Memory Effect in the Photoinduced Femtosecond Rotation of Magnetization in the Ferromagnetic Semiconductor GaMnAs

Terahertz conductivity of localized photoinduced carriers in Mott insulator YTiO$_{3}$ at low excitation density, contrasted with metallic nature in band semiconductor Si

We performed optical-pump terahertz-probe measurements of a Mott insulator YTiO$_{3}$ and a band semiconductor Si using a laser diode (1.47 eV) and a femtosecond pulse laser (1.55 eV). Both samples possess long energy-relaxation times (1.5 ms for YTiO$_{3}$ and 15 $μ$s for Si); therefore, it is possible to extract terahertz complex conductivities of photoinduced carriers under equilibrium. We observed highly contrasting behavior - Drude conductivity in Si and localized conductivity possibly obeying the Jonscher law in YTiO$_{3}$. The carrier number at the highest carrier-concentration layer in…

The photoinduced transition in magnetoresistive manganites: a comprehensive view

We use femtosecond x-ray diffraction to study the structural response of charge and orbitally ordered Pr$_{1-x}$Ca$_x$MnO$_3$ thin films across a phase transition induced by 800 nm laser pulses. By investigating the dynamics of both superlattice reflections and regular Bragg peaks, we disentangle the different structural contributions and analyze their relevant time-scales. The dynamics of the structural and charge order response are qualitatively different when excited above and below a critical fluence $f_c$. For excitations below $f_c$ the charge order and the superlattice is only partially…

Hole spin relaxation and coefficients in Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs

We investigate the temperature dependence of the coefficients in the Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs by employing the Zener model. We first calculate the hole spin relaxation time based on the microscopic kinetic equation. We find that the hole spin relaxation time is typically several tens femtoseconds and can present a nonmonotonic temperature dependence due to the variation of the interband spin mixing, influenced by the temperature related Zeeman splitting. With the hole spin relaxation time, we are able to calculate the coefficients in the Landau-Lifshitz-Gilbert …

钙钛矿纳米材料中光致反应的原位与实时超快光谱研究

In situ and real-time ultrafast spectroscopy of photoinduced reactions in perovskite nanomaterials

本研究开发了异步干涉瞬态吸收（AI-TA）方法，利用同步飞秒激光器实现了对钙钛矿纳米材料中光致反应的原位实时观察，揭示了光取代与光致合并过程中的载流子动力学变化。

Ultrafast Spectroscopy of Mid-Infrared Internal Exciton Transitions of Separated Single-Walled Carbon Nanotubes

We report a femtosecond mid-infrared study of the broadband low-energy response of individually separated (6,5) and (7,5) single-walled carbon nanotubes. Strong photoinduced absorption is observed around 200 meV, whose transition energy, oscillator strength, resonant chirality enhancement and dynamics manifest the observation of quasi-1D intra-excitonic transitions. A model of the nanotube 1s-2p cross section agrees well with the signal amplitudes. Our study further reveals saturation of the photoinduced absorption with increasing phase-space filling of the correlated e-h pairs.

Photoinduced Band Renormalization Effects in ZrSiS Topological Nodal-line Semimetal

Out-of-equilibrium effects provide an elegant pathway to probing and understanding the underlying physics of topological materials. Creating exotic states of matter using ultrafast optical pulses in particular has shown promise towards controlling electronic band structure properties. Of recent interest is band renormalization in Dirac and Weyl semimetals as it leads to direct physical observables through the enhancement of the effective mass, or, in the shift of resonant energies. Here we provide experimental and theoretical signatures of photo-induced renormalization of the electronic band s…

Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon

In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser micromachining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study …

飞秒级分子在二维材料界面的协同旋转

Femtosecond concerted rotation of molecules on a 2D material interface

该研究通过超快光谱技术揭示了分子在二维材料界面的协同旋转现象，展示了光诱导电荷转移对宏观分子旋转和手性排列的影响，为设计可调界面动力学的分子系统提供了新路径。

铁磁性薄膜上的手性血红素单分子层：杂化界面的超快光谱学

Chiral Porphyrin Monolayers on Ferromagnetic Thin Films: Ultrafast Spectroscopy of Hybrid Interfaces

研究铁磁多层结构对含血红素染料的分子动态的影响，发现表面吸附缩短激发态寿命，但未观察到长寿命电荷转移产物。

飞秒级自旋态切换动力学：薄 films 中的自旋交叉分子

Femtosecond spin-state switching dynamics of spin-crossover molecules condensed in thin films

研究通过超快X射线光谱揭示薄 films 中自旋交叉分子在超快时间尺度的自旋态切换动力学及三重态中间态的参与。

线偏振脉冲诱导的超快磁化在非磁性半导体中普遍存在

Ultrafast magnetization induced by linearly polarized pulses is widespread in nonmagnetic semiconductors

通过高通量第一性原理筛选，发现近440种非磁性半导体在线偏振飞秒脉冲光激发下可产生自旋极化，揭示了晶体场和能带边缘轨道特征对光致磁化的调控规律。

THz lattice vibrations for active plasmonics with light: Ultrafast optical response in gold/telluride hybrid plasmonic crystals

Excitation of coherent optical phonons in solids provides a pathway for ultrafast modulation of light on a sub-ps timescale. Here, we report on efficient 3.6 THz modulation of light reflected from hybrid metal/semiconductor plasmonic crystals caused by lattice vibrations in a few nm thick layer of elemental tellurium. We observe that surface plasmon polaritons contribute significantly to photoinduced formation of this thin layer at the interface between a telluride-based II-VI semiconductor, such as (Cd,Mg)Te or (Cd,Mn)Te, and a one-dimensional gold grating. The change in interface composition…