We investigate relaxation, formation, and recombination processes in monolayer MoSe2 using excitation correlation spectroscopy (ECS). Our findings highlight Auger recombination as the dominant nonradiative recombination path. Based on this observation, we introduce a three-level model to extract the effective timescale of free carrier relaxation and formation of neutral excitons. We find that free carrier relaxation occurs within 0.4-0.8 ps, depending on the excess energy under above-bandgap excitation. Extending the analysis to charged excitons, we observe a superlinear rise in charged exciton photoluminescence signal when the two excitation pulses overlap. Asymmetric-pulse ECS combined with gate-controlled carrier tuning resolves the relaxation ladder and demonstrates that charged exciton formation predominantly occurs directly in a high-density free-carrier environment, rather than via capture by thermalized neutral excitons.

We investigate the near-infrared optical response of the semiconducting van der Waals antiferromagnet CrPS4 and identify previously unreported spin-entangled optical resonances. The strong and anisotropic magnetic-field dependence of these resonances reflects the underlying magnetic order and confirms the biaxial antiferromagnetic nature of CrPS4. From the magnetic field evolution of the optical transition, we extract key magnetic parameters, including the spin-flop (≈0.9 T) and spin-saturation (≈8 T) fields. These results demonstrate a potential pathway for all-optical probing of spin states in van der Waals antiferromagnets, with relevance for spin-sensitive optoelectronic and magneto-optical devices.

Monolayers (MLs) of semiconducting transition metal dichalcogenides emit light very efficiently and display rich spin-valley physics, with gyromagnetic (g) factors of about -4. Here, we investigate how these properties can be tailored by alloying. Magneto-optical spectroscopy is used to reveal the peculiar properties of excitonic complexes in Mo_{x}W_{1-x}Se_{2} MLs with different metal concentrations. We show that the alloys feature extremely high g factors for neutral excitons, that change gradually with the composition up to reaching values of the order of -10 for x≈0.2. First-principles calculations quantitatively identify the alloy-induced mixing between different conduction band valleys as the underlying mechanism originating the anomalous composition dependence of the neutral exciton g factor. The theoretical framework also suggests a high strain sensitivity of the alloys, making them promising candidates for tailor-made optoelectronic devices.

Layered van der Waals magnetic semiconductors provide a versatile platform for exploring excitonic phenomena intertwined with spin and lattice degrees of freedom, enabling excitons to act as sensitive probes of magnetic order. CrPS 4 is a layered antiferromagnetic semiconductor that hosts rich excitonic features whose microscopic origin and connection to magnetic ordering remain incompletely understood. Here, we investigate the electronic and excitonic properties of bulk CrPS 4 using a combination of many-body perturbation theory, dynamical mean-field theory, and photoluminescence-based experiments. Our calculations establish CrPS 4 as a direct-gap semiconductor with a bandgap of 2.48 eV in the antiferromagnetic phase. Several subbandgap excitonic transitions are predicted by theory, comprising multiple spin-allowed excitons and an additional spin-flip excitation, predominantly localized on the Cr 3+ ions. Temperature- and magnetic-field-dependent optical measurements reveal thermally driven exciton redistribution among localized states and identify characteristic energy shifts that provide clear optical signatures of magnetic phase transitions in CrPS 4 . These results provide insights into the excitonic transitions of antiferromagnets and suggest potential routes for all-optical sensing and light-driven control of their magnetic order.

Recent research focused on single photon emitters (SPEs) hosted by layered semiconductors, particularly hexagonal boron nitride (hBN), has revealed a promising alternative to quantum dots (QDs) for generating single, indistinguishable photons. hBN-based SPEs offer lower material costs, room temperature emission, and easy integration into potential optoelectronic devices due to the layered structure of the host crystal. This work presents compact hybrid electroluminescence devices, in which GaN laser diodes (LDs) are used for bottom-to-top excitation of hBN nanoflakes deposited on the laser facets. This approach circumvents the issue of electroluminescence generation from hBN and provides access to the SPE's signal without optical driving by an external laser. Using laser diodes upgraded with Bragg reflectors a room-temperature generation of single photons from hBN is confirmed by an 80%-dip in their g(2) second-order correlation. The on-demand emission of single photons at room temperature is demonstrated by driving the laser diodes in pulsed operation, with confidence supported by a measured g(2)(0) value of 0.37.

The Raman scattering response of the biaxial antiferromagnet CoPS3 has been investigated as a function of both magnetic field and temperature. The peaks observed in the low-frequency spectral range (90-200 cm-1) have been identified as hybrid magnon-phonon excitations. The energies of the bare magnon and phonon modes and the effective coupling strengths between different excitation pairs have been determined. The strong and selective magnon-phonon interaction largely accounts for the pronounced splitting of two phononlike modes observed at 152 and 158 cm-1 in the antiferromagnetic phase of CoPS3. Based on the identification of bare magnon excitations and their magnetic-field dependence, we propose an updated set of parameters for the effective exchange (Jeff = 9.9 meV) and biaxial magnetic anisotropy (D = 4.3 meV and E = -0.7 meV) and advocate for an apparent anisotropic g factor (gx = gy = 2, gz = 4) in the CoPS3 antiferromagnet.

Magneto-optical excitations in antiferromagnetic d systems can originate from a multiplicity of light-spin and spin-spin interactions, as the light and spin degrees of freedom can be entangled. This is exemplified in van der Waals systems with attendant strong anisotropy between in-plane and out-of-plane directions, such as MnPS 3 ${\rm MnPS}_3$ and NiPS 3 ${\rm NiPS}_3$ films studied here. The rich interplay between the magnetic ordering and sub-bandgap optical transitions poses a challenge to resolve the mechanisms driving spin-entangled optical transitions, as well as the single-particle bandgap itself. Here, a high-fidelity ab initio theory is applied to find a realistic estimation of the bandgap by elucidating the atom- and orbital-resolved contributions to the fundamental sub-bands. It is further demonstrated that the spin-entangled excitations, observable as photoluminescence and absorption resonances, originate from an on-site spin-flip transition confined to a magnetic atom (Mn or Ni). The evolution of the spin-flip transition in a magnetic field is used to deduce the effective exchange coupling and anisotropy constants.

We present femtosecond pump-probe measurements of neutral and charged exciton optical response in monolayer MoSe2 to resonant photoexcitation of a given exciton state in the presence of 2D electron gas. We show that creation of charged exciton (X-) population in a given K+, K- valley requires the capture of available free carriers in the opposite valley and reduces the interaction of neutral exciton (X) with the electron Fermi sea. We also observe spectral broadening of the X transition line with the increasing X- population caused by efficient scattering and excitation induced dephasing. From the valley-resolved analysis of the observed effects we are able to extract the spin-valley relaxation times of free carriers as a function of carrier density. Moreover, we analyze the oscillator strength and energy shift of X in the regime of interaction with electron Fermi sea under resonant excitation. From this we can observe the process of X decay by radiative recombination paired with trion formation. We demonstrate an increase of neutral exciton relaxation rate with the introduction of Fermi sea of electrons. We ascribe the observed effect to the increased efficiency of the trion formation, as well as the radiative decay caused by the screening of disorder by the free carriers.

We experimentally demonstrate the creation of defects in monolayer WSe2 via nanopillar imprinting and helium ion irradiation. Based on the first method, we realize atomically thin vertical tunneling light-emitting diodes based on WSe2 monolayers hosting quantum emitters at deterministically specified locations. We characterize these emitters by investigating the evolution of their emission spectra in external electric and magnetic fields, as well as by inducing electroluminescence at low temperatures. We identify qualitatively different types of quantum emitters and classify them according to the dominant electron-hole recombination paths, determined by the mechanisms of intervalley mixing occurring in fundamental conduction and/or valence subbands.

The optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe_2 ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (X^D) and grey (X^G) excitons and the negative dark trion (T^D) differ substantially at a particular temperature. More importantly, they vanish considerably by about 3 – 4 orders of magnitude with the temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, i.e. neutral bright exciton (X^B) and spin-singlet (T^S) and spin-triplet (T^T) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X^D and T^D complexes and the corresponding bright X^B, T^S, and T^T ones vary with temperature rises from 4.2 K to 100 K. This can be explained in terms of the different exciton-phonon couplings for the bright and dark excitons stemming from their distinct symmetry properties.

FePSe3 is a collinear honeycomb antiferromagnet with an easy-axis anisotropy and large spins S=2. It belongs to a family of magnetic van der Waals materials, which recently attracted a considerable attention. In this work we present an experimental magneto-optical study of the low-energy excitation spectrum in FePSe3, together with its theoretical description. The observed response contains several types of magnon excitations. Two of them are conventional transverse magnons described by a classical theory of antiferromagnetic resonance. Two other modes are identified as multimagnon hexadecapole excitations with an anomalous g factor approximately equal to four times the g factor of a single Fe^2+ ion. These quasiparticles correspond to full reversals of iron spins that coherently propagate in the up-down antiferromagnetic structure. They constitute a novel type of collective excitations in anisotropic magnetic solids, called longitudinal magnons. Comparison between theory and experiment allows us to estimate the microscopic parameters of FePSe3 including exchange coupling constants and the single-ion anisotropy.

The interaction between condensed matter excitations and electromagnetic cavity fields serves as a rich playground for fundamental research and lies at the core of photonic and quantum technologies. Herein, the intriguing concept of composite states formed by distinct quasiparticles strongly coupled to the same optical cavity modes is experimentally and theoretically demonstrated. Specifically, magnons excited in a slab of an antiferromagnetic crystal and phonons excited in a distinct specimen of an insulating material are explored. The crystal slabs form an optical cavity with Fabry-P & eacute;rot oscillations in the terahertz range. Hybridized phonon-magnon polariton modes and their tunability by adjusting the distance between the slabs, showing that hybridization persists even at separations of up to several millimeters is demonstrated. The experimental results are interpreted using both classical and quantum electrodynamical models. The quantum description allows us to quantify the degree of hybridization linked to a topological behavior of the electric field phasor, in agreement with the classical electrodynamics expectations. The presented results are obtained at room temperature and cavities of millimeter size, paving the way for the engineering of realistic, frequency-tunable THz devices through the hybridization of electric (phononics) and magnetic (spintronics) elementary excitations of matter.

Abstract Magneto-spectroscopy methods have been employed to study the zero-wavevector magnon excitations in MnPSe3. Experiments carried out as a function of temperature and the applied magnetic field show that two low-energy magnon branches of MnPSe3 in its antiferromagnetic phase are gapped. The observation of two low-energy magnon gaps (at 1.70 ± 0.05 meV and 0.09 ± 0.01 meV) implies that MnPSe3 is a biaxial antiferromagnet. A relatively strong out-of-plane anisotropy imposes the spin alignment to be in-plane whereas the spin directionality within the plane is governed by a factor of 2.5 × 10−3 weaker in-plane anisotropy.

Optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe2 ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (XD) and grey (XG) excitons and the negative dark trion (TD) differ substantially at particular temperature. More importantly, they weaken considerably by about 3-4 orders of magnitude with temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, i.e. neutral bright exciton (XB) and spin-singlet (TS) and spin-triplet (TT) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark XD and TD complexes and the corresponding bright XB, TS, and TT ones vary with temperature rises from 4.2 K to 100 K. This is explained in terms of the different exciton-phonon coupling for the bright and dark excitons stemming from their distinct symmetry properties.

We present a time-resolved optical study of recently developed narrow-line MoSe2 monolayers grown on hexagonal boron nitride with means of molecular beam epitaxy. We find that the photoluminescence decay times are significantly shorter than in the case of the exfoliated samples, even below one picosecond. Such a short timescale requires measurements with better resolution than achievable with a streak camera. Therefore, we employ an excitation correlation spectroscopy pump-probe technique. This approach allows us to identify two distinct non-radiative recombination channels attributed to lattice imperfections. The first channel is active at helium temperatures. It reduces the lifetime of the neutral exciton to below one picosecond. The second channel becomes active at elevated temperatures, further shortening the lifetimes of both neutral and charged exciton. The high effectiveness of both radiative and non-radiative recombination makes epitaxial MoSe2 a promising material for ultrafast optoelectronics.

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

Alloying stands out as a pivotal technological method employed across various compounds, be they metallic, magnetic, or semiconducting, serving to fine-tune their properties to meet specific requirements. Ternary semiconductors represent a prominent example of such alloys. They offer fine-tuning of electronic bands, the band gap in particular, thus granting the technology of semiconductor heterostructures devices, key elements in current electronics and optoelectronics. In the realm of magnetically ordered systems, akin to electronic bands in solids, spin waves exhibit characteristic dispersion relations, featuring sizable magnon gaps in many antiferromagnets. The engineering of the magnon gap constitutes a relevant direction in current research on antiferromagnets, aiming to leverage their distinct properties for terahertz technologies, spintronics, or magnonics. In this study, we showcase the tunability of the magnon gap across the terahertz spectral range within an alloy comprising representative semiconducting van der Waals antiferromagnets FePS3 and NiPS3. These constituents share identical in-plane crystal structures, magnetic unit cells, and the direction of the magnetic anisotropy, but differ in the amplitude and sign of the latter. Altogether these attributes result in the wide tunability of the magnon gap in the Fe1-xNixPS3 alloy in which the magnetic order is imposed by the stronger, perpendicular anisotropy of iron.

Semiconducting transition metal dichalcogenides of group VI are well known for their prominent excitonic effects and the transition from an indirect to a direct band gap when reduced to monolayers. While considerable efforts have elucidated the Rydberg series of excitons in monolayers, understanding their properties in multilayers remains incomplete. In these structures, despite an indirect band gap, momentum-direct excitons largely shape the optical response. In this work, we combine magnetoreflectance experiments with theoretical modeling based on the kp approach to investigate the origin of excitonic resonances in WSe2 bi, tri, and quadlayers. For all investigated thicknesses, we observe a series of excitonic resonances in the reflectance spectra, initiated by a ground state with an amplitude comparable to the ground state of the 1s exciton in the monolayer. Higher-energy states exhibit a decrease in intensity with increasing energy, as expected for the excited states of the Rydberg series, although a significant increase in the diamagnetic shift is missing in tri and quadlayers. By comparing the experimental observations with theoretical predictions, we discover that the excitonic resonances observed in trilayers originate from two Rydberg series, while quadlayers exhibit four such series, and bilayers host a single Rydberg series similar to that found in monolayers.