Single Quantum Path Research Articles

Coherent Control of High Harmonic Generation Driven by Metal Nanotip PhotoemissionSupported by the National Key Research and Development Program of China (Grant Nos. 2019YFA0307700 and 2016YFA0401100), the National Natural Science Foundation of China (Grant Nos. 11774215, 11674209, 91950101, 11947243, 11334009, 11425414, and 11947080), Sino-German Mobility Programme (Grant No. M-0031), Department of Education of Guangdong Province (Grant No. 2018KCXTD011), High Level

Steering ultrafast electron dynamics with well-controlled laser fields is very important for generation of intense supercontinuum radiation. It can be achieved through coherent control of the symmetry of the interaction between strong-field laser fields and a metal nanotip. We employ a scheme of two-color laser pulses combined with a weak static field to realize the control of a single quantum path to generate high harmonic generation from a single solid-state nanoemitter. Moreover, a smooth and ultrabroad supercontinuum in the extreme ultraviolet region is obtained, which can produce a single attosecond pulse. Our findings are beneficial for efficient generation of isolated sub-100 as XUV pulses from solid-state sources.

Generation of intense single short attosecond pulse with a multicycle spatially inhomogeneous field

We theoretically study the high-order harmonics and single attosecond pulse generation from a pre-excited He+ ion in a multicycle two-color spatially inhomogeneous field. It is shown that the broadband supercontinuum spectrum and single quantum path selection can be realized, and then a single 15.2 as pulse is directly obtained. By appending a uv attosecond pulse to the two-color spatially inhomogeneous field at a proper time, the conversion efficiency of the high-energy part of the supercontinuum is improved by at least an order of magnitude, which enables the production of an intense single 17.3 as pulse.

Exploration of High-Harmonic Generation from the CS2 Molecule by the Lewenstein Method in Two-Color Circularly Polarized Laser Field**Supported by the National Natural Science Foundation of China under Grant Nos 11574117, 11271158, 61575077 and 11575071.

The high harmonic generation (HHG) from the CS2 molecule in intense laser fields is investigated using the extended Lewenstein method. The initial state is the highest-occupied molecular orbital of the CS2 molecule, which can be well described by Gaussian wave packet using GAMESS-UK package. Compared with the case of the elliptical laser, the HHG can be extended in two-color circularly polarized laser field. The time-frequency analysis and classical electron trajectory as well as the ionization yield curve are also presented to further explain the underlying mechanism. After adding a static electric field on the z-direction, the single quantum path control is realized and the supercontinuum spectra are obtained. Moreover, an isolated 110 as pulse can be obtained by superposing the harmonics from 130th to 180th order.

Molecular high-order harmonic spectra and its application to the generation of the isolated attosecond pulse.

Molecular high-order harmonic generation (HHG) spectra from H2+ and its application to the generation of the isolated attosecond pulses (IAPs) have been numerically investigated. Results show that (i) the 7th harmonic order is enhanced with the nuclei around the equilibrium internuclear, and as the internuclear distance increased, this enhanced harmonic produces a red-shift (even disappearance). Theoretical analyses show that the electronic transition between the ground and the 1st excited states is responsible for the red-shift enhanced harmonic. (ii) The harmonic spectra exhibit several maxima and minima, and a red-shift of these points is predicted as the internuclear distance increased. (iii) By properly choosing the internuclear distance, the harmonic yield is enhanced, and there is only the single quantum path contributing to the harmonic spectra. (iv) Further, by properly adding the half-cycle pulse as well as the spatial inhomogeneous effect, a 375eV supercontinuum with a pulse enhancement of 3.9dB and some attosecond X-ray pulses shorter than 60as can be produced.

Attosecond XUV sources generation from polarized gating two-color chirped pulse

A promising method to generate the attosecond XUV sources from the high-order harmonic has been theoretically presented by controlling the polarized gating two-color chirped pulse. The results show that with the introduction of the chirps, the harmonic has been remarkably extended. Moreover, the harmonic interferences are very sensitive to the polarization angle between the two lasers. Particularly, when the polarization angle is equal to [Formula: see text], the supercontinuum with a single quantum path contribution is achieved, and a series of isolated attosecond pulses with the duration of 33 as are directly obtained. Further, by testing the influences of other laser parameters on the supercontinuum, we found that this polarized two-color chirped scheme can also be achieved in the multi-cycle pulse region, which is much better for experimental realization.

Intense Isolated Ultrashort Attosecond Pulse Generation in a Multi-Cycle Three-Colour Laser Field

Abstract 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.

Quantum path selection in high-order harmonic generation from aligned molecules

We theoretically investigate high-order harmonic generation (HHG) from aligned N(2) molecules with a driving field composed of two-color circularly polarized laser pulses. It is shown that the combination of N(2) molecules and the waveform-controlled laser field allows us to select either long or short quantum path, depending on molecular alignment angles, while in atom Ar, two paths show comparable contribution to HHG. The selection of single quantum path in aligned N(2) molecules leads to an ultrabroad and smooth XUV supercontinuum, giving rise to isolated attosecond pulses generation. Moreover, we can control the intensity ratio of two attosecond pulses by adjusting the molecular alignment angles, providing an opportunity for attosecond pump-probe technique.

Isolated sub-30-attosecond pulse generation using a multicycle two-color chirped laser and a static electric field

We present a theoretical investigation of high-order harmonic generation in a chirped two-color laser field, which is synthesized by a 10-fs/800-nm fundamental chirped pulse and a 10-fs/1760-nm subharmonic pulse. It is shown that a supercontinuum can be produced using the multicycle two-color chirped field. However, the supercontinuum reveals a strong modulation structure, which is not good for the generation of an isolated attosecond pulse. By adding a static electric field to the multicycle two-color chirped field, not only the harmonic cutoff is extended remarkably, but also the quantum paths of the high-order harmonic generation (HHG) are modified significantly. As a result, both the extension of the supercontinuum and the selection of a single quantum path are achieved, producing an isolated 23-as pulse with a bandwidth of about 170.6 eV. Furthermore, the influences of the laser intensities on the supercontinuum and isolated attosecond pulse generation are investigated.

Generation of Broadband Supercontinuum and Isolated Attosecond Pulse in a Chirped Two-Colour Laser Field

We present a theoretical investigation of high-order harmonic generation in a chirped two-colour laser field, which is synthesized by an 800 nm fundamental chirped pulse and a 1200 nm subharmonic chirped pulse. With the introduction of a polarization angle, both the harmonic cutoff is significantly extended and the spectrum intensity is effectively enhanced compared with the orthogonally polarized chirped two-colour field. When the polarization angle between the two chirped pulses is less than or equal to 0:27p, the broadband supercontinuum with a single quantum path contribution is achieved, and then isolated attosecond pulses with the duration of about 40 as are directly obtained, which are all linearly polarized. In addition, the influences of other laser parameters on the supercontinuum are also investigated.

Isolated 38 as pulse generation from two-color pulse

We have investigated theoretically the high-order harmonics and attosecond pulse generation by numerically solving the one-dimensional time-dependent Schrödinger equation from a helium atom in a two-color laser field, which is synthesized by adding a 1330-nm infrared pulse with higher intensity to an 800-nm fundamental pulse with lower intensity. Our results clearly show that if the phase difference between the two pulses is selected suitably, the generation of high-order harmonics spectrum with a broadband supercontinuum characteristic can be achieved, and an isolated 38 as pulse can be realized. By the time-frequency analysis, we find that the isolated attosecond pulse comes from the contribution of the long and short quantum paths, and for these two quantum paths the change of emission time with the harmonics order is rather slow. This is different from the traditional isolated attosecond pulse generation that a single quantum path needs to be picked up to obtain an isolated attosecond pulse.

Single quantum path control by a fundamental chirped pulse combined with a subharmonic control pulse

In this paper, we study the issue of single quantum path control and its role in attosecond pulse generation. By carrying out the time-dependent Schrödinger equation analysis for the harmonic emission from a single He atom irradiated by the two-color laser field, consisting of a short 800 fundamental chirped pulse and a subharmonic 800–2400nm control pulse, we find that the most favorable condition for attosecond generation is at the fundamental chirp parameter β=4.55 together with the zero-phase 2000nm control pulse, in which the single quantum path (short quantum path) is selected to contribute to the harmonic spectrum exhibiting an ultrabroad supercontinuum of a 337eV bandwidth. Finally, an isolated attosecond pulse as short as 39as is thus generated directly.

Dramatic cutoff extension and broadband supercontinuum generation in multi-cycle two color pulses

We propose a method to markedly increase the electron-recollision kinetic energy, using the combination of a 0.8 μm/13 fs driving pulse with a much weaker multi-cycle mid-infrared pulse at 10.4 μm. The results show that the synthesized field effectively lengthens the accelerated distance of electron wave packet and the harmonic cutoff is significantly extended to IP+26UP, which is covered with the water-window spectral region. In addition, only one single quantum path contributes to harmonics, and those higher than IP+15UP become continuous. This supercontinuum can support the generation of an isolated sub-100 as pulse with tunable central wavelength and also the pulse with the duration below one atomic unit of time (24 as). Moreover, our scheme can further extend to more longer driving pulses, which provides a dramatic approach for cutoff extension and broadband supercontinuum generation with multi-cycle pump pulses.

Isolated sub-10 attosecond pulse generation by a 6-fs driving pulse and a 5-fs subharmonic controlling pulse

We theoretically study high-order harmonic generation by quantum path control in a special two-color laser field, which is synthesized by a 6 fs/800 nm fundamental pulse and a weaker 5 fs/1600 nm subharmonic controlling pulse. Single quantum path is selected without optimizing any carrier phase, which not only broadens the harmonic bandwidth to 400 eV, but also enhances the harmonic conversion efficiency in comparison with the short-plus-long scheme, which is based on 5 fs/800 nm driving pulse and 6 fs/1600 nm control pulse. An isolated 8-attosecond pulse is produced with currently available ultrafast laser sources.

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.

Isolated attosecond pulse emission in the plateau region of high-order harmonics driven by a 7-fs 800-nm laser field

We investigate theoretically the high-order harmonic generation in the argon gas cell driven with a 7-fs 800-nm intense laser pulse, by taking into account the phase-matching effect during the propagation process in the gas cell. We demonstrate a successful selection of single quantum path of the returning electrons and therefore the generation of an isolated xuv attosecond pulse in the plateau region of high-order harmonic spectrum, instead of the emission in the form of pulse trains if without optimizing the phase-matching process.

Quantum path control in few-optical-cycle regime

We theoretically show that selection of a single quantum path in high-order harmonics generation can be realized in a few-optical-cycle regime with two-color schemes. We also demonstrate, in theory as well, the generation of spectrally smooth and ultrabroad extreme ultraviolet supercontinuum in argon gas which can produce single similar to 79 as pulses with currently available ultrafast laser sources. Our finding can be beneficial for generating isolated sub-100 as extreme ultraviolet pulses.

Macroscopic control of high-order harmonics quantum-path components for the generation of attosecond pulses

We present measurements of the different quantum path contributions to the high-order harmonic emission. Through spatial and spectral filtering, we evidence the strong correlation between the spatial and spectral distributions, which allows us to quantify the contribution of each quantum path. A systematic analysis as a function of the generating parameters has been done to identify the conditions for efficient generation and selection of a single quantum path. We show that combining phase matching and spatial filtering allows maximizing and selecting the short quantum path contribution, condition for the generation of ``clean'' and intense attosecond pulses.

Strong field quantum path control using attosecond pulse trains.

We show that attosecond pulse trains have a natural application in the control of strong field processes. In combination with an intense infrared laser field, the pulse train can be used to microscopically select a single quantum path contribution to a process that would otherwise consist of several interfering components. We present calculations that demonstrate this by manipulating the time-frequency properties of high order harmonics at the single atom level. This quantum path selection can also be used to define a high resolution attosecond clock.

Selection of high-order harmonics from a single quantum path for the generation of an attosecond pulse train

This work is supported by the Minstry of Science and Technology of Korea Through the Creative Research Initiative Program.