Optical manipulation of a single Mn spin in a CdTe-based quantum dot

Low-Temperature Energy Transfer via Self-Trapped Excitons in Mn2+-Doped 2D Organometal Halide Perovskites

We investigate the mechanisms of energy transfer in Mn²⁺-doped ethylammonium lead bromide (EA₂PbBr₄:Mn²⁺), a two-dimensional layered perovskite (2DLP), using cryogenic optical spectroscopy. At temperature T > 120 K, photoluminescence (PL) is dominated by emission from Mn²⁺, with complete suppression of band edge (BE) emission and self-trapped exciton (STE) emission. However, for T < 120 K, in addition to Mn²⁺ emission, PL is observed from BE and STEs. Data further reveal that for 20 K < T < 120 K, STEs form the most dominant routes in assisting energy transfer (ET) from 2DLP to Mn²⁺ dopants. However, at higher Mn²⁺ concentration, higher activation energies indicate defect states come into play, successfully competing with STEs for ET both from BE to STE states and from STE to Mn²⁺. Finally, using polarization-resolved spectroscopy, we demonstrate optical spin orientation of the Mn²⁺ ions via ET from 2DLP excitons at zero magnetic field. Our results reveal fundamental insights on the interactions between quantum confined charge carriers and dopants in organometal halide perovskites.

Diffusion doping of cobalt in rod-shape anatase TiO2 nanocrystals leads to antiferromagnetism

Cobalt(ii) ions were adsorbed to the surface of rod-shape anatase TiO₂ nanocrystals and subsequently heated to promote ion diffusion into the nanocrystal. After removal of any remaining surface bound cobalt, a sample consisting of strictly cobalt-doped TiO₂ was obtained and characterized with powder X-ray diffraction, transmission electron microscopy, UV-visible spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy, SQUID magnetometry, and inductively-coupled plasma atomic emission spectroscopy. The nanocrystal morphology was unchanged in the process and no new crystal phases were detected. The concentration of cobalt in the doped samples linearly correlates with the initial loading of cobalt(ii) ions on the nanocrystal surface. Thin films of the cobalt doped TiO₂ nanocrystals were prepared on indium-tin oxide coated glass substrate, and the electrical conductivity increased with the concentration of doped cobalt. Magnetic measurements of the cobalt-doped TiO₂ nanocrystals reveal paramagnetic behavior at room temperature, and antiferromagnetic interactions between Co ions at low temperatures. Antiferromagnetism is atypical for cobalt-doped TiO₂ nanocrystals, and is proposed to arise from interstitial doping that may be favored by the diffusional doping mechanism.

Magnetic Circular Dichroism in Nanomaterials: New Opportunity in Understanding and Modulation of Excitonic and Plasmonic Resonances

The unique capability of magnetic circular dichroism (MCD) in revealing geometry and electronic information has provided new opportunities in exploring the relationship between structure and magneto-optical properties in nanomaterials with extraordinary optical absorption. Here, the representative studies referring to application of the MCD technique in semiconductor and noble metal nanomaterials are overviewed. MCD is powerful in elucidating the structural information of the excitonic transition in semiconductor nanocrystals, electronic transitions in noble metal nanoclusters, and plasmon resonance in noble metal nanostructures. By virtue of these advantages, the MCD technique shows its unrivalled ability in evaluating the magnetic modulation of excitonic and plasmonic optical activity of nanomaterials with varied chemical composition, geometry, assembly conformation, and coupling effect. Knowledge of the key factors in manipulating magneto-optical properties at the nanoscale acquired with the MCD technique will largely boost the application of semiconductor and noble nanomaterials in the fields of sensing, spintronic, nanophotonics, etc.

Ultra-long-working-distance spectroscopy of single nanostructures with aspherical solid immersion microlenses

In light science and applications, equally important roles are played by efficient light emitters/detectors and by the optical elements responsible for light extraction and delivery. The latter should be simple, cost effective, broadband, versatile and compatible with other components of widely desired micro-optical systems. Ideally, they should also operate without high-numerical-aperture optics. Here, we demonstrate that all these requirements can be met with elliptical microlenses 3D printed on top of light emitters. Importantly, the microlenses we propose readily form the collected light into an ultra-low divergence beam (half-angle divergence below 1°) perfectly suited for ultra-long-working-distance optical measurements (600 mm with a 1-inch collection lens), which are not accessible to date with other spectroscopic techniques. Our microlenses can be fabricated on a wide variety of samples, including semiconductor quantum dots and fragile van der Waals heterostructures made of novel two-dimensional materials, such as monolayer and few-layer transition metal dichalcogenides.

GexSi1-x virtual-layer enhanced ferromagnetism in self-assembled Mn0.06Ge0.94 quantum dots grown on Si wafers by molecular beam epitaxy

Self-assembled Mn_0.06Ge_0.94 quantum dots (QDs) on a Si substrate or Ge_xSi_1-x virtual substrate (VS) were grown by molecular beam epitaxy. The Ge_xSi_1-x VS of different thicknesses and Ge compositions x were utilized to modulate the ferromagnetic properties of the above QDs. The MnGe QDs on Ge_xSi_1-x VS show a significantly enhanced ferromagnetism with a Curie temperature above 220 K. On the basis of the microstructural and magnetization results, the ferromagnetic properties of the QDs on Ge_xSi_1-x VS are believed to come from the intrinsic MnGe ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. At the same time, we found that by increasing the Ge composition x of Ge_xSi_1-x VS, the ferromagnetism of QDs grown on VS will markedly increase due to the improvements of hole concentration and Ge composition inside the QDs. These results are fundamentally important in the understanding and especially in the realization of high Curie temperature MnGe diluted magnetic semiconductors.

Readout of a dopant spin in the anisotropic quantum dot with a single magnetic ion

Owing to exchange interaction between the exciton and magnetic ion, a quantum dot embedding a single magnetic ion is a great platform for optical control of individual spin. In particular, a quantum dot provides strong and sharp optical transitions, which give experimental access to spin states of an individual magnetic ion. We show, however, that physics of quantum dot excitons also complicate spin readout and optical spin manipulation in such a system. This is due to electron-hole exchange interaction in anisotropic quantum dots, which affects the polarisation of the emission lines. One of the consequences is that the intensity of spectral lines in a single spectrum are not simply proportional to the population of various spin states of magnetic ion. In order to provide a solution of the above problem, we present a method of extracting both the spin polarisation degree of a neutral exciton and magnetic dopant inside a semiconductor quantum dot in an external magnetic field. Our approach is experimentally verified on a system of CdSe/ZnSe quantum dot containing a single Fe²⁺ ion. Both the resonant and non-resonant excitation regimes are explored resulting in a record high optical orientation efficiency of dopant spin in the former case. The proposed solutions can be easily expanded to any other system of quantum dots containing magnetic dopants.

Carrier and spin dynamics of high-density exciton magnetic polarons in Cd0.8Mn0.2Te

We investigated the carrier and spin dynamics of high-density exciton magnetic polarons (HD-EMPs) in Cd_0.8Mn_0.2Te based on the measurement of their time-resolved photoluminescence (PL) spectra and polarization states, and the utilization of photo-induced Faraday rotation techniques. The PL from the HD-EMPs were collected in a forward scattering configuration, and was observed as a pulsed emission of a few picoseconds duration, exhibiting a blue-shift with time evolution. The blue shift originated from the refractive-index dispersion of the sample. By excluding the influence of the refractive-index dispersion on the time profile, it was revealed that the ultra-short pulsed emission with a time width smaller than 1 ps was initially radiated with a time delay of ~2.4 ps after photoexcitation. From the results of time evolution of the polarization states, it is concluded that the exciton-Mn spin interactions occurs immediately after the excitation, which causes the Mn ion spins to align to follow the spin states of photoexcited excitons. The alignment of the Mn ion spins through the formation of the HD-EMPs was significantly faster than that of the localized EMP. On the other hand, the time evolution of the photo-induced Faraday rotation showed two decay components attributed to spin relaxations of the excitons and Mn ions within the HD-EMP. The observation of the Faraday rotation signal due to the Mn ion spins further confirms that these spins were aligned by the photo-excited spin-aligned excitons. Our findings suggest a novel mechanism for the effective optical control of spins in a semimagnetic semiconductor, which is associated with a multi-exciton system and its localized state.

Photovoltaic Effect of Atomtronics Induced by an Artificial Gauge Field

We investigate the photovoltaic effect of atomtronics induced by an artificial gauge field in four optical potentials. Under an effective magnetic flux, the atom occupation probability would be polarized in a double-dot system, which gives rise to an atomic current. The relation between the atomic current and magnetic flux behaves like the current-phase property in a Josephson junction. A neutral particle photovoltaic cell is well defined by the atomic opened system that has an effective voltage and two different poles corresponding to two internal states of atomtronics. The atom flow is controllable by tuning the direction of incident light and other parameters. The detection of the atomic current intensity is available through an optical emission spectrum in experiments.

Mechanisms of optical orientation of an individual Mn2+ ion spin in a II-VI quantum dot

We provide a theoretical description of the optical orientation of a single Mn2+ ion spin under quasi-resonant excitation demonstrated experimentally by Goryca et al (2009 Phys. Rev. Lett. 103 087401). We build and analyze a hierarchy of models by starting with the simplest assumptions (transfer of perfectly spin-polarized excitons from Mn-free dot to the other dot containing a single Mn2+ spin, followed by radiative recombination) and subsequently adding more features, such as spin relaxation of electrons and holes. Particular attention is paid to the role of the influx of the dark excitons and the process of biexciton formation, which are shown to contribute significantly to the orientation process in the quasi-resonant excitation case. Analyzed scenarios show how multiple features of the excitonic complexes in magnetically-doped quantum dots, such as the values of exchange integrals, spin relaxation times, etc, lead to a plethora of optical orientation processes, characterized by distinct dependencies on light polarization and laser intensity, and occurring on distinct timescales. Comparison with experimental data shows that the correct description of the optical orientation mechanism requires taking into account Mn2+ spin-flip processes occurring not only when the exciton is already in the orbital ground state of the light-emitting dot, but also those that happen during the exciton transfer from high-energy states to the ground state. Inspired by the experimental results on energy relaxation of electrons and holes in nonmagnetic dots, we focus on the process of biexciton creation allowed by mutual spin-flip of an electron and the Mn2+ spin, and we show that by including it in the model, we obtain good qualitative and quantitative agreement with the experimental data on quasi-resonantly driven Mn2+ spin orientation.

Growth and optical investigations of high quality individual CdTe/(Cd,Mg)Te core/shell nanowires

CdTe nanowires with the average diameter of only 40 nm coated with (Cd,Mg)Te shells are grown using Au-catalyzed vapor-liquid-solid growth mechanism in a system for molecular beam epitaxy. High optical quality of individual nanowires is revealed by means of low temperature cathodoluminescence and micro-luminescence. It is found that, the optical emission spectrum consists mostly of the near band edge emission without any significant contribution of defect related luminescence. Moreover, the importance of surface passivation with (Cd,Mg)Te coating shells is demonstrated.

Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped with Individual Mn2+ Impurities

Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions-down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn²⁺ ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures.

Optically controlled spin-polarization memory effect on Mn delta-doped heterostructures

We investigated the dynamics of the interaction between spin-polarized photo-created carriers and Mn ions on InGaAs/GaAs: Mn structures. The carriers are confined in an InGaAs quantum well and the Mn ions come from a Mn delta-layer grown at the GaAs barrier close to the well. Even though the carriers and the Mn ions are spatially separated, the interaction between them is demonstrated by time-resolved spin-polarized photoluminescence measurements. Using a pre-pulse laser excitation with an opposite circular-polarization clearly reduces the polarization degree of the quantum-well emission for samples where a strong magnetic interaction is observed. The results demonstrate that the Mn ions act as a spin-memory that can be optically controlled by the polarization of the photocreated carriers. On the other hand, the spin-polarized Mn ions also affect the spin-polarization of the subsequently created carriers as observed by their spin relaxation time. These effects fade away with increasing time delays between the pulses as well as with increasing temperatures.

Magnetic ground state of an individual Fe(2+) ion in strained semiconductor nanostructure

Single impurities with nonzero spin and multiple ground states offer a degree of freedom that can be utilized to store the quantum information. However, Fe(2+) dopant is known for having a single nondegenerate ground state in the bulk host semiconductors and thus is of little use for spintronic applications. Here we show that the well-established picture of Fe(2+) spin configuration can be modified by subjecting the Fe(2+) ion to high strain, for example, produced by lattice mismatched epitaxial nanostructures. Our analysis reveals that high strain induces qualitative change in the ion energy spectrum and results in nearly doubly degenerate ground state with spin projection Sz= ± 2. We provide an experimental proof of this concept using a new system: a strained epitaxial quantum dot containing individual Fe(2+) ion. Magnetic character of the Fe(2+) ground state in a CdSe/ZnSe dot is revealed in photoluminescence experiments by exploiting a coupling between a confined exciton and the single-iron impurity. We also demonstrate that the Fe(2+) spin can be oriented by spin-polarized excitons, which opens a possibility of using it as an optically controllable two-level system free of nuclear spin fluctuations.

Designing quantum dots for solotronics

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

Single-atom dopants embedded in a semiconductor matrix are of potential use for optical, spintronics as well as information storage applications. Here, Kobak et al. realize CdTe and CdSe quantum dots with single cobalt and manganese ions and show how the quantum dot design influences single-spin relaxation time.

Coherent Precession of an Individual 5/2 Spin

We present direct observation of a coherent spin precession of an individual Mn^{2+} ion, having both electronic and nuclear spins equal to 5/2, embedded in a CdTe quantum dot and placed in a magnetic field. The spin state evolution is probed in a time-resolved pump-probe measurement of absorption of the single dot. The experiment reveals subtle details of the large-spin coherent dynamics, such as nonsinusoidal evolution of states occupation, and beatings caused by the strain-induced differences in energy levels separation. Sensitivity of the large-spin impurity on the crystal strain opens the possibility of using it as a local strain probe.

Ultranarrow and widely tunable Mn2+-Induced photoluminescence from single Mn-doped nanocrystals of ZnS-CdS alloys

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence over a narrow energy window of width ≤150  meV in the orange-red region and a surprisingly large spectral width (≥180  meV), contrary to its presumed atomic-like origin. Carrying out emission measurements on individual single nanocrystals and supported by ab initio calculations, we show that Mn PL emission, in fact, can (i) vary over a much wider range (∼370  meV) covering the deep green--deep red region and (ii) exhibit widths substantially lower (∼60-75  meV) than reported so far, opening newer application possibilities and requiring a fundamental shift in our perception of the emission from Mn-doped semiconductor nanocrystals.

Optical pump-probe detection of manganese hyperfine beats in (Cd,Mn)Te crystals

Optical pump-probe experiments reveal spin beats of manganese ions in (Cd,Mn)Te, due to hyperfine and crystal fields. At "magic" orientations of the magnetic field, the effect of local crystal field is strongly suppressed. In this case, the spin precession of Mn(2+) embedded in the lattice approaches the precession expected for the free ion. Following optical excitation, regular spin pulses show up, revealing the one-to-one correspondence between precession frequency and Mn(2+) nuclear spin state. The period of the spin pulses accurately determines the hyperfine constant |A|=705  neV. The manganese spin coherence time up to T(2)(Mn)≃15  ns is measured for a manganese concentration x=0.0011.

Diluted magnetic semiconductors. Copper joins the family

Magnetic circular dichroism on quantum dots doped with a small percentage of copper ions show the typical features of diluted magnetic semiconductors.

Quantum interference in exciton-Mn spin interactions in a CdTe semiconductor quantum dot

We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model, including electron-valence-hole correlations, the short- and long-range exchange interaction of a Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how the design of the electronic levels of a quantum dot enables the design of an exciton, control of the quantum interference, and hence engineering of light-Mn interaction.

Optical pumping and a nondestructive readout of a single magnetic impurity spin in an InAs/GaAs quantum dot

We report on the resonant optical pumping of the | ± 1⟩ spin states of a single Mn dopant in an InAs/GaAs quantum dot which is embedded in a charge tunable 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 X(0) energy. For a specific gate voltage, the redshifted photoluminescence of the charged exciton X- is observed, which allows a nondestructive readout of the spin polarization. An arbitrary spin preparation in the | + 1⟩ or |-1⟩ state characterized by a polarization near or above 50% is evidenced.

Monolithic ZnTe-based pillar microcavities containing CdTe quantum dots

Micropillars of different diameters have been prepared by focused ion beam milling out of a planar ZnTe-based cavity. The monolithic epitaxial structure, deposited on a GaAs substrate, contains CdTe quantum dots embedded in a ZnTe λ-cavity delimited by two distributed Bragg reflectors (DBRs). The high refractive index material of the DBR structure is ZnTe, while for the low index material a short-period triple MgTe/ZnTe/MgSe superlattice is used. The CdTe quantum dots are formed by a novel Zn-induced formation process and are investigated by scanning transmission electron microscopy. Micro-photoluminescence measurements show discrete optical modes for the pillars, in good agreement with calculations based on a vectorial transfer matrix method. The measured quality factor reaches a value of 3100.

Spin-spin indirect interaction at low-energy excitation in zero-dimensional cavities

We solve the low-energy part of the spectrum of a model that describes a circularly polarized cavity mode strongly coupled to two exciton modes, each of which is coupled to a localized spin of arbitrary magnitude. In the regime in which the excitons and the cavity modes are strongly coupled, forming polaritons, the low-energy part of the spectrum can be described by an effective spin model, which contains a magnetic field, an axial anisotropy, and an Ising interaction between the localized spins. For detunings such that the low-energy states are dominated by nearly degenerate excitonic modes, the description of the low-energy states by a simple effective Hamiltonian ceases to be valid and the effective interaction tends to vanish. Finally, we discuss a possible application to two-qubit quantum computing operations in a system of transition-metal impurities embedded in quantum dots inside a micropillar.

Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100  mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.

Spin-transfer torque on a single magnetic adatom

We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunneling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunneling. By changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunneling conductance.