A detailed study of the $g$-factor anisotropy of electrons and holes in InAs/${\mathrm{In}}_{0.53}{\mathrm{Al}}_{0.24}{\mathrm{Ga}}_{0.23}\mathrm{As}$ self-assembled quantum dots emitting in the telecom spectral range of $1.5–1.6\mu \mathrm{m}$ (around 0.8 eV photon energy) is performed by time-resolved pump-probe ellipticity technique using a superconducting vector magnet. All components of the $g$-factor tensors are measured, including their spread in the quantum dot (QD) ensemble. Surprisingly, the electron $g$ factor shows a large anisotropy changing from $g_{\mathrm{e},x}=-1.63$ to $g_{\mathrm{e},z}=-2.52$ between directions perpendicular and parallel to the dot growth axis, respectively, at an energy of 0.82 eV. The hole $g$-factor anisotropy at this energy is even stronger: $|g_{\text{h},x}|=0.64$ and $|g_{\text{h},z}|=2.29$. On the other hand, the in-plane anisotropies of electron and hole $g$ factors are small. The pronounced out-of-plane anisotropy is also observed for the spread of the $g$ factors, determined from the spin dephasing time. The hole longitudinal $g$ factors are described with a theoretical model that allows us to estimate the QD parameters. We find that the QD height-to-diameter ratio increases while the indium composition decreases with increasing QD emission energy.

We present results of optical experiments and theoretical analysis on the high-quality single-layer MoS₂ which reveal the fine structure of charged excitons, i.e., trions. In the emission spectra we resolve and identify two trion peaks, T₁ and T₂, resembling the pair of singlet and triplet trion peaks (T_S and T_T) in tungsten-based materials. However, in polarization-dependent photoluminescence measurements we identify these peaks as intra- and inter-valley singlet trions due to the trion fine structure distinct from that already known in bright and dark 2D materials with large conduction-band splitting induced by the spin-orbit coupling. We show that the trion energy splitting in MoS₂ is a sensitive probe of inter- and intra-valley carrier interaction. With additional support from theory we claim that the existence of these singlet trions combined with an anomalous excitonic g-factor together suggest that monolayer MoS₂ has a dark excitonic ground state, despite having "bright" singleparticle arrangement of spin-polarized conduction bands.

By comparing the worn and untouched locations of a tungsten-carbide/carbon surface of a dry-running twin-screw rotor, we demonstrate that tungsten-oxide Raman modes become observable only at worn locations and the integral intensity of the Raman line at 680 cm−1, which is related to the incipient oxidation of the tungsten-carbide stretching mode, is enhanced. Its frequency and width moreover change significantly, thus indicating the mechanical distortion of the bonding that has been occurred during the wearing process. The shape of the tungsten-oxide Raman lines, resembling the Voigt function, hints at a surface morphology that is a characteristic for an amorphous solid environment. Our Raman scattering results may be exploited to characterize the degree of wear of coated surfaces and to identify signatures of a tribological layer.

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe₂/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′₁ (WSe₂) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe₂ monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe₂ monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX⁰ at double resonance condition is observed for the first time.

Copper metaborate ${\mathrm{CuB}}_2{\mathrm{O}}_4$ crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below $T_{\text{N1}}=20$ K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic ${\mathrm{Cu}}^{2+}$ ($3d^9$, $S=1/2$) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the ${\mathrm{Cu}}^{2+}$ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split $d\text{−}d$ electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range $1.7–40$ K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets.

The electron g factor in an ensemble of InAs/InP quantum dots with emission wavelengths around 1.4 μm is measured using time-resolved pump-probe Faraday rotation spectroscopy in different magnetic field orientations. Thereby, we can extend recent single dot photoluminescence measurements significantly towards lower optical transition energies through 0.86 eV. This allows us to obtain detailed insight into the dispersion of the recently discovered g factor anisotropy in these infrared emitting quantum dots. We find with decreasing transition energy over a range of 50 meV a strong enhancement of the g factor difference between magnetic field normal and along the dot growth axis, namely, from 1 to 1.7. We argue that the g factor cannot be solely determined by the confinement energy, but the dot asymmetry underlying this anisotropy therefore has to increase with increasing dot size.