Highly nonlinear dipolar exciton-polaritons in bilayer MoS2.

Realizing nonlinear optical response in the low photon density limit in solid-state systems has been a long-standing challenge. Semiconductor microcavities in the strong coupling regime hosting exciton-polaritons have emerged as attractive candidates in this context. However, the weak interaction between these quasiparticles has been a hurdle in this quest. Dipolar excitons provide an attractive strategy to overcome this limitation but are often hindered by their weak oscillator strength. The interlayer dipolar excitons in naturally occurring homobilayer MoS2 alleviates this issue owing to their formation via hybridization of interlayer charge transfer exciton with intralayer B exciton. Here we demonstrate the formation of dipolar exciton polaritons in bilayer MoS2 resulting in unprecedented nonlinear interaction strengths. A ten-fold increase in nonlinearity is observed for the interlayer dipolar excitons compared to the conventional A excitons. These highly nonlinear dipolar polaritons will likely be a frontrunner in the quest for solid-state quantum nonlinear devices.

Stochastic scattering theory for excitation-induced dephasing: Time-dependent nonlinear coherent exciton lineshapes.

We develop a stochastic theory that treats time-dependent exciton-exciton s-wave scattering and that accounts for dynamic Coulomb screening, which we describe within a mean-field limit. With this theory, we model excitation-induced dephasing effects on time-resolved two-dimensional coherent optical lineshapes and we identify a number of features that can be attributed to the many-body dynamics occurring in the background of the exciton, including dynamic line narrowing, mixing of real and imaginary spectral components, and multi-quantum states. We test the model by means of multidimensional coherent spectroscopy on a two-dimensional metal-halide semiconductor that hosts tightly bound excitons and biexcitons that feature strong polaronic character. We find that the exciton nonlinear coherent lineshape reflects many-body correlations that give rise to excitation-induced dephasing. Furthermore, we observe that the exciton lineshape evolves with the population time over time windows in which the population itself is static in a manner that reveals the evolution of the multi-exciton many-body couplings. Specifically, the dephasing dynamics slow down with time, at a rate that is governed by the strength of exciton many-body interactions and on the dynamic Coulomb screening potential. The real part of the coherent optical lineshape displays strong dispersive character at zero time, which transforms to an absorptive lineshape on the dissipation timescale of excitation-induced dephasing effects, while the imaginary part displays converse behavior. Our microscopic theoretical approach is sufficiently flexible to allow for a wide exploration of how system-bath dynamics contribute to linear and non-linear time-resolved spectral behavior.

(4NPEA)2PbI4 (4NPEA = 4-Nitrophenylethylammonium): Structural, NMR, and Optical Properties of a 3 × 3 Corrugated 2D Hybrid Perovskite.

(4NPEA)2PbI4 (4NPEA = 4-nitrophenylethylammonium) is the first 3 × 3 corrugated 2D organic-Pb/I perovskite. The nitro groups are involved in cation-cation and cation-iodide interactions. The structure contains both highly distorted and near-ideal PbI6 octahedra, consistent with the observation of two 207Pb NMR resonances, while the optical properties resemble those of other 2D perovskites with distorted PbI6 octahedra.

Author Correction: Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites.

In the version of this Article originally published, the units of the Fig. 3a x axis were incorrectly given as meV. They should have been eV. This has now been corrected in all versions of the Article.

Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites.

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm-1) optical phonons involving motion in the lead iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight into the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

Spectroscopic studies of lanthanide complexes of varying nuclearity based on a compartmentalised ligand.

The synthesis, characterization and solid-state luminescence spectroscopy of mononuclear (f), heterodinuclear (d-f) and heterotrinuclear (d-f-d) coordination compounds with the compartmental ligand N,N'-bis(3-hydroxyl salicylidene)benzene-1,2-diamine (H2L) are reported. The trivalent lanthanide ions Nd(III), Sm(III), Eu(III), Gd(III), Tb(III) and Dy(III) as single metal centres or in combination with either Zn(II) or Ni(II) were coordinated. Compounds are characterised by elemental analyses, IR, 1D and 2D solution (1)H and (13)C NMR spectroscopy, measurements of magnetic moments and solid state UV-Vis-NIR reflectance, luminescence and Raman spectroscopy techniques. Crystal structures of the dinuclear compounds [SmZn(O2NO)3(L)(OH2)]·EtOH and [DyZn(O2NO)2(Cl)(L)(EtOH)]·3EtOH and the trinuclear compound [TbZn2(L)2(Cl)2(OH2)](NO3)·EtOH are presented, where samarium(iii) displays a coordination number of ten, with a bicapped cubic geometry, while for the dysprosium compound a nine-coordinated environment with a tricapped trigonal prismatic geometry is shown. Their crystals belong to the triclinic system and the P1[combining macron] space group. The coordination number for terbium(iii) in the trinuclear complex is nine, with a tricapped trigonal prismatic geometry, and its crystal belongs to the monoclinic system, space group C2/c. For these three compounds, the zinc ion stabilises a penta-coordinated environment with square pyramid geometry. All mononuclear and dinuclear compounds are neutral, whereas the trinuclear complexes are ionic. The results of DFT theoretical calculations for the ligand (H2L) are used to assign the ligand singlet and triplet excited state energy levels. Luminescence studies of the neodymium compounds indicate that the ligand is a sensitizer for NIR emitters.