Probing Ultrafast Dynamics of Ferroelectrics by Time-Resolved Pump-Probe Spectroscopy

Study of rutile TiO2(110) single crystal by transient absorption spectroscopy in the presence of Ce4+cations in aqueous environment. Implication on water splitting

Ce4+cations are commonly used as electron acceptors during the water oxidation to O2reaction over Ir- and Ru-based catalysts. They can also be reduced to Ce3+cations by excited electrons from the conduction band of an oxide semiconductor with a suitable energy level. In this work, we have studied their interaction with a rutile TiO2(110) single crystal upon band gap excitation by femtosecond transient absorption spectroscopy (TAS) in solution in the 350-900 nm range and up to 3.5 ns. Unlike excitation in the presence of water alone the addition of Ce4+resulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO2(ca. 400 nm) with a time constantt= 4-5 ps. This indicated that the Ce4+cations presence has quenched the e-h recombination rate when compared to water alone. In addition to GSB, two positive signals are observed and are attributed to trapped holes (in the visible region, 450-550 nm) and trapped electrons in the IR region (>700 nm). Contrary to expectation, the lifetime of the positive signal between 450 and 550 nm decreased with increasing concentrations of Ce4+. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ce4+at the surface of TiO2(110) and positively charged trapped holes. It was also found that at the very short time scale (<2-3 ps) the fast decaying TAS signal of excited electrons in the conduction band is suppressed because of the presence of Ce4+cations. Results point out that the presence of Ce4+cations increases the residence time (mobility) of excited electrons and holes at the conduction band and valence band energy levels (instead of being trapped). This might provide further explanations for the enhanced reaction rate of water oxidation to O2in the presence of Ce4+cations.

A hidden phase uncovered by ultrafast carrier dynamics in thin Bi2O2Se

Bi2O2Se has attracted intensive attention due to its potential in electronics, optoelectronics, and ferroelectric applications. Despite that, there have only been a handful of experimental studies based on ultrafast spectroscopy to elucidate the carrier dynamics in Bi2O2Se thin films. Besides, different groups have reported various ultrafast timescales and associated mechanisms across films of different thicknesses. A comprehensive understanding in relation to thickness and fluence is still lacking. In this work, we have systematically explored the thickness-dependent Raman spectroscopy and ultrafast carrier dynamics in chemical vapor deposition (CVD)-grown Bi2O2Se thin films on a mica substrate with thicknesses varying from 22.44 nm down to 4.62 nm in both low and high pump fluence regions. Combining the thickness dependence and fluence dependence of the slow decay time, we demonstrate a hidden photoinduced ferroelectric transition in the thinner (<8 nm) Bi2O2Se films below the material damage thresholds, influenced by substrate-induced compressive strain and far-from-equilibrium excitation. Moreover, this transition can be manifested at high electronic excitation densities. Our results deepen the understanding of the interplay between the ferroelectric phase and semiconducting characteristics of Bi2O2Se thin films, offering potential applications in optoelectronic devices that benefit from the ferroelectric transition.

Dynamical Control of Topology in Polar Skyrmions via Twisted Light

Twisted light carries a nonzero orbital angular momentum, that can be transferred from light to electrons and particles ranging from nanometers to micrometers. Up to now, the interplay between twisted light with dipolar systems has scarcely been explored, though the latter bear abundant forms of topologies such as skyrmions and embrace strong light-matter coupling. Here, using first-principles-based simulations, we show that twisted light can excite and drive dynamical polar skyrmions and transfer its nonzero winding number to ferroelectric ultrathin films. The skyrmion is successively created and annihilated alternately at the two interfaces, and experiences a periodic transition from a markedly "Bloch" to "Néel" character, accompanied with the emergence of a "Bloch point" topological defect with vanishing polarization. The dynamical evolution of skyrmions is connected to a constant jump of topological number between "0" and "1" over time. These intriguing phenomena are found to have an electrostatic origin. Our study thus demonstrates that, and explains why this unique light-matter interaction can be very powerful in creating and manipulating topological solitons in functional materials.

Ultrafast entropy production in pump-probe experiments

The ultrafast control of materials has opened the possibility to investigate non-equilibrium states of matter with striking properties, such as transient superconductivity and ferroelectricity, ultrafast magnetization and demagnetization, as well as Floquet engineering. The characterization of the ultrafast thermodynamic properties within the material is key for their control and design. Here, we develop the ultrafast stochastic thermodynamics for laser-excited phonons. We calculate the entropy production and heat absorbed from experimental data for single phonon modes of driven materials from time-resolved X-ray scattering experiments where the crystal is excited by a laser pulse. The spectral entropy production is calculated for SrTiO3 and KTaO3 for different temperatures and reveals a striking relation with the power spectrum of the displacement-displacement correlation function by inducing a broad peak beside the eigenmode-resonance.

Temporal and spatial tracking of ultrafast light-induced strain and polarization modulation in a ferroelectric thin film

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

High-Throughput Scanning Second-Harmonic-Generation Microscopy for Polar Materials

The Materials Genome Initiative aims to discover, develop, manufacture, and deploy advanced materials at twice the speed of conventional approaches. To achieve this, high-throughput characterization is essential for the rapid screening of candidate materials. In this study, a high-throughput scanning second-harmonic-generation microscope with automatic partitioning, accurate positioning, and fast scanning is developed that can rapidly probe and screen polar materials. Using this technique, typical ferroelectrics, including periodically poled lithium niobate crystals and PbZr_0.2 Ti_0.8 O₃ (PZT) thin films are first investigated, whereby the microscopic domain structures are clearly revealed. This technique is then applied to a compositional-gradient (100-x)%BaTiO₃ -x%SrTiO₃ film and a thickness-gradient PZT film to demonstrate its high-throughput capabilities. Since the second-harmonic-generation signal is correlated with the macroscopic remnant polarization over the probed region determined by the laser spot, it is free of artifacts arising from leakage current and electrostatic interference, while materials' symmetries and domain structures must be carefully considered in the data analysis. It is believed that this work can help promote the high-throughput development of polar materials and contribute to the Materials Genome Initiative.

Development of five-dimensional scanning transmission electron microscopy

By combining the scanning transmission electron microscopy with the ultrafast optical pump-probe technique, we improved the time resolution by a factor of ∼10¹² for the differential phase contrast and convergent-beam electron diffraction imaging. These methods provide ultrafast nanoscale movies of physical quantities in nano-materials, such as crystal lattice deformation, magnetization vector, and electric field. We demonstrate the observations of the photo-induced acoustic phonon propagation with an accuracy of 4 ps and 8 nm and the ultrafast demagnetization under zero magnetic field with 10 ns and 400 nm resolution, by utilizing these methods.

Light-Driven Ultrafast Polarization Manipulation in a Relaxor Ferroelectric

Relaxor ferroelectrics have been intensely studied for decades based on their unique electromechanical responses which arise from local structural heterogeneity involving polar nanoregions or domains. Here, we report first studies of the ultrafast dynamics and reconfigurability of the polarization in freestanding films of the prototypical relaxor 0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-0.32PT) by probing its atomic-scale response via femtosecond-resolution, electron-scattering approaches. By combining these structural measurements with dynamic phase-field simulations, we show that femtosecond light pulses drive a change in both the magnitude and direction of the polarization vector within polar nanodomains on few-picosecond time scales. This study defines new opportunities for dynamic reconfigurable control of the polarization in nanoscale relaxor ferroelectrics.