High-harmonic generation from a gas target exhibits sharp spectral features and rapid phase variation near the Cooper minimum. By applying spectral filtering, shaped isolated attosecond pulses can be generated where the pulse is split into two in the time domain. Using such shaped extreme-ultraviolet (XUV) pulses, we theoretically study attosecond transient absorption (ATA) spectra of helium 2s2p autoionizing state which is resonantly coupled to the 2s2 dark state by a time-delayed infrared laser. Our simulations show that the asymmetric 2s2p Fano line shape can be readily tuned into symmetric Lorentzian within the time delay of a few tens of attoseconds. Such efficient control is due to the destructive interference in the generation of the 2s2p state when it is excited by a strongly shaped XUV pulse. This is to be compared to prior experiments where tuning the line shape of a Fano resonance would take tens of femtoseconds. We also show that the predicted ATA spectral line shape can be observed experimentally after propagation in a gas medium. Our results suggest that strongly shaped attosecond XUV pulses offer the opportunity for controlling and probing fine features of narrow resonances on the few-ten attoseconds timescale.
In recent years, significant advancements in high-repetition-rate, high-average-power mid-infrared laser pulses have enabled the generation of tabletop high-flux coherent soft x-ray harmonics for photon-hungry experiments. However, for practical applications, it is crucial to effectively filter out the driving beam from the high harmonics. In this study, we leverage the distinctive properties of a Bessel–Gauss (BG) beam to introduce a novel approach for spatial filtering, specifically targeting soft x-ray harmonics, releasing with a high-photon flux simultaneously. Our simulations reveal that by finely adjusting the focus geometry and gas pressure, the BG beam naturally adopts an annular shape, emitting high harmonics with minimal divergence in the far field. To achieve complete spatial separation of the driving beam and harmonic emissions, we pinpoint the optimal gas pressure and focusing geometry, particularly under overdriven laser intensities, for achieving good phase matching of harmonic emissions from short-trajectory electrons within the gas medium when the exact ionization level is higher than the “critical” value. Additionally, we establish scaling relations for sustaining optimal phase-matching conditions crucial for spatially separating the driving laser and the high-harmonic field, especially as the wavelength of the driving laser increases. Furthermore, our analysis demonstrates a substantial enhancement of harmonic yields by at least one order of magnitude compared to a truncated Gaussian annular beam. We also show that under accessible experimental conditions, soft x-ray photon flux up to 1010 photons/s at 250 eV can be achieved. The utilization of the BG beam opens up a promising pathway for the development of high-flux attosecond soft x-ray light sources, poised to serve a wide range of applications.
Chromatic aberration occurs when high-order harmonic generation (HHG) is focused far away from its generation medium, exhibiting as the focal point being shifted with the harmonic order, which becomes an obstacle for HHG being a powerful table-top light source. Here, we demonstrate through simulations that chromatic aberration can be efficiently corrected by using a flat-top beam as the driving laser, which is induced by the plasma when the laser beam propagates in a gas medium. We show that the elimination of HHG chromatic aberration with the flat-top beam is robust in a gas-filled waveguide under proper gas pressure and waveguide length. We reveal that formation of the flat-top beam and reduction of HHG chromatic aberration in a gas cell is strongly dependent on its position with respect to the laser focus. HHG chromatic aberration can only be effectively reduced with the flat-top beam when the gas cell is located at the laser focus, which is attributed to the geometric phase of the driving laser. We also analyze that high harmonics are generated under the phase-matching conditions simultaneously when chromatic aberration is reduced. Finally, we show the limitation of a single-layer model by comparing its predictions with macroscopically propagated results in the gas medium. We expect that our method can be useful for focusing high harmonics in the extreme ultraviolet and soft x rays for various applications.
As a self-insulating building material which can meet the 65 percent energy-efficiency requirements in cold region of China, aerated concrete blocks often go moldy, frost heaving, or cause plaster layer hollowing at thermal bridge parts in the extremely cold regions due to the restrictions of environmental climate and construction technique. In this paper, partial insulation measures of the thermal-bridge position of these parts of aerated concrete walls are designed to weaken or even eliminate thermal bridge effect and improve the temperature of thermal-bridge position. A heat transfer calculation model for L-shaped wall and T-shaped wall is developed. Based on the simulation result, the influence of the thickness on the temperature field is analyzed. Consequently, the condensation inside self-thermal-insulating wall and frost heaving caused by condensation and low temperature will be reduced, avoiding damage to the wall body from condensation..
The concept of critical ionization fraction has been essential for high-harmonic generation, because it dictates the maximum driving laser intensity while preserving the phase matching of harmonics. In this work, we reveal a second, nonadiabatic critical ionization fraction, which substantially extends the phase-matched harmonic energy, arising because of the strong reshaping of the intense laser field in a gas plasma. We validate this understanding through a systematic comparison between experiment and theory for a wide range of laser conditions. In particular, the properties of the high-harmonic spectrum versus the laser intensity undergoes three distinctive scenarios: (i) coincidence with the single-atom cutoff, (ii) strong spectral extension, and (iii) spectral energy saturation. We present an analytical model that predicts the spectral extension and reveals the increasing importance of the nonadiabatic effects for mid-infrared lasers. These findings are important for the development of high-brightness soft x-ray sources for applications in spectroscopy and imaging.
Isolated attosecond pulses (IAPs) can be readily generated via high-order harmonic generation driven by an ultrashort laser pulse. Here, it is shown that the best way to obtain the ultrashort waveform for producing a short and intense IAP in the soft x rays is to optimize the three-color (TC) laser pulse consisting of the fundamental field and its second and third harmonic fields. To calibrate it, another way of constructing the ultrashort waveform directly in time using a truncated basis set of B-spline functions is first proposed. The calibration waveform (CW) contains more frequency components up to the eighth harmonic order. It is found that the IAP by the TC waveform has a shorter duration after macroscopic propagation in a nonlinear gas medium compared to that by the CW field. It is uncovered that the CW field is additionally modified by the higher-order frequency components during propagation, dominated by the neutral atom dispersion. The effect of phase jitter in the TC waveform and the extension of the TC scheme into higher photon energies are also discussed. Currently, precise control of TC laser waveform synthesis is already achievable in the labs, thus paving an effective way for generating a useful attosecond light source in the soft x rays.
Abstract Characterization of an isolated attosecond pulse (IAP) in the extreme ultraviolet (XUV) or soft x-ray (SXR) region is essential for its applications. Here we propose to retrieve an IAP in the time domain directly through the modulation of high-harmonic generation (HHG) spectra in the presence of a time-delayed intense few-cycle infrared or mid-infrared laser. The retrieval algorithm is derived based on the strong-field approximation and an extended quantitative rescattering model. We show that both isolated XUV pulses with a narrow spectral bandwidth and isolated SXR pulses with a broad bandwidth can be well characterized through the HHG streaking spectra. Such an all-optical method for characterizing the IAP differs from the commonly used approach based on the streaked photoelectron spectra that would require electron spectrometers. We check the robustness of the retrieval method by changing the dressing laser or by adjusting the steps of time delay. We also show that the XUV pulse can be accurately retrieved by treating the HHG streaking spectra calculated from solving the time-dependent Schrödinger equation for single atoms as the ‘experimental’ data.
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