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Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra-thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin film stacks cannot show tunable perfect absorption in a wide-angle range. Here, a tunable perfect absorption from normal incidence to a wide-angle range (0° to 70°) by utilizing a two-layer stack consisting of a high refractive index low-loss dielectric on a high reflecting metal is proposed. This is experimentally demonstrated by depositing a thin layer of low-loss phase change material such as stibnite (Sb 2 S 3 ) on a thin layer of silver. This structure shows tunable perfect absorption with large spectral tunability in the visible wavelength. Furthermore, the absorption enhancement in 2D materials by transferring monolayer molybdenum disulfide on the stack, which shows 96% light absorption with enhanced photoluminescence, is demonstrated. In addition, the thin film stack can work as a scalable phase modulator offering a maximum phase tunability of ≈140° by changing the structural state of Sb 2 S 3 from amorphous to crystalline., (© 2022 Wiley-VCH GmbH.)

Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light-emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier-phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI 6 octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two-step process leads to a unique pump- and probe-energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton-phonon coupling of 2D perovskites relevant for emergent optoelectronics development., (© 2021 Wiley-VCH GmbH.)

Pure 2D lead-iodide perovskites typically demonstrate poor charge transport and compromised visible light absorption, relative to their 3D congeners. This hinders their potential use as solar absorbers. Herein, the systematic tuning of pyridinium-based templating cations is reported to introduce intermolecular interactions that provide access to a series of new 2D lead-iodide perovskites with reduced inter-octahedral distortions (largest Pb-(μ-I)-Pb bond angles of 170-179°) and very short inorganic interlayer separations (shortest I⋅⋅⋅I contacts ≤4.278-4.447 Å). These features manifest in reduced band gaps (2.35-2.46 eV) and relaxed dielectric confinement (excitonic binding energies of 130-200 meV). As a consequence, they demonstrate (more than ten-fold) improved photo- and electrical conductivities relative to conventional 2D lead-iodide perovskites, such as that templated by 2-(1-naphthyl)ethylammonium. Through computational studies, the origin of this behavior was shown to derive from a combination of short iodoplumbate layer separations and the aromaticity of the organic dications., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self-assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton-photon coupling, etc. Detailed understanding into the strong light-matter interactions in these hybrid organic-inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C 6 H 5 C 2 H 4 NH 3 ) 2 PbI 4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton-phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin-quasiparticle interactions in these layered 2D halide perovskites with large spin-orbit coupling.