Strong spin-orbit coupling inducing Autler-Townes effect in lead halide perovskite nanocrystals

Compositionally Tunable Magneto-optical Properties of Lead-Free Halide Perovskite Nanocrystals

Inorganic lead-free metal halide perovskites have garnered much attention as low-toxicity alternatives to lead halide perovskites for luminescence and photovoltaic applications. However, the electronic structure and properties of these materials, including the composition dependence of the band structure, spin-orbit coupling, and Zeeman effects, remain poorly understood. Here, we investigated vacancy-ordered Cs3Bi2X9 (X= Cl, Br) perovskite nanocrystals using magnetic circular dichroism spectroscopy. Our results indicate that the excitonic spectra are predominantly composed of direct and indirect band gap transitions and that the Zeeman splitting energy of the direct exciton increases from 0.50 to 0.63 meV at 7 T by substituting Br for Cl. Comparison with analogous results for Cs2AgBiCl6 nanocrystals, obtained by cation substitution, suggests an important effect of charge distribution within electronic bands on the excitonic Zeeman splitting. This work demonstrates that the magneto-optical properties of these materials can be effectively manipulated via chemical composition, suggesting promising applications in photonics, spintronics, and optoelectronics.

A series of blue phosphorescent iridium complexes with thermally activated delayed fluorescence and efficiency roll-off properties

Six iridium complexes were designed and studied using the DFT approach, (ppy)2Ir(pic) (1), (f4ppy)2Ir(pic) (2), (ppy)2Ir(tmd) (3), (f4ppy)2Ir(tmd) (4), (ppy)2Ir(tpip) (5) and (f4ppy)2Ir(tpip) (6). Here ppy denotes phenylpyridine, f4ppy denotes 2-(2,3,4,5-tetrafluorophenyl) pyridine, pic denotes benzoic acid, tmd denotes 5-hydroxy-2,2,6,6-tetramethylhept-4-en-3-one and tpip denotes tetraphenylimido-diphosphinate. The geometries, absorptions, emissions, frontier molecular orbitals, and spin-orbit coupling (SOC) constants of the 6 complexes were evaluated. An intriguing phenomenon was observed during the excitation process of these molecules. It was discovered that, in the ground state, the Frontier Molecular Orbitals (FMOs) of these molecules were loosely arranged throughout the molecule. However, in the Lowest Unoccupied Molecular Orbitals (LUMO) of the triplet excited state, the FMOs become concentrated around the metal core and a maximum of two ligands. Furthermore, the analysis of the energy difference between the lowest singlet excited state and the lowest triplet excited state (ΔE S1T1 ) of these complexes in conjunction with their spin-orbit coupling performance indicated that complex 1 exhibits characteristics consistent with Thermally Activated Delayed (TAD) fluorescence. We hope that this research can serve as a reference for practical experimental synthesis.

Autler-Townes splitting and linear dichroism in colloidal CdSe nanoplatelets driven by near-infrared pulses

Coherent interaction between light fields and matter has led to many exotic physical phenomena, one of which is the Autler-Townes splitting originating from the optical Stark effect of a three-level system. It has been well documented for atoms, molecules, and low-dimensional, epitaxial-grown semiconductors but rarely for solution-processed samples. Here, we report on Autler-Townes splitting observed in CdSe nanoplatelets. The strong intersubband transition of these nanoplatelets situated in the near-infrared allows us to coherently drive it with femtosecond near-infrared pulses, and meanwhile, their strong interband transition in the visible records the resultant spectral changes. The splitting is most prominent with cross-polarized, linear pump-probe pulses, consistent with the orthogonal polarizations of interband and intersubband transition dipoles. When the nanoplatelets are driven from tilted incident angles, we observe anomalous spectral lineshapes arising from a competition between interband and intersubband drivings, which are nevertheless quantitatively captured by our dressed-state model simulation.

Origin of Dopant-Carrier Exchange Coupling and Excitonic Zeeman Splitting in Mn2+-Doped Lead Halide Perovskite Nanocrystals

Low-dimensional metal halide perovskites have unique optical and electrical properties that render them attractive for the design of diluted magnetic semiconductors. However, the nature of dopant-exciton exchange interactions that result in spin-polarization of host-lattice charge carriers as a basis for spintronics remains unexplored. Here, we investigate Mn2+-doped CsPbCl3 nanocrystals using magnetic circular dichroism spectroscopy and show that Mn2+ dopants induce excitonic Zeeman splitting which is strongly dependent on the nature of the band-edge structure. We demonstrate that the largest splitting corresponds to exchange interactions involving the excited state at the M-point along the spin-orbit split-off conduction band edge. This splitting gives rise to an absorption-like C-term excitonic MCD signal, with the estimated effective g-factor (geff) of ca. 70. The results of this work help resolve the assignment of absorption transitions observed for metal halide perovskite nanocrystals and allow for a design of new diluted magnetic semiconductor materials for spintronics applications.

Direct Observation of Circularly Polarized Nonlinear Optical Activities in Chiral Hybrid Lead Halides

Circularly polarized light emission is a crucial application in imaging, sensing, and photonics. However, utilizing low-energy photons to excite materials, as opposed to high-energy light excitation, can facilitate deep-tissue imaging and sensing applications. The challenge lies in finding materials capable of directly generating circularly polarized nonlinear optical effects. In this study, we introduce a chiral hybrid lead halide (CHLH) material system, R/S-DPEDPb3Br8·H2O (DPED = 1,2-diphenylethylenediammonium), which can directly produce circularly polarized second harmonic generation (CP-SHG) through linearly polarized infrared light excitation, exhibiting a polarization efficiency as high as 37% at room temperature. To understand the spin relaxation mechanisms behind the high polarization efficiency, we utilized two models, so-called D'yakonov-Perel' (DP) and Bir-Aronov-Pikus (BAP) mechanisms. The unique zigzag inorganic frameworks within the hybrid structure are believed to reduce the dielectric confinement and exciton binding energy, thus enhancing spin polarization, especially in regions with a high excitation pump fluence based on the DP mechanism. In the case of low excitation pump fluence, the BAP mechanism dominates, as evidenced by the observed decrease in the polarization ratio from CP-SHG measurement. Using density functional theory analysis, we elucidate how the distinctive 8-coordination environment of lead bromide building blocks effectively suppresses spin-orbit coupling at the conduction band minimum. This suppression significantly diminishes spin-splitting, thereby slowing the spin relaxation rate.

Floquet Engineering of Excitons in Two-Dimensional Halide Perovskites via Biexciton States

Layered two-dimensional halide perovskites (2DHPs) exhibit exciting non-equilibrium properties that allow the manipulation of energy levels through coherent light-matter interactions. Under the Floquet picture, novel quantum states manifest through the optical Stark effect (OSE) following intense subresonant photoexcitation. Nevertheless, a detailed understanding of the influence of strong many-body interactions between excitons on the OSE in 2DHPs remains unclear. Herein, we uncover the crucial role of biexcitons in photon-dressed states and demonstrate precise optical control of the excitonic states via the biexcitonic OSE in 2DHPs. With fine step tuning of the driven energy, we fully parametrize the evolution of exciton resonance modulation. The biexcitonic OSE enables Floquet engineering of the exciton resonance with either a blue-shift or a red-shift of the energy levels. Our findings shed new light on the intricate nature of coherent light-matter interactions in 2DHPs and extend the degree of freedom for ultrafast coherent optical control over excitonic states.

Room-temperature coherent optical manipulation of hole spins in solution-grown perovskite quantum dots

Manipulation of solid-state spin coherence is an important paradigm for quantum information processing. Current systems either operate at very low temperatures or are difficult to scale up. Developing low-cost, scalable materials whose spins can be coherently manipulated at room temperature is thus highly attractive for a sustainable future of quantum information science. Here we report ambient-condition all-optical initialization, manipulation and readout of hole spins in an ensemble of solution-grown CsPbBr₃ perovskite quantum dots with a single hole in each dot. The hole spins are initialized by sub-picosecond electron scavenging following circularly polarized femtosecond-pulse excitation. A transverse magnetic field induces spin precession, and a second off-resonance femtosecond-pulse coherently rotates hole spins via strong light-matter interaction. These operations accomplish near-complete quantum-state control, with a coherent rotation angle close to the π radian, of hole spins at room temperature.

A general one-step plug-and-probe approach to top-gated transistors for rapidly probing delicate electronic materials

The miniaturization of silicon-based electronics has motivated considerable efforts in exploring new electronic materials, including two-dimensional semiconductors and halide perovskites, which are usually too delicate to maintain their intrinsic properties during the harsh device fabrication steps. Here we report a convenient plug-and-probe approach for one-step simultaneous van der Waals integration of high-k dielectrics and contacts to enable top-gated transistors with atomically clean and electronically sharp dielectric and contact interfaces. By applying the plug-and-probe top-gate transistor stacks on two-dimensional semiconductors, we demonstrate an ideal subthreshold swing of 60 mV per decade. Using this approach on delicate lead halide perovskite, we realize a high-k top-gate CsPbBr₃ transistor with a low operating voltage and a very high two-terminal field-effect mobility of 32 cm² V^-1 s^-1. This approach can be extended to centimetre-scale MoS₂ and perovskite and generate top-gated transistor arrays, offering a rapid and convenient way of accessing intrinsic properties of delicate emerging materials.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.

Excitonic Bloch-Siegert shift in CsPbI3 perovskite quantum dots

Coherent interaction between matter and light field induces both optical Stark effect and Bloch-Siegert shift. Observing the latter has been historically challenging, because it is weak and is often accompanied by a much stronger Stark shift. Herein, by controlling the light helicity, we can largely restrict these two effects to different spin-transitions in CsPbI₃ perovskite quantum dots, achieving room-temperature Bloch-Siegert shift as strong as 4 meV with near-infrared pulses. The ratio between the Bloch-Siegert and optical Stark shifts is however systematically higher than the prediction by the non-interacting, quasi-particle model. With a model that explicitly accounts for excitonic effects, we quantitatively reproduce the experimental observations. This model depicts a unified physical picture of the optical Stark effect, biexcitonic optical Stark effect and Bloch-Siegert shift in low-dimensional materials displaying strong many-body interactions, forming the basis for the implementation of these effects to information processing, optical modulation and Floquet engineering.

Rapidly expanding spin-polarized exciton halo in a two-dimensional halide perovskite at room temperature

Monitoring of the spatially resolved exciton spin dynamics in two-dimensional semiconductors has revealed the formation of a spatial pattern and long-range transport of the spin-polarized excitons, which holds promise for exciton-based spin-optoelectronic applications. However, the spatial evolution has been restricted to cryogenic temperatures because of the short exciton spin relaxation times at room temperature. Here, we report that two-dimensional halide perovskites can overcome this limitation owing to their relatively long exciton spin relaxation times and substantial exciton-exciton interactions. We demonstrate the emergence of a halo-like spatial profile in spin-polarized exciton population and its ultrafast expansion at room temperature by performing time-resolved Faraday rotation imaging of spin-polarized excitons in two-dimensional perovskite (C₄H₉NH₃)₂(CH₃NH₃)₃Pb₄I₁₃. Exciton-exciton exchange interactions induce density-dependent nonlinear relaxation and ultrafast transport of exciton spins and give rise to a rapidly expanding halo-like spatial pattern. The density-dependent spatial control suggests the potential of using two-dimensional halide perovskites for spin-optoelectronic applications.

Room-Temperature Anomalous Coherent Excitonic Optical Stark Effect in Metal Halide Perovskite Quantum Dots

Nonresonant optical driving of confined semiconductors can open up exciting opportunities for experimentally realizing strongly interacting photon-dressed (Floquet) states through the optical Stark effect (OSE) for coherent modulation of the exciton state. Here we report the first room-temperature observation of the Floquet biexciton-mediated anomalous coherent excitonic OSE in CsPbBr₃ quantum dots (QDs). Remarkably, the strong exciton-biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency, similar to Autler-Townes splitting in atomic and molecular systems. The large biexciton binding energy of ∼71 meV and exciton-biexciton transition dipole moment of ∼25 D facilitate the hallmark observations, even at large detuning energies of >300 meV. This is accompanied by an unusual crossover from linear to nonlinear fluence dependence of the OSE as a function of the drive photon frequency. Our findings reveal crucial information on the unexplored many-body coherent interacting regime, making perovskite QDs suitable for room temperature quantum devices.

Enhanced Multiphoton Processes in Perovskite Metasurfaces

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.