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Su X, Wang Y, Su L, Liu S, Zheng L, Zhang B, He J, Zhang B. Sub-50-fs Kerr-lens mode-locked Yb-doped fluoride laser with 44% optical efficiency. APPLIED OPTICS 2024; 63:912-916. [PMID: 38437387 DOI: 10.1364/ao.509994] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/23/2023] [Accepted: 12/20/2023] [Indexed: 03/06/2024]
Abstract
Yb-doped fluoride has been demonstrated to be potential crystals for application in efficient ultrafast lasers. However, the trade-off between the shorter pulses with higher efficiencies is a challenge. In this work, using Y b,G d:C a S r F 2 crystal, we report on a sub-50-fs Kerr-lens mode-locked oscillator with an optical efficiency up to 44%. Pumped by a 976-nm diffraction-limited fiber laser and using chirped mirrors combined with prism pairs for the dispersion compensation, a pulse as short as 46 fs was obtained with 620-mW output power, corresponding to an optical efficiency more than 40%. Stable Kerr-lens mode-locking with RMS of output power lower than 0.3% and beam quality factors M 2<1.14 were achieved. Moreover, a maximum output power of 780 mW was obtained in continuous-wave operation with 55.3% optical efficiency. To the best of our knowledge, the results in this work represent the shortest pulses generated from Yb-doped fluoride lasers as well as the highest optical efficiencies ever reported in sub-100 fs Yb bulk lasers.
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Bai C, Wang G, Tian W, Zheng L, Tian X, Yu Y, Xu X, Wei Z, Zhu J. High average power amplification of femtosecond pulses based on the Yb:CaYAlO 4 crystal. OPTICS EXPRESS 2023; 31:12212-12219. [PMID: 37157385 DOI: 10.1364/oe.488013] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/10/2023]
Abstract
In this paper, we demonstrated the direct amplification of femtosecond pulses with the Yb:CaYAlO4 crystal for the first time. A compact and simple two-stage amplifier delivered amplified pulses with the average powers of 55.4 W for σ-polarization and 39.4 W for π-polarization at the center wavelengthes of 1032 nm and 1030 nm, corresponding to 28.3% and 16.3% optical-to-optical efficiencies, respectively. These are to the best of our knowledge the highest value achieved with a Yb:CaYAlO4 amplifier. Upon using a compressor consisting of prisms and GTI mirrors, a pulse duration of 166-fs was measured. Thanks to the good thermal management, the beam quality (M2) parameters <1.3 along each axis were maintained in each stage.
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Song J, Peng Y, Shen L, Sun J, Luo G, Xu X, Xu J, Leng Y. High-power femtosecond regenerative amplifier based on Yb:CaYAlO 4 dual-crystal configuration. OPTICS LETTERS 2023; 48:1395-1398. [PMID: 36946936 DOI: 10.1364/ol.484843] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2023] [Accepted: 02/06/2023] [Indexed: 06/18/2023]
Abstract
A thermal lens insensitive regenerative amplifier (RA) with a dual Yb:CaYAlO4 (Yb:CYA) crystal configuration for extending gain spectra is demonstrated for the first time, to the best of our knowledge. By orthogonalizing the orientation of two a-cut Yb:CYA crystals in one RA, the Q switched spectrum with a full width at half maximum of 15.4 nm is generated, which is 1.5 and 1.6 times of the Q switched spectral bandwidth with π- and σ-polarization, respectively. With chirped pulses injection, this RA can deliver laser pulses with an average power exceeding 10 W at the repetition rate of 20-800 kHz and pulse energy of 1.5 mJ at 1 kHz. This is the highest average power from the Yb:CYA RA to the best of our knowledge. Finally, compressed pulses of 163 fs with 92% overall efficiency are realized. Thanks to the heat insensitive cavity design and excellent thermodynamic properties of the Yb:CYA crystal, the output laser beam is close to the diffraction limit with an M2 value of 1.07 × 1.07.
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Liu F. Time- and angle-resolved photoemission spectroscopy (TR-ARPES) of TMDC monolayers and bilayers. Chem Sci 2023; 14:736-750. [PMID: 36755720 PMCID: PMC9890651 DOI: 10.1039/d2sc04124c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2022] [Accepted: 12/04/2022] [Indexed: 12/12/2022] Open
Abstract
Many unique properties in two-dimensional (2D) materials and their heterostructures rely on charge excitation, scattering, transfer, and relaxation dynamics across different points in the momentum space. Understanding these dynamics is crucial in both the fundamental study of 2D physics and their incorporation in optoelectronic and quantum devices. A direct method to probe charge carrier dynamics with momentum resolution is time- and angle-resolved photoemission spectroscopy (TR-ARPES). Such measurements have been challenging, since photoexcited carriers in many 2D monolayers reside at high crystal momenta, requiring probe photon energies in the extreme UV (EUV) regime. These challenges have been recently addressed by development of table-top pulsed EUV sources based on high harmonic generation, and the successful integration into a TR-ARPES and/or time-resolved momentum microscope. Such experiments will allow direct imaging of photoelectrons with superior time, energy, and crystal momentum resolution, with unique advantage over traditional optical measurements. Recently, TR-ARPES experiments of 2D transition metal dichalcogenide (TMDC) monolayers and bilayers have created unprecedented opportunities to reveal many intrinsic dynamics of 2D materials, such as bandgap renormalization, charge carrier scattering, relaxation, and wavefunction localization in moiré patterns. This perspective aims to give a short review of recent discoveries and discuss the challenges and opportunities of such techniques in the future.
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Affiliation(s)
- Fang Liu
- Department of Chemistry and the PULSE Institute, Stanford University Stanford California 94305 USA
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5
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Cucini R, Pincelli T, Panaccione G, Kopic D, Frassetto F, Miotti P, Pierantozzi GM, Peli S, Fondacaro A, De Luisa A, De Vita A, Carrara P, Krizmancic D, Payne DT, Salvador F, Sterzi A, Poletto L, Parmigiani F, Rossi G, Cilento F. Coherent narrowband light source for ultrafast photoelectron spectroscopy in the 17-31 eV photon energy range. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2020; 7:014303. [PMID: 32039283 PMCID: PMC6994270 DOI: 10.1063/1.5131216] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/10/2019] [Accepted: 01/13/2020] [Indexed: 05/25/2023]
Abstract
Here, we report on a novel narrowband High Harmonic Generation (HHG) light source designed for ultrafast photoelectron spectroscopy (PES) on solids. Notably, at 16.9 eV photon energy, the harmonics bandwidth equals 19 meV. This result has been obtained by seeding the HHG process with 230 fs pulses at 515 nm. The ultimate energy resolution achieved on a polycrystalline Au sample at 40 K is ∼22 meV at 16.9 eV. These parameters set a new benchmark for narrowband HHG sources and have been obtained by varying the repetition rate up to 200 kHz and, consequently, mitigating the space charge, operating with ≈ 3 × 10 7 electrons/s and ≈ 5 × 10 8 photons/s. By comparing the harmonics bandwidth and the ultimate energy resolution with a pulse duration of ∼105 fs (as retrieved from time-resolved experiments on bismuth selenide), we demonstrate a new route for ultrafast space-charge-free PES experiments on solids close to transform-limit conditions.
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Affiliation(s)
| | | | | | | | | | | | | | - Simone Peli
- Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, Trieste, Italy
| | | | | | - Alessandro De Vita
- Dipartimento di Fisica, Università di Milano, via Celoria 16, Milano, Italy
| | - Pietro Carrara
- Dipartimento di Fisica, Università di Milano, via Celoria 16, Milano, Italy
| | | | - Daniel T. Payne
- Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, Trieste, Italy
| | | | - Andrea Sterzi
- Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, Trieste, Italy
| | | | | | | | - Federico Cilento
- Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, Trieste, Italy
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6
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Liu Y, Beetar JE, Hosen MM, Dhakal G, Sims C, Kabir F, Etienne MB, Dimitri K, Regmi S, Liu Y, Pathak AK, Kaczorowski D, Neupane M, Chini M. Extreme ultraviolet time- and angle-resolved photoemission setup with 21.5 meV resolution using high-order harmonic generation from a turn-key Yb:KGW amplifier. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2020; 91:013102. [PMID: 32012559 DOI: 10.1063/1.5121425] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/24/2019] [Accepted: 12/15/2019] [Indexed: 06/10/2023]
Abstract
Characterizing and controlling electronic properties of quantum materials require direct measurements of nonequilibrium electronic band structures over large regions of momentum space. Here, we demonstrate an experimental apparatus for time- and angle-resolved photoemission spectroscopy using high-order harmonic probe pulses generated by a robust, moderately high power (20 W) Yb:KGW amplifier with a tunable repetition rate between 50 and 150 kHz. By driving high-order harmonic generation (HHG) with the second harmonic of the fundamental 1025 nm laser pulses, we show that single-harmonic probe pulses at 21.8 eV photon energy can be effectively isolated without the use of a monochromator. The on-target photon flux can reach 5 × 1010 photons/s at 50 kHz, and the time resolution is measured to be 320 fs. The relatively long pulse duration of the Yb-driven HHG source allows us to reach an excellent energy resolution of 21.5 meV, which is achieved by suppressing the space-charge broadening using a low photon flux of 1.5 × 108 photons/s at a higher repetition rate of 150 kHz. The capabilities of the setup are demonstrated through measurements in the topological semimetal ZrSiS and the topological insulator Sb2-xGdxTe3.
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Affiliation(s)
- Yangyang Liu
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - John E Beetar
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Md Mofazzel Hosen
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Gyanendra Dhakal
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Christopher Sims
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Firoza Kabir
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Marc B Etienne
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Klauss Dimitri
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Sabin Regmi
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Yong Liu
- Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011-3020, USA
| | - Arjun K Pathak
- Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011-3020, USA
| | - Dariusz Kaczorowski
- Institute of Low Temperature and Structure Research, Polish Academy of Sciences, PL-50-950 Wroclaw, Poland
| | - Madhab Neupane
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
| | - Michael Chini
- Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
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Saule T, Heinrich S, Schötz J, Lilienfein N, Högner M, deVries O, Plötner M, Weitenberg J, Esser D, Schulte J, Russbueldt P, Limpert J, Kling MF, Kleineberg U, Pupeza I. High-flux ultrafast extreme-ultraviolet photoemission spectroscopy at 18.4 MHz pulse repetition rate. Nat Commun 2019; 10:458. [PMID: 30692528 PMCID: PMC6349926 DOI: 10.1038/s41467-019-08367-y] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2018] [Accepted: 01/04/2019] [Indexed: 11/23/2022] Open
Abstract
Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 × 105 photons per pulse in the 25-to-60-eV range, releasing 1 × 1010 photoelectrons per second from a 10-µm-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.
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Affiliation(s)
- T Saule
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - S Heinrich
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - J Schötz
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - N Lilienfein
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - M Högner
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - O deVries
- Fraunhofer-Institut für Angewandte Optik und Feinmechanik (IOF), Albert-Einstein-Str. 7, 07745, Jena, Germany
| | - M Plötner
- Fraunhofer-Institut für Angewandte Optik und Feinmechanik (IOF), Albert-Einstein-Str. 7, 07745, Jena, Germany
| | - J Weitenberg
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Fraunhofer-Institut für Lasertechnik (ILT), Steinbachstr. 15, 52074, Aachen, Germany
| | - D Esser
- Fraunhofer-Institut für Lasertechnik (ILT), Steinbachstr. 15, 52074, Aachen, Germany
| | - J Schulte
- Fraunhofer-Institut für Lasertechnik (ILT), Steinbachstr. 15, 52074, Aachen, Germany
| | - P Russbueldt
- Fraunhofer-Institut für Lasertechnik (ILT), Steinbachstr. 15, 52074, Aachen, Germany
| | - J Limpert
- Friedrich-Schiller-Universität Jena, Institut für Angewandte Physik (IAP), Albert-Einstein-Str. 15, 07745, Jena, Germany
- Helmholtz-Institut Jena, Fröbelstieg 3, 07743, Jena, Germany
- Active Fiber Systems GmbH (AFS), Wildenbruchstr. 15, 07745, Jena, Germany
| | - M F Kling
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - U Kleineberg
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany
- Ludwig-Maximilians-Universität München (LMU), Am Coulombwall 1, 85748, Garching, Germany
| | - I Pupeza
- Max-Planck-Institut für Quantenoptik (MPQ), Hans-Kopfermann-Str. 1, 85748, Garching, Germany.
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Bigler N, Pupeikis J, Hrisafov S, Gallmann L, Phillips CR, Keller U. High-power OPCPA generating 1.7 cycle pulses at 2.5 µm. OPTICS EXPRESS 2018; 26:26750-26757. [PMID: 30469755 DOI: 10.1364/oe.26.026750] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/07/2018] [Accepted: 09/18/2018] [Indexed: 06/09/2023]
Abstract
We present a high-power mid-infrared (mid-IR) optical parametric chirped-pulse amplifier (OPCPA) generating 14.4 fs pulses centered at 2.5 µm with an average power of 12.6 W and a repetition rate of 100 kHz. The short pulses are obtained without nonlinear pulse compression. This is in contrast to most few-cycle systems operating in the mid-IR. In our case, the ultrashort pulse duration is enabled by a careful design of the gain profile of each amplification stage as well as a precise control of the signal dispersion throughout the system. A pulse shaper is used in the seed beam to adjust the spectral phase at the output of the OPCPA system. This approach allows for a clean temporal profile leading to a high peak power of 6.3 GW.
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Major B, Horváth ZL, Varjú K. Spatial characterization of light beams analyzed by cylindrical-grating slit-less spectrometers. APPLIED OPTICS 2018; 57:738-745. [PMID: 29400741 DOI: 10.1364/ao.57.000738] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/03/2017] [Accepted: 12/28/2017] [Indexed: 06/07/2023]
Abstract
In this work, we theoretically analyze the spatial information provided by cylindrical-grating slit-less spectrometers. We raise attention on the often not considered property that the spatial features acquired using these spectrometers are different from what can be obtained using a spectrometer with an entrance slit. In relation to this, we also highlight that they do not provide information directly on the real spatial beam profile. It is important to consider this fact in spatio-spectral analysis of extreme ultraviolet radiation, often carried out using cylindrical-grating slit-less spectrometers. Since the models used are based on the Fresnel diffraction integral and ideal optical systems, the results are valid also for other spectral regions.
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10
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Pincelli T, Grasselli F, Petrov VN, Torelli P, Rossi G. Performance of photoelectron spin polarimeters with continuous and pulsed sources: from storage rings to free electron lasers. JOURNAL OF SYNCHROTRON RADIATION 2017; 24:175-187. [PMID: 28009557 DOI: 10.1107/s1600577516017513] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/31/2016] [Accepted: 11/01/2016] [Indexed: 06/06/2023]
Abstract
In this work the experimental uncertainties concerning electron spin polarization (SP) under various realistic measurement conditions are theoretically derived. The accuracy of the evaluation of the SP of the photoelectron current is analysed as a function of the detector parameters and specifications, as well as of the characteristics of the photoexcitation sources. In particular, the different behaviour of single counter or twin counter detectors when the intensity fluctuations of the source are considered have been addressed, leading to a new definition of the SP detector performance. The widely used parameter called the figure of merit is shown to be inadequate for describing the efficiency of SP polarimeters, especially when they are operated with time-structured excitation sources such as free-electron lasers. Numerical simulations have been performed and yield strong implications in the choice of the detecting instruments in spin-polarization experiments, that are constrained in a limited measurement time. Our results are therefore applied to the characteristics of a wide set of state-of-the-art spectroscopy facilities all over the world, and an efficiency diagram for SP experiments is derived. These results also define new mathematical instruments for handling the correct statistics of SP measurements in the presence of source intensity fluctuations.
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Affiliation(s)
- T Pincelli
- Dipartimento di Fisica, Università degli studi di Milano, Via Celoria 16, 20133 Milano, Italy
| | - F Grasselli
- Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università degli Studi di Modena e Reggio Emilia, Via Campi 213/a, Modena, Italy
| | - V N Petrov
- Saint Petersburg State Polytechnical University, Politechnicheskaya Street 29, 195251, Russian Federation
| | - P Torelli
- Laboratorio TASC, IOM-CNR, SS 14 km 163.5, Basovizza, 34149 Trieste, Italy
| | - G Rossi
- Dipartimento di Fisica, Università degli studi di Milano, Via Celoria 16, 20133 Milano, Italy
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11
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Larsen EW, Carlström S, Lorek E, Heyl CM, Paleček D, Schafer KJ, L'Huillier A, Zigmantas D, Mauritsson J. Sub-cycle ionization dynamics revealed by trajectory resolved, elliptically-driven high-order harmonic generation. Sci Rep 2016; 6:39006. [PMID: 27991521 PMCID: PMC5171819 DOI: 10.1038/srep39006] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2016] [Accepted: 11/16/2016] [Indexed: 12/02/2022] Open
Abstract
The sub-cycle dynamics of electrons driven by strong laser fields is central to the emerging field of attosecond science. We demonstrate how the dynamics can be probed through high-order harmonic generation, where different trajectories leading to the same harmonic order are initiated at different times, thereby probing different field strengths. We find large differences between the trajectories with respect to both their sensitivity to driving field ellipticity and resonant enhancement. To accurately describe the ellipticity dependence of the long trajectory harmonics we must include a sub-cycle change of the initial velocity distribution of the electron and its excursion time. The resonant enhancement is observed only for the long trajectory contribution of a particular harmonic when a window resonance in argon, which is off-resonant in the field-free case, is shifted into resonance due to a large dynamic Stark shift.
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Affiliation(s)
- E W Larsen
- Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
| | - S Carlström
- Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
| | - E Lorek
- Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
| | - C M Heyl
- Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
| | - D Paleček
- Department of Chemical Physics, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.,Department of Chemical Physics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague, Czech Republic
| | - K J Schafer
- Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana, 70803, United States of America
| | - A L'Huillier
- Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
| | - D Zigmantas
- Department of Chemical Physics, Lund University, P.O. Box 124, SE-22100 Lund, Sweden
| | - J Mauritsson
- Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
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