This PDF file contains the front matter associated with SPIE Proceedings Volume 12132, including the Title Page, Copyright information, Table of Contents, and Committee Page.

Excitons, quasi-particles generated by the Coulomb interaction between an excited electron and a hole, have been proposed as a possibility to overcome the limits of electronics. To fully exploit them, a deeper understanding of their interaction with light is required. In this study, the interaction of a core exciton with an intense, few-femtosecond infrared pulse is investigated with attosecond transient reflection spectroscopy in a bulk monocrystalline MgF₂ sample. The distinct few- and sub-femtosecond optical responses attest the dual, atomic- and solid-like, nature of core excitons. Sub-femtosecond dynamics, in particular, are dictated by the interplay between the exciton and the conduction band of the crystal. Theoretical simulations allow to propose the exciton binding energy as a lever to control exciton dynamics on an ultrafast timescale.

Studying magnetization configurations of ever more complex magnetic structures has become a major challenge in the past decade, especially at ultrashort timescales. Most of current approaches are based on the analysis of polarization and magnetization-dependent reflectivity. We introduced a different concept, centered on the coupling of magnetic structures with light beams carrying orbital angular momentum (OAM), which was recently tested it in an experiment with magnetic vortices. Upon reflection by a magnetic vortex, an incoming beam with a well-defined OAM ℓ gets enriched in the neighboring OAM modes ℓ ± 1. It results in anisotropic far-field profiles, which leads to a magnetic helicoidal dichroism (MHD) signal. In this paper we provide a detailed analysis of MHD for the case of a magnetic vortex, providing an intuitive explanation in terms of transverse MOKE. The analysis of MHD allows to retrieve the complex magneto-optical constants. This method, which does not require any polarimetric measurement, is a new promising tool for the identification and analysis of magnetic configurations such as vortices, with a possible extension to the femtosecond to attosecond time resolution.

In this work, we revisit the DFT-based results for the electron-phonon scattering in highly excited silicon. Using state-of-the-art ab initio methods, we examine the main scattering channels which contribute to the total electron-phonon scattering rate and to the energy loss rate of photoexcited electrons in silicon as well as their temperature dependence. Both temperature dependence and the main scattering channels are shown to strongly differ for the total electron-phonon scattering rate and for the energy loss rate of photoexcited electrons. Whereas the total electron-phonon scattering rate increases strongly with temperature, the temperature dependence of the energy loss rate is negligible. Also, while acoustic phonons dominate the total electron-phonon scattering rate at 300 K, the main contribution to the energy loss rate comes from optical modes.

The control over the longitudinal and transverse wave functions of free electrons has recently experienced an impressive boost thanks to advances in electron microscope instrumentation, particularly in combination with ultrafast light pulses and the ability to synthesize femtosecond electron wave packets. In this presentation, we overview key concepts describing the associated interactions between free electrons, light, and photonic nanostructures, making emphasis on quantum aspects and exploring several remaining challenges and emerging opportunities. We further discuss potential applications in noninvasive spectroscopy and microscopy, the possibility of sampling the nonlinear optical response at the nanoscale, the manipulation of the density matrices associated with free electrons and optical sample modes, optical modulation of electron beams, and improved schemes for electron-driven light emission over a wide range of photon energies.

Ultra-short laser material processing has received much attention due to the broad applications across nearly all manufacturing sectors. Ultra-short laser ablation is a complex phenomenon involving laser energy spatial distribution, energy absorption on the irradiated surface, transient changes in optical response, and ablation. In order to determine the ablation characteristics and performance, a fundamental study of the interaction between ultra-short laser pulses and the material will be valuable. A theoretical analysis of ultra-short laser-matter interaction can be represented by the two-temperature model which describes the temperature of the electron or carrier and lattice in non-equilibrium conditions when ultra-short laser pulses are applied. During ultrafast irradiation, due to peculiarities between the metal energy absorption to in contrast to semiconductor, a comparative study of silicon and gold ablation mechanism presented. A 2D axial symmetry simulated ablation profiles were compared with the experimental result at fluence ranging from 1 J/cm² to 9 J/cm² at the wavelength of 515 nm and 180 fs laser on the silicon and gold sample. The concordance between model calculations and experimental data demonstrates that fs laser ablation of silicon is thermal in nature in a low fluence regime, whereas it is non-thermal in a high-fluence regime. On the other hand, the phase explosion mechanism is prevalent to understand the ablation characteristics of gold with fs pulses. Fundamental information such as the time evolution of the carrier density in silicon, carrier or electron temperature evolution, and lattice temperature evolution can be obtained from the simulation results.

The generation of spin current pulses by laser-driven demagnetization links the field of ultrafast magnetism to spintronics. So far, this spin transport and its cause could only be observed indirectly. We demonstrate that femtosecond spin injection can be observed on the femtosecond time scale by spin and time resolved photoemission experiments.

We study thin, epitaxial iron films which are excited by a 800 nm pump laser beam. Photoemission by a higher harmonic generation source (photon energy: 21 eV) in combination with an electron spin polarimeter is used to measure the chemical potentials of the minority and majority electrons. This way, we observe the spin voltage, which acts as the driving force for the spin current.

If we deposit a thin gold film onto the iron sample and excite the iron film through the transparent substrate, we can study spin injection and accumulation. The spin polarization in Au rises on the femtosecond time scale and decays within < 1 ps. The decay time depends on the Au film thickness. This thickness dependence can be described by a "spin capacitance," which is similar to the capacitance in charge-based electronics.

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions among others. The presentation will focus on the behavior of chiral DWs at ultrashort timescale after optical pumping in perpendicularly magnetized asymmetric multilayers. The magnetization dynamics is probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). In the first 2 picosecond, a transient reduction of the CD-XRMS asymmetry ratio is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. On the one hand, this time-varying change of the DW texture shortly after the laser pulse is identified as a distortion of the homochiral Néel shape toward a transient mixed Bloch-Néel-Bloch texture along a direction transverse to the DW due to the coherent torque. On the other hand, the overall effect of the spin current incoherent torque results in an average loss of angular momentum that induces an increase of the spin relaxation processes within the DW at the ps timescale. It leads to a faster remagnetization inside the DWs compared to domains.

Exploring the ultrafast dynamics of quantum materials is crucial to understand their out-of-equilibrium behavior and the properties of their excited states. By using multi-dimensional time-resolved photoemission, we show that the dichroism in the photoemission yield is related to their orbital and spin texture, both for the filled and the empty electronic states. This is possible thanks to the use of an angle-resolved time-of-flight detector, which makes it possible to effectively measure the overall photoelectron yield and electronic band structure in reciprocal space. We present some relevant examples of this spectroscopic approach, which exploits the polarization control of ultrafast laser sources, applied to the study of Dirac fermions in prototype topological materials such as Bi₂Se₃ and Bi₂Te₂Se.

Two-dimensional electron gases are an essential building block of today’s technology and attract broad interest in the context of material science or nanoengineering. Nowadays, 2DEGs are employed for computing, metrology, spin to charge conversion, and optoelectronics. Usually, these low-dimensional channels spontaneously form or can be electrostatically induced at the interface of polar materials. The polar scattering can limit the electron mobility if electrons attain a temperature comparable to the LO phonon frequency. The remote LO coupling is of major concern in gate insulators with a high relative dielectric constant. In order to uncover the relevance of remote phonon coupling, we perform here the ultrafast spectroscopy of an accumulation layer. The quasi-2DEG is obtained by evaporating cesium (Cs) atoms on the surface of indium selenide (InSe) at low temperature and in ultrahigh vacuum conditions. This doping method simulates, with good accuracy, the electrostatic gating and can be easily implemented in our experiment. The choice of polar material is motivated by the fact that InSe is one of the best van der Waals structures for the fabrication of FET devices. It has an electronic gap comparable to silicon, small effective mass, layered structure, and carrier mobility higher than transition metal dichalcogenides. The electronic states and distribution function of hot electrons in the accumulation layer is directly monitored by time- and angle resolved photoelectron spectroscopy (tr-ARPES). We find that, hot electrons in quasi-2DEGs display a remote coupling to polar optical phonons persisting up to high electronic density. The accurate modeling of such interaction should include the wavefunctions of confined 2D electrons, dynamical screening effects, surface plasmons polaritons, and interface phonons. Nonetheless, the static screening of bulk phonons by 3D electrons can quantitatively reproduce the experimental cooling rate. This finding highlights that electrons in the accumulation layers or 2D conductors at the interface with a polar medium experience 3D dissipation channels. The outcome is of high relevance for the carriers’ mobility in FET devices with high  dielectric gates, van der Waals heterostructures, and 2DEGs at the interface between oxides.

The interplay between various degrees of freedom in laser induced ultrafast magnetization dynamics (LIUMD) of magnetic alloys is intricate due to the competition between different mechanisms and processes. In this work, we resolve the element specific magnetization dynamics of FePd alloy and further elucidate the dependency of the OISTR mechanism on the laser pulse parameters using ultrashort, short and relatively longer pulse duration with weak and strong fluence. Remarkably, our results illustrate potential discrepancies in experiments measuring the optical inter site spin transfer (OISTR) effect in magnetic alloys.

A fundamental study of the interaction of ultrashort pulses and metal will be useful for predicting the ablation morphology and optimizing the process parameters. To study the ultrashort laser pulse interaction on gold, a set of coupled partial differential equations of the two-temperature model was solved in the spatial and time domains with dynamic optical properties and phase explosion mechanism. In an extended Drude model which also takes into account inter-band transitions, the reflectivity and absorption coefficient are contemplated based on the electron relaxation time. The laser energy deposition and phase explosion ablation mechanism are analyzed in the case of succession of laser pulses on the gold with experimental results for fundamental wavelength 1030 nm and fluence ranging from 3 J/cm² to 18 J/cm². Electron-lattice thermal relaxation time and separation time are important factors for multi-pulse laser ablation and have been studied. The simulation results demonstrate that by increasing the number of pulses with a shorter separation time compared to electron-lattice thermal relaxation time, lattice temperature can be considerably increased without a noticeable increase in ablation depth. In the study of multiple pulses femtosecond laser ablation, the computational model indicates that succession of laser pulses with a pulse separation time of 50 ps or longer can significantly boost the ablation rate at the same laser fluence. Thus, the deviation from experimental and simulation results gives rise to the conclusion that temporal pulse manipulation with separation time greater than the electron-lattice relaxation time is a useful technique for increasing ablation rate in industrial fast femtosecond laser processing.