Direct evidence of correlation between the second harmonic generation anisotropy patterns and the polarization orientation of perovskite ferroelectric

Supramolecular Metal Halide Complexes for High-Temperature Nonlinear Optical Switches

Nonlinear optical (NLO) switching materials, which exhibit reversible intensity modulation in response to thermal stimuli, have found extensive applications across diverse fields including sensing, photoelectronics, and photonic applications. While significant progress has been made in solid-state NLO switching materials, these materials typically showcase their highest NLO performance near room temperature. However, this performance drastically deteriorates upon heating, primarily due to the phase transition undergone by the materials from noncentrosymmetric to centrosymmetric phase. Here, we introduce a new class of NLO switching materials, solid-state supramolecular compounds 18-Crown-6 ether@Cu2Cl4·4H2O (1·4H2O), exhibiting reversible and stable NLO switching when subjected to near-infrared (NIR) photoexcitation and/or thermal stimuli. The reversible crystal structure in response to external stimuli is attributed to the presence of a weakly coordinated bridging water molecule facilitated by hydrogen bonding/chelation interactions between the metal halide and crown-ether supramolecules. We observed an exceptionally high second-harmonic generation (SHG) signal under continuous photoexcitation, even at temperatures exceeding 110 °C. In addition, the bridging water molecules within the complex can be released and recaptured in a fully reversible manner, all without requiring excessive energy input. This feature allows for precise control of SHG signal activation and deactivation through structural transformations, resulting in a high-contrast off/on ratio, reaching values in the million-fold range.

Local measurement of terahertz field-induced second harmonic generation in plasma filaments

The concept of Terahertz Field-Induced Second Harmonic (TFISH) Generation is revisited to introduce a single-shot detection scheme based on third order nonlinearities. Focused specifically on the further development of THz plasma-based sources, we begin our research by reimagining the TFISH system to serve as a direct plasma diagnostic. In this work, an optical probe beam is used to mix directly with the strong ponderomotive current associated with laser-induced ionization. A four-wave mixing (FWM) process then generates a strong second-harmonic optical wave because of the mixing of the probe beam with the nonlinear current components oscillating at THz frequencies. The observed conversion efficiency is high enough that for the first time, the TFISH signal appears visible to the human eye. We perform spectral, spatial, and temporal analysis on the detected second-harmonic frequency and show its direct relationship to the nonlinear current. Further, a method to detect incoherent and coherent THz inside plasma filaments is devised using spatio-temporal couplings. The single-shot detection configurations are theoretically described using a combination of expanded FWM models with Kostenbauder and Gaussian Q-matrices. We show that the retrieved temporal traces for THz radiation from single- and two-color laser-induced air-plasma sources match theoretical descriptions very well. High temporal resolution is shown with a detection bandwidth limited only by the spatial extent of the probe laser beam. Large detection bandwidth and temporal characterization is shown for THz radiation confined to under-dense plasma filaments induced by < 100 fs lasers below the relativistic intensity limit.

Local symmetry breaking drives picosecond spin domain formation in polycrystalline halide perovskite films

Photoinduced spin-charge interconversion in semiconductors with spin-orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. However, in structurally disordered polycrystalline semiconductors, which are being widely explored for device applications, the presence and role of spin-associated charge currents remains unclear. Here, using femtosecond circular-polarization-resolved pump-probe microscopy on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin domains on the micrometre scale formed through lateral spin currents. Micrometre-scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spin-domain formation is driven by the presence of strong local inversion symmetry breaking via structural disorder. We propose that this leads to spatially varying Rashba-like spin textures that drive spin-momentum-locked currents, leading to local spin accumulation. Ultrafast spin-domain formation in polycrystalline halide perovskite films provides an optically addressable platform for nanoscale spin-device physics.

Enhanced second harmonic generation in laser-induced air plasma

We report a systematic investigation into the processes behind a near hundred-fold enhanced second harmonic wave generated from a laser-induced air plasma, by examining the temporal dynamics of the frequency conversion processes, and the polarization of the emitted second harmonic beam. Contrary to typical nonlinear optical processes, the enhanced second harmonic generation efficiency is only observed within a sub-picosecond time window and found to be nearly constant across fundamental pulse durations spanning from 0.1 ps to over 2 ps. We further demonstrate that with the adopted orthogonal pump-probe configuration, the polarization of the second harmonic field exhibits a complex dependence on the polarization of both input fundamental beams, contrasting with most of the previous experiments with a single-beam geometry.

Octahedral Distortion and Displacement-Type Ferroelectricity with Switchable Photovoltaic Effect in a 3d^{3}-Electron Perovskite System

Because of the half-filled t_{2g}-electron configuration, the BO_{6} octahedral distortion in a 3d^{3} perovskite system is usually very limited. In this Letter, a perovskitelike oxide Hg_{0.75}Pb_{0.25}MnO_{3} (HPMO) with a 3d^{3} Mn^{4+} state was synthesized by using high pressure and high temperature methods. This compound exhibits an unusually large octahedral distortion enhanced by approximately 2 orders of magnitude compared with that observed in other 3d^{3} perovskite systems like RCr^{3+}O_{3} (R=rare earth). Essentially different from centrosymmetric HgMnO_{3} and PbMnO_{3}, the A-site doped HPMO presents a polar crystal structure with the space group Ama2 and a substantial spontaneous electric polarization (26.5  μC/cm^{2} in theory) arising from the off-center displacements of A- and B-site ions. More interestingly, a prominent net photocurrent and switchable photovoltaic effect with a sustainable photoresponse were observed in the current polycrystalline HPMO. This Letter provides an exceptional d^{3} material system which shows unusually large octahedral distortion and displacement-type ferroelectricity violating the "d^{0}-ness" rule.

Optical properties of Pb0.52Zr0.48TiO3 nanorod arrays: second harmonic generation and multiphoton carrier dynamics

Nonlinear optical properties of poled and unpoled, lead zirconate-titanate (Pb0.52Zr0.48TiO3) nanorod arrays, grown on Pt-coated Si with ∼200 nm diameter and ∼600 nm height, were investigated. Clear signatures of second harmonic generations (SHG), from 490–525 nm (2.38–2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties of photoexcited carriers in the range of 690–1000 nm where multiphoton processes were responsible for the photo-excitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼3.1 eV) was close to the band gap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications.