Relaxation time increases as the magnetic field is reduced, which is consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions.     

Blinking statistics of silicon quantum dots

The blinking statistics of numerous single silicon quantum dots fabricated by electron-beam lithography, plasma etching, and oxidation have been analyzed. Purely exponential on- and off-time distributions were found consistent with the absence of statistical aging. This is in contrast to blinking reports in the literature where power-law distributions prevail as well as observations of statistical aging in nanocrystal ensembles. A linear increase of the switching frequency with excitation power density indicates a domination of single-photon absorption processes, possibly through a direct transfer of charges to trap states without the need for a bimolecular Auger mechanism. Photoluminescence saturation with increasing excitation is not observed; however, there is a threshold in excitation (coinciding with a mean occupation of one exciton per nanocrystal) where a change from linear to square-root increase occurs. Finally, the statistics of blinking of single quantum dots in terms of average on-time, blinking frequency and blinking amplitude reveal large variations (several orders) without any significant correlation demonstrating the individual microscopic character of each quantum dot.     

Ultrafast spin dynamics in colloidal ZnO quantum dots

Time-resolved Faraday rotation measurements in the ultraviolet have been performed to reveal the ultrafast spin dynamics of electrons in colloidal ZnO quantum dots. Oscillating Faraday rotation signals are detected at frequencies corresponding to an effective g factor of g = 1.96. Biexponential oscillation decay is observed that is due to (i) rapid depopulation of the fundamental exciton (tau = 250 ps) and (ii) slow electron spin dephasing ( T 2 = 1.2 ns) within a metastable state formed by hole-trapping at the quantum dot surface.     

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

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

Coherent spin transport through dynamic quantum dots

Spin transport and manipulation in semiconductors have been studied intensively with the ultimate goal of realizing spintronic devices. Previous work in GaAs has focused on controlling the carrier density, crystallographic orientation and dimensionality to limit the electron spin decoherence and allow transport over long distances. Here, we introduce a new method for the coherent transport of spin-polarized electronic wave packets using dynamic quantum dots (DQDs) created by the piezoelectric field of coherent acoustic phonons. Photogenerated spin carriers transported by the DQDs in undoped GaAs (001) quantum wells exhibit a spin coherence length exceeding 100 microm, which is attributed to the simultaneous control of the carrier density and the dimensionality by the DQDs during transport. In the absence of an applied magnetic field, we observe the precession of the electron spin induced by the internal magnetic field associated with the spin splitting of the conduction band (Dresselhaus term). The coherent manipulation of the precession frequency is also achieved by applying an external magnetic field.     

Quantum oscillations in magnetically doped colloidal nanocrystals

Progress in the synthesis of colloidal quantum dots has recently provided access to entirely new forms of diluted magnetic semiconductors, some of which may find use in quantum computation. The usefulness of a spin qubit is defined by its Rabi frequency, which determines the operation time, and its coherence time, which sets the error correction window. However, the spin dynamics of magnetic impurity ions in colloidal doped quantum dots remain entirely unexplored. Here, we use pulsed electron paramagnetic resonance spectroscopy to demonstrate long spin coherence times of ～0.9 μs in colloidal ZnO quantum dots containing the paramagnetic dopant Mn(2+), as well as Rabi oscillations with frequencies ranging between 2 and 20 MHz depending on microwave power. We also observe electron spin echo envelope modulations of the Mn(2+) signal due to hyperfine coupling with protons outside the quantum dots, a situation unique to the colloidal form of quantum dots, and not observed to date.     

Giant fluctuations and gate control of the g-factor in InAs nanowire quantum dots

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2 Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.     

Hanle Effect of Charged and Neutral Excitons in Quantum Wells

We measured magnetic field depolarization of charged and neutral exciton cw photoluminescence of a system consisting of two coupled quantum wells with a residual concentration of holes. Using the Hanle expression, we obtained exciton lifetime and electron spin relaxation time, which are in agreement with results of time-resolved experiments. We suppose that the tunneling takes place via an emission of an LO phonon and we find this process spin conserving.     

硫化铜量子点的研究进展

硫化铜量子点作为一种p型半导体纳米晶,具有很强的表面等离子体共振效应、低的毒性以及独特的光学和电学性能,在光催化、生物技术、光电转换材料领域受到了极大关注。由于单分散的硫化铜量子点的制备过程复杂,效率较低,并且纯的硫化铜量子点电导率较低,这极大地限制了其在能量存储器件方面的应用。此外,由于硫化铜量子点复杂的能带结构和独特的p型半导体特性,针对硫化铜量子点的光学性能调控尚不成熟。基于此,本文综述了硫化铜量子点在制备方面的研究现状与取得的进展,介绍了硫化铜量子点的能带结构、晶体结构,及其在量子点敏化太阳能电池、光催化降解污染物、肿瘤细胞诊断与治疗等方面的研究进展,并对硫化铜量子点或Cu系量子点更进一步的研究、开发应用提出了几点建议。     

Ultrafast room temperature single-photon source from nanowire-quantum dots

Epitaxial semiconductor quantum dots are particularly promising as realistic single-photon sources for their compatibility with manufacturing techniques and possibility to be implemented in compact devices. Here, we demonstrate for the first time single-photon emission up to room temperature from an epitaxial quantum dot inserted in a nanowire, namely a CdSe slice in a ZnSe nanowire. The exciton and biexciton lines can still be resolved at room temperature and the biexciton turns out to be the most appropriate transition for single-photon emission due to a large nonradiative decay of the bright exciton to dark exciton states. With an intrinsically short radiative decay time (≈300 ps) this system is the fastest room temperature single-photon emitter, allowing potentially gigahertz repetition rates.     

Highly directional emission and photon beaming from nanocrystal quantum dots embedded in metallic nanoslit arrays

We demonstrate a directional beaming of photons emitted from nanocrystal quantum dots that are embedded in a subwavelength metallic nanoslit array with a divergence angle of less than 4°. We show that the eigenmodes of the structure result in localized electromagnetic field enhancements at the Bragg cavity resonances, which could be controlled and engineered in both real and momentum space. The photon beaming is achieved using the enhanced resonant coupling of the quantum dots to these Bragg cavity modes, which dominates the emission properties of the quantum dots. We show that the emission probability of a quantum dot into the narrow angular mode is 20 times larger than the emission probability to all other modes. Engineering nanocrystal quantum dots with subwavelength metallic nanostructures is a promising way for a range of new types of active optical devices, where spatial control of the optical properties of nanoemitters is essential, on both the single and many photons level.     

Landau–Zener spin transitions in Fe2+–Fe2+ quantum dots controlling dislocation mobility in NaCl:Fe crystals

Magnetic field induces transition in the distorted Fe²⁺–Fe²⁺ pairs (quantum dots) from the initial bonding singlet state to the high spin antibonding state providing decay of the pairs for two separated Fe²⁺ ions. Dislocations moving under internal stresses easily overcome separated Fe²⁺ ions in comparison with Fe²⁺–Fe²⁺ pairs lying close to the glide plane. Non-monotonous field dependence of dislocation displacements under internal stresses governed by short (100 μs) impulse of high magnetic fields up to 31 T was revealed in NaCl:Fe crystals. This non-typical dependence is the fingerprint of the Landau–Zener non-adiabatic spin transition between singlet and high spin states in quantum dots distorted by mechanical stresses of moving dislocations.     

Spin filling of valley-orbit states in a silicon quantum dot

We report the demonstration of a low-disorder silicon metal-oxide-semiconductor (Si MOS) quantum dot containing a tunable number of electrons from zero to N = 27. The observed evolution of addition energies with parallel magnetic field reveals the spin filling of electrons into valley-orbit states. We find a splitting of 0.10?meV between the ground and first excited states, consistent with theory and placing a lower bound on the valley splitting. Our results provide optimism for the realisation in the near future of spin qubits based on silicon quantum dots.