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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) - XUV Coherence Tomography with Nanoscale Resolution Driven by High Harmonic Generation
摘要： Optical coherence tomography (OCT) is a well-established method to retrieve three-dimensional, cross-sectional images of biological samples in a non-invasive way using near-infrared radiation. The axial resolution of OCT is on the order of the coherence length lc ∝ λ 2 /Δλ which depends on the central wavelength λ0 and the spectral width Δλ of the light source. As a consequence, the axial resolution only depends on the spectrum rather than the geometrical properties of the radiation. OCT with broadband visible and near-infrared sources typically reaches axial (depth) resolutions on the order of a few micrometers [1]. Here we present extreme ultra violet (XUV) coherence tomography (XCT) [2], which takes advantage of the fact that the coherence length can be signi?cantly reduced if broadband XUV and soft X-ray (SXR) radiation is used. XCT can display its full capabilities when used in the spectral transmission windows of the sample materials. For instance, the silicon transmission window (30-99 eV) corresponds to a coherence length of about 12 nm, thus suggesting applications for semiconductor inspection. In the water window at 280-530 eV, a coherence length as short as 3 nm can be achieved and highlights possible applications of XCT for life sciences. XCT utilizes a variant of the Fourier-domain OCT scheme that completely avoids a beam splitter. In the experiment, broadband XUV from a lab-based high-harmonic generation light source [3] is focused onto the surface of the sample. The re?ected spectrum is measured with a grating spectrometer (see Fig. 1 left side). The top layer re?ection assumes the role of a reference beam. Previously, this simpli?cation led to artifacts [2]. Here we show a novel one-dimensional phase retrieval algorithm (PR) to mitigate such disadvantages and enable artifact-free so-called PR-XCT [4]. The right side of Fig. 1 shows a 3D-tomogram of a nondestructive XCT scan of two nanometer thin laterally structured gold layers embedded in silicon. An axial resolution of 24 nm could be reached in the silicon transmission window with the table-top system. Another remarkable result is the high material sensitivity of XCT. At a depth of about 160 nm, a Silicon dioxide layer (blue) was detected which developed during the production process of the sample and has a thickness of a few nanometers only. This layer could not be detected with a SEM in a thin slice cut out of the sample, and even in a TEM image it is only barely visible. Furthermore, the PR-XCT algorithm is capable of extracting material information about the materials inside of the sample. We will present ?rst results on material-resolved XCT.
关键词： optical coherence tomography,high harmonic generation,XUV coherence tomography,nanoscale resolution,life sciences,semiconductor inspection
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) - Electric-Dipole Based Chiral Sensitivity in High Harmonic Generation by Dynamical Symmetry Breaking Spectroscopy
摘要： Chirality is a fundamental asymmetry property that appears abundantly in nature [1]. A system is chiral if and only if it is distinct from its mirror image (its opposite handedness chiral-partner), e.g. circularly polarized light or chiral molecules. Such systems are unique in that their properties are completely independent of their handedness up until the moment they interact with another chiral object. For instance, partner chiral molecules have identical cross-sections for absorption of linearly-polarized light, but not for absorption of circularly polarized light, leading to circular dichroism (CD) [1]. Standardly, chirality is analyzed by chiroptical techniques that measure the medium’s response to elliptically polarized light. However, such techniques rely on magnetic-dipole or higher electric-moment transitions, because electric-dipole interactions average-out to zero in isotropic media (circularly polarized light has a helical pitch that is negligible in the dipole approximation) [1]. Consequently, standard chiroptical approaches lead to very weak signals, especially in the gas phase. In recent years, several seminal electric-dipole based methods were developed that lead to much larger chiral signals, including photoelectron CD [1–4], coulomb explosion imaging [5], and microwave three-wave mixing [6]. Importantly, high harmonic generation (HHG) was shown to be chirality-sensitive, leading to relatively large (up to 10%) femtosecond-resolved chiral signals [7,8]. Still, the chiral signal in HHG is based on magnetic-dipole interactions (same as in the standard techniques), and the signal is relatively large only due to the non-perturbative nature of the process. Extending HHG to produce an electric-dipole chiral response could open-up many possibilities for optically exploring ultrafast chirality and weakly-chiral systems. Here we propose and theoretically explore a novel HHG-based chiroptical approach that relies solely on electric-dipole interactions. The method is implemented through bi-chromatic non-collinear HHG, where the beams’ properties are chosen from group theory symmetry-based considerations to exhibit reflection or inversion dynamical symmetries (DSs) [9]. This scheme leads to forbidden harmonic selection rules from isotropic achiral media which are broken in chiral media, because it does not exhibit reflection and inversion symmetries. As a result, ‘forbidden’ harmonics are emitted only if the medium is chiral, and their intensity is correlated to the enantiomeric excess (ee), providing a single-shot background free signal. We analytically derive the general conditions that allow an electric-dipole based chiral response, and numerically demonstrate several feasible geometries [10]. For instance, using DS group theory considerations [9], we numerically demonstrate that the bi-chromatic non-collinear chiral HHG scheme presented in Fig. 1(a) – two intense non-collinear bi-chromatic (3:5 carrier frequency ratios), counter rotating, elliptically polarized beams propagate with a relative opening angle of 2α – leads to an electric-dipole based ‘forbidden harmonic’ signal. When this field interacts with an achiral medium (e.g. a non-oriented racemic mixture of chiral molecules), even harmonic emission is forbidden due to a dynamical inversion symmetry selection rule (the pump is invariant under the DS: (cid:1870)?→-(cid:1870)?, t→t+T/2) [9]. However, when this field interacts with a chiral medium, even harmonics are emitted in all polarizations, and the electric-dipole response does not average-out to zero [10]. This results from the medium breaking the pump’s inversion DS; hence, the intensity of the even harmonics is correlated to the ee, while the odd harmonics are chirality-independent and can be used as a reference (see Fig. 1(c)). This scheme leads to a nearly background free chiral/achiral signal, reaching as high as 97% for the normalized harmonic response from chiral media compared to achiral media (Fig. 1(d)).
关键词： Electric-Dipole,Dynamical Symmetry Breaking Spectroscopy,Chirality,High Harmonic Generation
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) - Quasi-Phase Matching in High Harmonic Generation using Structured Plasmas
摘要： High harmonic generation (HHG) is an important process in attosecond science. Its efficiency is, however, limited to values below 10-5 of the incident power into a single harmonic. Phase matching can increase the efficiency but deals with low ionization rates. Furthermore HHG from laser-produced plasmas has shown some promising advantages in increasing the response of the conversion. These are the appearance of resonantly enhanced harmonics or the efficient generation in carbon or nanoparticle containing plasmas. On the contrary laser-produced plasmas inherently possess a high degree of ionization which hampers any phase-matching scheme with these sources. This dilemma can be circumvented by quasi-phase matching (QPM). In this case the conversion process is suppressed when the harmonics are generated out of phase leading to a quadratic growth of the harmonic intensity with interaction distance. Experimentally QPM can be achieved e.g. by applying a structured conversion medium in a focused driving beam geometry. Here, we show how such a scheme is realized with laser-produced plasmas. The idea of using a structured plasma for QPM is not new [1], but in none of these earlier studies QPM has been optimized for maximum efficiency nor controlled for different regions of the harmonic spectrum.
关键词： laser-produced plasmas,quasi-phase matching,attosecond science,High harmonic generation
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) - Square Pulse and Harmonic Ultrashort Pulse Generation in Semiconductor Optical Amplifier-Based Mamyshev Oscillator
摘要： Modulation instability is responsible for the symmetry breaking of homogeneous spatio-temporal states or wave envelopes and the formation of stable patterns in a variety of physical media. Recently, by using this phenomenon in laser physics, remarkable developments have been made based on the use of the effect of self-phase modulation in Mamyshev oscillator [1-3]. The oscillator is designed using a spectral filter and a gain to compensate for filtering losses. By mismatching spectral filter offsets, entire cavity works as an element with the power-dependent transmission, suppressing the continuous wave lasing and favouring the formation of ultrashort pulses. Recently application of semiconductor optical amplifiers (SOA) as a gain media in fibre lasers as a gain medium has drawn a new wave of research attention due to their potential applications in telecommunications, all-optical square-wave clocks, range finding, and optical sensing. In this work, we developed a Mamyshev oscillator based on SOA operating in square pulses and high harmonic generation at 1550 nm.
关键词： high harmonic generation,Mamyshev oscillator,semiconductor optical amplifiers,square pulses,Modulation instability
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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Non-Markovian pure dephasing in a dielectric excited by a few-cycle laser pulse
摘要： We develop the theory of pure dephasing in a solid exposed to an ultrashort laser pulse beyond the commonly used Markov approximation. This approach takes into account the finite cutoff energy of the bath and can be applied to both many-particle and phonon environments. With numerical simulations performed with the time-dependent Hartree-Fock equations, we investigate how the excitation probability and high-harmonic generation are described by different models of decoherence. It is shown that the time-dependent rates allow for temporally high dephasing to successfully reproduce the main features of high-harmonics spectrum and avoid an overestimation of the charge carrier population after the pulse, which is a common problem of the Markov approximation.
关键词： pure dephasing,time-dependent Hartree-Fock,non-Markovian,ultrashort laser pulse,high-harmonic generation
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) - All-Optical Background-Free Detection of Ring Currents by Dynamical Symmetry Breaking High Harmonic Spectroscopy
摘要： Excited atoms and molecules can carry long-lived currents that circulate in the microscopic media. From a quantum mechanical perspective, these currents can be understood as a coherent wave-packet comprising a superposition of bound- states that oscillates in time [1–3]. When the wave-packet has a non-zero angular momentum expectation value, ring-currents circulate in the medium. For instance, a hydrogen atom excited to a 2p-state with non-zero magnetic quantum number m (e.g. by interaction with circularly polarized light) carries a steady-state ring current [2]. More complex systems can also carry persistent ring currents, e.g. spin-orbit wave-packets in Xenon [4], or multi-electron wave-packets in larger molecules [1]. This phenomenon is general to any quantum system and is especially interesting because it occurs on the natural time-scale of electronic motion – attoseconds to femtoseconds. Understanding ring currents is thus fundamentally important for manipulating and controlling ultrafast processes on the nanoscale, including chemical bond formation and topologically protected surface currents [5], as well as for the generation of intense attosecond-duration magnetic fields [1,6]. However, ring currents are very difficult to detect, particularly in a time-resolved manner. Only very recently were ring currents directly experimentally resolved in Argon through pump-probe angularly-resolved incidence photoelectron spectrum measurements [3]. Here we propose and theoretically explore an all-optical technique for ultrafast time-resolved detection of ring currents in atoms, molecules and solids, based on high harmonic generation (HHG). In this technique a microscopic medium interacts with a bi-chromatic (ω-2ω) bi-elliptical laser pulse, generating high harmonic photons. We show that the harmonic spectra emitted from current-carrying media differs from that of current-free media. We use dynamical symmetry (DS) considerations [7] to derive conditions for a maximal (background-free) signal in the harmonic ellipticity, which occurs when the pump beams are cross-linearly polarized [8]. In this configuration the bi-chromatic laser field exhibits a dynamical reflection symmetry that leads to linearly polarized harmonic selection rules [7]; however, this selection rule is broken when the medium carries a current, because ring-currents are not reflection-symmetric (similar to chiral systems [9], but where the current can be described in 2D). Thus, current-carrying media emit elliptically polarized harmonics, where the harmonic ellipticity is correlated to the intensity and sign of the current in the system. We numerically demonstrate the approach by pump-probe HHG calculations from Neon noble gas atoms, and from both aligned and un-oriented aromatic molecules (benzene and furan), using a non-interacting electron quantum model, and time-dependent density functional theory calculations. The presented work could be useful for ultrafast spectroscopy of current-carrying processes (chemical reactions, topological currents, etc.), as well as for manipulation and control of ring currents, paving the way for their table-top all- optical detection.
关键词： high harmonic generation,ultrafast spectroscopy,ring currents,dynamical symmetry breaking
更新于2025-09-11 14:15:04
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[IEEE 2019 IEEE Photonics Conference (IPC) - San Antonio, TX, USA (2019.9.29-2019.10.3)] 2019 IEEE Photonics Conference (IPC) - High-Repetition-Rate High-Harmonic Generation Driven by Infrared Free-Electron Lasers
摘要： We present for high-harmonic-generation (HHG) driven by infrared free-electron lasers (FEL), which can be operated at MHz-repetition. Wavelength tunability of FEL enables one to explore HHG from mid-infrared pulses for higher cutoff energies above 1 keV. Plans and status of R&D are also presented.
关键词： x-ray,attosecond,free electron laser,high harmonic generation
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) - Multi-Scale Symmetries and Selection Rules in High Harmonic Generation
摘要： Symmetry-based analysis is very useful in many fields of science. In the case of time-dependent systems, the symmetries of interest are dynamical, and are maintained during the system’s evolution in time. Interestingly, such symmetries may manifest over several length scales, e.g. both microscopically and macroscopically. For example, in high harmonic generation (HHG), micro-scale dynamical symmetries (DSs) yield selection rules that forbid some harmonic orders and determine the polarizations of the allowed harmonics [1], as was formulated recently in a consistent group theory [2]. Macroscopically, DSs of the wave equation may induce selection rules with respect to phase-matching conditions and conservation of angular or linear momentum [3]. However, the DSs of these different length scales are generally thought to be non-interacting, and have so far only been described separately. Here we formulate and explore plethora of multi-scale dynamical symmetries (MSDS), where the symmetry operations involve both microscopic and macroscopic length scales. We derive all possible MSDS operations and their resulting selection rules in HHG, and experimentally demonstrate non-trivial selection rules of the new theory.
关键词： selection rules,dynamical symmetries,high harmonic generation,multi-scale dynamical symmetries,symmetry-based analysis
更新于2025-09-11 14:15:04
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High-order harmonic generation with controllable temporal coherence generated from a coherently excited medium
摘要： High harmonic generation has been considered for many years to be a coherent source of upconverted laser radiation. We propose a method for the generation of essentially incoherent high harmonic generation with a controlled degree of temporal coherence, obeying random-walk statistics. This is achieved by a unique combination of single-atom and macroscopic effects involving preparation of the medium in an excited state with coordinate-dependent population inversion and using an intense off-resonant and phase-mismatched pumping.
关键词： excited medium,temporal coherence,high harmonic generation,phase-mismatched pumping,random-walk statistics
更新于2025-09-10 09:29:36