Decoding ultrafast polarization responses in lead halide perovskites by the two-dimensional optical Kerr effect.

The ultrafast polarization response to incident light and ensuing exciton/carrier generation are essential to outstanding optoelectronic properties of lead halide perovskites (LHPs). A large number of mechanistic studies in the LHP field to date have focused on contributions to polarizability from organic cations and the highly polarizable inorganic lattice. For a comprehensive understanding of the ultrafast polarization response, we must additionally account for the nearly instantaneous hyperpolarizability response to the propagating light field itself. While light propagation is pivotal to optoelectronics and photonics, little is known about this in LHPs in the vicinity of the bandgap where stimulated emission, polariton condensation, superfluorescence, and photon recycling may take place. Here we develop two-dimensional optical Kerr effect (2D-OKE) spectroscopy to energetically dissect broadband light propagation and dispersive nonlinear polarization responses in LHPs. In contrast to earlier interpretations, the below-bandgap OKE responses in both hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites are found to originate from strong hyperpolarizability and highly anisotropic dispersions. In both materials, the nonlinear mixing of anisotropically propagating light fields results in convoluted oscillatory polarization dynamics. Based on a four-wave mixing model, we quantitatively derive dispersion anisotropies, reproduce 2D-OKE frequency correlations, and establish polarization-dressed light propagation in single-crystal LHPs. Moreover, our findings highlight the importance of distinguishing the often-neglected anisotropic light propagation from underlying coherent quasiparticle responses in various forms of ultrafast spectroscopy.

Electron Dynamics in Hybrid Perovskites Reveal the Role of Organic Cations on the Screening of Local Charges.

The large tolerance of hybrid perovksites to the trapping of electrons by defects is a key asset in photovoltaic applications. Here, the ionic surface terminations of CH3NH3PbI3 are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterize the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. When the orientation of organic cations is frozen in the orthorhombic phase, the positively charged termination induces a massive accumulation of excited electrons at the surface of the sample. Conversely, no electron accumulation is observed in the tetragonal phase. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ε = 120.

Solvated Electrons in Solids-Ferroelectric Large Polarons in Lead Halide Perovskites.

Solvation plays a pivotal role in chemistry and biology. A solid-state analogy of solvation is polaron formation, but the magnitude of Coulomb screening is typically an order of magnitude weaker than that of solvation in aqueous solutions. Here, we describe a new class of polarons, the ferroelectric large polaron, proposed initially by Miyata and Zhu in 2018 (Miyata, K.; Zhu, X.-Y. Ferroelectric Large Polarons. Nat. Mater. 2018, 17 (5), 379-381). This type of polaron allows efficient Coulomb screening of an electron or hole by extended ordering of dipoles from symmetry-broken unit cells. The local ordering is reflected in the ferroelectric-like THz dielectric responses of lead halide perovskites (LHPs) and may be partially responsible for their exceptional optoelectronic performances. Despite the likely absence of long-range ferroelectricity in LHPs, a charge carrier may be localized to and/or induce the formation of nanoscale domain boundaries of locally ordered dipoles. Based on the known planar nature of energetically favorable domain boundaries in ferroelectric materials, we propose that a ferroelectric polaron localizes to planar boundaries of transient polar nanodomains. This proposal is supported by dynamic simulations showing sheet-like transient electron or hole wave functions in LHPs. Thus, the Belgian-waffle-shaped ferroelectric polaron in the three-dimensional LHP crystal structure is a large polaron in two dimensions and a small polaron in the perpendicular direction. The ferroelectric large polaron may form in other crystalline solids characterized by dynamic symmetry breaking and polar fluctuations. We suggest that the ability to form ferroelectric large polarons can be a general principle for the efficient screening of charge carriers from scattering with other charge carriers, with charged defects and with longitudinal optical phonons, thus contributing to enhanced optoelectronic properties.

The ultrafast Kerr effect in anisotropic and dispersive media.

The ultrafast optical Kerr effect (OKE) is widely used to investigate the structural dynamics and interactions of liquids, solutions, and solids by observing their intrinsic nonlinear temporal responses through nearly collinear four-wave mixing. Non-degenerate mixing schemes allow for background free detection and can provide information on the interplay between a material's internal degrees of freedom. Here, we show a source of temporal dynamics in the OKE signal that is not reflective of the internal degrees of freedom but arises from a group index and momentum mismatch. It is observed in two-color experiments on condensed media with sizable spectral dispersion, a common property near an optical resonance. In particular, birefringence in crystalline solids is able to entirely change the character of the OKE signal via the off-diagonal tensor elements of the nonlinear susceptibility. We develop a detailed description of the phase-mismatched ultrafast OKE and show how to extract quantitative information on the spectrally resolved birefringence and group index from time-resolved experiments in one and two dimensions.

Nonlinear terahertz control of the lead halide perovskite lattice.

Lead halide perovskites (LHPs) have emerged as an excellent class of semiconductors for next-generation solar cells and optoelectronic devices. Tailoring physical properties by fine-tuning the lattice structures has been explored in these materials by chemical composition or morphology. Nevertheless, its dynamic counterpart, phonon-driven ultrafast material control, as contemporarily harnessed for oxide perovskites, has not yet been established. Here, we use intense THz electric fields to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 to 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Fröhlich polaron. Our work opens the door to selective control of LHP's vibrational degrees of freedom governing phase transitions and dynamic disorder.

Optical Anisotropy and Phase Transitions in Lead Halide Perovskites.

Optical anisotropy originates from crystalline structures with low symmetry and governs the polarization-dependent light propagation. Optical anisotropy is particularly important to lead halide perovskites that have been under intense investigation for optoelectronic and photonic applications, as this group of materials possesses rich structural phases that deviate from the high-symmetry cubic phase. Here we apply 2D optical Kerr effect spectroscopy to quantify the optical anisotropy in single-crystal methylammonium lead bromide (MAPbBr3). We determine the strong photon energy dependence of optical anisotropy near the band gap and show the dramatic change in optical anisotropy across phase transitions. We correlate the optical anisotropy with the structural anisotropy and demonstrate the tuning of optical anisotropy by alloyed CsxMA1-xPbBr3 perovskite crystals.

Broad-Band Near-Infrared Doublet Emission in a Tetrathiafulvalene-Based Metal-Organic Framework.

The upper limit in LED quantum efficiency from conventional closed-shell molecules is 25% as dictated by singlet and triplet spin statistics. Spin-doublet organic molecules are attractive candidates to exceed this limit, thanks to their 100% theoretical quantum efficiency in radiative recombination. However, examples of stable spin-doublet molecules in the solid state are rare. Here we show broad-band near-infrared emission in the columnar π-π stacked tetrathiafulvalene (TTF) in a metal organic framework (MOF) single crystal. The broad emission is similar to known TTF+• doublet emission and is stabilized in the MOF crystal. This interpretation is supported by the observation of enhanced PL emission following UV oxidation of the MOF crystal to increase the doublet concentration. The findings suggest tetrathiafulvalene-based MOFs as promising materials for near-IR light emission and the MOF structure may be a general strategy to stabilize radical cation species in the solid state.

Dynamic Screening and Slow Cooling of Hot Carriers in Lead Halide Perovskites.

Among the exceptional properties of lead halide perovskites (LHPs) is the ultraslow cooling of hot carriers. Carrier densities below the Mott density for large polarons (≤ ≈1018 cm-3 ) are focused on here. As in other semiconductors, a nascent hot electron distribution initially cools down via emission of longitudinal optical (LO) phonons on the 10-14 -10-13 s timescale. What distinguishes LHPs from conventional semiconductors is the exceptionally efficient screening in the former. The dielectric screening in LHPs on the 10-13 s timescale results in an order-of-magnitude reduction in the Coulomb potential upon the formation of a large polaron, likely with ferroelectric-like local ordering. Further LO-phonon emission is inhibited, and this leads to partial retention of hot electron energy on the 10-12 s timescale, more so in hybrid LHPs than in their all-inorganic counterparts. Further cooling of hot polarons occurs on the 10-10 s timescale, and this can be attributed to the slow diffusion of heat out of the large polaron volume due to the low thermal conductivity of LHPs. Like other carrier properties, slow hot carrier cooling in LHPs can be intimately related to efficient screening in a soft, anharmonic, and dynamically disordered lattice.

Hierarchical Coherent Phonons in a Superatomic Semiconductor.

The coupling of phonons to electrons and other phonons plays a defining role in material properties, such as charge and energy transport, light emission, and superconductivity. In atomic solids, phonons are delocalized over the 3D lattice, in contrast to molecular solids where localized vibrations dominate. Here, a hierarchical semiconductor that expands the phonon space by combining localized 0D modes with delocalized 2D and 3D modes is described. This material consists of superatomic building blocks (Re6 Se8 ) covalently linked into 2D sheets that are stacked into a layered van der Waals lattice. Using transient reflectance spectroscopy, three types of coherent phonons are identified: localized 0D breathing modes of isolated superatom, 2D synchronized twisting of superatoms in layers, and 3D acoustic interlayer deformation. These phonons are coupled to the electronic degrees of freedom to varying extents. The presence of local phonon modes in an extended crystal opens the door to controlling material properties from hierarchical phonon engineering.

Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation.

To gain control over magnetic order on ultrafast time scales, a fundamental understanding of the way electron spins interact with the surrounding crystal lattice is required. However, measurement and analysis even of basic collective processes such as spin-phonon equilibration have remained challenging. Here, we directly probe the flow of energy and angular momentum in the model insulating ferrimagnet yttrium iron garnet. After ultrafast resonant lattice excitation, we observe that magnetic order reduces on distinct time scales of 1 ps and 100 ns. Temperature-dependent measurements, a spin-coupling analysis, and simulations show that the two dynamics directly reflect two stages of spin-lattice equilibration. On the 1-ps scale, spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. This mechanism leads to identical demagnetization of the ferrimagnet's two spin sublattices and to a ferrimagnetic state of increased temperature yet unchanged total magnetization. Finally, on the much slower, 100-ns scale, the excess of spin angular momentum is released to the crystal lattice, resulting in full equilibrium. Our findings are relevant for all insulating ferrimagnets and indicate that spin manipulation by phonons, including the spin Seebeck effect, can be extended to antiferromagnets and into the terahertz frequency range.

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy.

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current js arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.