Nuclear magnetic resonance (NMR) is particularly relevant for studies of internuclear spin coupling at zero and ultra-low fields (ZULF), where spin-spin interactions dominate over Zeeman ones. Here we report on ZULF NMR in CdTe. In this semiconductor all magnetic isotopes have spin $I = 1/2$, so that internuclear interactions are never overshadowed by quadrupole effects. Our experiments rely on warm-up spectroscopy, a technique that combines optical pumping, additional cooling via adiabatic demagnetisation, and detection of the oscillating magnetic field-induced warm-up of the nuclear spin system via Hanle effect. We show that NMR spectra exhibit a rich fine structure, consistent with the low abundance of magnetic isotopes in CdTe, their zero quadrupole moments, as well as direct and indirect interactions between them. A model assuming that the electromagnetic radiation is absorbed by nuclear spin clusters composed of up to 5 magnetic isotopes allows us to reproduce the shape of a major part of the measured spectra.

The spectra of photoluminescence and polarized reflection under normal and oblique incidence of light from structures with quantum wells with symmetric Cd0.9Zn0.1Te/CdTe/Cd0.9Zn0.1Te and asymmetric Cd0.9Zn0.1Te/CdTe/Cd0.4Mg0.6Te barriers have been studied. Due to the mechanical stresses caused by the mismatch of the crystal lattices of the wells and barriers, the energy of light holes in quantum wells was higher than in barriers, i.e., the band structure for them was of type II. However, in the reflection spectra, the lines of heavy and light excitons had comparable intensities. In structures with symmetric barriers, exciton resonances, which do not appear in the photoluminescence spectra, were found in the reflection spectra. A detailed calculation of the energy levels and reflection spectra has been carried out.

The nuclear spin systems in CdTe/(Cd,Zn)Te and CdTe/(Cd,Mg)Te quantum wells (QW) are studied using a multistage technique combining optical pumping and Hanle effect-based detection. The samples demonstrate drastically different nuclear spin dynamics in zero and weak magnetic fields. In CdTe/(Cd,Zn)Te, the nuclear spin relaxation time is found to strongly increase with the magnetic field, growing from 3 s in zero field to tens of seconds in a field of 25 G. In CdTe/(Cd,Mg)Te the relaxation is an order of magnitude slower, and it is field-independent up to at least 70 G. The differences are attributed to the nuclear spin relaxation being mediated by different kinds of resident electrons in these QWs. In CdTe/(Cd,Mg)Te, a residual electron gas trapped in the QW largely determines the relaxation dynamics. In CdTe/(Cd,Zn)Te, the fast relaxation in zero field is due to interaction with localized donor-bound electrons. Nuclear spin diffusion barriers form around neutral donors when the external magnetic field exceeds the local nuclear field, which is about $B_L\approx $0.4 G in CdTe. This inhibits nuclear spin diffusion towards the donors, slowing down relaxation. These findings are supported by theoretical modeling. In particular, we show that the formation of the diffusion barrier is made possible by several features specific to CdTe: (i) the large donor binding energy (about 10 meV), (ii) the low abundance of magnetic isotopes (only $\approx$30% of nuclei have nonzero spin), and (iii) the absence of nuclear quadrupole interactions between nuclei. The two latter properties are also favorable to nuclear spin cooling via optical pumping followed by adiabatic demagnetization. Under non-optimized conditions we have reached sub-microkelvin nuclear spin temperatures in both samples, lower than all previous results obtained in GaAs.

The spectra of photoluminescence and polarized reflection under normal and oblique incidence of light from structures with quantum wells with symmetric Cd0.9Zn0.1Te/CdTe/Cd0.9Zn0.1Te and asymmetric Cd0.9Zn0.1Te/CdTe/Cd0.4Mg0.6Te barriers have been studied. Due to the mechanical stresses caused by the mismatch of the crystal lattices of the wells and barriers, the energy of light holes in quantum wells was higher than in barriers, i. e., the zone structure for them was of type II. However, in the reflection spectra, the lines of heavy and light excitons had comparable intensities. In structures with symmetric barriers, exciton resonances, which do not appear in the photoluminescence spectra, were found in the reflection spectra. A detailed calculation of the energy levels and reflection spectra has been carried out.

The spectra of polarized reflection from a structure with a single quantum well and asymmetric Cd 0.9 Zn 0.1 Te/CdTe/Cd 0.4 Mg 0.6 Te barriers are studied in this work. The Stokes parameters of reflected light were measured. In the structure with asymmetric barriers under study in the region of exciton resonances, the phenomenon of light birefringence caused by a reduced symmetry of the interfaces compared to the bulk was found.

Photoluminescence and polarized reflection spectra of quantum well structures with symmetric Cd0.9Zn0.1Te/CdTe/Cd0.9Zn0.1Te and asymmetric Cd0.9Zn0.1Te/CdTe/Cd0.4Mg0.6Te barriers were studied. The Stokes parameters of the reflected light from these structures were measured. In structures with asymmetric barriers, in the region of exciton resonances, the phenomenon of light birefringence was detected, caused by a lower symmetry of the interfaces compared with the symmetry of bulk crystals. A discussion of this phenomenon is given.

Exciton energy structure and population dynamics in a wide CdTe/CdZnTe quantum well are studied by spectrally-resolved pump-probe spectroscopy. Multiple excitonic resonances in reflectance spectra are observed and identified by solving numerically three-dimensional Schr\"odinger equation. The pump-probe reflectivity signal is shown to be dominated by the photoinduced nonradiative broadening of the excitonic resonances, while pump-induced exciton energy shift and reduction of the oscillator strength appear to be negligible. This broadening is induced by the reservoir of dark excitons with large in-plane wave vector, which are coupled to the the bright excitons states. The dynamics of the pump-induced nonradiative broadening observed experimentally is characterised by three components: signal build up on the scale of tens of picoseconds (i) and bi-exponential decay on the scale of one nanosecond (ii) and ten nanosecons (iii). Possible mechanisms of the reservoir population and depletion responsible for this behaviour are discussed.

Photoluminescence and polarized reflection spectra of quantum well structures with symmetric Cd$_{0.9}$Zn$_{0.1}$Te/CdTe/Cd$_{0.9}$Zn$_{0.1}$Te and asymmetric Cd$_{0.9}$Zn$_{0.1}$Te/CdTe/Cd$_{0.4}$Mg$_{0.6}$Te barriers were studied. The Stokes parameters of the reflected light from these structures were measured. In the structures with symmetric barriers, exciton resonances were found in the reflection spectra and were not present in the photoluminescence spectra. In structures with asymmetric barriers, in the region of exciton resonances, the phenomenon of light birefringence was detected, caused by a lower symmetry of the interfaces compared to the symmetry of bulk crystals. A discussion of both phenomena was given.

Motional narrowing is a phenomenon by which a quantum state can be entangled with a noisy environment and still retain its intrinsic coherence. Using two optically induced motional forces driving the environmental electrical field amplitude and fluctuations, we present a compelling illustration of the effects of motional narrowing on the energy, line shape, and line width of a single quantum emitter, a Te2 molecule embedded in ZnSe, subject to spectral diffusion. Motional narrowing is achieved in several regimes, irrespectively of the inhomogeneous disorder initially present and the charge reservoir state sourcing the field. The optimal coherence limit set by the radiative rate can be approached by accelerating spectral diffusion into the THz regime. Motional narrowing applies to any quantum systems for which environmental fluctuations can be deliberately accelerated and alleviates the need for perfected materials and devices.

We report on the precise determination, in Ag2ZnSnSe4 epitaxial layer, of both the band gap E-g and the characteristic Urbach energy U that describes the density of localized, defect states in the gap. Various origins for these defect band tail states have been considered, together with the corresponding modeling for their density of states, in order to fit the whole of the optical spectral data. The interest of the methodology developed here is to account quantitatively not only for the absorption and steady-state photoluminescence data but also for the time-resolved photoluminescence spectra. We compare the different origins of localized band tail states to select the standard textbook, Urbach tail model that corresponds to short-range band gap fluctuations. Such an approach is different from the one most often used to evaluate the energy extent of the localized states, which is the Stokes shift between the energies of the photoluminescence emission and the absorption threshold. The advantage of the present method is that no arbitrary choice of the low power excitation has to be done to select the photoluminescence emission spectrum and its peak energy. Thanks to this systematic study of both photoluminescence excitation and time-resolved photoluminescence spectra at low temperature (6 K), the values E-g = 1226 +/- 5 meV and U = 20 +/- 3 meV are found for this promising absorber for thin films photovoltaics.

The propagation of sidewall steps during the growth of nanowires is calculated in the frame of the Burton-Cabrera-Frank model. The stable shape of the nanowire comprises a cylinder section on top of a cone section: their characteristics are obtained as a function of the radius of the catalyst-nanowire area, the desorption-limited diffusion length of adatoms on the terraces, and the sticking of adatoms at step edges. The comparison with experimental data allows us to evaluate these last two parameters for InP and ZnTe nanowires; it reveals a different behavior for the two materials, related to a difference by an order of magnitude of the desorption-limited diffusion length.

A whole series of complementary studies have been performed on the same, single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive X-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si 3 N 4 membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn,Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of $s$ and $p$ polarized light in vicinity of exciton resonances. We demonstrate that this contribution to the reflection is caused by magnetogyrotropy of quantum wells, i.e. by the terms in the optical response which are linear in both the magnetic field strength and light wavevector. Theory of magnetogyrotropic effects in light reflection is developed with account for linear in momentum BIA induced terms in the electron and hole effective Hamiltonians. Theoretical estimates agree with the experimental findings. We have found the electron BIA splitting constant in both GaAs and CdTe based quantum wells is about three times smaller than that for heavy holes.

We report on ultrafast time-resolved pump-probe studies in a CdZnTe/CdMgTe planar guiding structure covered with a metallic grating. The one-dimensional periodic gold structure allows for efficient coupling into the guiding layer for p-polarized 30 fs optical pulses with a large spectral bandwidth of about 60 nm. The resulting spectral width of optical pulses propagating inside the guiding layer corresponds to 20-30 nm. We demonstrate that the excitation of exciton-polariton modes in the guiding layer leads to a modulation of the optical response in the vicinity of the excitonic resonance. Spatially resolved pump-probe measurements show an asymmetric behavior in the optical response when the relative position of the pump and probe spots is varied on the scale of ten micrometers perpendicular to the metal ridges. This is attributed to the excitation of resonant and off-resonant exciton-polariton modes which propagate in opposite directions inside the guiding layer in accordance with their dispersion relations. Two main mechanisms are considered and evaluated, namely, Pauli blocking and excitation-induced dephasing, which are shown to be responsible for the pump-induced changes in the exciton absorption spectrum. While both of these processes lead to the generation of photoexcited carriers in the guiding layer, their impact on the optical properties (transmission and reflection) are different which leads to the asymmetric behavior of the spatially resolved transients.

Effects of spatial dispersion in quantum wells are discovered and investigated in detail in reflection experiments. We studied oblique incidence of pure s and p polarized light which has been reflected elliptically polarized. The polarization degree of the reflected light is governed by the in‐plane photon momentum which is the distinctive feature of the spatial dispersion effects. The effects of spatial dispersion are allowed by symmetry in inversion‐asymmetric systems only. Therefore we investigated bulk‐inversion asymmetric ZnSe/ZnMgSSe and structure‐asymmetric GaAs/AlGaAs and CdZnTe/CdTe/CdMgTe quantum wells. We studied the reflected light polarization state in the vicinity of the heavy‐ and light‐exciton resonances where the spatial dispersion effects are resonantly enhanced.

The role of the sublimation of the compound and of the evaporation of the constituents from the gold nanoparticle during the growth of semiconductor nanowires is exemplified with CdTe-ZnTe heterostructures. Operating close to the upper temperature limit strongly reduces the amount of Cd present in the gold nanoparticle and the density of adatoms on the nanowire sidewalls. As a result, the growth rate is small and strongly temperature dependent, but a good control of the growth conditions allows the incorporation of quantum dots in nanowires with sharp interfaces and adjustable shape, and it minimizes the radial growth and the subsequent formation of additional CdTe clusters on the nanowire sidewalls, as confirmed by photoluminescence. Uncapped CdTe segments dissolve into the gold nanoparticle when interrupting the flux, giving rise to a bulblike (pendant-droplet) shape attributed to the Kirkendall effect.

The correlative use of atom probe tomography (APT) and energy dispersive x-ray spectroscopy in scanning transmission electron microscopy (STEM) allows us to characterize the structure of ZnTe/CdSe superlattices at the nanometre scale. Both techniques reveal the segregation of zinc along [111] stacking faults in CdSe layers, which is interpreted as a manifestation of the Suzuki effect. Quantitative measurements reveal a zinc enrichment around 9 at. % correlated with a depletion of cadmium in the stacking faults. Raw concentration data were corrected so as to account for the limited spatial resolution of both STEM and APT techniques. A simple calculation reveals that the stacking faults are almost saturated in Zn atoms (∼66 at. % of Zn) at the expense of Cd that is depleted.

Abstract—Effects implying violation of the Fresnel light reflection laws in semiconductor structures with quantum wells (QWs) have been investigated. This violation is related to the manifestation of spatial dispersion caused by spin–orbit coupling in structures without inversion centers. The spin–orbit coupling constants characterizing polarization conversion in symmetric and asymmetric structures with QWs have been measured.

Polarization conversion of light reflected from quantum wells governed by both magnetic field and light propagation direction is observed. We demonstrate that the polarization conversion is caused by the magnetospatial dispersion in quantum wells which manifests itself in the reflection coefficient contribution bilinear in the in-plane components of the magnetic field and the light wave vector. The magnetospatial dispersion is shown to arise due to structure inversion asymmetry of the quantum wells. The effect is resonantly enhanced in the vicinity of the heavy-hole exciton. We show that microscopically the magnetospatial dispersion is caused by the mixing of heavy- and light-hole states in the quantum well due to both orbital effect of the magnetic field and the in-plane hole motion. The degree of the structure inversion asymmetry is determined for GaAs/AlGaAs and CdTe quantum wells.

Abstract —Effects implying violation of the Fresnel light reflection laws in semiconductor structures with quantum wells (QWs) have been investigated. This violation is related to the manifestation of spatial dispersion caused by spin–orbit coupling in structures without inversion centers. The spin–orbit coupling constants characterizing polarization conversion in symmetric and asymmetric structures with QWs have been measured.