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Single-crystal layered perovskite heterostructures provide a scalable platform for potentially realizing emergent properties recently seen in mechanically stacked monolayers. We report two new layered perovskite heterostructures M2(PbCl2)(AMCHC)2(PbCl4)·2H2O (1_M where M = Na+, Li+; AMCHC = +NH3CH2C6H10COO‒) crystallizing in the chiral, polar space group C2. The heterostructures exhibit alternating layers of a lead-chloride perovskite and an intergrowth comprising corner-sharing PbCl4(η2-COO)2 polyhedra with bridging equatorial chlorides and terminal axial oxygen ligands. Small alkali metal cations and water molecules occupy the cavities between the polyhedra in the intergrowth layer. The heterostructures display wide bandgaps and two closely spaced excitonic features in their optical spectra and strong second harmonic generation. The calculated band structure of 1_Na features a Type-I quantum-well structure, where the electron-hole correlation function corresponding to the lowest excited state points to electron-hole pairs localized within a single inorganic layer (intralayer excitons), as seen in typical layered halide perovskites. In contrast, calculations show that 1_Li adopts a Type II quantum-well structure, with electrons and holes in the lowest-excited state residing in different inorganic layers (interlayer excitons). Calculations on model complexes suggest that these changes in band alignment, between Type-I and Type-II quantum-well structures, are driven by the placement of the alkali metal and the orientation of the water molecules changing the electrostatic potential-energy profiles of the heterostructures. Thus, this study sets the stage for accessing different alignments of the perovskite and intergrowth bands in bulk perovskite heterostructures that self-assemble in solution. 

Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS₂/WS₂at 4° twist angle and WSe₂/MoSe₂heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations. 

Interactions of quantum materials with strong laser fields can induce exotic non-equilibrium electronic states. Monolayer transition metal dichalcogenides, a new class of direct-gap semiconductors with prominent quantum confinement, offer exceptional opportunities for the Floquet engineering of excitons, which are quasiparticle electron–hole correlated states8. Strong-field driving has the potential to achieve enhanced control of the electronic band structure and thus the possibility of opening a new realm of exciton light–matter interactions. However, a full characterization of strong-field driven exciton dynamics has been difficult. Here we use mid-infrared laser pulses below the optical bandgap to excite monolayer tungsten disulfide and demonstrate strong-field light dressing of excitons in excess of a hundred millielectronvolts. Our high-sensitivity transient absorption spectroscopy further reveals the formation of a virtual absorption feature below the 1s-exciton resonance, which we assign to a light-dressed sideband from the dark 2p-exciton state. Quantum-mechanical simulations substantiate the experimental results and enable us to retrieve real-space movies of the exciton dynamics. This study advances our understanding of the exciton dynamics in the strong-field regime, showing the possibility of harnessing ultrafast, strong-field phenomena in device applications of two-dimensional materials. 

Abstract Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe 2 ) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼ $${g}_{{pol}-{pol}}^{2s} \sim 46.4\pm 13.9\,\mu {eV}\mu {m}^{2}$$ g p o l − p o l 2 s ~ 46.4 ± 13.9 μ e V μ m 2 which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies.