In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial—tens of teslas or more—due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer $${{\rm{MoS}}}_{2},{{\rm{MoSe}}}_{2},{{\rm{MoTe}}}_{2}$$ MoS 2 , MoSe 2 , MoTe 2 , and $${{\rm{WS}}}_{2}$$ WS 2 in very high magnetic fields to 91 T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton’s $$1s$$ 1 s ground state but also its excited $$2s,3s,\ldots ,ns$$ 2 s , 3 s , … , n s Rydberg states. This provides a direct experimental measure of the effective (reduced) exciton masses and dielectric properties. Exciton binding energies, exciton radii, and free-particle bandgaps are also determined. The measured exciton masses are heavier than theoretically predicted, especially for Mo-based monolayers. These results provide essential and quantitative parameters for the rational design of opto-electronic van der Waals heterostructures incorporating 2D semiconductors.

A system of two coupled CdTe quantum dots, one of them containing a single Mn ion, was studied in continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information in the spin state of the Mn ion has been demonstrated, using orientation of the Mn spin by spin-polarized carriers transferred from the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns were measured, depending on the excitation power. Storage time of the information in the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly static and dynamical properties of the system.One of important research directions that may influence the future of information processing, especially of spintronics [1], is focused on physical phenomena occurring in nanoscale-size quantum objects. One of such objects, close to the ultimate limit of information storage miniaturization, is a single Mn atom in a semiconductor quantum dot (QD) [2,3]. After intensive studies of semimagnetic QD containing many magnetic ions [4,5,6,7], single Mn atoms in CdTe [8] and InAs [9] QDs have been observed in photoluminescence (PL) experiments. Many experiments supplied substantial knowledge on physical properties of single Mn atoms, especially in CdTe QDs. In particular, they revealed a strong influence of the position of the Mn atom in the QD, reflecting the symmetry of the system [10]. They demonstrated an efficient optical read-out of the Mn spin state [8]. Furthermore, the dynamics of this state has been studied in photon correlation experiments [11], revealing an important influence of photo-created carriers on Mn spin relaxation. The writing and storing of the information in the Mn spin state has received less attention so far. These issues represent the focus of the present work.In particular, we demonstrate optical writing of information in the spin state of a single Mn ion and we test the stability of this state in the time range up to 0.2 ms.CdTe QDs containing single Mn ions were grown by molecular beam epitaxy. A single layer of self-assembled QDs was deposited in a ZnTe matrix. Manganese was added by briefly opening the Mn effusion cell during the growth of the CdTe layer [12]. The opening time and the Mn flux were adjusted to achieve a large probability of growth of QDs with a single Mn ion in each dot. The selection of single QDs was done by spatial limitation of PL excitation and detection to an area smaller than 0.5 micrometer in diameter, with microscope objective immersed in pumped liquid helium. Continuous wave excitation was used either above the ZnTe barrier gap (at 457 nm) or by a tunable dye laser in the range 570 -600 nm. Well separated photoluminescence lines from individual QDs were observed in the low energy part of the PL spectrum. We were able to select numerous lines showing a PL pattern characteristic for the presence of a...

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.

In monolayers of semiconducting transition metal dichalcogenides, the light helicity (σ þ or σ − ) is locked to the valley degree of freedom, leading to the possibility of optical initialization of distinct valley populations. However, an extremely rapid valley pseudospin relaxation (at the time scale of picoseconds) occurring for optically bright (electric-dipole active) excitons imposes some limitations on the development of opto-valleytronics. Here, we show that valley pseudospin relaxation of excitons can be significantly suppressed in a WSe 2 monolayer, a direct-gap two-dimensional semiconductor with the exciton ground state being optically dark. We demonstrate that the already inefficient relaxation of the exciton pseudospin in such a system can be suppressed even further by the application of a tiny magnetic field of about 100 mT. Time-resolved spectroscopy reveals the pseudospin dynamics to be a two-step relaxation process. An initial decay of the pseudospin occurs at the level of dark excitons on a time scale of 100 ps, which is tunable with a magnetic field. This decay is followed by even longer decay (>1 ns), once the dark excitons form more complex pseudo-particles allowing for their radiative recombination. Our findings of slow valley pseudospin relaxation easily manipulated by the magnetic field open new prospects for engineering the dynamics of the valley pseudospin in transition metal dichalcogenides.

Systematic measurements of auto-and cross-correlations of photons emitted from individual CdTe/ZnTe quantum dots under pulsed excitation were used to elucidate non-resonant excitation mechanisms in this self-assembled system. Memory effects extending over a few excitation pulses have been detected in agreement with previous reports and quantitatively described by a rate equation model, fitting a complete set of correlation and PL intensity results. The important role of single carrier trapping in the quantum dot was established. An explanation was suggested for the unusually wide antibunching dip observed previously in X-X autocorrelation experiments on quantum dots under cw excitation.