The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Photophysics and its application in photon upconversion

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interaction, where the energy of the emitted photons is higher than that of the incident photons. PL upconversion has promising applications in optoelectronic devices, displays, photovoltaics, imaging, diagnosis and treatment. In this review, we summarize the mechanism of PL upconversion and ultrafast PL physical processes. In particular, we highlight the advances in laser cooling, biological imaging, volumetric displays and photonics.

How cation nature controls the bandgap and bulk Rashba splitting of halide perovskites

Because of instability issues presented by metal halide perovskites based on methylammonium (MA), its replacement to Cs $$ \mathrm{Cs} $$ has emerged as an alternative to improve the materials' durability. However, the impact of this replacement on electronic properties, especially gap energy and bulk Rashba splitting remains unclear since electrostatic interactions from organic cations can play a crucial role. Through first-principles calculations, we investigated how organic/inorganic cations impact the electronic properties of MAPbI 3 $$ {\mathrm{MAPbI}}_3 $$ and CsPbI 3 $$ {\mathrm{CsPbI}}_3 $$ perovskites. Although at high temperatures the organic cation can assume spherical-like configurations due to its rotation into the cages, our results provide a complete electronic mechanism to show, from a chemical perspective based on ab initio calculations at 0 K $$ 0\ \mathrm{K} $$ , how the MA $$ \mathrm{MA} $$ dipoles suppression can reduce the MAPbI 3 $$ {\mathrm{MAPbI}}_3 $$ gap energy by promoting a degeneracy breaking in the electronic states from the PbI 3 $$ {\mathrm{PbI}}_3 $$ framework, while the dipole moment reinforcement is crucial to align theory ↔ $$ \leftrightarrow $$ experiment, increasing the bulk Rashba splitting through higher Pb $$ \mathrm{Pb} $$ off-centering motifs. The lack of permanent dipole moment in Cs $$ \mathrm{Cs} $$ results in CsPbI 3 $$ {\mathrm{CsPbI}}_3 $$ polymorphs with a pronounced Pb $$ \mathrm{Pb} $$ on-centering-like feature, which causes suppression in their respective bulk Rashba effect.

Nonlinear Two-Photon Absorption in the Near-Infrared Band for Lead Bromide Perovskite Films Using an F-Scan Nonlinear Spectrometer

An F-scan nonlinear spectrometer is used to measure the two-photon absorption coefficient for CH₃NH₃PbBr₃ perovskite films from 690 to 995 nm. This spectrometer uses an electrically focused tunable lens and a tunable femtosecond-pulse laser (Mai Tai-HP) with a resolution of 5 nm. Two-band models and saturation irradiance corrections are used to fit the experimental data. The nonlinear absorption in this wavelength range is of the order of cm/MW. We found that the best agreement between the experimental data, the reported values in the literature, and the theoretical model is obtained for an extended two-band model with irradiance saturation correction.

Third-order optical nonlinearity of CsPb(Br/I)3 metal halide perovskites nano-crystals embedded chalcogenide glass

The nonlinear optical properties of emerging metal halide perovskites (MHP) materials are sufficiently intriguing that this topic has become the hotspots in the realm of material science. Hence, we investigate the third-order nonlinear optical properties of CsPbBrx/I_3-x (x = 1, 2, 3) MHP nano-crystals (NCs) embedded chalcogenide glass (ChG) within a GeS₂-Sb₂S₃ pseudo-binary system, by monitoring the composition, excitation wavelength and intensity dependencies via femtosecond Z-scan technique. We have found that the intrinsic large optical nonlinearity of ChG can be further enhanced because of the incorporation of MHP NCs, and that the optical nonlinearity of MHP-ChG containing pristine Br NCs is more pronounced compared to its counterparts with mixed Br/I NCs, due to a combination of multiple factors.

Advances of Nonlinear Photonics in Low-Dimensional Halide Perovskites

Hybrid halide perovskites emerging as a highly promising class of functional materials for semiconductor optoelectronic applications have drawn great attention from worldwide researchers. In the past few years, prominent nonlinear optical properties have been demonstrated in perovskite bulk structures indicating their bright prospect in the field of nonlinear optics (NLO). Following the surge of 3D perovskites, more recently, the low-dimensional perovskites (LDPs) materials ranging from two-, one-, to zero-dimension such as quantum-wells or colloidal nanostructures have displayed unexpectedly attractive NLO response due to the strong quantum confinement, remarkable exciton effect, and structural diversity. In this perspective, the current state of the art is reviewed in the field of NLO for LDP materials. The relationship between confinement effect and NLO is analyzed systematically to give a comprehensive understanding of the function of dimension reduction. Furthermore, future directions and challenges toward the improvement of the NLO in LDP materials are discussed to provide an outlook in this rapidly developing field.

Low-field onset of Wannier-Stark localization in a polycrystalline hybrid organic inorganic perovskite

Methylammonium lead iodide perovskite (MAPbI₃) is renowned for an impressive power conversion efficiency rise and cost-effective fabrication for photovoltaics. In this work, we demonstrate that polycrystalline MAPbI₃s undergo drastic changes in optical properties at moderate field strengths with an ultrafast response time, via transient Wannier Stark localization. The distinct band structure of this material - the large lattice periodicity, the narrow electronic energy bandwidths, and the coincidence of these two along the same high-symmetry direction - enables relatively weak fields to bring this material into the Wannier Stark regime. Its polycrystalline nature is not detrimental to the optical switching performance of the material, since the least dispersive direction of the band structure dominates the contribution to the optical response, which favors low-cost fabrication. Together with the outstanding photophysical properties of MAPbI₃, this finding highlights the great potential of this material in ultrafast light modulation and novel photonic applications.

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

Manipulation of excitons via coherent light-matter interaction is a promising approach for quantum state engineering and ultrafast optical modulation. Various excitation pathways in the excitonic multilevel systems provide controllability more efficient than that in the two-level system. However, these control schemes have been restricted to limited control-light wavelengths and cryogenic temperatures. Here, we report that lead halide perovskites can lift these restrictions owing to their multiband structure induced by strong spin-orbit coupling. Using CsPbBr₃ perovskite nanocrystals, we observe an anomalous enhancement of the exciton energy shift at room temperature with increasing control-light wavelength from the visible to near-infrared region. The enhancement occurs because the interconduction band transitions between spin-orbit split states have large dipole moments and induce a crossover from the two-level optical Stark effect to the three-level Autler-Townes effect. Our finding establishes a basis for efficient coherent optical manipulation of excitons utilizing energy states with large spin-orbit splitting.

Here, Yumoto et al. demonstrate that for a halide perovskite with large spin-orbit splitting the optical Stark effect can give way to a three level Autler-Townes effect in the near-infrared region. The multiband nature of the effect potentially allows for further optical control over quantum states.

Water Triggered Synthesis of Highly Stable and Biocompatible 1D Nanowire, 2D Nanoplatelet, and 3D Nanocube CsPbBr3 Perovskites for Multicolor Two-Photon Cell Imaging

Two-photon imaging in the near-infrared window holds huge promise for real life biological imaging due to the increased penetration depth. All-inorganic CsPbX3 nanocrystals with bright luminescence and broad spectral tunability are excellent smart probes for two-photon bioimaging. But, the poor stability in water is a well-documented issue for limiting their practical use. Herein, we present the development of specific antibody attached water-resistant one-dimensional (1D) CsPbBr3 nanowires, two-dimensional (2D) CsPbBr3 nanoplatelets, and three-dimensional (3D) CsPbBr3 nanocubes which can be used for selective and simultaneous two-photon imaging of heterogeneous breast cancer cells in the near IR biological window. The current manuscript reports the design of excellent photoluminescence quantum yield (PLQY), biocompatible and photostable 1D CsPbBr3 nanowires, 2D CsPbBr3 nanoplatelets, and 3D CsPbBr3 nanocubes through an interfacial conversion from zero-dimensional (0D) Cs4PbBr6 nanocrystals via a water triggered strategy. Reported data show that just by varying the amount of water, one can control the dimension of CsPbBr3 perovskite crystals. Time-dependent transition electron microscopy and emission spectra have been reported to find the possible pathway for the formation of 1D, 2D, and 3D CsPbBr3 nanocrystals from 0D Cs4PbBr6 nanocrystals. Biocompatible 1D, 2D, and 3D CsPbBr3 nanocrystals were developed by coating with amine-poly(ethylene glycol)-propionic acid. Experimental data show the water-driven design of 1D, 2D, and 3D CsPbBr3 nanocrystals exhibits strong single-photon PLQY of ∼66-88% as well as excellent two-photon absorption properties (σ2) of ∼8.3 × 105-7.1 × 104 GM. Furthermore, reported data show more than 86% of PL intensity remains for 1D, 2D, and 3D CsPbBr3 nanocrystals after 35 days under water, and they exhibit excellent photostability of keeping 99% PL intensity after 3 h under UV light. The current report demonstrates for the first time that antibody attached 1D and 2D perovskites have capability for simultaneous two-photon imaging of triple negative breast cancer cells and human epidermal growth factor receptor 2 positive breast cancer cells. CsPbBr3 nanocrystals exhibit very high two-photon absorption cross-section and good photostability in water, which are superior to those of commonly used organic probes (σ2 = 11 GM for fluorescein), and therefore, they have capability to be a better probe for bioimaging applications.

Photoredox Organic Synthesis Employing Heterogeneous Photocatalysts with Emphasis on Halide Perovskite

Lately, heterogeneous semiconductor materials have been explored as an emerging type of efficient photocatalyst for photoredox organic synthesis. Among these semiconductors, lead halide perovskite materials demonstrate unique properties towards excellent charge separation and charge transfer, extremely long charge carrier migration, high efficiency in visible light absorption, and long excited states lifetimes, etc., as proved in ground-breaking solar cell applications, garnering necessary merits for an efficient catalytic system for photoredox organic reactions. Here, the latest progress in heterogeneous semiconductor materials towards this endeavor is examined, with particular emphasis on lead halide perovskite nanocrystals (NCs) in photocatalytic organic synthesis.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH₃NH₃)PbI₃ methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

2D materials towards ultrafast photonic applications

Two-dimensional materials are now excelling in yet another arena of ultrafast photonics, including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices, etc.