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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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Observation of Laser-Induced Spark in the Density Jump in a Gas-Jet Target
摘要： In the design of powerful laser-plasma sources of extreme ultraviolet radiation with a gas jet as the target, the problem of degradation of gas nozzles is topical. Degradation is observed due to insufficient distance from the generation zone of the laser-induced spark to the nozzle exit. The comprehensive solution to this problem is proposed by forming the laser-induced spark in the density jumps that are generated upon deceleration of the supersonic gas jet. In such a density jump, the laser-induced spark is obtained and measurements of emission radiation intensity are performed.
关键词： density jumps,laser-induced spark,degradation of gas nozzles,extreme ultraviolet radiation
更新于2025-09-12 10:27:22
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Radiation transport and scaling of optical depth in Nd:YAG laser-produced microdroplet-tin plasma
摘要： Experimental scaling relations of the optical depth are presented for the emission spectra of a tin-droplet-based, 1-lm-laser-produced plasma source of extreme-ultraviolet (EUV) light. The observed changes in the complex spectral emission of the plasma over a wide range of droplet diameters (16–65 lm) and laser pulse durations (5–25 ns) are accurately captured in a scaling relation featuring the optical depth of the plasma as a single, pertinent parameter. The scans were performed at a constant laser intensity of 1.4 (cid:2) 1011 W/cm2, which maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for industrial EUV lithography. Using a one-dimensional radiation transport model, the relative optical depth of the plasma is found to linearly increase with the droplet size with a slope that increases with the laser pulse duration. For small droplets and short laser pulses, the fraction of light emitted in the 2% bandwidth around 13.5 nm relative to the total spectral energy is shown to reach high values of more than 14%, which may enable conversion ef?ciencies of Nd:YAG laser light into—industrially—useful EUV radiation rivaling those of current state-of-the-art CO2-laser-driven sources.
关键词： microdroplet-tin plasma,Nd:YAG laser,optical depth,radiation transport,extreme-ultraviolet (EUV) light
更新于2025-09-12 10:27:22
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Characterization of radiative and kinetic properties of laser produced plasmas for efficient extreme ultraviolet nanolithography sources
摘要： Increasing extreme ultraviolet (EUV) photon power in laser-produced plasma (LPP) sources is critically important for e?cient future nanolithography devices. Enhancing the lifetime of the optical collector system in these devices is another important challenge for reliable and economically feasible devices. In this work, various mechanisms a?ecting ion acceleration in LPP were investigated to predict the maximum ion energies and ?ux arriving at the collecting mirror surfaces. Plasma evolution produced by an Nd:YAG laser from Sn targets was studied in detail to predict the dynamics of EUV producing ions and their contribution to the EUV power. The multiphysics fully 3D integrated HEIGHTS computer package was used in this analysis. HEIGHTS simulations of detail plasma evolution and ion kinetic energies were compared with various worldwide experimental data. Excellent agreement was shown regarding the range of ion kinetic energies and their angular distribution as well as recombination processes and their e?ect on the temporal output of EUV photons. Spatial and charge distributions were predicted for EUV producing ions and ionic debris. The analysis showed that mainly two Sn ions, i.e., Sn XII and Sn XIII, determine the EUV source intensity and spatial location. It was also shown that reducing the laser spot size and increasing the pulse duration allow a signi?cant reduction in ion kinetic energies that is important for a longer lifetime of the optical collection system.
关键词： laser-produced plasma,ion acceleration,extreme ultraviolet,nanolithography,EUV photon power
更新于2025-09-12 10:27:22
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irradiated by a co-rotating two-color circularly polarized laser field
摘要： The high-order harmonic generation process of H2+ in a co-rotating two-color circularly polarized laser pulse was investigated by numerically solving the time-dependent Schr?dinger equation. Compared with the target of the atom, the harmonic intensity of H2+ is remarkably enhanced. Moreover, the harmonic intensity of H2+ approaches its maximum when the electric field amplitude ratio of the fundamental frequency and the second one is approximately 1:2.3. By analyzing the evolution of the time-dependent wave packets, the enhancement of harmonic intensity may be attributed to an increase in the probability of the ionized electron recolliding with the parent ion. In addition, the dependence of harmonic intensity on the internuclear distance and the alignment angle of H2+ is explored. We found that the ellipticity of the particular harmonic order changes with the molecular alignment angle, which provides a chance to generate an extreme ultraviolet coherent light pulse with controllable polarization.
关键词： time-dependent Schr?dinger equation,H2+,harmonic intensity,alignment angle,extreme ultraviolet coherent light pulse,controllable polarization,co-rotating two-color circularly polarized laser field,internuclear distance,high-order harmonic generation
更新于2025-09-12 10:27:22
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Resonance Enhancement in Laser-Produced Plasmas (Concepts and Applications) || Resonance Enhancement of Harmonics in Metal-Ablated Plasmas: Early Studies
摘要： In this subsection, the first observation of an exceptionally strong resonance enhancement of a single harmonic in a plateau region is described. This phenomenon was observed during investigations of harmonic generation from femtosecond pulse interacting with indium plasma. A very strong 13th harmonic generation (λ = 61.2 nm), almost 2 orders of magnitude, exceeding the intensities of neighboring harmonics in the plateau range was observed in those studies [1].
关键词： harmonics,femtosecond pulse,extreme ultraviolet,indium plasma,resonance enhancement
更新于2025-09-11 14:15:04
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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) - Interferometric Attosecond Lock-in Measurement of Extreme Ultraviolet Circular Diehroism
摘要： Probing vectorial properties of light-matter interactions requires control over the polarization state of light. The generation of extreme-ultraviolet (XUV) attosecond pulses opened new perspectives in measurements of chiral phenomena. Recently, new methods for polarization control in the XUV range, which are based on manipulation of the high harmonic generation (HHG) process were demonstrated [1-4]. However, the limited polarization control in this regime prevents the development of advanced measurement schemes for weak vectorial signals, which require polarization modulation. In our work [5], we establish an XUV lock-in detection scheme, allowing the isolation and amplification of extremely weak chiral signals, by achieving a dynamical control over the polarization state of the XUV light. We demonstrate a time-domain approach to control and modulate the polarization state. This scheme enables us to characterize the polarization state via an in-situ measurement (see Figure 1a,d,e). Our approach, resembling a birefringent crystal for the visible range, is based on the collinear superposition of two independent, phase-locked, orthogonally polarized XUV sources and the control of their relative delay with sub-cycle accuracy (see Figure 1b). We achieve lock-in detection of XUV magnetic circular dichroism (XMCD) in cobalt, transferring weak amplitude variations into a phase modulation (see Figure 1c), by controlling the relative angle, θ , between the two linearly polarized sources. This approach holds the potential of significantly extending the scope of vectorial measurements to the attosecond and nanometer frontiers.
关键词： attosecond pulses,polarization control,high harmonic generation,extreme-ultraviolet,chiral signals,XUV magnetic circular dichroism,XUV lock-in detection
更新于2025-09-11 14:15:04
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Intense vortex high-order harmonics generated from laser-ablated plume
摘要： In this study, we demonstrate intense extreme-ultraviolet optical vortices generated using laser-ablation plume as the nonlinear medium. We used two types of plumes that are known to generate intense high-order harmonics for driving lasers with Gaussian beam profiles, but through different mechanisms, namely, carbon (diatomic carbon molecules) and tin (resonance with the autoionizing state). We find that the harmonic fluxes for diatomic carbon molecules are similar for Gaussian and vortex driving fields. However, for harmonics from the autoionizing state of tin (~26.3 eV), the enhancement factor of the resonant harmonic intensity decreases by ~50% when using the vortex driving field. The intense extreme-ultraviolet optical vortices demonstrated in this study will be useful for many applications including a material characterization technique known as optical angular momentum dichroism as well as the spectroscopy of spin-forbidden electronic transitions.
关键词： optical angular momentum dichroism,laser-ablation plume,spin-forbidden electronic transitions,extreme-ultraviolet optical vortices,high-order harmonics,diatomic carbon molecules,autoionizing state
更新于2025-09-11 14:15:04
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Symmetry of molecular Rydberg states revealed by XUV transient absorption spectroscopy
摘要： Transient absorption spectroscopy is utilized extensively for measurements of bound- and quasibound-state dynamics of atoms and molecules. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses has the potential to attain unprecedented time resolution. Here we apply this technique to aligned-in-space molecules. The XUV pulses are much shorter than the time during which the molecules remain aligned, typically <100 fs. However, transient absorption is not an instantaneous probe, because long-lived coherences re-emit for picoseconds to nanoseconds. Due to dephasing of the rotational wavepacket, it is not clear if these coherences will be evident in the absorption spectrum, and whether the properties of the initial excitations will be preserved. We studied Rydberg states of N2 and O2 from 12 to 23 eV. We were able to determine the polarization direction of the electronic transitions, and hence identify the symmetry of the final states.
关键词： transient absorption spectroscopy,extreme ultraviolet,attosecond pulses,molecular alignment,Rydberg states
更新于2025-09-11 14:15:04