Ultrafast Optical Pumping Research Articles

X-ray and photoelectron spectroscopy of surface chemistry; from bonding via femtosecond to operando

For the 60th anniversary of Surface Science, I present here a personal account of some of the most significant contributions I have made to the field over the past three decades. The utilisation of X-rays serves as the foundation for these studies, encompassing X-ray spectroscopy for the mapping of surface chemical bonds, probing of surface reactions on ultrafast timescales, and X-ray photoelectron spectroscopy under operando conditions. The direct projection of electronic states onto the adsorbed atom allowed the detection of bonding and anti-bonding states within the d-band model. The selective probing of orbitals of different symmetries on the two atoms in adsorbed N2 provided a fundamental understanding of the nature of diatomic bonding to surfaces. Ultrafast optical pumping and X-ray laser techniques allowed the study of CO undergoing desorption leading to the observation of the precursor state. Pump-probed studies of co-adsorbed CO and O on Ru enabled the means to detect transition state species during catalytic CO oxidation. The use of operando X-ray photoelectron spectroscopy at near-atmospheric pressures opened the door to probe the surface chemistry and gain insight into the reaction mechanism during hydrogenation reactions to produce ammonia, hydrocarbons, methanol and ethanol. By inserting an electrochemical cell into the spectroscopic chamber, both fuel cell and water splitting electrocatalysis could be studied giving insight about the reaction mechanism.

Ultrafast optical pumping induced polarity and chirality reversal of Néel skyrmions in amorphous GdFeCo ferrimagnet

The development of next-generation data storage devices relies on the efficient control of topological spin textures at ultrafast timescales with minimal energy consumption. Here, we theoretically investigate the switching of the magnetic skyrmion in ferrimagnetic GdFeCo utilizing the helicity-independent all-optical switching (HI-AOS) driven by femtosecond laser pulses. Our study demonstrates the switching of Néel skyrmion between the two degenerate skyrmion ground states having opposite polarity and chirality. A systematic study was done by varying the laser fluence, and we found that single-shot skyrmion switching is observed for a range of fluence values, where optically induced magnetization switching is observed. The present study proves that HI-AOS is a potential mechanism for switching magnetic skyrmion at ultrafast timescales. Our results offer significant insight into implementing optical writing skyrmion-based memory devices.

Ultrafast perturbation of magnetic domains by optical pumping in a ferromagnetic multilayer

Ultrafast optical pumping of spatially nonuniform magnetic textures is known to induce far-from-equilibrium spin transport effects. Here, we use ultrafast x-ray diffraction with unprecedented dynamic range to study the laser-induced dynamics of labyrinth domain networks in ferromagnetic CoFe/Ni multilayers. We detected azimuthally isotropic, odd order, magnetic diffraction rings up to fifth order. The amplitudes of all three diffraction rings quench to different degrees within 1.6 ps. In addition, all three of the detected diffraction rings both broaden by 15% and radially contract by 6% during the quench process. We are able to rigorously quantify a 31% ultrafast broadening of the domain walls via Fourier analysis of the order-dependent quenching of the three detected diffraction rings. The broadening of the diffraction rings is interpreted as a reduction in the domain coherence length, but the shift in the ring radius, while unambiguous in its occurrence, remains unexplained. In particular, we demonstrate that a radial shift explained by domain-wall broadening can be ruled out. With the unprecedented dynamic range of our data, our results provide convincing evidence that labyrinth domain structures are spatially perturbed at ultrafast speeds under far-from-equilibrium conditions, albeit the mechanism inducing the perturbations remains yet to be clarified.7 MoreReceived 15 June 2022Revised 15 September 2022Accepted 26 September 2022DOI:https://doi.org/10.1103/PhysRevB.106.144422©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDomain wallsMagnetic domainsMagnetic textureMagnetismMagnetization dynamicsUltrafast demagnetizationUltrafast magnetic effectsUltrafast magnetization dynamicsCondensed Matter, Materials & Applied Physics

Light-Tunable Charge Density Wave Orders in MoTe2 and WTe2 Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure and bandgap. Moreover, optical pumping of the semimetallic 1T' phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

On the preservation of coherence in the electronic wavepacket of a neutral and rigid polyatomic molecule

We present various types of reduced models including five vibrational modes and three electronic states for the pyrazine molecule in order to investigate the lifetime of electronic coherence in a rigid and neutral system. Using ultrafast optical pumping in the ground state (11Ag), we prepare a coherent superposition of two bright excited states, 11B2u and 11B1u, and reveal the effect of the nuclear motion on the preservation of the electronic coherence induced by the laser pulse. More specifically, two aspects are considered: the anharmonicity of the potential energy surfaces and the dependence of the transition dipole moments (TDMs) with respect to the nuclear coordinates. To this end, we define an ‘ideal model’ by making three approximations: (i) only the five totally symmetric modes move, (ii) which correspond to uncoupled harmonic oscillators, and (iii) the TDMs from the ground electronic state to the two bright states are constant (Franck–Condon approximation). We then lift the second and third approximations by considering, first, the effect of anharmonicity, second, the effect of coordinate-dependence of the TDMs (first-order Herzberg–Teller contribution), third, both. Our detailed numerical study with quantum dynamics is meant to be realistic for pyrazine over about 20 femtoseconds, and was further extended so as to probe the effect of such approximations on a model system. We show that long-term revivals of the electronic coherence persist up to the picosecond time range even for the most realistic model.

All-optical tuning of symmetry protected quasi bound states in the continuum

We demonstrate all-optical switching of high quality factor quasibound states in the continuum resonances in broken symmetry GaAs metasurfaces. By slightly breaking the symmetry of the GaAs nanoresonators, we enable leakage of symmetry protected bound states in the continuum (BICs) to free space that results in sharp spectral resonances with high quality factors of ∼500. We tune the resulting quasi-BIC resonances with ultrafast optical pumping at 800 nm and observe a 10 nm spectral blue shift of the resonance with pump fluences of less than 100 μJ cm−2. The spectral shift is achieved in an ultrafast time scale (<2.5 ps) and is caused by a shift in the refractive index mediated by the injection of free carriers into the GaAs resonators. An absolute reflectance change of 0.31 is measured with 150 μJ cm−2. Our results demonstrate a proof-of-concept that these broken symmetry metasurfaces can be modulated or switched at ultrafast switching speeds with higher contrast at low optical fluences (<100 μJ cm−2) than conventional Mie-metasurfaces.

Spin-current-mediated rapid magnon localisation and coalescence after ultrafast optical pumping of ferrimagnetic alloys

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 107 A cm−2. Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.

Optical control, selection and analysis of population dynamics in ultrafast protein X-ray crystallography.

Ultrafast pump-probe X-ray crystallography has now been established at X-ray free electron lasers that operate at hard X-ray energies. We discuss the performance and development of current applications in terms of the available data quality and sensitivity to detect and analyse structural dynamics. A discussion of technical capabilities expected at future high repetition rate applications as well as future non-collinear multi-pulse schemes focuses on the possibility to advance the technique to the practical application of the X-ray crystallographic equivalent of an impulse time-domain Raman measurement of vibrational coherence. Furthermore, we present calculations of the magnitude of population differences and distributions prepared with ultrafast optical pumping of single crystals in the typical serial femtosecond crystallography geometry, which are developed for the general uniaxial and biaxial cases. The results present opportunities for polarization resolved anisotropic X-ray diffraction analysis of photochemical populations for the ultrafast time domain. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Design and Study of Structural Linear and Nonlinear Optical Properties of Chiral [Fe(phen)3]2+ Complexes.

The dependence of nonlinear optical properties upon the spin state in molecular switches is still an unexplored area. Chiral [Fe( phen)3]2+ complexes are excellent candidates for those studies because they are expected to show nonlinear optical properties of interest and at the same time show photoconversion to a short-lived metastable high-Spin state by ultrafast optical pumping. Herein, we present the synthesis, crystallographic, and spectroscopic comparison of chiral [Fe( phen)3]2+ complexes obtained with chiral anions, a new lipophilic derivative of the D2-symmetric (As2(tartrate)2)2-, and D3-symmetric tris(catechol)phosphate(V) (TRISCAT), tris(catechol)arsenate(V) (TRISCAS), and 3,4,5,6-tetrachlorocatechol phosphate(V) (TRISPHAT). Complexes [Fe( phen)3]( rac-TRISCAT)2 (2) and [Fe( phen)3](X-TRISCAS)2 (X = rac (3), Δ (4), Λ (5)) were found to be isomorphous in the R32 Sohncke space group with twinning by inversion correlated with the starting chiral anion optical purity. The structures show the [Fe( phen)3]2+ complex interacting strongly along its 3-fold axis with two anions. Only the structure of a [Fe( phen)3]( rac-TRISPHAT)2 solvate (6) could be obtained, which showed no particular anion/cation interaction contrary to what was observed previously in solution. The [Fe( phen)3](X-As2(tartrate)2) (X = Δ (7), Λ (8), and racemic mixture (9)) crystallizes in enantiomorphic space groups P3121/ P3221 with the same solid-state packing. Dichroic electronic absorption studies evidenced racemization for all chiral complexes in solution due to ion pair dissociation, whereas the asymmetric induction is conserved in the solid state in KBr pellets. We evidenced on chiral complexes 4 and 5 strong nonlinear second harmonic generation, the intensity of which could be correlated with the complex electronic absorption.

Ultrafast carrier dynamics and optical pumping of lasing from Ar-plasma treated ZnO nanoribbons

It is a well-known fact that ZnO has been one of the most studied wide bandgap II–VI materials by the scientific community specifically due to its potential for being used as exciton-related optical devices. Hence, realizing ways to increase the efficiency of these devices is important. We discuss a plasma treatment technique to enhance the near-band-edge (NBE) excitonic emission from ZnO based nanoribbons. We observed an enhancement of the NBE peak and simultaneous quenching of the visible emission peak resulting from the removal of surface traps on these ZnO nanoribbons. More importantly, we report here the associated ultrafast carrier dynamics resulting from this surface treatment. Femtosecond transient absorption spectroscopy was performed using pump-probe differential transmission measurements shedding new light on these improved dynamics with faster relaxation times. The knowledge obtained is important for improving the application of ZnO based optoelectronic devices. We also observed how these improved carrier dynamics have a direct effect on the threshold and efficiency of random lasing from the material.

A 10-mJ-level compact CPA system based on Yb:KGW for ultrafast optical parametric amplifier pumping

A compact diode-pumped Yb:KGW regenerative amplifier producing 10 Hz, 10-mJ-level picosecond pulses at 1,040 nm is demonstrated. The system is used at the new front end of the PHELIX petawatt laser system to pump an ultrafast optical parametric amplifier for temporal contrast enhancement. Before frequency doubling, a pulse length of ∼1 ps is obtained by using a stretcher/compressor system based on a single large-aperture chirped volume Bragg grating.

Ultrafast above-transition-temperature resurrection of spin density wave driven by coherent phonon generation in BaFeAs

A model incorporating the time-dependent Ginzburg–Landau theory of ultrafast cooperative interplay among charge, spin, and lattice is presented for the pnictide superconductor BaFeAs. Under the ultrafast optical pumping, a pronounced spin–phonon coupling is found to drive the reconstruction of spin density wave (SDW) at (: SDW transition temperature) through the coherent phonon generation. First-principles electronic structure calculation confirms that the macroscopically coherent oscillation of As over a new equilibrium displacement offers a route to the antiferromagnetic SDW state. Our finding explains the recent experimental observation.

Ultrafast interband pumping of quantum-cascade structures: A feasibility study of a THz pulse amplifier

Gain spectra of a three-well GaAs/AlGaAs quantum-cascade structure under interband pumping by ultrashort optical pulses are studied within the framework of the density matrix theory. A mathematical model for intersubband kinetics is derived taking into account many-body interactions and ultrafast interband optical pumping. It is found that pulsed interband pumping leads to an increase in intersubband gain by an order of magnitude, which is characterised by 1 ps rise time and 8 ps recovery time under pumping by 100 fs pulses with a peak intensity of 100 MW/cm2. Possible implementations of the concept are highlighted. The results uncover a new possibility for ultrafast switching of the material gain in quantum-cascade structures.

Sub-250 nm light emission and optical gain in AlGaN materials

We investigate the deep-UV optical emission and gain properties of AlxGa1−xN/AlyGa1−yN multiple quantum wells structures. These structures were grown by plasma-assisted molecular-beam epitaxy on 6H-SiC substrates, under a growth mode which promotes various degrees of band-structure potential fluctuations in the form of cluster-like features within the wells. The degree of inhomogeneities in these samples was determined by cathodoluminescence mapping. We measured the TE-polarized amplified spontaneous emission in the sample with cluster-like features and quantified the optical absorption/gain coefficients and gain spectra by the variable stripe length technique under ultrafast optical pumping. A maximum net modal gain of about 120 cm−1 is measured at 4.9 eV. On the other hand, we found that samples with homogeneous quantum wells lead to absorption. Numerical simulations are performed to support our experimental findings.