Oleh Hordiichuk

Understanding the fine structure of excitons is crucial for optoelectronic and quantum photonic applications of lead halide perovskites. It is demonstrated that polarization‐sensitive photon echo spectroscopy in magnetic field provides a powerful method to access coherent exciton dynamics and reveal their energy level structure hidden by inhomogeneous broadening. In particular, exciton quantum beats in Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1 exciton states. Applying this technique to bulk mixed halide perovskite crystals reveals a bright‐dark exciton splitting of meV, along with electron and hole Landé factors and , respectively. The quantum beats persist on timescales of 20–50 ps, demonstrating remarkably robust spin and optical coherences at cryogenic temperature of 2 K. The decay of the quantum beats of the outer doublet is governed by dephasing due to dispersion of the bright‐dark splitting of meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of meV due to anisotropic potentials.

Plasmonic cavities provide a powerful platform for nanoscale light confinement far beyond the diffraction limit, enabling exceptionally strong light-matter interactions. Recent advances in plasmonic architectures and active materials enable stable, scalable routes toward fully integrated on-chip photonics. In this work, we demonstrate the strong coupling of 2D layered perovskites with a transferable plasmonic cavity directly fabricated on top of hexagonal boron nitride (hBN). Under optical excitation, we observe clear interaction between the exciton and cavity modes leading to strong and even ultrastrong coupling, with coupling factors g up to 10% of the resonance energy. Combined with the polariton mode hybridization and a waveguiding effect, this results in pronounced line width narrowing of the polariton modes well below both the bare cavity and exciton line widths. Electrical excitation of the hybrid modes is achieved via electron tunneling and simultaneous energy transfer, making this design an attractive configuration for on-chip integration.

InP-based quantum dots (QDs) represent the major commercial success of colloidal semiconductor nanocrystals (NCs). A combination of the robust, mostly covalent, structure and nontoxic nature of the constituent elements makes them a QD material of choice for display and LED technologies. Despite successful commercial realization, InP NCs still lack synthetic versatility and robustness, seen, for instance, as a continued quest to substitute a commonly used pyrophoric and expensive tris(trimethylsilyl)phosphine precursor. Herein, we propose solid-state, nonpyrophoric, and synthetically readily accessible acylphosphines as convenient phosphorus precursors for the synthesis of InP NCs. When combined with suitable anionic nucleophiles, such as arylthiolates, both tris(acyl)phosphines and indium complexes of bis(acyl)phosphines act as efficient sources of the P^3- anion, as corroborated by NMR spectroscopy and powder X-ray diffraction studies. This type of reactivity is utilized in colloidal synthesis of uniform InP QDs with well-defined excitonic features in their optical absorption spectra, spanning 460-600 nm. The conversion kinetics and therefore the final NC size are controlled by the nature of acyl substituents and by the use of either indium or zinc long-chain carboxylates as ligands. The proposed acylpnictide route is anticipated to foster the development of other metal phosphide and metal arsenide NCs.

The great variety of lead halide perovskite semiconductors represents an outstanding platform for studying crystal symmetry effects on the spin‐dependent properties. Access to them is granted through the optical orientation of exciton and carrier spins by circularly polarized photons. Here, the exciton spin polarization is investigated at 1.6 K cryogenic temperature in four lead halide perovskite crystals with different symmetries: (almost) cubic in FA_0.9Cs_0.1PbI_2.8Br_0.2 and FAPbBr₃, and orthorhombic in MAPbI₃ and CsPbBr₃. Giant optical orientation of 85% is found for the excitons in FA_0.9Cs_0.1PbI_2.8Br_0.2, MAPbI₃, and CsPbBr₃, while it amounts to 20% in FAPbBr₃. For all studied crystals, the optical orientation is robust to detuning of the laser photon energy from the exciton resonance, remaining constant for high energy detunings up to 0.3 eV, above which it continuously decreases to zero for detunings exceeding 1 eV. No acceleration of the spin relaxation for excitons with large kinetic energy is found in the cubic and orthorhombic crystals. This evidences the absence of the Dyakonov‐Perel spin relaxation mechanism, which is based on the Rashba‐Dresselhaus splitting of spin states at finite k‐vectors. This indicates that the spatial inversion symmetry is maintained in perovskite crystals, independent of the cubic or orthorhombic phase.

Quantum technologic and spintronic applications require reliable material platforms that enable significant and long‐living spin polarization of excitations, the ability to manipulate it optically in external fields, and the possibility to implement quantum correlations between spins, i.e., entanglement. Here it is demonstrated that these conditions are met in bulk crystals of lead halide perovskites. A giant optical orientation of 85% of excitons, approaching the ultimate limit of unity, in FA_0.9Cs_0.1PbI_2.8Br_0.2 crystals is reported. The exciton spin orientation is maintained during the exciton lifetime of 55 ps resulting in high circular polarization of the exciton emission. The optical orientation is robust to detuning of the excitation energy up to 0.3 eV above the exciton resonance and remains larger than 20% up to detunings of 0.9 eV. It evidences pure chiral selection rules and suppressed spin relaxation of electrons and holes, even with large kinetic energies. The exciton and electron–hole recombinations are distinguished by means of the spin dynamics detected via coherent spin quantum beats in magnetic field. Further, electron–hole spin correlations are demonstrated through linear polarization beats after circularly polarized excitation. These findings are supported by atomistic calculations. All‐in‐all, the results establish lead halide perovskite semiconductors as suitable platform for quantum technologies.

Coherent spin dynamics of electrons and holes are studied in hybrid organic-inorganic lead halide perovskite FAPbBr₃ bulk single crystals using the time-resolved Kerr ellipticity technique at cryogenic temperatures. The Larmor spin precession of the carrier spins in a magnetic field is monitored to measure the Landé g-factors of electrons (+2.44) and holes (+0.41). These g-factors are highly isotropic. The measured spin dephasing times amount to a few nanoseconds, and the longitudinal hole spin relaxation time is 470 ns. The important role of the strong hyperfine interaction between carrier spins and nuclear spins is demonstrated via dynamic nuclear polarization. At low temperatures, electron and hole spin relaxation predominantly occurs via the hyperfine interaction, whose importance significantly decreases at temperatures above 12 K. We overview the spin dynamics in various lead halide perovskite crystals and polycrystalline films and conclude on their common features provided by charge carrier localization at cryogenic temperatures.

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA_0.9Cs_0.1PbI_2.8Br_0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T₂ = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.