@article{zhao_new_2017,
	title = {A new method for accurate retrieval of atomic dipole phase or photoionization group delay in attosecond photoelectron streaking experiments},
	volume = {19},
	issn = {2040-8978, 2040-8986},
	url = {https://iopscience.iop.org/article/10.1088/2040-8986/aa8fb6},
	doi = {10.1088/2040-8986/aa8fb6},
	abstract = {In recent years, attosecond streaking experiments have been used to extract the phase of photoionization dipole transition matrix element (or the photoionization group delay) in atoms, molecules and condensed materials. The most accurate retrieval method so far is based on the frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROGCRAB). However, the FROG-CRAB employs a number of approximations that would cause errors in the retrieved results. In this article, we applied our recently proposed attosecond pulse characterization method phase retrieval of broadband pulses (PROBP) to the retrieval of photoionization group delay difference between Ne(2p) and Ne(2s) ionization channels, and between Ar(3p) and Ne(2p) channels. Our simulations demonstrate that more accurate results can be retrieved using PROBP than using FROG-CRAB, and cast doubts on group delays reported in previous streaking measurements.},
	language = {en},
	number = {11},
	urldate = {2020-06-16},
	journal = {Journal of Optics},
	author = {Zhao, Xi and Wei, Hui and Wei, Changli and Lin, C D},
	month = nov,
	year = {2017},
	pages = {114009},
}

In recent years, attosecond streaking experiments have been used to extract the phase of photoionization dipole transition matrix element (or the photoionization group delay) in atoms, molecules and condensed materials. The most accurate retrieval method so far is based on the frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROGCRAB). However, the FROG-CRAB employs a number of approximations that would cause errors in the retrieved results. In this article, we applied our recently proposed attosecond pulse characterization method phase retrieval of broadband pulses (PROBP) to the retrieval of photoionization group delay difference between Ne(2p) and Ne(2s) ionization channels, and between Ar(3p) and Ne(2p) channels. Our simulations demonstrate that more accurate results can be retrieved using PROBP than using FROG-CRAB, and cast doubts on group delays reported in previous streaking measurements.

@article{gebert_michelson-type_2014,
	title = {Michelson-type all-reflective interferometric autocorrelation in the {VUV} regime},
	volume = {16},
	issn = {1367-2630},
	url = {https://iopscience.iop.org/article/10.1088/1367-2630/16/7/073047},
	doi = {10.1088/1367-2630/16/7/073047},
	abstract = {We demonstrate second-order interferometric autocorrelation of a pulse in the vacuum-ultraviolet (VUV) spectral range using an optical arrangement equivalent to a Michelson interferometer. In an all-reflective design, wavefront splitting is realized with two moveable interdigitated reflective gratings forming a diffraction pattern with well separated orders and an intensity distribution depending on the precisely adjustable path-length difference. An imaging timeof-flight spectrometer is able to spatially select ions created by nonlinear twophoton absorption in the focus of the zeroth diffraction order. This arrangement is used to demonstrate interferometric autocorrelation in krypton with femtosecond VUV pulses at 160 nm wavelength. In addition to the pulse duration, which is already accessible with non-collinear intensity autocorrelation, the full interferometric contrast of the presented approach enables us to extract also information on temporal phases.},
	language = {en},
	number = {7},
	urldate = {2020-06-16},
	journal = {New Journal of Physics},
	author = {Gebert, T and Rompotis, D and Wieland, M and Karimi, F and Azima, A and Drescher, M},
	month = jul,
	year = {2014},
	pages = {073047},
}

We demonstrate second-order interferometric autocorrelation of a pulse in the vacuum-ultraviolet (VUV) spectral range using an optical arrangement equivalent to a Michelson interferometer. In an all-reflective design, wavefront splitting is realized with two moveable interdigitated reflective gratings forming a diffraction pattern with well separated orders and an intensity distribution depending on the precisely adjustable path-length difference. An imaging timeof-flight spectrometer is able to spatially select ions created by nonlinear twophoton absorption in the focus of the zeroth diffraction order. This arrangement is used to demonstrate interferometric autocorrelation in krypton with femtosecond VUV pulses at 160 nm wavelength. In addition to the pulse duration, which is already accessible with non-collinear intensity autocorrelation, the full interferometric contrast of the presented approach enables us to extract also information on temporal phases.

@article{li_characterization_2012,
	title = {Characterization of elliptically polarized femtosecond pulses by molecular-alignment-based frequency resolved optical gating},
	volume = {108},
	issn = {0946-2171, 1432-0649},
	url = {http://link.springer.com/10.1007/s00340-012-5132-4},
	doi = {10.1007/s00340-012-5132-4},
	abstract = {We demonstrated that molecular-alignmentbased cross-correlation frequency resolved optical gating (M-XFROG) could be used for complete characterization of elliptically polarized femtosecond pulses by measuring the orthogonal linear polarization components and the additional polarization projection at 45 degree of the target pulse. The electric field orientation, polarization ellipticity angles, and phase information of the target pulse were also obtained. The transiently aligned air molecules functioned as a linear optical gating function in the measurement processes. The validity and robustness of M-XFROG were confirmed by the comparison between the retrieved optical gating function and measured molecular alignment signal in air.},
	language = {en},
	number = {4},
	urldate = {2020-06-16},
	journal = {Applied Physics B},
	author = {Li, Hao and Li, Wenxue and Liu, Jia and Pan, Haifeng and Wu, Jian and Zeng, Heping},
	month = sep,
	year = {2012},
	pages = {761--766},
}

We demonstrated that molecular-alignmentbased cross-correlation frequency resolved optical gating (M-XFROG) could be used for complete characterization of elliptically polarized femtosecond pulses by measuring the orthogonal linear polarization components and the additional polarization projection at 45 degree of the target pulse. The electric field orientation, polarization ellipticity angles, and phase information of the target pulse were also obtained. The transiently aligned air molecules functioned as a linear optical gating function in the measurement processes. The validity and robustness of M-XFROG were confirmed by the comparison between the retrieved optical gating function and measured molecular alignment signal in air.

@article{chen_reconstruction_2011,
	title = {Reconstruction of attosecond pulses using two-color pumping},
	volume = {28},
	issn = {0740-3224, 1520-8540},
	url = {https://www.osapublishing.org/abstract.cfm?URI=josab-28-9-2195},
	doi = {10.1364/JOSAB.28.002195},
	language = {en},
	number = {9},
	urldate = {2020-06-16},
	journal = {Journal of the Optical Society of America B},
	author = {Chen, Jun and Itakura, Ryuji and Nakajima, Takashi},
	month = sep,
	year = {2011},
	pages = {2195},
}

@article{zuo_spectral_2010,
	title = {Spectral phase transfer to ultrashort {UV} pulses through four-wave mixing},
	volume = {18},
	issn = {1094-4087},
	url = {https://www.osapublishing.org/oe/abstract.cfm?uri=oe-18-15-16183},
	doi = {10.1364/OE.18.016183},
	abstract = {Transfer of spectral phase from near infrared ultrashort pulses to deep ultraviolet (UV) sub-30-fs pulses through four-wave mixing process is demonstrated. Micro joule UV pulses at 237 nm were generated by nonlinear mixing of second harmonic pulses of Ti:sapphire laser output and near infrared pulses from a noncollinear optical parametric amplifier. Chirp of the near infrared pulse was transfered to the UV pulse with the opposite sign. A positively chirped near infrared pulse was used for generating a negatively chirped UV pulse, which was compressed down to 25 fs by a magnesium fluoride window.},
	language = {en},
	number = {15},
	urldate = {2020-06-16},
	journal = {Optics Express},
	author = {Zuo, P. and Fuji, T. and Suzuki, T.},
	month = jul,
	year = {2010},
	pages = {16183},
}

Transfer of spectral phase from near infrared ultrashort pulses to deep ultraviolet (UV) sub-30-fs pulses through four-wave mixing process is demonstrated. Micro joule UV pulses at 237 nm were generated by nonlinear mixing of second harmonic pulses of Ti:sapphire laser output and near infrared pulses from a noncollinear optical parametric amplifier. Chirp of the near infrared pulse was transfered to the UV pulse with the opposite sign. A positively chirped near infrared pulse was used for generating a negatively chirped UV pulse, which was compressed down to 25 fs by a magnesium fluoride window.

@article{chen_characterization_2010,
	title = {Characterization of attosecond {XUV} pulses utilizing a broadband {UV}{\textasciitilde}{VUV} pumping},
	volume = {18},
	issn = {1094-4087},
	url = {https://www.osapublishing.org/oe/abstract.cfm?uri=oe-18-3-2020},
	doi = {10.1364/OE.18.002020},
	abstract = {We propose a simple scheme to characterize attosecond extreme ultraviolet (XUV) pulses. A broadband ultraviolet (UV) ∼ vacuum ultraviolet (VUV) pump pulse creates a coherent superposition of atomic bound states, from which photoionization takes place by the time-delayed attosecond XUV probe pulse. Information on the spectral phase of the XUV pulse can be extracted from the phase offset of the interference beating in the photoelectron spectra using a standard SPIDER (spectral phase interferometry for direct electric-field reconstruction) algorithm. We further discuss the influence of the chirp and polychromaticity of the pump pulse, and show that they do not spoil the reconstruction process. Since our scheme is applicable for various simple atoms such as H, He, and Cs, etc., and capable of characterizing attosecond XUV pulses with a pulse duration of a few hundred attoseconds or even less, it can be an alternative technique to characterize attosecond XUV pulses. Specific numerical examples are presented for the H atom utilizing the 2p and 3p states.},
	language = {en},
	number = {3},
	urldate = {2020-06-16},
	journal = {Optics Express},
	author = {Chen, Jun and Itakura, Ryuji and Nakajima, Takashi},
	month = feb,
	year = {2010},
	pages = {2020},
}

We propose a simple scheme to characterize attosecond extreme ultraviolet (XUV) pulses. A broadband ultraviolet (UV) ∼ vacuum ultraviolet (VUV) pump pulse creates a coherent superposition of atomic bound states, from which photoionization takes place by the time-delayed attosecond XUV probe pulse. Information on the spectral phase of the XUV pulse can be extracted from the phase offset of the interference beating in the photoelectron spectra using a standard SPIDER (spectral phase interferometry for direct electric-field reconstruction) algorithm. We further discuss the influence of the chirp and polychromaticity of the pump pulse, and show that they do not spoil the reconstruction process. Since our scheme is applicable for various simple atoms such as H, He, and Cs, etc., and capable of characterizing attosecond XUV pulses with a pulse duration of a few hundred attoseconds or even less, it can be an alternative technique to characterize attosecond XUV pulses. Specific numerical examples are presented for the H atom utilizing the 2p and 3p states.

@article{power_xfrog_2010,
	title = {{XFROG} phase measurement of threshold harmonics in a {Keldysh}-scaled system},
	volume = {4},
	issn = {1749-4885, 1749-4893},
	url = {http://www.nature.com/articles/nphoton.2010.38},
	doi = {10.1038/nphoton.2010.38},
	language = {en},
	number = {6},
	urldate = {2020-06-16},
	journal = {Nature Photonics},
	author = {Power, Erik P. and March, Anne Marie and Catoire, Fabrice and Sistrunk, Emily and Krushelnick, Karl and Agostini, Pierre and DiMauro, Louis F.},
	month = jun,
	year = {2010},
	pages = {352--356},
}

@article{faucher_four-dimensional_2009,
	title = {Four-dimensional investigation of the 2nd order volume autocorrelation technique},
	volume = {97},
	issn = {0946-2171, 1432-0649},
	url = {http://link.springer.com/10.1007/s00340-009-3559-z},
	doi = {10.1007/s00340-009-3559-z},
	abstract = {The 2nd order volume autocorrelation technique, widely utilized in directly measuring ultra-short light pulses durations, is examined in detail via model calculations that include three-dimensional integration over a large ionization volume, temporal delay and spatial displacement of the two beams of the autocorrelator at the focus. The effects of the inherent displacement to the 2nd order autocorrelation technique are demonstrated for short and long pulses, elucidating the appropriate implementation of the technique in tight focusing conditions. Based on the above investigations, a high accuracy 2nd order volume autocorrelation measurement of the duration of the 5th harmonic of a 50 fs long laser pulse, including the measurement of the carrier wavelength oscillation, is presented.},
	language = {en},
	number = {2},
	urldate = {2020-06-16},
	journal = {Applied Physics B},
	author = {Faucher, O. and Tzallas, P. and Benis, E. P. and Kruse, J. and Peralta Conde, A. and Kalpouzos, C. and Charalambidis, D.},
	month = oct,
	year = {2009},
	pages = {505--510},
}

The 2nd order volume autocorrelation technique, widely utilized in directly measuring ultra-short light pulses durations, is examined in detail via model calculations that include three-dimensional integration over a large ionization volume, temporal delay and spatial displacement of the two beams of the autocorrelator at the focus. The effects of the inherent displacement to the 2nd order autocorrelation technique are demonstrated for short and long pulses, elucidating the appropriate implementation of the technique in tight focusing conditions. Based on the above investigations, a high accuracy 2nd order volume autocorrelation measurement of the duration of the 5th harmonic of a 50 fs long laser pulse, including the measurement of the carrier wavelength oscillation, is presented.

@article{monmayrant_real_2006,
	title = {Real time quantum state holography using coherent transients},
	volume = {264},
	issn = {00304018},
	url = {https://linkinghub.elsevier.com/retrieve/pii/S0030401806004858},
	doi = {10.1016/j.optcom.2006.03.073},
	abstract = {In a two-level atom, real-time quantum state holography is performed through interferences between quantum states created by a reference pulse and a chirped pulse resulting in coherent transients. A sequence of several measurements allows one to measure the real and imaginary parts of the excited state wave function. These measurements are performed during the interaction with the ultrashort laser pulse. The extreme sensitivity of this method to the pulse shape provides a tool for electric field measurement.},
	language = {en},
	number = {2},
	urldate = {2020-06-16},
	journal = {Optics Communications},
	author = {Monmayrant, Antoine and Chatel, Béatrice and Girard, Bertrand},
	month = aug,
	year = {2006},
	keywords = {\_tablet},
	pages = {256--263},
}

In a two-level atom, real-time quantum state holography is performed through interferences between quantum states created by a reference pulse and a chirped pulse resulting in coherent transients. A sequence of several measurements allows one to measure the real and imaginary parts of the excited state wave function. These measurements are performed during the interaction with the ultrashort laser pulse. The extreme sensitivity of this method to the pulse shape provides a tool for electric field measurement.

@article{kosuge_frequency-resolved_2006,
	title = {Frequency-{Resolved} {Optical} {Gating} of {Isolated} {Attosecond} {Pulses} in the {Extreme} {Ultraviolet}},
	volume = {97},
	issn = {0031-9007, 1079-7114},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.97.263901},
	doi = {10.1103/PhysRevLett.97.263901},
	language = {en},
	number = {26},
	urldate = {2020-06-16},
	journal = {Physical Review Letters},
	author = {Kosuge, A. and Sekikawa, T. and Zhou, X. and Kanai, T. and Adachi, S. and Watanabe, S.},
	month = dec,
	year = {2006},
	pages = {263901},
}

@article{nabekawa_interferometric_2006,
	title = {Interferometric {Autocorrelation} of an {Attosecond} {Pulse} {Train} in the {Single}-{Cycle} {Regime}},
	volume = {97},
	issn = {0031-9007, 1079-7114},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.97.153904},
	doi = {10.1103/PhysRevLett.97.153904},
	language = {en},
	number = {15},
	urldate = {2020-06-16},
	journal = {Physical Review Letters},
	author = {Nabekawa, Yasuo and Shimizu, Toshihiko and Okino, Tomoya and Furusawa, Kentaro and Hasegawa, Hirokazu and Yamanouchi, Kaoru and Midorikawa, Katsumi},
	month = oct,
	year = {2006},
	pages = {153904},
}

@article{corsi_direct_2006,
	title = {Direct {Interferometric} {Measurement} of the {Atomic} {Dipole} {Phase} in {High}-{Order} {Harmonic} {Generation}},
	volume = {97},
	issn = {0031-9007, 1079-7114},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.97.023901},
	doi = {10.1103/PhysRevLett.97.023901},
	language = {en},
	number = {2},
	urldate = {2020-06-16},
	journal = {Physical Review Letters},
	author = {Corsi, Chiara and Pirri, Angela and Sali, Emiliano and Tortora, Alessandra and Bellini, Marco},
	month = jul,
	year = {2006},
	pages = {023901},
}

@article{cormier_self-referencing_2005,
	title = {Self-{Referencing}, {Spectrally}, or {Spatially} {Encoded} {Spectral} {Interferometry} for the {Complete} {Characterization} of {Attosecond} {Electromagnetic} {Pulses}},
	volume = {94},
	issn = {0031-9007, 1079-7114},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.94.033905},
	doi = {10.1103/PhysRevLett.94.033905},
	language = {en},
	number = {3},
	urldate = {2020-06-16},
	journal = {Physical Review Letters},
	author = {Cormier, Eric and Walmsley, Ian A. and Kosik, Ellen M. and Wyatt, Adam S. and Corner, Laura and DiMauro, Louis F.},
	month = jan,
	year = {2005},
	keywords = {\_tablet},
	pages = {033905},
}

@article{kienberger_single_2005,
	title = {Single sub-fs soft-{X}-ray pulses: generation and measurement with the atomic transient recorder},
	volume = {52},
	issn = {0950-0340, 1362-3044},
	shorttitle = {Single sub-fs soft-{X}-ray pulses},
	url = {http://www.tandfonline.com/doi/abs/10.1080/09500340412331315114},
	doi = {10.1080/09500340412331315114},
	abstract = {The change from a zero transition to the maximum amplitude of the electric field of visible light lasts shorter than one femtosecond (1 fs ¼ 10À15 s). By precisely controlling the hyperfast electric field oscillations in a short laser pulse we developed a measuring apparatus—the atomic transient recorder—like an ultrafast stopwatch. This apparatus is capable of measuring the duration of atomic processes with an accuracy of less than 100 as (1 as ¼ 10À18 s), which is the typical duration of electronic processes (transients) deep inside atoms. A 250 as X-ray pulse initiates the atomic process to be measured and the attosecond stopwatch at the same time. For the first time it is now possible with this new measuring method to observe ultrafast processes in the electron shell of atoms.},
	language = {en},
	number = {2-3},
	urldate = {2020-06-16},
	journal = {Journal of Modern Optics},
	author = {Kienberger, R. and Uiberacker, M. and Goulielmakis, E. and Baltuska, A. and Drescher, M. and Krausz, F.},
	month = jan,
	year = {2005},
	pages = {261--275},
}

The change from a zero transition to the maximum amplitude of the electric field of visible light lasts shorter than one femtosecond (1 fs ¼ 10À15 s). By precisely controlling the hyperfast electric field oscillations in a short laser pulse we developed a measuring apparatus—the atomic transient recorder—like an ultrafast stopwatch. This apparatus is capable of measuring the duration of atomic processes with an accuracy of less than 100 as (1 as ¼ 10À18 s), which is the typical duration of electronic processes (transients) deep inside atoms. A 250 as X-ray pulse initiates the atomic process to be measured and the attosecond stopwatch at the same time. For the first time it is now possible with this new measuring method to observe ultrafast processes in the electron shell of atoms.

@article{mauritsson_probing_2005,
	title = {Probing temporal aspects of high-order harmonic pulses via multi-colour, multi-photon ionization processes},
	volume = {38},
	issn = {0953-4075, 1361-6455},
	url = {https://iopscience.iop.org/article/10.1088/0953-4075/38/13/018},
	doi = {10.1088/0953-4075/38/13/018},
	abstract = {High-order harmonics generated through the interaction of atoms and strong laser fields are a versatile, laboratory-scale source of extreme ultraviolet (XUV) radiation on a femtosecond or even attosecond time-scale. In order to be a useful experimental tool, however, this radiation has to be well characterized, both temporally and spectrally. In this paper we discuss how multi-photon, multi-colour ionization processes can be used to completely characterize either individual harmonics or attosecond pulse trains. In particular, we discuss the influence of the intensity and duration of the probe laser, and how these parameters effect the accuracy of the XUV characterization.},
	language = {en},
	number = {13},
	urldate = {2020-06-16},
	journal = {Journal of Physics B: Atomic, Molecular and Optical Physics},
	author = {Mauritsson, J and Johnsson, P and López-Martens, R and Varjú, K and L'Huillier, A and Gaarde, M B and Schafer, K J},
	month = jul,
	year = {2005},
	pages = {2265--2278},
}

High-order harmonics generated through the interaction of atoms and strong laser fields are a versatile, laboratory-scale source of extreme ultraviolet (XUV) radiation on a femtosecond or even attosecond time-scale. In order to be a useful experimental tool, however, this radiation has to be well characterized, both temporally and spectrally. In this paper we discuss how multi-photon, multi-colour ionization processes can be used to completely characterize either individual harmonics or attosecond pulse trains. In particular, we discuss the influence of the intensity and duration of the probe laser, and how these parameters effect the accuracy of the XUV characterization.

@article{varju__frequency_2005,
	title = {Frequency chirp of harmonic and attosecond pulses},
	volume = {52},
	issn = {0950-0340, 1362-3044},
	url = {http://www.tandfonline.com/doi/abs/10.1080/09500340412331301542},
	doi = {10.1080/09500340412331301542},
	abstract = {We study the phase of the atomic polarization in the process of high-order harmonic generation. Its dependence on the laser intensity and the harmonic order induce a frequency variation in time (chirp) respectively of the harmonic pulses and attosecond pulses. We review the recent experimental results on the temporal characterization of the harmonic emission and show that measurements performed using very different techniques (like XFROG and RABITT), probing the phase in different parameter spaces, can be connected through the mixed phase derivatives, demonstrating the common underlying physics.},
	language = {en},
	number = {2-3},
	urldate = {2020-06-16},
	journal = {Journal of Modern Optics},
	author = {Varjú ‖, K. and Mairesse, Y. and Carré, B. and Gaarde, M. B. and Johnsson, P. and Kazamias, S. and López-Martens, R. and Mauritsson, J. and Schafer, K. J. and Balcou, Ph. and L'huillier, A. and Salières, P.},
	month = jan,
	year = {2005},
	pages = {379--394},
}

We study the phase of the atomic polarization in the process of high-order harmonic generation. Its dependence on the laser intensity and the harmonic order induce a frequency variation in time (chirp) respectively of the harmonic pulses and attosecond pulses. We review the recent experimental results on the temporal characterization of the harmonic emission and show that measurements performed using very different techniques (like XFROG and RABITT), probing the phase in different parameter spaces, can be connected through the mixed phase derivatives, demonstrating the common underlying physics.

@article{tzallas_attosecond-science_2005,
	title = {The attosecond-science frontiers: generation, metrology and paths to applications},
	volume = {144-147},
	issn = {03682048},
	shorttitle = {The attosecond-science frontiers},
	url = {https://linkinghub.elsevier.com/retrieve/pii/S0368204805002823},
	doi = {10.1016/j.elspec.2005.01.267},
	abstract = {Recent developments in the generation, metrology and potential applications of XUV-attosecond (as) pulses, formed by the coherent superposition of harmonics of an infrared femtosecond (fs) laser pulse, are reviewed. Particular emphasis is given to the recently achieved second-order autocorrelation (AC) measurement of a sub-femtosecond (sub-fs) pulse train. The method is a non-trivial extension in the XUV spectral range of the well-established technique routinely used in ‘fs’ pulse metrology. Besides the direct visualization of a periodic attosecond structure and the quantitative information it supplies, the approach introduces a unique technique for the implementation of XUV-pump XUVprobe type of experimental studies of ultra-fast phenomena. It further relates to the metrology and time domain application interests of other advanced XUV radiation sources, such as the XFEL-based installations. While the implementation of the method and its refined versions with laser harmonics is a challenge at the limits of the realizable, the profoundly high scheduled intensities of XFEL sources are expected to sufficiently enhance its robustness and range of applicability.},
	language = {en},
	urldate = {2020-06-16},
	journal = {Journal of Electron Spectroscopy and Related Phenomena},
	author = {Tzallas, P. and Tsakiris, G.D. and Witte, K. and Nikolopoulos, L.A.A. and Benis, E.P. and Charalambidis, D.},
	month = jun,
	year = {2005},
	pages = {1129--1135},
}

Recent developments in the generation, metrology and potential applications of XUV-attosecond (as) pulses, formed by the coherent superposition of harmonics of an infrared femtosecond (fs) laser pulse, are reviewed. Particular emphasis is given to the recently achieved second-order autocorrelation (AC) measurement of a sub-femtosecond (sub-fs) pulse train. The method is a non-trivial extension in the XUV spectral range of the well-established technique routinely used in ‘fs’ pulse metrology. Besides the direct visualization of a periodic attosecond structure and the quantitative information it supplies, the approach introduces a unique technique for the implementation of XUV-pump XUVprobe type of experimental studies of ultra-fast phenomena. It further relates to the metrology and time domain application interests of other advanced XUV radiation sources, such as the XFEL-based installations. While the implementation of the method and its refined versions with laser harmonics is a challenge at the limits of the realizable, the profoundly high scheduled intensities of XFEL sources are expected to sufficiently enhance its robustness and range of applicability.

@article{agostini_physics_2004,
	title = {The physics of attosecond light pulses},
	volume = {67},
	issn = {0034-4885, 1361-6633},
	url = {https://iopscience.iop.org/article/10.1088/0034-4885/67/6/R01},
	doi = {10.1088/0034-4885/67/6/R01},
	abstract = {The word ‘attosecond’ (1 as = 10−18 s) officially entered the vocabulary of physics when sub-femtosecond pulses of UV/XUV light produced either by nonlinear frequency conversion of a ultra-short infrared pump pulse or Fourier synthesis of broad bandwidth radiation were established. The physics of these pulses is based on nonlinear, nonperturbative laser–atom interaction: stimulated Raman scattering or high harmonic generation (HHG) is used to generate the necessary bandwidth, which naturally encompasses the visible and UV/XUV spectral range. However, the crucial element for attosecond pulse generation is the control of the spectral phase. New methods of temporal characterization at frequencies lying in the UV/XUV had to be elaborated. These methods rely on the energy/momentum analysis of photoelectrons produced by XUV attosecond flashes in the presence of an intense infrared field whose optical cycle itself becomes the basic clock. Single 650 as pulses have been produced and applied to trace the dynamics of electrons inside atoms following the creation of an inner-shell hole. Periodic combs of 250 as pulses have been synthesized by superposing just four harmonics and applying to the attosecond timing of the electron motion in HHG. Although it is easy to increase the bandwidth by coupling more harmonics, a fundamental limit to the duration of the light bursts produced has been discovered. It is imposed by the lack of synchronization of the different harmonic orders. The current limit is estimated to be 130 as. The latest advances include a direct autocorrelation of an attosecond pulse train and the production of a single 250 as soft x-ray pulse. This paper offers a snapshot of the state-ofthe-art in the production and characterization of attosecond light pulses, with a glimpse at the first steps in attophysics.},
	language = {en},
	number = {6},
	urldate = {2020-06-16},
	journal = {Reports on Progress in Physics},
	author = {Agostini, Pierre and DiMauro, Louis F},
	month = jun,
	year = {2004},
	pages = {813--855},
}

The word ‘attosecond’ (1 as = 10−18 s) officially entered the vocabulary of physics when sub-femtosecond pulses of UV/XUV light produced either by nonlinear frequency conversion of a ultra-short infrared pump pulse or Fourier synthesis of broad bandwidth radiation were established. The physics of these pulses is based on nonlinear, nonperturbative laser–atom interaction: stimulated Raman scattering or high harmonic generation (HHG) is used to generate the necessary bandwidth, which naturally encompasses the visible and UV/XUV spectral range. However, the crucial element for attosecond pulse generation is the control of the spectral phase. New methods of temporal characterization at frequencies lying in the UV/XUV had to be elaborated. These methods rely on the energy/momentum analysis of photoelectrons produced by XUV attosecond flashes in the presence of an intense infrared field whose optical cycle itself becomes the basic clock. Single 650 as pulses have been produced and applied to trace the dynamics of electrons inside atoms following the creation of an inner-shell hole. Periodic combs of 250 as pulses have been synthesized by superposing just four harmonics and applying to the attosecond timing of the electron motion in HHG. Although it is easy to increase the bandwidth by coupling more harmonics, a fundamental limit to the duration of the light bursts produced has been discovered. It is imposed by the lack of synchronization of the different harmonic orders. The current limit is estimated to be 130 as. The latest advances include a direct autocorrelation of an attosecond pulse train and the production of a single 250 as soft x-ray pulse. This paper offers a snapshot of the state-ofthe-art in the production and characterization of attosecond light pulses, with a glimpse at the first steps in attophysics.

@article{bloembergen_nanosecond_1999,
	title = {From nanosecond to femtosecond science},
	volume = {71},
	language = {en},
	number = {2},
	journal = {Rev. Mod. Phys.},
	author = {Bloembergen, N and Hall, Pierce},
	year = {1999},
	pages = {5},
}

@article{siders_multipulse_1999,
	title = {Multipulse interferometric frequency-resolved optical gating},
	volume = {35},
	issn = {00189197},
	url = {http://ieeexplore.ieee.org/document/753648/},
	doi = {10.1109/3.753648},
	abstract = {We review multipulse interferometric frequencyresolved optical gating (MI-FROG) as a technique, uniquely suited for pump-probe coherent spectroscopy using amplified visible and near-infrared short-pulse systems and/or emissive targets, for time-resolving ultrafast phase shifts and intensity changes. Application of polarization-gate MI-FROG to the study of ultrafast ionization in gases is presented.},
	language = {en},
	number = {4},
	urldate = {2020-06-16},
	journal = {IEEE Journal of Quantum Electronics},
	author = {Siders, C.W. and Siders, J.L.W. and Omenetto, F.G. and Taylor, A.J.},
	month = apr,
	year = {1999},
	pages = {432--440},
}

We review multipulse interferometric frequencyresolved optical gating (MI-FROG) as a technique, uniquely suited for pump-probe coherent spectroscopy using amplified visible and near-infrared short-pulse systems and/or emissive targets, for time-resolving ultrafast phase shifts and intensity changes. Application of polarization-gate MI-FROG to the study of ultrafast ionization in gases is presented.

@article{weinkauf_time_1997,
	title = {Time multiplexing: a new single shot femtosecond pump-probe technique},
	volume = {64},
	issn = {0946-2171, 1432-0649},
	shorttitle = {Time multiplexing},
	url = {http://link.springer.com/10.1007/s003400050208},
	doi = {10.1007/s003400050208},
	abstract = {We propose a new multiplex and fast readout technique for gaining a complete femtosecond pump-probe spectrum in a single shot which can be monitored on a conventional oscilloscope. The technique is based on counterpropagating fs laser pulses in gaseous samples of low density and probe detection by ion or fragment ion formation. Along the laser interaction path, each site in the sample corresponds to a particular pump-probe delay: The complete molecular response is multiplexed to a one dimensional spatial map of ion density. We propose a time-of-flight mass spectrometer for the fast point-wise readout e.g. the demultiplexing of this ion map. By calculation and experiments we find that a linear demultiplexing is possible and that in a reflectron time-of-flight instrument a 100 fs pump-probe delay interval translates to an easily resolved time-of-flight spacing of 10 ns. The range of pump-probe delays that is estimated to be accommodated covers 100 fs up to 10 ps. The technique is combined with mass selection providing mass resolution of 100.},
	language = {en},
	number = {5},
	urldate = {2020-06-16},
	journal = {Applied Physics B: Lasers and Optics},
	author = {Weinkauf, R. and Lehr, L. and Georgiev, D. and Schlag, E.W.},
	month = may,
	year = {1997},
	pages = {515--519},
}

We propose a new multiplex and fast readout technique for gaining a complete femtosecond pump-probe spectrum in a single shot which can be monitored on a conventional oscilloscope. The technique is based on counterpropagating fs laser pulses in gaseous samples of low density and probe detection by ion or fragment ion formation. Along the laser interaction path, each site in the sample corresponds to a particular pump-probe delay: The complete molecular response is multiplexed to a one dimensional spatial map of ion density. We propose a time-of-flight mass spectrometer for the fast point-wise readout e.g. the demultiplexing of this ion map. By calculation and experiments we find that a linear demultiplexing is possible and that in a reflectron time-of-flight instrument a 100 fs pump-probe delay interval translates to an easily resolved time-of-flight spacing of 10 ns. The range of pump-probe delays that is estimated to be accommodated covers 100 fs up to 10 ps. The technique is combined with mass selection providing mass resolution of 100.

@article{ross_simple_1994,
	title = {A simple technique to assist in temporal profile diagnostics of short pulses},
	volume = {107},
	issn = {00304018},
	url = {https://linkinghub.elsevier.com/retrieve/pii/0030401894901120},
	doi = {10.1016/0030-4018(94)90112-0},
	abstract = {Two-photon absorption (TPA) can be a useful nonlinear process in the diagnosis of very short pulses. One technique enables a simple measurement o f pulse duration. A second technique is useful in the measurement of pulse contrast ratio.},
	language = {en},
	number = {1-2},
	urldate = {2020-06-16},
	journal = {Optics Communications},
	author = {Ross, I.N. and Hooker, C.J. and Houliston, J.R.},
	month = apr,
	year = {1994},
	pages = {111--114},
}

Two-photon absorption (TPA) can be a useful nonlinear process in the diagnosis of very short pulses. One technique enables a simple measurement o f pulse duration. A second technique is useful in the measurement of pulse contrast ratio.