Extreme Ultraviolet Supercontinuum Research Articles

Broadband XUV supercontinuum generation by time-dependent phase-matching

We investigate the influence of the time-dependent on-axis phase matching on broadband extreme ultraviolet (XUV) supercontinuum generation with an intense laser field. By using the strong-field approximation model and solving Maxwell’s equations we show that time-dependent on-axis phase matching can lead to broadening of the XUV supercontinuum. Moreover, we also demonstrate that the bandwidth of the XUV supercontinuum can be controlled by adjusting the beam waist of the laser field and the position of the gas medium.

High-order harmonics in a quantum dot and metallic nanorod complex.

We investigate the high-order harmonic generation (HHG) in a semiconductor quantum dot (SQD) and metallic nanorod (MNR) complex driven by a moderate intensity (<10(12) W/cm(2)) frequency-chirped Gaussian few-cycle pulse. Our numerical results indicate that the cutoff energy of the HHG can be controlled by optimizing the shape of the MNR and surface-to-surface distance between the SQD and the MNR. We also show that the extreme ultraviolet supercontinuum harmonics (25 eV maximal photon energy) and isolated ultrashort pulses (2.67-4.36 fs FWHM) are achievable.

Generation of an extreme ultraviolet supercontinuum in a two-color spatially inhomogeneous field

We theoretically investigate the high-order harmonic generation from the helium ion in a two-color spatially inhomogeneous laser field. The numerical calculations show that this spatiotemporally synthesized laser field can not only significantly extend the harmonic cut-off, but also generate an ultrabroad extreme ultraviolet supercontinuum with a 1737 eV bandwidth. In addition, the spatiotemporal combination can eliminate the long quantum path and select the short one, and then an ultrashort isolated 12.3 as pulse with a bandwidth of 310.2 eV is obtained directly. Further, the dependences of the harmonic spectrum on the parameters including spatial inhomogeneity, time delay, carrier-envelope phase, and intensity are further discussed.

Isolated few-attosecond emission in a multi-cycle asymmetrically nonhomogeneous two-color laser field

By solving the three-dimensional time-dependent Schrödinger equation, we present a theoretical investigation of helium-based high-order harmonic generation (HHG) in both symmetric and asymmetric spatially nonhomogeneous laser fields. It is determined that the harmonic cutoff is extended in both symmetric and asymmetric nonhomogeneous laser fields, and the spatial symmetry of the nonhomogeneous field greatly influences the movement of electronic wave packets, which subsequently affect harmonic emission. In particular, the spatially asymmetric distribution of the laser field will induce asymmetric recombination of electronic wave packets between the two sides of a helium atom, which could reduce the interference in HHG emission for every optical cycle. In combination with the two-color technique, multi-cycle laser pulses with asymmetric spatially nonhomogeneous characteristics are capable of generating a coherent extreme ultraviolet supercontinuum, which can be used to produce isolated few-attosecond pulses.

Coherent contribution of consecutive electron rescatterings in the extreme ultraviolet supercontinuum

In this article, coherent contributions of high-order quantum paths are proposed to generate a highly efficient and fine appearance attosecond pulse. We show how the electron dynamics of the high-order harmonics evolves so that harmonics are generated by a helium atom exposed to the two-color waveform composed of sub-one-cycle pulses. The leading and the trailing of the pulse is constructed by the near-infrared driving pulse and a midinfrared control pulse, respectively, thereby tunnel ionization and recombination are effectively manipulated. This waveform not only broadens the bandwidth but also leads to the contribution of high-order rescatterings in the generated supercontinuum and thereby enhances the harmonic yields. Whereas the effective duration of the waveform is lesser than the midinfrared pulse, with prolonging the pulse trailing, the long path of first-order rescattering is completely suppressed and high-order rescatterings of different electron trajectories appear. The most interesting feature is that under optimized conditions, the short paths of consecutive rescatterings coherently contribute in the high-order-harmonic generation process that leads to a fine appearance in the attosecond pulses.

Isolated ultrashort attosecond pulse generation from a coherent superposition state in a few-cycle chirped laser pulse

We theoretically demonstrate an efficient method for producing an isolated ultrashort attosecond pulse by high-order harmonic generation in an intense few-cycle chirped laser pulse. Our simulation calculations show that the harmonic spectrum reveals an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared in a coherent superposition of the ground and first excited states. It is also shown that the method can enhance the short quantum path and eliminate the long one, and then an isolated 23-attosecond pulse is produced. By properly adjusting the chirp parameter, a clean single attosecond pulse as short as 11 attoseconds is directly obtained.

Generation of isolated sub-40-as pulses from gas-phase CO molecules using an intense few-cycle chirped laser and a unipolar pulse

We theoretically investigate the high-order-harmonic generation (HHG) from the oriented gas-phase CO molecule exposed to the combination of an intense few-cycle chirped laser and a unipolar pulse. We show that the HHG is dramatically extended and an ultrabroad extreme ultraviolet supercontinuum spectra in the plateau is generated by adding a unipolar pulse to the few-cycle chirped laser at a proper time. Both the classical trajectory simulation and quantum time-frequency analysis show that a dominant short quantum path is selected to contribute the HHG spectra in the plateau. By superposing the supercontinuum harmonics centered at several different central frequencies, we can directly produce several isolated pulses in which the shortest one achieves 38 as.

Generation of an extreme ultraviolet supercontinuum with a multicycle chirped laser and a static electric field

We theoretically present a method for generating an ultrabroad extreme ultraviolet (XUV) supercontinuum by using the combination of a multicycle chirped laser and a static electric field. At a low laser intensity, the spectral cutoff is extended to the 495th order harmonic, and the bandwidth of the supercontinuum spectrum is broadened to 535 eV. At a high laser intensity, the harmonic cutoff is enlarged to the 667th order, and a supercontinuum covering a bandwidth of 1035 eV is generated. In these two cases, the long quantum path is removed, and the short quantum path is selected. Especially for the relatively high laser intensity, an isolated 23-attosecond pulse with a bandwidth of about 170.6 eV is directly obtained. Finally, we also analyze the influences of the chirp parameter and the duration of the chirped pulse as well as the static field strength on the supercontinuum.

Direct generation of intense extreme-ultraviolet supercontinuum with 35-fs, 11-mJ pulses from a femtosecond laser amplifier

We report on the generation of intense extreme ultraviolet (EUV) supercontinuum with photon energies spanning from 35 eV to 50 eV (i. e., supporting an isolated attosecond pulse with a duration of ~271 as) by loosely focusing 35 fs, 11 mJ pulses from a femtosecond laser amplifier into a 10-mm long gas cell filled with krypton gas. The dramatic change of spectral and temporal properties of the driver pulses after passing through the gas cell indicates that propagation effects play a significant role in promoting the generation of the EUV supercontinuum.

Propagation effects of isolated attosecond pulse generation with a multicycle chirped and chirped-free two-color field

We present a theoretical study of isolated attosecond pulse generation with a multicycle chirped and chirped-free two-color field. We show that the bandwidth of the extreme ultraviolet supercontinuum can be extended by combining a multicycle chirped pulse and a multicycle chirped-free pulse. Also, the broadband supercontinuum can still be generated when the macroscopic effects are included. Furthermore, the macroscopic effects can ameliorate the temporal characteristic of the broadband supercontinuum of the single atom, and eliminate the modulations of the broadband supercontinuum. Thus a very smooth broadband supercontinuum and a pure isolated 102-as pulse can be directly obtained. Moreover, the structure of the broadband supercontinuum can be steadily maintained for a relative long distance after a certain distance.

Isolated short attosecond pulse generation in an orthogonally polarized multicycle chirped laser field

We theoretically demonstrate the generation of a high-order harmonic and isolated attosecond pulse in an orthogonally polarized laser field, which is synthesized by an 800-nm chirped laser pulse and an 800-nm chirp-free laser pulse. Owing to the instantaneous frequency increasingly reducing close to the center of the driving pulse, the extreme ultraviolet supercontinuum for the chirped synthesized field is even broader than that for an orthogonal chirp-free two-color laser field. It is found that the broadband supercontinuum spectrum can be achieved for the driving pulse with ten and above optical cycles. After phase compensation an isolated attosecond pulse with a duration of \ensuremath{\sim}16 as is produced. Furthermore, the optimization of the chirping rate parameters is investigated to achieve cutoff extension and an isolated short attosecond pulse.

Extension of harmonic cutoff in a multicycle chirped pulse combined with a chirp-free pulse

We demonstrate high-order harmonic generation in a wave form synthesized by a multicycle 800-nm chirped laser pulse and a chirp-free laser pulse. Compared with the case of using only a chirped pulse, both the harmonic cutoff and the extreme ultraviolet supercontinuum can be extended when a weak chirp-free pulse is combined with the chirped pulse. When chirp-free pulse intensity grows, the cutoff energy and bandwidth of the supercontinuum grow as well. It is found that the broad supercontinuum can be achieved for a driving pulse with long duration even though the driving pulse reaches 10 optical cycles. An isolated attosecond pulse with duration of about 59 as is obtained, and after appropriate phase compensation with a duration of about $11$ as. In addition, by performing time-frequency analyses and the classical trajectory simulation, the difference in supercontinuum generation between the preceding wave form and a similar wave form synthesized by an $800$-nm fundamental pulse and a $1600$-nm subharmonic pulse is investigated.

ENHANCEMENT AND EXTENSION OF HIGH-ORDER HARMONIC EMISSION AND AN ISOLATED SUB-100 AS PULSE GENERATION FROM TWO-COLOR LASER FIELDS

We theoretically propose a method to generate a coherent and ultrabroad extreme ultraviolet supercontinuum using Ar + ions. When the medium is exposed to a two-color laser field, which is synthesized by a few-cycle fundamental laser pulse and its second-harmonics pulse, the harmonics spectrum presents a two-plateau structure. For the selection of the short quantum path utilizing the two-color scheme, the supercontinuum in the second plateau is almost synchronously emitted and a single 58 attosecond (as) pulse can be directly obtained. By increasing the intensity of the controlling field or adjusting the delay time of the two laser fields, a more intense isolated as pulse will be generated.

Extension of harmonic cut-off in a waveform controlled laser field by prolonging the recombining period

AbstractWe present a novel waveform synthesized by a half-cycle fundamental wave (800 nm) and a half-cycle subharmonic wave (2400 nm) with an appropriate carrier-envelope phase and a phase delay, in which more ionized electrons can recombine with its parent ions to emit high-order harmonic, giving rise to both the extension of the extreme ultraviolet supercontinuum and the enhancement of harmonic conversion efficiency. An isolated attosecond pulse with considerable energy is obtained as well. By performing time-frequency analyses, it is revealed that the prolonged opposite electric field increases the recombining probability of high-energy electrons.

Isolated short attosecond pulse generated using a two-color laser and a high-order pulse

An efficient method to generate an isolated short attosecond pulse is investigated theoretically. A broadband extreme ultraviolet supercontinuum harmonics can be generated when a model He${}^{+}$ ion is exposed to the combination of an intense few-cycle laser pulse and a low-frequency field. By properly adding a $27$th harmonics pulse to resonantly excite the He${}^{+}$ ion, the intensity of the high-order harmonic generation (HHG) plateau is enhanced by $3\ensuremath{-}4$ orders of magnitude. As a result, an isolated $24$-as pulse with a bandwidth of $138$ eV can be obtained directly from the supercontinuum around the cutoff of HHG.

Generation of an isolated sub-100 attosecond pulse in the water-window spectral region

We propose a scheme to generate isolated attosecond pulses in the water-window spectral region. Based on the numerical solutions of the single active electron model, we investigate high-order harmonic generation in helium atoms driven by a multi-cycle two-colour optical field synthesized by an intense 2000 nm, 20 fs pulse and its frequency-doubled pulse. When the latter is slightly detuned and properly phase shifted with respect to the fundamental laser pulse, an ultra-broad extreme ultraviolet supercontinuum with a spectral width of 130 eV can be generated in the 270–400 eV spectral regions. A supercontinuum from 280–340 eV in the water window can be selected to yield an isolated 67 attosecond pulse without employing any phase compensation. This water window coherent x-ray pulse with less than 100 attosecond duration is a potential tool for studying the ultrafast electronic dynamics of biological samples in water.

Demonstration of extreme ultraviolet supercontinuum at the high harmonic plateau with a 6.5 fs/800 nm driving laser pulse

We demonstrate experimentally the extreme ultraviolet supercontinuum in the plateau of high-order harmonic emission from the argon gas cell driven by a 6.5 fs, 800 nm phase-stabilized laser pulse. The result is interpreted as a selection of single quantum path of the returning electrons due to the propagation process in the gas cell. Moreover we predict the possibility to generate isolated 100 as pulses using a driving laser pulse longer than 2 cycles.

Generation of an extreme ultraviolet supercontinuum and isolated sub-50 as pulse in a two-colour laser field

We theoretically study high-order harmonic generation when a helium ion is exposed to a two-colour laser field, which is synthesized by a 5 fs/800 nm laser pulse and a 64 fs/2400 nm laser pulse. Our numerical results show that the harmonic spectrum exhibits an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared as a coherent superposition of the ground state and the first excited state. By superposing a series of properly selected harmonics, an isolated attosecond pulse with a duration of 47 as is obtained. Compared with the case of the ground state in a one-colour field, the intensity of this isolated attosecond pulse is six orders of magnitude higher. We also demonstrate these results in terms of the time–frequency analysis and the semiclassical three-step model.

Pulse shaping to generate an xuv supercontinuum in the high-order harmonic plateau region

Generation of extreme ultraviolet supercontinuum in the plateau region of high-order harmonics has been demonstrated by shaping a fundamental-wave laser pulse with its second- and third-harmonic pulses. Such a Fourier synthesis of the optical waveforms could confine the harmonic emissions mainly in the center peak of the driving pulse, which suppresses those on other half-cycles efficiently, and as a consequence avoids or reduces spectral interference of harmonic emissions at different half-cycles, resulting in harmonic emissions free of interference within a certain spectral span in the frequency domain that forms supercontinuum in the high-order harmonic plateau region. This supercontinuum is significant for the efficient generation and further control of single attosecond pulses.

Multi-cycle laser-driven broadband supercontinuum with a modulated polarization gating

We propose a method to generate broadband supercontinuum with a modulated polarization gating in the multi-cycle regime. The polarization gating at 1600 nm is employed to intensify the ellipticity dependence of high harmonic signal, and the adding second harmonic (SH) field is used for modulating ionization ratio between the driving field half-cycles. It is shown that longer driving pulses (6 optical cycles, 32.4 fs) can be applied to confine the efficient recollision of electron wave packet to half an optical cycle. The produced bandwidth of the extreme ultraviolet (XUV) supercontinuum is about 280 eV, corresponding to a fourier-transform-limited pulse of 10 as. In addition, an attosecond pulse of about 100 as with tunable wavelength can be straightforwardly obtained just by filtering different range of the spectrum.