This PDF file contains the front matter associated with SPIE Proceedings Volume 12992, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The quest for manipulation of magnetization on ultrafast timescales faces many technological challenges. Successful achievement thereof could shed light on novel fundamental phenomena, such as inertial magnetization dynamics, as well as accelerate technological advancements towards higher information processing rates. One of the recent approaches towards this end concerns excitation of magnetization dynamics via laser-induced picosecond acoustic pulses, which has given birth to the field of ultrafast magneto-acoustics. Considerable progress has been made in the field from an experimental point of view, as well as from the perspective of theoretical modelling. In this talk, we aim to review some of the aforementioned progress and propose the frequency dependent cooperativity parameter (strong coupling regime) to measure the efficiency of resonantly enhanced phonon-magnon interactions in the GHz-to-THz frequency range.

Nanometer-thick multilayer structures, characterized by contrasts in elastic properties, present promising avenues for engineering and manipulating acoustic phonons at the nanoscale. Semiconductor nano-acoustic cavities, particularly those based on Distributed Bragg Reflectors (DBRs), have demonstrated unique capabilities in simultaneously confining light and acoustic phonons. This dual confinement enhances the generation and detection phononic fields, making these structures attractive for ultra-high-frequency applications and as platforms for simulating solid-state systems. In this study, we further explore the possibilities of hybrid nanostructures that could be both tunable and responsive to ultrafast changes in elastic properties induced by external stimuli such as temperature, humidity, and electrical fields. Building upon our theoretical simulations, our experimental investigation focuses on the dynamics of acoustic phonons spanning the frequency range of 5-500 GHz, utilizing near-infrared pump and probe ultrafast transient reflectivity experiments. The materials under investigation include mesoporous SiO₂/TiO₂ multilayers with a Nickel transducer, GaAs/AlAs DBR incorporating mesoporous SiO₂ as an open cavity layer, YBCO/STO multilayers, and other potential responsive materials. Our long-term objective is to uncover the interplay between these nanostructures and external stimuli through systematic experimentation, shedding light on their tunability and responsiveness. Our experimental findings pave the way for developing nanoacoustic sensing technologies and reconfigurable optoacoustic nanodevices.

We employ attosecond core-level x-ray absorption spectroscopy to unravel the intricate many-body interactions occurring in strongly photo-excited semi-metallic graphite. This allows us to unveil remarkably high conductivity and delineate deviations from the single particle bandstructure.

Ultrafast photoreactions are governed by multidimensional excited-state Potential Energy Surfaces (PESs), which describe how the molecular potential varies with the nuclear coordinates. Nature has tailored electronically excited PESs, in which the molecular geometry is specifically modified from the ground state equilibrium configuration to efficiently convert the absorbed light energy into specific nuclear rearrangements, driving the system photochemistry. This can be rationalized by the displacements between two different PESs, crucial quantities that are encoded in the Franck-Condon overlap integrals. Conventional spectroscopic approaches probe transition amplitudes, only accessing the absolute value of nuclear displacements; herein we introduce an experimental technique, based on broadband impulsive Raman response to directly measure the magnitude and the sign of excited state displacements, revealing the first steps of photoreaction processes.

This work presents findings from femtosecond time-resolved spectroscopy on LaVS3, revealing interactions between carriers and phonon modes. It elucidates the effects on thermal conductivity and the emergence of metastable states due to carrier trapping within vanadium clusters.

The progress in DFT-based description of the electron-phonon scattering allowed to describe the relaxation dynamics of hot or photoexcited electrons in several materials, in very good agreement with time-resolved spec- troscopy experiments [1-3]. As hot carriers also start to attract attention in the context of emerging concepts for energy conversion, here we present our first results related to the coupling of ab initio data with device-oriented Monte Carlo simulation methods [4]. We show that DFT-based description of the electron-phonon intervalley scattering in GaAs, coupled with stochastic Monte Carlo method, allows to describe the energy transfer from electrons to phonons in transient regime, in good agreement with previous time-resolved photoemission experiments. [1] J. Sjakste et al, J. Phys: Cond. Mat. 30, 353001 (2018). [2] Chen, Sjakste et al, PNAS 117, 21962-21967 (2020). [3] H. Tanimura et al, Phys. Rev. B 100, 035201 (2019). [4] R. Sen, N. Vast, J. Sjakste, Appl. Phys. Lett. 120, 082101 (2022).

We present findings on High Harmonic Generation (HHG) in solids utilizing a high-energy fiber laser system operating at 1550 nm. The driving laser source comprises an Erbium-Doped Fiber chirped pulse Amplifier (EDFA) combined with a post-compression stage employing a hollow-core photonic crystal fiber (HC-PCF) filled with noble gases. Nonlinear self-compression in the HC-PCF enables the generation of ultrashort pulses with a duration of 50 fs and energy of 0.91 μJ at a repetition rate of 660 kHz. In a first step, harmonics up to H7 were observed when focusing the laser into small bandgap materials such as Zinc Oxide (ZnO). Subsequently, the system was enhanced to measure high harmonics in the extreme ultraviolet (XUV) range, with harmonics up to H25 observed using a large bandgap material, magnesium oxide (MgO). To the best of our knowledge, this represents the first solid-state HHG source driven by a high-energy few-cycle fiber laser in the telecom region.

High-order harmonics were generated from mono- and polycrystaline molybdenum disulfide (MoS₂) monolayers with an infrared femtosecond pulse. We control the Orbital Angular Momentum (OAM) and spatial polarization distribution of the generation beam by using a liquid crystal Q-plate. We then measure the OAM and the full polarization map of the emitted harmonics. We observe that monocrystaline MoS₂ behaves as a polarization converter, while polycrystaline MoS₂ may be used as a phase mask.

We investigate here the influence of pulse duration on high harmonic spectra in a wide band gap dielectric, MgO. Employing a Ti:S pulse compressed in a hollow core fiber, we can effectively tune the output pulse duration from 30 fs to 4.5 fs. By systematically varying both intensity and pulse duration, we explore macroscopic scaling laws governing harmonic generation in solids. We examine the cut-off energy, divergence, and emission wavelength. Notably, we show in the measured harmonics spectra differences in the highest energy reached, the shape of the harmonics and their center emitted energy. Pulse duration significantly impacts the emission process, thereby modulating the spectro-spatial characteristics of the generated harmonics, even at constant intensity levels.

We investigate Anomalous High-Harmonic Generation (AHHG) within a model undergoing a transition from a Weyl semimetal with broken time-reversal symmetry to a semi-Dirac regime. The latter represents a gapless semimetal with a parabolic dispersion in one direction and conical dispersion in the other two. We highlight the broadening of the distribution of excitations in the Brillouin zone and peaks in the AHHG response, observed as a function of the parameter governing the separation of Weyl nodes, as the frequency of the laser pulse increases. Furthermore, we explain the splitting of these peaks upon an increase in the frequency when multiple semi-Dirac points coexist in the Brillouin zone.

High-harmonic spectroscopy is an all-optical nonlinear technique with inherent attosecond temporal resolution. It has been applied to a variety of systems in the gas phase and solid state. Here we extend its use to liquid samples. By studying high-harmonic generation over a broad range of wavelengths and intensities, we show that the cut-off energy is independent of the wavelength beyond a threshold intensity and that it is a characteristic property of the studied liquid. We explain these observations with a semi-classical model based on electron trajectories that are limited by the electron scattering. This is further confirmed by measurements performed with elliptically polarized light and with ab-initio time-dependent density functional theory calculations. Our results propose high-harmonic spectroscopy as an all-optical approach for determining the effective mean free paths of slow electrons in liquids.

High-Harmonic Generation (HHG) is a highly non-linear frequency up conversion process, mostly studied from a classical point of view. Recently, independent theoretical investigations about the quantum nature of HHG predicted several, non-classical effects in the high-harmonic radiation. In addition to the fundamental interest in understanding the physics behind HHG, a better understanding of the quantum nature of this process could potentially have a broad impact on the rapidly developing field of quantum technologies. It is in this context that present here our experimental photon statistics investigations showing the quantum nature of the HHG process.

We demonstrate non-resonant optical control with bi-circular laser fields. Having the pump polarization simultaneously break time and space inversion symmetry allows for switching between K, K’ valleys of MoS₂ even in its inversion symmetric bulk phase. This strong field effect enables a universal and material agnostic approach to valleytronics.

Optophononic resonators based on GaAs/AlAs multilayer structures can confine near-infrared photons and sub-terahertz phonons. Previous works have studied the generation and detection of coherent acoustic phonons in planar and circular micropillar cavities. However, these structures exhibit only one optical cavity mode, which makes it challenging to reach the efficient generation and sensitive detection simultaneously in a standard pump-probe experiment. Here, we propose the use of elliptical micropillars to reach an improved condition of these two processes. The elliptical cross-section of the microcavity lifts the degeneracy of the fundamental optical modes, related to each of its major and minor axes. By tuning the pump pulses in resonance with one optical mode, the generation efficiency is enhanced by maximizing the electromagnetic field inside the cavity. Meanwhile, the probe pulses at the same wavelength and with orthogonal polarization detects phonons at the slope of the other mode, where it is sensitive to reflectivity changes. We experimentally demonstrated that the phonon amplitude is enhanced by introducing the ellipticity compared to the circular micropillar. This improvement is promising for future developments in constructing efficient phonon transducers.

We have studied the Hubbard model on a dimer under the effect of an intense ultra-short laser pulse using the Dynamical Projective Operatorial Approach (DPOA). This shows how DPOA, stemming from the Composite Operator Method, can effectively tackle strongly correlated systems and real materials. To study the effect of the pump pulse on the anti-ferromagnetic-like order, we have analyzed the behavior of the double occupancy. It turns out that the pulse, in a specific range of its parameters, can drive the system in and out of the otherwise very robust anti-ferromagnetic-like phase that naturally emerges at half-filling and low temperatures. In such a situation, the system undergoes Rabi-like oscillations.

Recent experimental advancements in ultrafast pump-probe setups drive a growing need for efficient and insightful theoretical methods. This study will delve into the intricacies of our newly developed approach, the Dynamical Projective Operatorial Approach (DPOA). DPOA stands out as a highly efficient model-Hamiltonian method capable of calculating a wide range of single/multi-time single/multi-particle properties, such as excitation populations, out-of-equilibrium Green's functions, TR-ARPES signals, and various response functions. When dealing with transient optical properties, utilizing the DPOA formulation and capitalizing on the weak intensity of the probe pulse, we efficiently compute the linear response of the pumped system to a generic probe pulse, significantly accelerating calculations. Alongside the theoretical framework, we present noteworthy results that experimentalists can utilize to establish connections between observed effects and their underlying physical phenomena.