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Waveform control of three kinds of chirped pulses (i.e. βt, βt 2 and βt 3) to produce harmonic spectra and attosecond pulses has been investigated. It is found that by properly choosing the chirps, the chirp delays and the other laser parameters, not only the instantaneous frequency of some specific half profiles can be decreased, but also its intensity can be increased. As a result, the free electron can receive more energy when it accelerates in these regions, thus leading to the extension of the harmonic cutoff and harmonic plateau. Finally, through the Fourier transformation of the harmonic spectra and by superposing some harmonics, three single attosecond pulses with the durations of 30 as, 33 as and 39 as can be obtained.

In the literature it is well known that the produced high-order harmonics inherit certain properties from the driving laser especially in terms of spectral characteristics [1] , therefore introducing modulation in the generating laser spectrum we can have significant impact on the high harmonic generation process [2] , [3] .

A potential method to produce isolated attosecond pulses (IAPs) by using low-intensity chirped-UV combined field has been investigated. The results can be separated into three parts. First, by properly introducing the mid-chirp or down-chirp of the low-intensity laser field, the harmonic cutoff can be extended and achieve the referenced value, which is produced from the high-intensity referenced field. Moreover, the spectral continuum is contributed by a single harmonic emission peak, which is beneficial to produce IAPs. However, the harmonic yield is very low due to the lower driven laser intensity. Second, by properly adding a UV pulse, the harmonic yield can be enhanced and achieve the referenced value due to the UV resonance ionisation. The intensity of the combined field is lower than that of the referenced field, which reduces the experimental requirements for producing high-intensity spectral continuum. Third, with the introduction of the positive or negative inhomogeneous effect of the mid-chirped combined field or down-chirped combined field, respectively, the similar harmonic cutoff and harmonic yield can also be obtained but with a much lower driven laser intensity. Finally, by superposing the harmonics on the spectral continuum, the IAPs with the durations of sub-38 as can be obtained.

To control the polarization of attosecond pulse trains (APTs), the most widely used way is to adjust the intensity ratio between the two components in a bicircular laser field. Here we demonstrate ...

When using a multilayer mirror to reflect an attosecond pulse, the temporal broadening effect always happens, and enlarges the duration of incident attosecond pulse. In this paper, the temporal broadening of an isolated attosecond pulse induced by a periodic Mo/Si multilayer mirror reflection is analyzed. In order to evaluate the effect of pulse broadening quantitatively, two temporal parameters, the pulse broadening coefficient and the pulse reflection coefficient, are used. By studying two temporal parameters of periodic Mo/Si multilayer mirrors with different bi-layers for incident isolated attosecond pulses with various durations, we find that the incident isolated attosecond pulse with smaller duration suffers from more serious pulse broadening and energy loss, when reflected by a periodic Mo/Si multilayer mirror. In addition, a periodic Mo/Si multilayer mirror with more bi-layers causes more serious pulse broadening for an incident isolated attosecond pulse, even though it can provide larger pulse reflection efficiency. Furthermore, by analyzing of the reflectivity and the dispersion of periodic multilayer mirrors in the spectral region of incident pulses, we demonstrate that the spectrum filtering and dispersion inducing by periodic Mo/Si multilayer mirrors are responsible for the pulse broadening of incident isolated attosecond pulses. The study of pulse broadening induced by periodic Mo/Si multilayer mirrors reflection could improve the understanding of the performances of periodic multilayer mirrors for pulse reflection, and provide a guide for high-performance attosecond multilayer mirrors design.

Single image measurement of an attosecond pulse is demonstrated for its complete temporal characterization. To determine the spectrally resolved emission time of the attosecond pulse, we image the dipole modulated by a time-dependent optical grating., CLEO: QELS_Fundamental Science, May 9–14, 2021, San Jose, Ca. USA

A GW-scale isolated attosecond pulse is demonstrated by a fully stabilized 50-mJ three-channel waveform synthesizers. Isolated pulse duration with 213-as, 0.24-μJ is experimentally characterized by the FROG-CRAB method with a 10-Hz repetition rate.

We propose a new self-contained method to characterize an attosecond pulse. Our method employs the carrier-envelop-ephase dependence of an attosecond pulse with respect to a driving pulse where the carrier-envelope-phase dependence is equivalent to a time-delayed gating function. It simplifies attosecond measurements of ultrafast phenomena., IPC 2020 (2020 IEEE Photonics Conference), Sept. 28th - Oct. 1st, 2020, Held Virtually

Isolated attosecond pulses can be generated by the polarization gating method, for which the gating time window and the polarization direction are key parameters. In this work, we propose methods to characterize these parameters with an imaging polarimeter based on the division of the focal plane in a single-shot manner. The gating time window and the polarization direction can be characterized using the degree of linear polarization and the angle of linear polarization, respectively. Spatially resolved evaluation of gating parameters is important for generating high-intensity isolated attosecond pulses. The robust generation of isolated attosecond pulses by evaluating the gating parameters with the proposed method will enable us to perform the attosecond-pump and attosecond-probe of electron dynamics in atoms and molecules.

We propose a mechanism to generate a single intense circularly polarized attosecond x-ray pulse from the interaction of a circularly polarized relativistic few-cycle laser pulse with an ultrathin foil at normal incidence. Analytical modeling and particle-in-cell simulation demonstrate that a huge charge-separation field can be produced when all the electrons are displaced from the target by the incident laser, resulting in a high-quality relativistic electron mirror that propagates against the tail of the laser pulse. The latter is efficiently reflected as well as compressed into an attosecond pulse that is also circularly polarized.

Attosecond pulses are fundamental for the investigation of valence and core-electron dynamics on their natural timescale. At present the reproducible generation and characterisation of attosecond waveforms has been demonstrated only through the process of high-order harmonic generation. Several methods for the shaping of attosecond waveforms have been proposed, including metallic filters, multilayer mirrors and manipulation of the driving field. However, none of these approaches allow for the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free Electron Lasers, on the contrary, deliver femtosecond, extreme ultraviolet and X-ray pulses with energies ranging from tens of $\mathrm{\mu}$J to a few mJ. Recent experiments have shown that they can generate sub-fs spikes, but with temporal characteristics that change shot-to-shot. Here we show the first demonstration of reproducible generation of high energy ($\mathrm{\mu}$J level) attosecond waveforms using a seeded Free Electron Laser. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with a novel approach for its temporal reconstruction. The results presented here open the way to perform attosecond time-resolved experiments with Free Electron Lasers.

Rotational nonlinearity of molecular gases can be harnessed to generate more than two octaves of bandwidth and compress 280 fs, 400 µJ pulses from a commercial Yb:KGW laser amplifier to 1.6 cycles for attosecond pulse generation.

We control the spectral and temporal characteristics of attosecond pulses by tailoring the HHG-driving optical waveform via parametric waveform synthesis. Attosecond streaking measurements are performed for full characterization of the XUV pulses and synthesized fields.

We demonstrate supercontinuum spectral phase interferometry with an isolated attosecond pulse (IAP). The interferogram indicates a high degree of temporal coherence over the full IAP continuous spectrum, which will contribute to exploring dispersive electron dynamics. 1. Introduction Combining spatial and temporal detection by means of self-referencing interferometry with high-order harmonics has a variety of applications in, for instance, molecular orbital tomography [1], nanoscale sample imaging [2], and the characterization of complex electron dynamics [3]. Meanwhile, an isolated attosecond pulse (IAP) with short duration and broadband supercontinuum spectrum have contributed to observing ultrafast electronic motions in various targets [4]. The self-referencing interferometry with two IAPs will be an important tool for observing electron dynamics through the phase-based spectroscopy. Here, we demonstrated the supercontinuum spectral phase interferometry with IAP [5]. The coherence over broadband spectral region will contribute to studying the dispersive electron dynamics through Fourier spectroscopy.

A method for the carrier envelope phase (CEP) measurement of the multi-cycle mid-infrared field has been theoretically presented by using the CEP dependence of the harmonic cutoffs. It shows that by properly adding a CEP-known few-cycle near-infrared pulse to the multi-cycle mid-infrared field, the CEP dependence of the harmonic cutoffs from the combined field become much more distinct in comparison with those from the single mid-infrared field, which provides a route to a new CEP measurement technique. Further, by properly controlling the combined field, three supercontinua of 230, 242, and 266 eV with less modulation can be obtained. Finally, by selecting the generated harmonics, a series of sub-35 as pulses in the X-rays region can be produced.

Light pulses of sub-100 as (1 as=10-18 s) duration, with photon energies in the extreme-ultraviolet (XUV) spectral domain, represent the shortest event in time ever made and controlled by human beings. Their first experimental observation in 2001 has opened the door to investigating the fundamental dynamics of the quantum world on the natural time scale for electrons in atoms, molecules and solids and marks the beginning of the scientific field now called attosecond science.

High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase (CEP) of few-cycle laser pulse. We find that short pulse duration will cause the negative chirp for the high harmonics. When the laser pulse is shortened to a few cycles, the influence of the laser CEP on the chirp of the harmonics will also become more prominent.

High harmonic generation (HHG) is strongly modified near plasmonic nanostructures due to confinement and inhomogeneity of the electromagnetic field. Previous studies have revealed low-intensity generation of HHG and extension of the plateau; however, the roles of potential shape and a combination of inhomogeneous infrared (IR) and blue fields on HHG have not been studied. In this work, we study HHG driven by inhomogeneous two-color (800–400 nm) IR and blue femtosecond pulses by numerically solving the time-dependent Schrödinger equation. HHG spectra are computed for two different models: for a short-range potential, which supports a single-bound state, and for a long-range potential, which supports a Rydberg series, to show potential dependence on inhomogeneous two-color HHG. A substantial enhancement in the value of the cut-off resulting from inhomogeneity up to the ∼600th order, extending beyond the water window, is found for both the models. The HHG spectra are highly sensitive to the relative phase of the two-color fields and this sensitivity increases with increasing inhomogeneity. Possibilities of efficiently generating and controlling attosecond pulse train and isolated attosecond pulse are discussed.

Laser control on the generations of the molecular high-order harmonic generation (MHHG) and the isolated attosecond pulse (IAP) from H2+ have been theoretically investigated through solving the non-Bohn–Oppenheimer time-dependent Schrödinger equation. It is found that (i) in the middle laser intensity (i.e., I = 4.0 × 1014 W/cm2), the contribution from negative-H to the MHHG is higher than that from the positive-H. With the decrease (i.e., I = 2.0 × 1014 W/cm2) or the increase (i.e., I = 7.0 × 1014 W/cm2) in laser intensity, the asymmetric contributions from the two H nuclei to the MHHG are decreased. (ii) Pulse duration investigation shows that the distributions of the MHHG in two H nuclei present similar contributions as the pulse duration is enhanced. (iii) Laser phase investigation shows that when the laser phase is chosen from 0.0π to 0.6π and from 1.7π to 2.0π, the contribution from the negative-H plays the main role in MHHG. When the laser phase is chosen from 0.7π to 1.6π, the contribution from the positive-H to the MHHG is remarkably enhanced and becomes greater than that from the negative-H. The time–frequency analyses of the MHHG and the time-dependent wave function are shown to explain the harmonic emission process and the electron motion in H2+. (iv) With the introduction of one or two half-cycle controlling pulses, the cutoff and the intensity of the MHHG spectra can be controlled. Finally, by selecting the harmonics generated during a single harmonic emission event, an IAP with the full width at half maximum of 48 as can be produced.

Photo-induced molecular dynamics can now be investigated using free electron lasers (FELs) whose attributes are unprecedented brightness, few femtosecond pulses duration and in the near future few hundreds of attosecond pulse duration. These relatively new tools are utilized to investigate photon-induced physical and chemical processes by ionizing inner-shell electrons and thus unlocking molecular dynamics from within.

A method is proposed to generate an isolated attosecond X-ray free-electron laser pulse with the peak power beyond 1 TW., A method is proposed to generate an isolated attosecond X-ray pulse in free-electron lasers, using irregularly spaced current peaks induced in an electron beam through interaction with an intense short-pulse optical laser. In comparison with a similar scheme proposed in a previous paper, the irregular arrangement of current peaks significantly improves the contrast between the main and satellite pulses, enhances the attainable peak power and simplifies the accelerator layout. Three different methods are proposed for this purpose and achievable performances are computed under realistic conditions. Numerical simulations carried out with the best configuration show that an isolated 7.7 keV X-ray pulse with a peak power of 1.7 TW and pulse length of 70 as can be generated. In this particular example, the contrast is improved by two orders of magnitude and the peak power is enhanced by a factor of three, when compared with the previous scheme.

Femtosecond enhancement cavities have enabled multi-10-MHz-repetition-rate coherent extreme ultraviolet (XUV) sources with photon energies exceeding 100 eV - albeit with rather severe limitations of the net conversion efficiency and of the duration of the XUV emission. Here, we explore the possibility of circumventing both these limitations by harnessing spatiotemporal couplings in the driving field, similar to the "attosecond lighthouse," in theory and experiment. Our results predict dramatically improved output coupling efficiencies and efficient generation of isolated XUV attosecond pulses.

Carrier-envelope phase (CEP) stabilization plays an important role in the field of attosecond pulse generation and high-field physics [1]. Low-jitter CEP stabilization of ultrashort pulses typically relies on active stabilization schemes, either employing feedback or feed-forward control. However, both established methods come with a detrimental side effect. Feedback control directly modulates pump power or intracavity loss, which causes unwanted power fluctuations of the laser output. While feed-forward control does not corrupt the output stability, it nevertheless induces beam pointing variations in the acousto-optic frequency shifter employed. Here we demonstrate a novel method for modulating the CEP of a laser oscillator, which is based on the Doppler effect and displays neither of the above-mentioned side-effects.

Harmonic generation from solid surfaces is a promising tool for producing high energy attosecond pulses. We report shaping of the harmonic spectrum to achieve the bandwidth necessary for attosecond pulse generation. The shaping is demonstrated for lower as well as for higher harmonics using single and two-pulse pumping. The measured harmonic field autocorrelation function exhibits attosecond spikes in good agreement with the harmonic spectrum. Double slit experiments reveal a high spatial coherence of the harmonic beam., Comment: 9 pages, 8 figures

Many experiments in attosecond science will benefit from attosecond pulses at high repetition rates with sufficient photon flux for pump-probe experiments. We use 7fs, 800nm pulses from a non-collinear optical parametric chirped pulse amplification (NOPCPA) laser system to generate few-pulse attosecond pulse trains (APTs) in the extreme ultraviolet (XUV) at a repetition rate of 100kHz. The pulse trains have been fully characterized by recording FROG-CRAB (Frequency-Resolved Optical Gating for Complete Reconstruction of Attosecond Bursts) traces with a velocity map imaging spectrometer., Comment: 7 pages, 4 figures, submitted version

New theoretical and numerical results are presented regarding isolated attosecond XUV – soft X-ray pulses, that can be generated by Thomson-backscattering of a high-intensity single-cycle near-infrared laser pulse on a suitable nanobunch of MeV electons. A simple approximate formula is derived for the cut-off frequency of the collective radiation spectrum, which is then employed to find the length of the nanobunch which emits an isolated pulse of 16 as length. Detailed analysis of the spectral, temporal and spatial features of this attosecond pulse is given. It is also shown that the 100 nJ pulse energy, corresponding to 2.1 × 1018W/cm2 peak intensity of the laser pulse, can be increased to reach the μJ pulse energy both by increasing the intensity or by setting a suitable down-chirp of the laser pulse.

The attosecond lighthouse is a method of using ultrafast wavefront rotation with high-harmonic generation to create a series of coherent, spatially separated attosecond pulses. Previously, temporal measurements by photoelectron streaking characterized isolated attosecond pulses created by manipulating the single-atom response. The attosecond lighthouse, in contrast, generates a series of pulses that spatially separate and become isolated by propagation. Here, we show that ultrafast wavefront rotation maintains the single-atom response (in terms of temporal character) of an isolated attosecond pulse over two octaves of bandwidth. Moreover, we exploit the unique property of the attosecond lighthouse—the generation of several isolated pulses—to measure the three most intense pulses. These pulses each have a unique spectrum and spectral phase.

The ellipticity of high harmonics driven by bi-chromatic (e.g. ω − 2 ω ) co-propagating fields can be fully tuned by varying the polarization of the pump components. In order to start revealing the underlying mechanism of this control, we explore a relatively simple regime of this scheme that still gives rise to full control over the harmonics ellipticities. In this regime, the pumps are only slightly elliptical and the high harmonic radiation consists of two (different) interlocked attosecond pulse trains (APTs). We formulate a semi-analytic model that maps the high harmonic ellipticity to properties of the APTs harmonic decompositions. Utilizing this model, we reconstruct these APTs variables from measurements of the high harmonics ellipticities. This ellipticity-resolved spectroscopy of interlocked APTs may be useful for ultrafast probing of chiral degrees of freedom.

A potential scheme to obtain intense harmonic spectra from X[Formula: see text] ([Formula: see text] and T) has been studied by using chirped-delay effect and half-cycle pulse. First, for the shorter pulse duration case, the main contributions of harmonic spectra of H[Formula: see text] and T[Formula: see text] are both coming from the falling part of laser field. While, for the longer pulse duration case, the rising and falling parts of laser field play the main role in harmonic spectra of H[Formula: see text] and T[Formula: see text], respectively. Second, according to the changed law of harmonic yields of H[Formula: see text] and T[Formula: see text] and by properly introducing the chirp-delay effect at some specific timing, the intense harmonic emission peak (HEP) can be obtained from both H[Formula: see text] and T[Formula: see text]. Third, with the help of the half-cycle pulse, the selected HEP can be further extended, showing the intense and broad spectral continua. Finally, by Fourier transformation of some harmonics on the spectral continua, the intense attosecond pulses with durations of 35 as can be produced.

Synopsis We present an efficient scheme for the generation of broadband supercontinuum harmonics and isolated utrashort attosecond pulse by the optimized three-color laser fileds. The optimized laser pulse is synthesized by three one-color laser pulses with proper relative phases. The broadband supercontinuum harmonics are obtained by solving accurately the time-dependent Schrödinger equation using the time-dependent generalized pseudospectral method. Our scheme proposed produces a broadband supercontinuum spectrum with the photon energies near 0.3 keV. As a result, an isolated 17 as pulse can be obtained directly by superposing the supercontinuum harmonics.

Attosecond technology is based on a non-linear optical process named high-order harmonic generation, which allows sub-femtosecond light transients in the XUV spectral region to be produced. The possibility to follow in real time the electron dynamics occurring in matter has raised a continuous growing interest in this extremely fast laser technology. This article introduces the principles of attosecond pulse generation, discusses the femtosecond technology required to drive this extremely non-linear process and presents a general overview of attosecond metrology.

International audience; Processes in atoms or molecules on the attosecond timescale have been measured using XUV attosecond and IR femtosecond pulses overlapping in time and controlled with attosecond accuracy. Within this general framework, many strategies have been developed using the harmonics of the fundamental pulse. In this paper, we focus on a specific configuration where the attosecond pulse train is composed by odd harmonics and is dressed by the second harmonic of the fundamental light. Measuring the angularly resolved photoelectron spectrum as a function of the delay between the pulses, a clear oscillation of the anisotropy parameters appears revealing attosecond controlled interferences. This process, is assigned to interferences between two quantum paths involving one XUV photon, on one path, and a XUV+UV photons on the other path. The XUV-UV delay dependent up–down asymmetry can be interpreted following the usual RABBITT formalism, where the dressing photon energy corresponds to the energy separation between the XUV photons of the attosecond pulse train. This approach allows an intuitive analysis of the interference and provides a well suited method for the study of complex valence band systems thanks to the limited congestion of the resulting spectrum.

We have proposed a metallic dipole-ellipsoid-bagel nanostructure, which could effectively control the enhanced plasmonic fields within two-color laser for producing isolated attosecond pulses. This nanostructure enables the multiple resonances leading to broad overall bandwidth, which benefits high harmonic generation (HHG) and the production of isolated attosecond pulses via enhanced plasmonic field. The nanostructure could enhance the field intensity exceed 103 in the volume of 10 × 5 × 5 nm3. The isolated attosecond pulse with a duration of 155 as is obtained in the two-color 15-fs incidence. This nanostructure would benefit for the application of the ultra-sensitive multi-color sensor and isolated attosecond pulses generation.

An efficient method for generating an intense isolated ultrashort attosecond pulse is presented theoretically. By adding a 267 nm controlling pulse to a multi-cycle two-colour field, not only the spectral cutoff and the yields of the harmonic spectrum are evidently enhanced, but also the selection of the single quantum path is realised. Then a high-efficiency supercontinuum with a 504 eV bandwidth and smooth structure is obtained, which enables the production of an intense isolated 30 as pulse. In addition, the influences of the laser parameters on the supercontinuum and isolated attosecond pulse are investigated.

Combining the concept of temporal field synthesis with optical parametric chirped pulse amplification (OPCPA) holds promise for generation of multi-mJ, watt-level light transients, which may extends isolated attosecond pulse to multi-keV photon energies [1, 2]. The details of our proposed conceptual architecture of such a field synthesizer is described in [3]. The apparatus consists of (i) an Yb:YAG thin-disk regenerative amplifier, followed by (ii) phase-stable multi-octave seed generation, (iii) amplification of three complementary portions of the supercontinuum in OPCPA channels, and (iv) coherent combination of the output of these channels.

Attosecond pulses propagating in different directions, generated in a rotating wavefront of a driving laser field, can provide a source of multiple isolated attosecond pulses. Clear spatial separation of the attosecond pulses is attained if the divergence of the individual attosecond pulse is smaller than their angular separation, which is limited by the bandwidth of the driving laser pulse. Here we demonstrate both experimentally and numerically that an additional wavefront rotation is imposed during the propagation of the driving laser pulse in a highly ionizing medium. This dynamic wavefront rotation enables the generation of the isolated attosecond pulse even in the case when the conditions derived from a linear diffraction theory do not permit the angular separation. The described nonlinear phenomenon has its roots in the half-cycle ionization events, and may open up new ways to study strong field processes in highly ionizing media. (C) 2017 Optical Society of America

We report on the first streaking measurement of water-window attosecond pulses generated via high harmonic generation, driven by sub-2-cycle, CEP-stable, 1850 nm laser pulses. Both the central photon energy and the energy bandwidth far exceed what has been demonstrated thus far, warranting the investigation of the attosecond streaking technique for the soft X-ray regime and the limits of the FROGCRAB retrieval algorithm under such conditions. We also discuss the problem of attochirp compensation and issues regarding much lower photo-ionization cross sections compared with the XUV in addition to the fact that several shells of target gases are accessed simultaneously. Based on our investigation, we caution that the vastly different conditions in the soft X-ray regime warrant a diligent examination of the fidelity of the measurement and the retrieval procedure., Comment: 14 Pages, 12 figures

We combine harmonics from solids and gases to generate isolated attosecond pulses with central frequency controlled by over an octave in the XUV. Spatio-temporal coupling and the carrier-envelope-phase significantly affect the generated half-cycle optical transient field., CLEO: QELS_Fundamental Science, 14-19 May 2017, San Jose, California, USA

Coherent synchrotron emission from relativistic laser-plasma interactions is an exciting route towards bright attosecond pulse production. We demonstrate experimentally the critical role that laser contrast on 1-2 picosecond timescales plays in the detailed generation mechanism.

We present a high-power, MHz-repetition-rate, phase-stable femtosecond laser system based on a phase-stabilized Ti:Sa oscillator and a multi-stage Yb-fiber chirped-pulse power amplifier. A 10-nm band around 1030 nm is split from the 7-fs oscillator output and serves as the seed for subsequent amplification by 54 dB to 80 W of average power. The µJ-level output is spectrally broadened in a solid-core fiber and compressed to ~30 fs with chirped mirrors. A pulse picker prior to power amplification allows for decreasing the repetition rate from 74 MHz by a factor of up to 4 without affecting the pulse parameters. To compensate for phase jitter added by the amplifier to the feed-forward phase-stabilized seeding pulses, a self-referencing feed-back loop is implemented at the system output. An integrated out-of-loop phase noise of less than 100 mrad was measured in the band from 0.4 Hz to 400 kHz, which to the best of our knowledge corresponds to the highest phase stability ever demonstrated for high-power, multi-MHz-repetition-rate ultrafast lasers. This system will enable experiments in attosecond physics at unprecedented repetition rates, it offers ideal prerequisites for the generation and field-resolved electro-optical sampling of high-power, broadband infrared pulses, and it is suitable for phase-stable white light generation.

We have been developing two types of intense attosecond beam lines for investigating attosecond wavepackt dynamics in atoms and molecules. One produces an attosecond pulse train for attosecond Fourier transform spectroscopy by attosecond-pump/attosecond-probe scheme, while the other produces an intense isolated attosecond pulse for attosecond nonlinear optics.

Generation of high-order harmonic spectrum and isolated attosecond pulse (IAP) from frequency-chirping combined field has been investigated. It is found that by adding the middle-chirp form of the laser field, the extension of the harmonic cutoff comes from the middle region of the laser field. However, the intensity of the higher-order spectral continuum is very low. By adding the asymmetric negative chirp form of the laser field, the extension of the harmonic cutoff is attributed to the falling region of the laser field. Moreover, the intensity of the higher-order spectral continuum presents similar value as that produced from the chirp-free pulse. Further, by properly adding a second controlling pulse (i.e., infrared field or ultraviolet pulse) into the fundamental chirped pulse, the intensity of the harmonic spectrum can be enhanced by 2–3 and 1–2 orders of magnitudes for the cases of the middle-chirp form and the asymmetric negative chirp form, respectively. Finally, by using the best laser conditions, a series of isolated attosecond pulses with the durations of 38 as can be produced.

A potential scheme to obtain the high-intensity KeV isolated attosecond pulses (IAPs) from the high-order harmonic generation (HHG) has been proposed and studied via the superposition of the multi-color inhomogeneous laser beams. The scheme can be separated into three steps. Firstly, with the superposition of two-color mid-infrared field and a few-cycle infrared field at the proper delay time and the carrier envelope phase, the optimal HHG spectrum can be found. Secondly, with the help of the positive inhomogeneous effect in space, the harmonic emission peaks (HEPs) from the positive half-cycle can be well selected. Secondly, by properly adding a unipolar pulse with the positive peak, the harmonic cutoff can be extended to the KeV region and the harmonic plateau is only contributed by the single HEP. Thirdly, with the introduction of the UV pulse, the signal of the harmonic plateau can be enhanced by at least 500 dB due to the UV-resonance-enhancement-ionization. Finally, through the Fourier transformation of some selected harmonics, the sub-35 as IAPs in the KeV region can be obtained.