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| Magneto-optical spectroscopy of X and X À in a QD with a single Fe 2 þ ion. Magnetic field dependence of the photoluminescence spectrum of a X: (a) experimental data and (b) simulation assuming strain-induced magnetism of the Fe 2 þ ion, as described in the text. The spectra were measured and simulated in s À circular polarization. (c) Schematic field dependence of the initial and final energy levels of the X recombination together with the indicated s À -polarized X optical transitions observed in photoluminescence measurements. The upper pair of levels corresponds to "+ j i exciton coupled with the ion spin (where m and + represent the spin projection of the electron and the heavy hole on the growth axis, respectively), while the bottom pair represents the energies of the ion states in the empty dot. The excitonic transitions preserving (altering) the ion spin projection are marked with solid (dashed) arrows. (d) Magnetic field evolution of a X À photoluminescence spectrum measured in s À circular polarization. Red and orange dashed lines indicate magnetic field values B ¼ 3A h /(2g Fe m B ) and B ¼ A e /(2g Fe m B ), respectively, which correspond to the end points of the cross-like feature in X À magneto-photoluminescence. (e) Histogram of experimentally determined ratio of ion-electron to ion-hole exchange integrals for the Fe 2 þ in a QD. Dashed line indicates the ratio |a/b| of s-d and p-d exchange constants known from the bulk Cd 1 À x Fe x Se.
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Single impurities with nonzero spin and multiple ground states offer a degree of freedom that can be utilized to store the quantum information. However, Fe2+ 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 pic...
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... þ ion. In order to provide the final proof of the magnetic character of the Fe 2 þ ion in a QD, we measured the evolution of the X photoluminescence spectrum in external magnetic field applied along the growth direction (quantization axis of the magnetic ion and QD excitons). Typical results obtained in s À polarization of detection are shown in Fig. 3a. We note that the observed pattern is quite similar to the one obtained for InAs/GaAs QD containing a complex of a single Mn 2 þ ion and a bound hole 27,31,39,52,53 , despite different electronic and spin ...
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... results in Fig. 3a seem complex, however, they originate from quite simple behaviour of the initial and final energy levels of the transitions, as illustrated in Fig. 3c. First effect of the magnetic field is the Zeeman splitting between S z ¼ 2 and S z ¼ À 2 states of the Fe 2 þ ion. Unfortunately, the photoluminescence spectrum does not show this ...
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... results in Fig. 3a seem complex, however, they originate from quite simple behaviour of the initial and final energy levels of the transitions, as illustrated in Fig. 3c. First effect of the magnetic field is the Zeeman splitting between S z ¼ 2 and S z ¼ À 2 states of the Fe 2 þ ion. Unfortunately, the photoluminescence spectrum does not show this splitting directly, since, in general, exciton recombination does not affect the ion spin state and thus the energy of emitted photon does not depend on the ...
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... spectrum does not show this splitting directly, since, in general, exciton recombination does not affect the ion spin state and thus the energy of emitted photon does not depend on the ion Zeeman splitting. However, in the vicinity of the level anticrossings the Fe 2 þ spin states are mixed and this selection rule is relaxed. Indeed, data in Fig. 3a feature several weaker lines in the anticrossing range (that is, 0-2 T). Before we discuss the origin of the anticrossings, let us analyse the behaviour of these weak lines, in particular the cross-like feature. The two Spin-orbit ...
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... corresponds to g-factor g Fe ¼ 2.0, exactly as expected for the Fe 2 þ spin. Let us now focus on the nature of the observed anticrossings. The first, relatively weak anticrossing occurs around 0 T. It is a signature of the fact that the S z ¼ ±2 states of the Fe 2 þ ion are not perfectly degenerate, but are split by a small energy a, as shown in Fig. 3c. This splitting varies between different studied dots and its typical value yields about 50 meV. Consequently, this splitting is much smaller than the X-Fe 2 þ exchange or Zeeman energy above 1 T. Such a splitting arises because of the spin-orbit coupling, acting either in the fourth order, or lower orders in the presence of an ...
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... exchange field of this exciton increases the energy of the state corresponding to S z ¼ À 2 ion spin projection (Fig. 3c), the anticrossing of the Fe 2 þ ion is effectively shifted from 0 T to a higher ...
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... in Fig. 3b, such a model reproduces all features of the experimental results. The simulation correctly captures even the observed thermalization of the ion spin at increasing magnetic field by taking into account the effective Fe 2 þ spin temperature of 15 K. Such a good overall agreement provides a strong proof of correct identification of all ...
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... the X to probe the properties of the Fe 2 þ ion. To determine both exchange integrals between the ion and each confined carrier (the electron and the hole), one needs to probe the Fe 2 þ ion using single carriers. Experimentally, it is realized with a negatively charged exciton (refs 27,53), the magneto-photoluminescence of which is presented in Fig. 3d. In the initial state of X À recombination the Fe 2 þ interacts only with the hole (owing to spin-pairing of the two electrons), while in the final state the ion interacts only with the remaining electron. As such, the Fe 2 þ ion experiences an exchange field in both of these states, which reduces a mixing between the Fe 2 þ spin ...
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... such, the Fe 2 þ ion experiences an exchange field in both of these states, which reduces a mixing between the Fe 2 þ spin states S z ¼ ±2 at B ¼ 0 T. To bring these two spin states into an anticrossing, one needs to apply a magnetic field, which compensates either ion-hole or ion-electron exchange interaction (in the initial or final state of the X À optical transitions). Such values of the magnetic field correspond to the end points of the cross-like feature in X À magneto-photolumi- nescence, which is thus shifted with respect to previously considered case of the X, as seen in Figs 3a,d. Indeed, the cross-pattern for the X begins at zero magnetic field, while the end points of this pattern for the X À are given by B ¼ A e /(2g Fe m B ) and B ¼ 3A h /(2g Fe m B ), where A e and 3A h denote the ion-electron and ion-hole exchange integrals, respectively. ...
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... e /(2g Fe m B ) and B ¼ 3A h /(2g Fe m B ), where A e and 3A h denote the ion-electron and ion-hole exchange integrals, respectively. Both of these integrals might be thus directly determined based on the magnetic field dependence of the X À photoluminescence spectrum. The results of such an analysis performed for a number of QDs are presented in Fig. 3e. They evidence that ion- hole exchange constant clearly dominates over one of ion- electron interaction. It can be directly related to the differences in values of bulk Cd 1 À x Fe x Se s-d and p-d exchange constants 55 N 0 a ¼ 0.26 eV and N 0 b ¼ À 1.53 eV. In particular, the ratio A e / 3A h should be equal to ratio a/b if only the ...
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... be directly related to the differences in values of bulk Cd 1 À x Fe x Se s-d and p-d exchange constants 55 N 0 a ¼ 0.26 eV and N 0 b ¼ À 1.53 eV. In particular, the ratio A e / 3A h should be equal to ratio a/b if only the local densities of electron and hole wave functions at the Fe 2 þ site in the QD are equal. Centring of the results in the Fig. 3e around a marked a/b ratio is fully consistent with this ...
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... of magneto-photoluminescence. The X magneto-photoluminescence from Fig. 3a is quantitatively described by a model of a X inside a QD with a single Fe 2 þ ion. Our simulation is based on the standard procedure of finding the eigenstates of the exciton complex and the empty (that is, without the exciton) QD and subsequent calculation of allowed optical transitions. For simplicity, we assume that the spatial ...
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... The g z factor values determined by Kittel's formula are shown in figure 4(d) as a function of field strength with a linear regression, predicting a zero-field g-factor along the z-axis of the thin film to be g z = 2.26. A typical value for a metal ion with six electrons in the d-orbital is g z = 2.20, while previously reported g factor values for Fe(II) range from 2.2 to 2.3 [3,44,45]. The g factor for a Fe(II) ion deviates from the spin-only value of g = 2.00 due to the presence of a small amount of orbital moments. ...
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... 36,37 To bring the benefits of quantum optics and related tools to solotronics, one first needs to introduce the dopant ion into the QD 38−40 and enclose it within a photonic structure to enhance the light−matter coupling. 41,42 Recently, this requirement was fulfilled by molecular beam epitaxy (MBE) of the II−VI semiconductor CdTe, nowadays offering QD systems hosting various magnetic ions 37,43,44 and reliable fabrication of optical microcavities. 45 Nevertheless, the progress in coherent spectroscopy of single excitons in CdTe QDs has been laborious 22,42 due to the restricted availability of femtosecond laser sources emitting in the visible range. ...
... For such purposes, transition-metal (TM) doped semiconducting crystals have for a long time been recognized as relevant candidates. Depending on site symmetry and 3d-electrons number, these embedded TMs can bear a local electronic high-spin state, laying within the energy band gap, below the Fermi level [11][12][13]. The required temperature for observing such states mainly depends on the spin-lattice relaxation, for which embedded manganese and cobalt ions offer weak-and strong-coupling examples, respectively resulting in high and low working temperatures. ...
Cobalt-doped ZnO nanoparticles (NPs) with different Co concentrations are investigated by means of Xand
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... 36,37 To bring the benefits of quantum optics and related tools to solotronics, one first needs to introduce the dopant ion into the QD [38][39][40] and enclose it within a photonic structure to enhance the light-matter coupling. 41,42 Recently, this requirement was fulfilled by molecular beam epitaxy (MBE) of the II-VI semiconductor CdTe, nowadays offering QD systems hosting various magnetic ions 37,43,44 and reliable fabrication of optical microcavities. 45 Nevertheless, the progress in coherent spectroscopy of single excitons in CdTe QDs has been laborious, 22,42 due to the restricted availability of femtosecond laser sources emitting in the visible range. ...
For future quantum technologies the combination of a long quantum state lifetime and an efficient interface with external optical excitation are required. In solids, the former is for example achieved by individual spins, while the latter is found in semiconducting artificial atoms combined with modern photonic structures. One possible combination of the two aspects is reached by doping a single quantum dot, providing a strong excitonic dipole, with a magnetic ion, that incorporates a characteristic spin texture. Here, we perform four-wave mixing spectroscopy to study the system's quantum coherence properties. We characterize the optical properties of the undoped CdTe quantum dot and find a strong photon echo formation which demonstrates a significant inhomogeneous spectral broadening. Incorporating the Mn$^{2+}$ ion introduces its spin-5/2 texture to the optical spectra via the exchange interaction, manifesting as six individual spectral lines in the coherent response. The random flips of the Mn-spin result in a special type of spectral wandering between the six transition energies, which is fundamentally different from the quasi-continuous spectral wandering that results in the Gaussian inhomogeneous broadening. Here, the discrete spin-ensemble manifests in additional dephasing and oscillation dynamics.
... 40 More recently, a controllable occupation transfer between bright and dark excitons in a QD-cavity system was suggested. 41 Our method facilitates QDs with a single magnetic dopant, which can be deterministically fabricated for several years, 42 with dopand atoms like Manganese (Mn), 43,44 Cromium, 45 Iron, 46 or Cobalt. 47 Interestingly, the spin state of the dopant can be changed via optical control. ...
Light trapping is a crucial mechanism for synchronization in optical communication. Especially on the level of single photons, control of the exact emission time is desirable. In this paper, we theoretically propose a single-photon buffering device composed of a quantum dot doped with a single Mn atom in a cavity. We present a method to detain a single cavity photon as an excitation of the dot. The storage scheme is based on bright to dark exciton conversion performed with an off-resonant external optical field and mediated via a spin-flip with the magnetic ion. The induced Stark shift brings both exciton states to resonance and results in an excitation transfer to the optically inactive one. The stored photon can be read out on demand in the same manner by repopulating the bright state, which has a short lifetime. Our results indicate the possibility to suspend a photon for almost two orders of magnitude longer than the lifetime of the bright exciton.
... 5,6 However, the spin of the Fe dopant is very sensitive to the local environment, and CdS nanocrystals (NCs) have the potential to strongly hybridize with magnetic ions, beyond the classical description of diluted magnetic semiconductors. 5,7 Until now, however, the power of spatial confinement of the wave function as well as the role of dopant−host interactions, free of dopant−dopant interactions, have not been explored. Here we combine ultrafast transient absorption spectroscopy and density functional theory within the ground and excited states to demonstrate that the uneven interaction of the two spin states for uniformly doped Fe 2+ ions in spatially confined CdS NCs produces two magnetically inequivalent excitonic states upon mild optical perturbation. ...
Fe 2+ doping in II-VI semiconductors, due to the absence of energetically accessible multiple spin state configurations, has not given rise to interesting spintronic applications. In this work, we demonstrate for the first time that the interaction of homogeneously doped Fe 2+ ions with the host CdS nanocrystal with no clustering is different for the two spin states and produces two magnetically inequivalent excitonic states upon optical perturbation. We combine ultrafast transient absorption spectroscopy and density functional theoretical analysis within the ground and excited states to demonstrate the presence of the magneto-optical Stark effect (MOSE). The energy gap between the spin states arising due to MOSE does not decay within the time frame of observation, unlike optical and electrical Stark shifts. This demonstration provides a stepping-stone for spin-dependent applications.
... II-VI semiconductors can be doped with transition metal magnetic elements offering a large choice of localized spins. Individual magnetic atoms can be optically probed and to some extent controlled when they are incorporated in a quantum dot (QD) [8][9][10][11][12][13] . Among these magnetic elements, atoms featuring both spin and orbital degrees of freedom present a large spin-to-strain coupling. ...
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... II-VI semiconductors can be doped with transition metal magnetic elements offering a large choice of localized spins. Individual magnetic atoms can be optically probed and to some extent controlled when they are incorporated in a quantum dot (QD) [8][9][10][11][12][13] . Among these magnetic elements, atoms featuring both spin and orbital degrees of freedom present a large spin-to-strain coupling. ...
We demonstrate radio-frequency tuning of the energy of individual CdTe/ZnTe quantum dots (QDs) by Surface Acoustic Waves (SAWs). Despite the very weak piezoelectric coefficient of ZnTe, SAW in the GHz range can be launched on a ZnTe surface using interdigitated transducers deposited on a c-axis oriented ZnO layer grown on ZnTe containing CdTe QDs. The photoluminescence (PL) of individual QDs is used as a nanometer-scale sensor of the acoustic strain field. The energy of QDs is modulated by SAW in the GHz range and leads to characteristic broadening of time-integrated PL spectra. The dynamic modulation of the QD PL energy can also be detected in the time domain using phase-locked time domain spectroscopy. This technique is in particular used for monitoring complex local acoustic fields resulting from the superposition of two or more SAW pulses in a cavity. Under magnetic field, the dynamic spectral tuning of a single QD by SAW can be used to generate single photons with alternating circular polarization controlled in the GHz range.
... The CdTe/ZnTe and CdSe/ZnSe QD systems were selected to demonstrate the application of the elliptical µlenses in the field of optoelectronics based on solitary dopant-solotronics 42,43 . When doped with single magnetic ions, such QDs can be used to efficiently probe and manipulate the spin of the magnetic ion they contain [44][45][46] , which otherwise is not optically active. ...
... In each case, we were able to find a µ-lens printed over a singly doped QD, including a CdTe/ZnTe QD with a single Co 2+ ion (Fig. 4b), a CdTe/ZnTe QD with a single Mn 2+ ion (Fig. 4c), and a CdSe/ZnSe QD with a single Fe 2+ ion (Fig. 4e). For all of them, we clearly observed a characteristic splitting of the exciton line into components corresponding to different spin projections onto the quantisation axis [42][43][44] . In particular, Fig. 4f displays the magnetic-field evolution of the excitonic emission lines highlighted in Fig. 4e Fig. 4 Benefits of using elliptical µ-lenses in the photoluminescence study of single QDs. ...
... More details on the growth procedures of CdTe/ZnSe and CdTe/ZnTe QDs with and without magnetic ions can be found in refs. 40,42,43 . ...
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.
