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Photoinduced valence tautomerism of a cobalt-dioxolene complex revealed with femtosecond M-edge XANES
摘要： Cobalt complexes that undergo charge-transfer induced spin-transitions or valence tautomerism from low spin CoIII to high spin (HS) CoII are potential candidates for magneto-optical switches. We use M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy with 40 fs time resolution to measure the excited-state dynamics of CoIII(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p → 3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin CoII ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin CoII in 67 fs. Vibrational cooling from this hot HS CoII state competes on the hundreds-of-fs time scale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS CoII state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M2,3-edge XANES to measure ultrafast photophysics of molecular Co complexes.
关键词： valence tautomerism,ultrafast spectroscopy,M-edge XANES,Cobalt complexes,high-harmonic generation
更新于2025-09-19 17:13:59
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Cavity-Enhanced Noncollinear High-Harmonic Generation
摘要： The generation of extreme-ultraviolet (XUV) attosecond pulse trains (APTs) by the process of high harmonic generation (HHG) has enabled the investigation of electronic dynamics in atoms, molecules and solids. Enhancing the pulse energy impinging on the target gas by usually one to two orders of magnitude by means of an external, passive resonator (enhancement cavity, EC) permits efficient HHG at repetition rates exceeding 10 MHz, enabling applications in XUV precision spectroscopy and space-charge-limited regimes of photoelectron or coincidence spectroscopy.
关键词： attosecond pulse trains,high harmonic generation,extreme-ultraviolet,XUV precision spectroscopy,enhancement cavity
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Ultrafast Extreme-Ultraviolet Photoemission Spectroscopy at 18.4 MHz
摘要： In the past two decades, attosecond science has led to important insights into ultrafast electron dynamics. Whilst extreme-ultraviolet (XUV) photon flux attained from high harmonic generation (HHG) has increased significantly over this time, space-charge effects limit the maximum allowable number of photons per pulse for many attosecond experiments, especially in multidimensional – i.e., either angularly (ARPES) or spatially (PEEM) resolved – photoelectron spectroscopy (PES) on solids [1,2]. To allow for higher average flux without increasing space-charge impairments, HHG sources at higher repetition rates are needed. We combine a fibre-based ytterbium amplifier system with 4.5-μJ, 40-fs and 1030-nm pulses at 18.4 MHz repetition rate with intra-cavity HHG with an enhancement of 35 (Fig. 1a). Geometric output coupling through a pierced mirror affords high photon energies of 25-60 eV at a flux of 9×1012 XUV photons per second [3], which is unprecedented at this photon energy and MHz repetition rates. The HHG beam is focused on a tungsten target and the generated photoelectrons are detected by an angle-resolving time-of-flight (ToF) spectrometer. Simultaneous stabilization of cavity length and oscillator repetition rate result in excellent intensity stability of the system and enable long-term measurements with three orders of magnitude less integration time at equal space charge conditions in comparison to state-of-the-art kHz HHG sources. An evaluation of the photoelectron statistics of the setup by observing the temporal evolution of the relative standard deviation σ of the photoelectron spectrum proves the aptitude of our setup for high-precision, long-term PES experiments, with a statistical behaviour for up to 3×109 shots (see Fig. 1b). By measuring photoelectron spectra at different XUV flux, we show that no space charge within the instrumental resolution of 0.3 eV can be observed in a 10-μm-diameter focus spot. Our experiments reveal the XUV-IR-delay dependence of sidebands in the photoelectron spectrum generated by laser-assisted photoemission. This represents the first measurement with attosecond precision at MHz repetition rate.
关键词： High Harmonic Generation,Extreme-Ultraviolet,Ultrafast,Photoemission Spectroscopy,MHz repetition rates
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - High Efficiency, High Energy Few-Cycle Driver at 1-μm
摘要： The recent development of high repetition rate lasers based on ytterbium-doped fiber amplifiers (YDFA), has paved the way to increase the repetition rate (>100 kHz) of coherent extreme ultra violet (XUV) sources generated by high harmonic generation (HHG). High repetition rate HHG driver comes with several advantages, such as increased photon flux [1], reduction of the acquisition time in coincidence experiments to study molecular dynamics such as COLTRIMS, and the possibility to study the electronic structure of matter via photoemission spectroscopy and microscopy, where low doses are needed to avoid space-charge effects [2]. Up to now, the overall majority of HHG studies and applications has been restricted to the low repetition rates of Ti:sapphire lasers. Commonly, Ti:sapphire lasers delivers 20 fs pulses at a central wavelength λ = 800 nm, with pulse energies up to hundreds of mJ. However, the average power of these laser systems cannot easily be scaled beyond 10 W, restricting HHG at low repetition rates (up to 10 kHz). Currently, the most mature and powerful ultrafast source technology is undoubtedly ytterbium-based systems, with average power levels beyond 1 kW [3] and numerous industrial applications. However, the long pulse duration of around >200 fs delivered by YDFA sources limits their relevance to this application field. Therefore, nonlinear compression setups have been used successfully to reduce the pulse duration and obtain XUV photon flux among the highest ever reported for HHG-based sources [1]. However, to reach sub-3 cycles regime (< 10 fs at 1030 nm), which is typically required in combination with gating techniques to obtain isolated attosecond pulses, two stages of compression must usually be implemented [4]. This reduces the energy efficiency of the systems dedicated to attosecond physics to typically less than 30% of the overall YDA energy. Here, we demonstrate a two-cycle-source based on a high-energy femtosecond YDFA followed by a hybrid two-stage nonlinear compression setup. The association of a multipass cell-based stage and large-diameter capillary stage provides a compression factor of 48 with an overall transmission of 61%. This source is, to the best of our knowledge, the most efficient few cycle, high energy and high repetition rate laser demonstrated to date. It is very compact with an overall footprint of 1.8 m × 1.0 m and provides a stable train of few-cycle pulses at a central wavelength of 1030 nm that has been continuously characterized over more than 8h. The delivered 6.8 fs (see Fig. 1) 140 μJ pulses at 150 kHz repetition rate, corresponding to 21 W average power, are ideally suited to drive high-photon flux XUV sources [5] through HHG. The described laser system is robust, compact, and power efficient, making it an ideal driver laser for application-ready high flux XUV and attosecond sources.
关键词： attosecond pulses,high harmonic generation,ytterbium-doped fiber amplifiers,extreme ultra violet,high repetition rate lasers
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Extreme-Ultraviolet Pulses with Self-Torque
摘要： Light beams carrying orbital angular momentum (OAM) are well known due to their powerful capabilities for applications in many fields, such as optical communications, microscopy, quantum optics, quantum information or microparticle manipulation. In this work we introduce a new class of light beams that possess a unique property associated with a temporal variation of their OAM: the self-torque of light. Despite the recent progress in the generation of designer ultrafast light waveforms with OAM, up to now there is no evidence for the creation of pulses with time-dependent OAM in any spectral regime. We define the self-torque of light as ?ξ = ?d?(t)/dt, where ??(t) represents the inherent time-variation of a beam’s OAM. This definition has an analogy with mechanical systems that self-induce a variation of their angular momentum. Here, we theoretically predict, and experimentally validate, that self-torque can be naturally imprinted onto extreme-ultraviolet (EUV) beams through the extreme nonlinear process of high-harmonic generation (HHG). By driving HHG with two time-delayed infrared pulses with different OAM, EUV pulses are produced with an inherent, time-varying OAM. Moreover, the dynamical process of HHG imprints a continuous OAM distribution that varies on the attosecond timescale, where all intermediate OAM components are present. The excellent agreement between our simulations and our experimental characterization confirms the creation of EUV self-torqued beams. We show that the properties of the driving pulses—duration and time delay—provide for exquisite control of the amount of self-torque imprinted into a light pulse. This new class of beams can serve as a unique tool for imaging magnetic and topological excitations, for launching selective excitation of quantum matter, and for nano-manipulation on unprecedented time and length scales.
关键词： high-harmonic generation,self-torque,extreme-ultraviolet pulses,orbital angular momentum
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Transfer of Orbital Angular Momentum in High Harmonic Generation using two Driving Beams
摘要： Light beams may carry both a spin and an orbital angular momentum (OAM). While the former is associated to their polarization state, the latter stems from the geometrical properties of their wavefront. In their prototypical form, beams with OAM have “donuts-like” intensity profile and helicoidal wavefront, carrying integral multiples of ? as angular momenta. Since their “rediscovery” in the late 90’s, beams with OAM of visible wavelengths have found innumerable applications in optics, microscopy, quantum or information transfer. A major recent development was the generation of such beams with much smaller wavelengths – in the extreme ultraviolet (XUV) - using synchrotron sources, free electron lasers as well as high harmonic sources (HHG). In this latter case, it creates ultrashort XUV sources of beams with OAM, for time-resolved and femtosecond applications attosecond time scales. Remarkably, we showed that even though HHG is a non-perturbative process, the OAM transfer from the driving beam to the harmonics was purely parametric; i.e., the q-th harmonic – which can be thought as the upconversion of q infrared photons – carries q times the OAM of the driver [3, 4]. However, this rule limits severely the flexibility in choosing the OAM of the XUV emission. In this communication we present experiments that go beyond this simple scheme. If OAM transfer does behave in a parametric way, using two driving beams of different OAM should give us the control knob to fully tailor the harmonic emission. We show that this is indeed the case, using an extra “perturbing” beam to adjust at will the value of the OAM of the outgoing harmonics[2] (see figures). In a second part, we study how far this perturbative description can be taken. We observe that when increasing the perturbation, the yield of the higher perturbation orders can overwhelm the yield of the zeroth order. To explain this result, which seems to go against describing HHG as a parametric process, we derive a quantitative theory of HHG with two beams, based on the analysis of the wavefront of the global electric field at focus [1]. This reproduces the experimental results strikingly well, allowing us to gain a deeper understanding of the complex interplay of phenomena underlying the transfer of OAM in high-harmonic generation.
关键词： parametric process,orbital angular momentum,extreme ultraviolet,high harmonic generation,active grating
更新于2025-09-12 10:27:22
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Attosecond Pulse Amplification in a Plasma-Based X-Ray Laser Dressed by an Infrared Laser Field
摘要： High-harmonic generation (HHG) of laser radiation has led to attosecond pulse formation which offers unprecedented temporal resolution in observing and controlling electron and nuclear dynamics. But the energy of attosecond pulses remains quite small, especially for photon energies exceeding 100 eV, which limits their practical applications. We propose a method for amplification of attosecond pulses in the active medium of a plasma-based x-ray laser dressed by a replica of the laser field used for HHG. The experimental implementation is suggested in hydrogenlike C5t x-ray laser at 3.4 nm wavelength in the “water window” range.
关键词： attosecond pulse amplification,infrared laser field,plasma-based x-ray laser,high-harmonic generation,water window
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Theoretical Investigation of Resonant High-Harmonic Generation in Transition Metal Plasma
摘要： High-harmonic generation (HHG) is a promising tool for generating XUV and soft X-ray radiations using table-top experimental setup. Whereas the conversion efficiency of HHG is usually low, one alternative for increasing HH yield lies in the use of transition metal plasma as a generating medium, from which several resonantly enhanced harmonic peaks have been observed [1]. In this work, we numerically investigate HHG from transition metal elements. We use state-of-the-art all-electron first-principles simulation methods, TD-CASSCF and TD-ORMAS [2-4], which we have recently developed. The calculated harmonic spectra from Mn and its cation (“active 3p” in Fig. 1) exhibit an enhanced peak at 51 eV, successfully reproducing the experimental observations [5, 6]. The arrows in Fig. 1 mark the cutoff positions (cid:7)(cid:15) of the indicated ionic species predicted by the cutoff law, (cid:7)(cid:15) (cid:20) (cid:8)(cid:16) (cid:17) (cid:5)(cid:2)(cid:4)(cid:6)(cid:9)(cid:16), where (cid:8)(cid:16) and (cid:9)(cid:16) denote the ionization potential and ponderomotive energy, respectively. The extension of the cutoff beyond that corresponding to the starting ionic species indicates a significant production of higher ionic states, i.e., plasma. One of the advantages of our methods is that we can freeze the motion of given orbitals to get physical insights into the underlying mechanism. If we freeze the 3p orbitals, the enhanced peak disappears (“frozen 3p” in Fig. 1). This unambiguously demonstrates that the origin of the peak is the 3p-3d giant resonance, as implied by its position [5], and, thus, the manifestation of multielectron effects. Further, we have analyzed how each of the three 3p-3d transition lines ( (cid:10) (cid:20) (cid:18)(cid:4)(cid:1)(cid:3)(cid:21) contributes to the peak and revealed that they constructively interfere to form the enhanced peak at 51 eV.
关键词： High-harmonic generation,TD-CASSCF,soft X-ray,transition metal plasma,TD-ORMAS,XUV
更新于2025-09-12 10:27:22
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Topological strong-field physics on sub-laser-cycle timescale
摘要： The sub-laser-cycle timescale of the electronic response to strong fields enables attosecond dynamical imaging in atoms, molecules and solids, with optical tunnelling and high-harmonic generation the hallmarks of attosecond optical spectroscopy. Topological insulators are intimately linked with electron dynamics, as manifested via the chiral edge currents, but it is unclear if and how topology leaves its mark on optical tunnelling and sub-cycle electronic response. Here, we identify distinct bulk topological effects on directionality and timing of currents arising during electron injection into conduction bands. We show that electrons tunnel differently in trivial and topological insulators, for the same band structure, and identify the key role of the Berry curvature in this process. These effects map onto topologically dependent attosecond delays and helicities of emitted harmonics that record the phase diagram of the system. Our findings create new roadmaps in studies of topological systems, building on the ubiquitous properties of the sub-laser-cycle strong-field response—a unique mark of attosecond science.
关键词： high-harmonic generation,attosecond science,Berry curvature,topological insulators,strong-field physics
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Development of an XUV Frequency Comb for Precision Spectroscopy of Highly Charged Ions
摘要： Highly charged ions (HCI) have a few tightly bound electrons and many interesting properties for probing fundamental physics and developing new frequency standards [1, 2]. Many optical transitions of HCI are located in the extreme ultraviolet (XUV) and conventional light sources do not allow to study these transistions with highest precision. For this reason, we are developing an XUV frequency comb by transfering the coherence and stability of a near infrared frequency comb to the XUV by means of high-harmonic generation (HHG) [3-4]. Reaching intensity levels necessary for HHG (～ 1013W/cm2), while operating at high repetition rates (100 MHz) for large comb line spacing, is challenging. Therefore, the laser pulses are ?rst ampli?ed in a rod-type ?ber to 70 W and compressed to sub-200 fs in a grating and prism compressor. Afterwards, pulses are resonantly overlapped in an astigmatism-compensated femtosecond enhancement cavity, which is locked to the frequency comb. To achieve high stability and low-noise performance, the cavity is built on a rigid titanium structure with vibrational decoupling from the vacuum pumps. High-harmonics will then be generated in a target gas in the tight focus of the cavity and coupled out of the cavity by minus-?rst order diffraction from a small-period grating etched into a high-re?ective cavity mirror [5]. Mirror degradation due to contamination and hydrocarbon aggregation is prevented by operating the whole cavity under ultra-high vacuum conditions. A differential pumping scheme will enable high target gas pressures in the laser focus without impairing the pressure elsewhere in the chamber [6]. Finally, the XUV light will be guided to trapped and sympathetically cooled HCI [7] in a cryogenic superconducting linear Paul trap to drive narrow transitions with individual comb lines. A schematic overview of the experiment is shown in Fig. 1. Recent progress and ?rst experiments with intra-cavity multiphoton ionization are presented.
关键词： femtosecond enhancement cavity,highly charged ions,precision spectroscopy,high-harmonic generation,XUV frequency comb
更新于2025-09-12 10:27:22